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Sample records for detect strong excitonic

  1. Two-photon excited fluorescence from higher electronic states of chlorophylls in photosynthetic antenna complexes a new approach to detect strong excitonic chlorophyll a/b coupling

    CERN Document Server

    Leupold, D; Ehlert, J; Irrgang, K D; Renger, G; Lokstein, H

    2002-01-01

    Stepwise two-photon excitation of chlorophyll a and b in the higher plant main light-harvesting complex (LHC II) and the minor complex CP29 (as well as in organic solution) with 100-fs pulses in the Q/sub y/ region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength offers a new approach to elucidate the long-standing problem of the origin of spectral "chlorophyll forms" in pigment-protein complexes, in particular the characterization of chlorophyll a/b-heterodimers. As a first result we present evidence for the existence of strong chlorophyll a/b-interactions (excitonically coupled transitions at 650 and 680 nm) in LHC II at ambient temperature. In comparison with LHC II, the experiments with CP29 provide further evidence that the lowest energy chlorophyll a transition (at ~680 nm) is not excitonically coupled to chlorophyll b. (22 refs).

  2. Strong-field terahertz-optical mixing in excitons

    CERN Document Server

    Su, M Y; Sherwin, M S; Huntington, A S; Coldren, L A

    2002-01-01

    Driving a double-quantum-well excitonic intersubband resonance with a terahertz (THz) electric field of frequency \\omega_{THz} generated terahertz optical sidebands \\omega=\\omega_{THz}+\\omega_{NIR} on a weak NIR probe. At high THz intensities, the intersubband dipole energy which coupled two excitons was comparable to the THz photon energy. In this strong-field regime the sideband intensity displayed a non-monotonic dependence on the THz field strength. The oscillating refractive index which gives rise to the sidebands may be understood by the formation of Floquet states, which oscillate with the same periodicity as the driving THz field.

  3. Dynamic Stark effect in strongly coupled microcavity exciton polaritons.

    Science.gov (United States)

    Hayat, Alex; Lange, Christoph; Rozema, Lee A; Darabi, Ardavan; van Driel, Henry M; Steinberg, Aephraim M; Nelsen, Bryan; Snoke, David W; Pfeiffer, Loren N; West, Kenneth W

    2012-07-20

    We present experimental observations of a nonresonant dynamic Stark shift in strongly coupled microcavity quantum well exciton polaritons--a system which provides a rich variety of solid-state collective phenomena. The Stark effect is demonstrated in a GaAs/AlGaAs system at 10 K by femtosecond pump-probe measurements, with the blueshift approaching the meV scale for a pump fluence of 2  mJ cm(-2) and 50 meV red detuning, in good agreement with theory. The energy level structure of the strongly coupled polariton Rabi doublet remains unaffected by the blueshift. The demonstrated effect should allow generation of ultrafast density-independent potentials and imprinting well-defined phase profiles on polariton condensates, providing a powerful tool for manipulation of these condensates, similar to dipole potentials in cold-atom systems.

  4. Strong Exciton-photon Coupling in Semiconductor Microcavities

    DEFF Research Database (Denmark)

    Jensen, Jacob Riis; Borri, Paola; Hvam, Jørn Märcher

    1999-01-01

    directionality of the spontaneous emission in the diodes.At low temperatures, a strong coupling between the excitons in the quantum well and the electromagnetic field in the cavity may be achieved. This coupling leads to new quantum mechanical states, so-called polaritons, the properties of which are still......The basic building block of vertical cavity surface emitting lasers (VCSELs) and high efficiency diodes, is a quantum well embedded in a semiconductor microcavity. The high finesse that may be achieved in such a cavity is utilised to get a low threshold current in the VCSELs and a high...... place. Due to the steep dispersion, the amount of phase space available for polariton scattering is reduced, yielding longer dephasing times and hence narrower lines. The possibility of tailoring the polariton dispersion in order to reduce the line width is very interesting, for instance for all...

  5. Effect of Strong Orbital Magnetic Field on the Exciton Condensation in an Extended Falicov Kimball Model

    Science.gov (United States)

    Pradhan, S.; Taraphder, A.

    2016-10-01

    A spinless, extended Falicov-Kimball model in the presence of a perpendicular magnetic field is investigated employing a self-consistent mean-field theory in two dimensions. In the presence of the field the excitonic average Δ = is modified: the exciton responds in subtle different ways for different values of the magnetic flux. We examine the effects of Coulomb interaction and hybridization between the localized and itinerant electrons on the excitonic average, for rational values of the applied magnetic field. The excitonic average is found to get enhanced exponentially with the Coulomb interaction while it saturates at large hybridization. The orbital magnetic field suppresses the excitonic average in general, though a strong commensurability effect of the magnetic flux on the behaviour of the excitonic order parameter is observed.

  6. Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities

    Science.gov (United States)

    Graf, Arko; Tropf, Laura; Zakharko, Yuriy; Zaumseil, Jana; Gather, Malte C.

    2016-10-01

    Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix and a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (>110 meV), efficient polariton relaxation and narrow band emission (devices operating at telecommunication wavelengths.

  7. Strong Photocurrent from Two-Dimensional Excitons in Solution-Processed Stacked Perovskite Semiconductor Sheets.

    Science.gov (United States)

    Ahmad, Shahab; Kanaujia, Pawan K; Beeson, Harry J; Abate, Antonio; Deschler, Felix; Credgington, Dan; Steiner, Ullrich; Prakash, G Vijaya; Baumberg, Jeremy J

    2015-11-18

    Room-temperature photocurrent measurements in two-dimensional (2D) inorganic-organic perovskite devices reveal that excitons strongly contribute to the photocurrents despite possessing binding energies over 10 times larger than the thermal energies. The p-type (C6H9C2H4NH3)2PbI4 liberates photocarriers at metallic Schottky aluminum contacts, but incorporating electron- and hole-transport layers enhances the extracted photocurrents by 100-fold. A further 10-fold gain is found when TiO2 nanoparticles are directly integrated into the perovskite layers, although the 2D exciton semiconducting layers are not significantly disrupted. These results show that strong excitonic materials may be useful as photovoltaic materials despite high exciton binding energies and suggest mechanisms to better understand the photovoltaic properties of the related three-dimensional perovskites.

  8. Photon echo in exciton-plasmon nanomaterials: a signature of strong coupling

    CERN Document Server

    Blake, Adam

    2016-01-01

    The results of rigorous numerical simulations of photon echoes in exciton-plasmon systems are presented. Using a self-consistent model based on coupled Maxwell-Bloch equations we investigate femtosecond time dynamics of ensembles of interacting molecules and molecular aggregates optically coupled to surface-plasmon supporting materials. It is shown that observed photon echoes under two pulse pump-probe sequence are highly dependent on various material parameters such as molecular concentration and periodicity. Simulations of photon echoes in exciton-plasmon materials reveal a unique signature of the strong exciton-plasmon coupling, namely a double-peak structure in spectra of recorded echo signals. This phenomenon is shown to be related to hybrid states (upper and lower polaritons) in exciton-plasmon systems under strong coupling conditions. It is also demonstrated that the double-peak echo is highly sensitive to mild deviations of the coupling from the resonance between molecules and plasmons making it a gre...

  9. Density of states of Frenkel excitons in strongly disordered two-dimensional systems

    Science.gov (United States)

    Siemann, Robert; Boukahil, Abdelkrim

    2014-03-01

    We present the calculation of the density of states of Frenkel excitons in strongly disordered two-dimensional systems. A random distribution of transition frequencies with variance σ2 characterizes the disorder. The Coherent Potential Approximation (CPA) calculations show a strong dependence of the density of states (DOS) on the disorder parameter σ.

  10. Strong coupling of two interacting excitons confined in a nanocavity-quantum dot system

    Energy Technology Data Exchange (ETDEWEB)

    Cardenas, Paulo C; RodrIguez, Boris A [Instituto de Fisica, Universidad de Antioquia, AA 1226 MedellIn (Colombia); Quesada, Nicolas [McLennan Physical Laboratories, University of Toronto, 60 St George Street, Toronto, ON, M5S 1A7 (Canada); Vinck-Posada, Herbert, E-mail: pcardenas@fisica.udea.edu.co [Departamento de Fisica, Universidad Nacional de Colombia, Ciudad Universitaria, Bogota (Colombia)

    2011-07-06

    We present a study of the strong coupling between radiation and matter, considering a system of two quantum dots, which are in mutual interaction and interact with a single mode of light confined in a semiconductor nanocavity. We take into account dissipative mechanisms such as the escape of the cavity photons, decay of the quantum dot excitons by spontaneous emission, and independent exciton pumping. It is shown that the mutual interaction between the dots can be measured off-resonance only if the strong coupling condition is reached. Using the quantum regression theorem, a reasonable definition of the dynamical coupling regimes is introduced in terms of the complex Rabi frequency. Finally, the emission spectrum for relevant conditions is presented and compared with the above definition, demonstrating that the interaction between the excitons does not affect the strong coupling.

  11. Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities

    Science.gov (United States)

    Graf, Arko; Tropf, Laura; Zakharko, Yuriy; Zaumseil, Jana; Gather, Malte C.

    2016-01-01

    Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix and a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (>110 meV), efficient polariton relaxation and narrow band emission (<15 meV). Given their high charge carrier mobility and excellent photostability, SWCNTs represent a promising new avenue towards practical exciton-polariton devices operating at telecommunication wavelengths. PMID:27721454

  12. Radiative recombination of localized excitons and mobility edge excitons in GaInNAs/GaAs quantum wells with strong carrier localization

    Energy Technology Data Exchange (ETDEWEB)

    Zhao, Q.X. [Physical Electronics and Photonics, Department of Physics, Chalmers University of Technology and Goeteborg University, SE-412 96 Goeteborg (Sweden)]. E-mail: zhao@fly.chalmers.se; Wang, S.M. [Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Wei, Y.Q. [Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Sadeghi, M. [Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Larsson, A. [Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden); Willander, M. [Physical Electronics and Photonics, Department of Physics, Chalmers University of Technology and Goeteborg University, SE-412 96 Goeteborg (Sweden)

    2005-06-20

    The radiative recombination in In{sub x}Ga{sub 1-x}N{sub 0.01}As{sub 0.99}/GaAs quantum well structures exhibiting strong carrier localization was investigated by optical spectroscopy. For In-concentration from 0 to 30%, the results indicate that the degree of carrier localization decreases with increasing In-concentration. At temperatures below 100 K, the mobility edge excitons as well as localized excitons are identified and their transitions energies strongly depend on the excitation intensity. At elevated temperatures the localized excitons become quenched. The temperature dependence of the photoluminescence emission energy shows different behaviors at different excitation intensities.

  13. Communication: Strong excitonic and vibronic effects determine the optical properties of Li₂O₂

    DEFF Research Database (Denmark)

    García Lastra, Juan Maria; Bass, J. D.; Thygesen, Kristian Sommer

    2011-01-01

    The band structure and optical absorption spectrum of lithium peroxide (Li2O2) is calculated from first-principles using the G0W0 approximation and the Bethe-Salpeter equation, respectively. A strongly localized (Frenkel type) exciton corresponding to the π*→σ* transition on the O2 −2 peroxide io...

  14. Strong coupling in porphyrin J-aggregate excitons and plasmons in nano-void arrays

    Science.gov (United States)

    Ferdele, Stefano; Jose, Bincy; Foster, Robert; Keyes, Tia E.; Rice, James H.

    2017-10-01

    Active plasmonic nano-void arrays made through colloidal lithography (a cost effective and rapid process) potentially offers opportunities for scalable device design. In this work we demonstrate strong coupling between Bragg-like quadrupole surface plasmon modes in nano-void substrate designs with Frankel excitons in a molecular J-aggregate layer though angular tuning. The enhanced exciton-plasmon coupling creates a Fano like line shape in the differential reflection spectra associated with the formation of new hybrid states, leading to anti-crossing of the upper and lower polaritons with a Rabi frequency of 120 meV.

  15. Strong coupling between Tamm plasmon polariton and two dimensional semiconductor excitons

    CERN Document Server

    Hu, Tao; Wu, Lin; Zhang, Long; Shan, Yuwei; Lu, Jian; Wang, Jun; Luo, Song; Zhang, Zhe; Liao, Liming; Wu, Shiwei; Shen, S C; Chen, Zhanghai

    2016-01-01

    Two dimensional (2D) semiconductor materials of transition-metal dichalcogenides (TMDCs) manifest many peculiar physical phenomena in the light-matter interaction. Due to their ultrathin property, strong interaction with light and the robust excitons at room temperature, they provide a perfect platform for studying the physics of strong coupling in low dimension and at room temperature. Here we report the strong coupling between 2D semiconductor excitons and Tamm plasmon polaritons (TPPs). We observe a Rabi splitting of about 54 meV at room temperature by measuring the angle resolved differential reflectivity spectra and simulate the theoretical results by using the transfer matrix method. Our results will promote the realization of the TPP based ultrathin polariton devices at room temperature.

  16. Dynamical Linear Response of TDDFT with LDA+U Functional: Strongly Hybridized Frenkel Excitons in NiO

    Energy Technology Data Exchange (ETDEWEB)

    Lee, C.C.; Ku, W.; Hsueh, H.C.

    2010-08-30

    Within the framework of time-dependent density-functional theory (TDDFT), we derive the dynamical linear response of local-density approximation plus U functional and benchmark it on NiO, a prototypical Mott insulator. Formulated using real-space Wannier functions, our computationally inexpensive framework gives detailed insights into the formation of tightly bound Frenkel excitons with reasonable accuracy. Specifically, a strong hybridization of multiple excitons is found to significantly modify the exciton properties. Furthermore, our study exposes a significant generic limitation of adiabatic approximation in TDDFT with hybrid functionals and in existing Bethe-Salpeter-equation approaches, advocating the necessity of strongly energy-dependent kernels in future development.

  17. Influences of strong exciton-phonon interaction on two coupled quantum dots within cavity QED

    Energy Technology Data Exchange (ETDEWEB)

    Yuan Xiaozhong [Department of Physics, Institute of Quantum Optics and Quantum Information, Shanghai Jiao Tong University, Shanghai 200240 (China)]. E-mail: yxz@sjtu.edu.cn; Zhu Kadi [Department of Physics, Institute of Quantum Optics and Quantum Information, Shanghai Jiao Tong University, Shanghai 200240 (China); Li Waisang [Department of Electronic and Information Engineering, Hong Kong Polytechnic University, Hong Kong (China)

    2004-08-30

    For two coupled quantum dots within cavity QED, we show that the exciton-phonon interaction reduces the Rabi frequency and Foerster interaction even at absolute zero temperature. The exciton-phonon interaction also makes an additional contribution to the static exciton-exciton dipole interaction energy.

  18. Influences of strong exciton-phonon interaction on two coupled quantum dots within cavity QED

    Science.gov (United States)

    Yuan, Xiao-Zhong; Zhu, Ka-Di; Li, Wai-Sang

    2004-08-01

    For two coupled quantum dots within cavity QED, we show that the exciton-phonon interaction reduces the Rabi frequency and Förster interaction even at absolute zero temperature. The exciton-phonon interaction also makes an additional contribution to the static exciton-exciton dipole interaction energy.

  19. Observation of Mode Splitting in Photoluminescence of Individual Plasmonic Nanoparticles Strongly Coupled to Molecular Excitons.

    Science.gov (United States)

    Wersäll, Martin; Cuadra, Jorge; Antosiewicz, Tomasz J; Balci, Sinan; Shegai, Timur

    2017-01-11

    Plasmon-exciton interactions are important for many prominent spectroscopic applications such as surface-enhanced Raman scattering, plasmon-mediated fluorescence, nanoscale lasing, and strong coupling. The case of strong coupling is analogous to quantum optical effects studied in solid state and atomic systems previously. In plasmonics, similar observations have been almost exclusively made in elastic scattering experiments; however, the interpretation of these experiments is often cumbersome. Here, we demonstrate mode splitting not only in scattering, but also in photoluminescence of individual hybrid nanosystems, which manifests a direct proof of strong coupling in plasmon-exciton nanoparticles. We achieved these results due to saturation of the mode volume with molecular J-aggregates, which resulted in splitting up to 400 meV, that is, ∼20% of the resonance energy. We analyzed the correlation between scattering and photoluminescence and found that splitting in photoluminescence is considerably less than that in scattering. Moreover, we found that splitting in both photoluminescence and scattering signals increased upon cooling to cryogenic temperatures. These findings improve our understanding of strong coupling phenomena in plasmonics.

  20. Strong photocurrent from 2D excitons in solution-processed stacked perovskite semiconductor sheets

    OpenAIRE

    Ahmad, Shahab; Kanaujia, Pawan Kumar; Beeson, Harry J.; Abate, Antonio; Deschler, Felix; Credgington, Dan; Steiner, Ullrich; Prakash, G. Vijaya; Baumberg, Jeremy

    2015-01-01

    This is the final version of the article. It was first available from ACS via http://dx.doi.org/10.1021/acsami.5b07026 Room-temperature photocurrent measurements in two-dimensional inorganic-organic perovskite devices reveal that excitons strongly contribute to the photocurrents despite possessing binding energies over ten times larger than thermal energies. The p-type (C?H?C?H4NH?)?PbI? [CHPI] liberates photocarriers at metallic Schottky Al contacts, but incorporating electron and hole tr...

  1. Transformation Optics Approach to Plasmon-Exciton Strong Coupling in Nanocavities

    Science.gov (United States)

    Li, Rui-Qi; Hernángomez-Pérez, D.; García-Vidal, F. J.; Fernández-Domínguez, A. I.

    2016-09-01

    We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. Inspired by transformation optics ideas, a quasianalytical approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity.

  2. Mode Modification of Plasmonic Gap Resonances induced by Strong Coupling with Molecular Excitons

    CERN Document Server

    Chen, Xingxing; Qin, Jian; Zhao, Ding; Ding, Boyang; Blaikie, Richard J; Qiu, Min

    2016-01-01

    Plasmonic cavities can be used to control the atom-photon coupling process at the nanoscale, since they provide ultrahigh density of optical states in an exceptionally small mode volume. Here we demonstrate strong coupling between molecular excitons and plasmonic resonances (so-called plexcitonic coupling) in a film-coupled nanocube cavity, which can induce profound and significant spectral and spatial modifications to the plasmonic gap modes. Within the spectral span of a single gap mode in the nanotube-film cavity with a 3-nm wide gap, the introduction of narrow-band J-aggregate dye molecules not only enables an anti-crossing behavior in the spectral response, but also splits the single spatial mode into two distinct modes that are easily identified by their far-field scattering profiles. Simulation results confirm the experimental findings and the sensitivity of the plexcitonic coupling is explored using digital control of the gap spacing. Our work opens up a new perspective to study the strong coupling pr...

  3. Active control of the strong coupling regime between porphyrin excitons and surface plasmon polaritons

    NARCIS (Netherlands)

    Berrier, A.; Cools, R.; Arnold, C.; Offermans, P.; Crego-Calama, M.; Brongersma, S.H.; Gomez-Rivas, J.

    2011-01-01

    We experimentally demonstrate the active control of the coupling strength between porphyrin dyes and surface plasmon polaritons supported by a thin gold layer. This control is externally exerted by a gas flow and is reversible. The hybridized exciton-polariton branches resulting from the

  4. Interband Optical Transitions due to Donor Bound Excitons in Wurtzite InGaN Strained Coupled Quantum Dots: Strong Built-in Electric Field Effects

    Institute of Scientific and Technical Information of China (English)

    CHI Yue-Meng; SHI Jun-Jie

    2006-01-01

    @@ Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field (BEF) in the wurtzite InGaN strained coupled quantum dots (QDs), the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF,and the structure parameters of the QD system.

  5. Bose-Einstein condensation of dipolar excitons in quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Timofeev, V B; Gorbunov, A V, E-mail: timofeev@issp.ac.r [Institute of Solid State Physics, Russian Academy of Sciences, 142432, Chernogolovka, Moscow region (Russian Federation)

    2009-02-01

    The experiments on Bose-Einstein condensation (BEC) of dipolar (spatially-indirect) excitons in the lateral traps in GaAs/AlGaAs Schottky-diode heterostructures with double and single quantum wells are presented. The condensed part of dipolar excitons under detection in the far zone is placed in k-space in the range which is almost two orders of magnitude less than thermal exciton wave vector. BEC occurs spontaneously in a reservoir of thermalized excitons. Luminescence images of Bose-condensate of dipolar excitons exhibit along perimeter of circular trap axially symmetrical spatial structures of equidistant bright spots which strongly depend on excitation power and temperature. By means of two-beam interference experiments with the use of cw and pulsed photoexcitation it was found that the state of dipolar exciton Bose-condensate is spatially coherent and the whole patterned luminescence configuration in real space is described by a common wave function.

  6. Forbidden singlet exciton transitions induced by localization in polymer light-emitting diodes in a strong electric field.

    Science.gov (United States)

    Sun, Zheng; Xu, Yuan-Ping; Li, Sheng; George, Thomas F

    2011-02-10

    Through combining the electron transition process and dipole moment evolution as well as electron-phonon coupling, molecular dynamics calculations show that the radiative decay of singlet excitons in a conjugated polymer, such as a polymer light-emitting diode (PLED), is largely determined by the evolution of the dipole moment. Without an electric field, the decay life of a singlet exciton is about 1 ns. Once an electric field is applied and exceeds a critical value, with electron-phonon coupling, the original lattice structure evolves into two new localized lattice distortions, consistent with the experimental results. Owing to the new lattice structure and self-trapping, the dipole moment rapidly decreases to zero within 5 fs, eliminating the radiative decay of the singlet exciton.

  7. Van der Waals Interactions and Exciton Condensation

    Science.gov (United States)

    Handel, P. H.; Kittel, C.

    1971-01-01

    It is shown that the van der Waals interaction can lead at low temperatures to a condensed state of excitons with properties in qualitative agreement with the observations of exciton droplets. Our calculation gives a binding energy of the correct sign and magnitude for the exciton condensate. In a diclectric medium, the strong enhancement of the exciton polarizability leads to a giant van der Waals interaction, and this interaction appears to make possible a condensed exciton phase. PMID:16591958

  8. Editorial on indirect excitons: Physics and applications

    Science.gov (United States)

    2017-08-01

    This special issue contains 9 original review papers, research papers and discussion papers on indirect excitons. An exciton is a Coulomb-correlated electron-hole pair. Frenkel excitons dominate optical properties of organic semiconductors, while Wannier-Mott excitons are responsible for the hydrogen-like absorption spectra of inorganic semiconductors at low temperatures. The interest to the physics of excitons has strongly increased in the new century. This interest is motivated by unique bosonic properties of excitons that lead to the phenomena of exciton-polariton lasing and stimulated scattering, build-up of the spontaneous coherence and polarisation in cold exciton gases. In addition to the rich fundamental physics, excitons offer the perspective of applications in opto-electronic devices such as exciton transistors, switches, optical integrated circuits, etc.

  9. Kinetic theory of exciton-exciton annihilation.

    Science.gov (United States)

    May, Volkhard

    2014-02-07

    Weakly excited states of dye aggregates and supramolecular complexes can be characterized by single or two exciton states. Stronger excitation results in the presence of multiple excited molecules, and complex processes of internal energy transfer dynamics take place. The direct consideration of all excited states is limited to systems with a few molecules only. Therefore, an approach is used based on transition operators among the molecular states of interest and resulting in a dynamic theory for excitation energy transfer in strongly excited molecular systems. As a first application of this theory a detailed description of exciton-exciton annihilation is given. The obtained novel nonlinear theory is related to the standard description. Possible further approximation schemes in the offered theoretical framework are discussed.

  10. Strong terahertz field generation, detection, and application

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Ki-Yong [Univ. of Maryland, College Park, MD (United States)

    2016-05-22

    This report describes the generation and detection of high-power, broadband terahertz (THz) radiation with using femtosecond terawatt (TW) laser systems. In particular, this focuses on two-color laser mixing in gases as a scalable THz source, addressing both microscopic and macroscopic effects governing its output THz yield and radiation profile. This also includes the characterization of extremely broad THz spectra extending from microwaves to infrared frequencies. Experimentally, my group has generated high-energy (tens of microjoule), intense (>8 MV/cm), and broadband (0.01~60 THz) THz radiation in two-color laser mixing in air. Such an intense THz field can be utilized to study THz-driven extremely nonlinear phenomena in a university laboratory.

  11. Strong terahertz field generation, detection, and application

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Ki-Yong [University of Maryland, College Park

    2016-05-15

    This report describes the generation and detection of high-power, broadband terahertz (THz) radiation with using femtosecond terawatt (TW) laser systems. In particular, this focuses on two-color laser mixing in gases as a scalable THz source, addressing both microscopic and macroscopic effects governing its output THz yield and radiation profile. This also includes the characterization of extremely broad THz spectra extending from microwaves to infrared frequencies. Experimentally, my group has generated high-energy (tens of microjoule), intense (>8 MV/cm), and broadband (0.01~60 THz) THz radiation in two-color laser mixing in air. Such an intense THz field can be utilized to study THz-driven extremely nonlinear phenomena in a university laboratory.

  12. Exciton luminescence in BaFCl crystal

    Energy Technology Data Exchange (ETDEWEB)

    Radzhabov, E

    1998-05-01

    The luminescence spectra and decay characteristics of both 5.45 and 3.4 eV bands in BaFCl oxygen-free crystals were investigated at 5-300 K temperature range using vacuum ultraviolet excitation as well as X-ray excitation. The similarities with excitons in alkali halides allow us to consider both excitons in BaFCl as on-center excitons and strong off-center excitons.

  13. Exciton Transport in Organic Semiconductors

    Science.gov (United States)

    Menke, Stephen Matthew

    Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (L D) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on L D are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.

  14. Multiscale photosynthetic exciton transfer

    CERN Document Server

    Ringsmuth, A K; Stace, T M; 10.1038/nphys2332

    2012-01-01

    Photosynthetic light harvesting provides a natural blueprint for bioengineered and biomimetic solar energy and light detection technologies. Recent evidence suggests some individual light harvesting protein complexes (LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction centers (RCs) via an interplay between excitonic quantum coherence, resonant protein vibrations, and thermal decoherence. The role of coherence in vivo is unclear however, where excitons are transferred through multi-LHC/RC aggregates over distances typically large compared with intra-LHC scales. Here we assess the possibility of long-range coherent transfer in a simple chromophore network with disordered site and transfer coupling energies. Through renormalization we find that, surprisingly, decoherence is diminished at larger scales, and long-range coherence is facilitated by chromophoric clustering. Conversely, static disorder in the site energies grows with length scale, forcing localization. Our results suggest s...

  15. Relaxation of nonthermal hh and lh excitons in ZnSe quantum wells

    DEFF Research Database (Denmark)

    Kalt, H.; Hoffmann, J.; Umlauff, M.

    1998-01-01

    The strong exciton-LO phonon coupling in ZnSe QWs gives a direct access to the relaxation dynamics of nonthermal, free heavy-hole and light-hole excitons. Narrow hot-exciton distributions can be generated by LO-phonon assisted exciton formation. The thermalization of these excitons is monitored...

  16. Exciton size and quantum transport in nanoplatelets

    Energy Technology Data Exchange (ETDEWEB)

    Pelzer, Kenley M., E-mail: kpelzer@anl.gov; Gray, Stephen K. [Center for Nanoscale Materials, Argonne National Laboratory, 9700 Cass Ave., Argonne, Illinois 60439 (United States); Darling, Seth B. [Center for Nanoscale Materials, Argonne National Laboratory, 9700 Cass Ave., Argonne, Illinois 60439 (United States); Institute for Molecular Engineering, University of Chicago, 5747 S. Ellis Ave., Chicago, Illinois 60637 (United States); Schaller, Richard D. [Center for Nanoscale Materials, Argonne National Laboratory, 9700 Cass Ave., Argonne, Illinois 60439 (United States); Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208 (United States)

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  17. Exciton size and quantum transport in nanoplatelets.

    Science.gov (United States)

    Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  18. Exciton-polaritons in Bragg gratings

    Energy Technology Data Exchange (ETDEWEB)

    Creatore, C [Department of Physics ' A. Volta' , Universita di Pavia, via Bassi 6, I-27100, Pavia (Italy); Mouchliadis, L; Langbein, W [School of Physics and Astronomy, Cardiff University, The Parade, CF24 3AA, Cardiff (United Kingdom); Biancalana, F [Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1/Bau 24, 91058 Erlangen (Germany); Osborne, S, E-mail: creatore@fisicavolta.unipv.i [Tyndall National Institute, Lee Maltings, Prospect Row, Cork (Ireland)

    2010-02-01

    We study the strong coupling between photons and bulk excitons in a one-dimensional Bragg grating. The dispersion of the resulting Bragg-polariton states resembles the dispersion of quantum-well microcavity polaritons. We report on a parametric scattering process at two 'magic frequencies' occurring due to the strong excitonic nonlinearity.

  19. Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

    Science.gov (United States)

    Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.

    2016-06-01

    We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

  20. Tight-binding approach to excitons bound to monolayer impurity planes: strong radiative properties of InAs in GaAs

    OpenAIRE

    Iotti, Rita Claudia

    1998-01-01

    A theory of Wannier-Mott excitons bound to monolayer (ML) impurity planes in semiconductors, which is based on Green's function tight-binding calculations of the single-particle states, is presented. Binding energies and oscillator strengths for one and two MLs of InAs in GaAs are predicted to be much larger than in the usual InxGa1-xAs/GaAs thick quantum wells. The reason is the increase of effective mass of both carriers due to folding of the InAs bands along the growth direction. The resul...

  1. Giant exciton Fano resonance in quasi-one-dimensional Ta2NiSe5

    Science.gov (United States)

    Larkin, T. I.; Yaresko, A. N.; Pröpper, D.; Kikoin, K. A.; Lu, Y. F.; Takayama, T.; Mathis, Y.-L.; Rost, A. W.; Takagi, H.; Keimer, B.; Boris, A. V.

    2017-05-01

    We report the complex dielectric function of the quasi-one-dimensional chalcogenide Ta2NiSe5 , which undergoes a structural phase transition presumably associated with exciton condensation below Tc=326 K [Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009), 10.1103/PhysRevLett.103.026402; Y. F. Lu et al., Nat. Commun. 8, 14408 (2017), 10.1038/ncomms14408], and of the isostructural Ta2NiS5 , which does not exhibit such a transition. Using spectroscopic ellipsometry, we have detected exciton doublets with pronounced Fano line shapes in both the compounds. The exciton Fano resonances in Ta2NiSe5 display an order-of-magnitude higher intensity than those in Ta2NiS5 . In conjunction with prior theoretical work [E. Rashba, Sov. Phys. Semicond. 8, 807 (1975)], we attribute this observation to the giant oscillator strength of spatially extended exciton-phonon bound states in Ta2NiSe5 . The formation of exciton-phonon complexes in Ta2NiS5 and Ta2NiSe5 is confirmed by the pronounced temperature dependence of sharp interband transitions in the optical spectra, the peak energies and widths of which scale with the thermal population of optical phonon modes. The description of the optically excited states in terms of strongly overlapping exciton complexes is in good agreement with the hypothesis of an exciton insulator ground state.

  2. Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons

    Science.gov (United States)

    Zhou, You; Scuri, Giovanni; Wild, Dominik S.; High, Alexander A.; Dibos, Alan; Jauregui, Luis A.; Shu, Chi; de Greve, Kristiaan; Pistunova, Kateryna; Joe, Andrew Y.; Taniguchi, Takashi; Watanabe, Kenji; Kim, Philip; Lukin, Mikhail D.; Park, Hongkun

    2017-09-01

    Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin-orbit coupling and spin-valley degrees of freedom. Depending on the spin configuration of the electron-hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication.

  3. Plasmon-Exciton-Polariton Lasing

    CERN Document Server

    Ramezani, Mohammad; Fernández-Domínguez, Antonio I; Feist, Johannes; Rodriguez, Said Rahimzadeh-Kalaleh; Garcia-Vidal, Francisco J; Gómez-Rivas, Jaime

    2016-01-01

    Strong coupling of Frenkel excitons with surface plasmons leads to the formation of bosonic quasi-particles known as plasmon-exciton-polaritons (PEPs).Localized surface plasmons in nanoparticles are lossy due to radiative and nonradiative decays, which has hampered the realization of polariton lasing in a plasmonic system, i.e., PEP lasing. These losses can be reduced in collective plasmonic resonances supported by arrays of nanoparticles. Here we demonstrate PEP lasing in arrays of silver nanoparticles by showing the emergence of a threshold in the photoluminescence accompanied by both a superlinear increase of the emission and spectral narrowing. We also observe a reduction of the threshold by increasing the coupling between the molecular excitons and the resonances supported by the array despite the reduction of the quantum efficiency of the emitters. The coexistence of bright and dark collective modes in this plasmonic system allows for a 90?-change of polarization in the emission beyond the threshold.

  4. Excitonic polaritons in Fibonacci quasicrystals.

    Science.gov (United States)

    Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M

    2008-09-29

    The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission.

  5. Biexcitonic effects in the coherent control of the excitonic polarization detected in six-wave-mixing signals

    Science.gov (United States)

    Voss, T.; Breunig, H. G.; Rückmann, I.; Gutowski, J.; Axt, V. M.; Kuhn, T.

    2002-10-01

    Coherently controlled six-wave-mixing experiments on a ZnSe single quantum well are performed with different polarizations of the excitation pulses to enable or disable the creation of a biexcitonic polarization. A significant contribution of the biexcitonic polarization and other two-pair correlations is observed in the signal which is mainly modulated with the exciton transition frequency. A microscopic theory based on the dynamics controlled truncation scheme including six-point correlations is able to reproduce the experimental results.

  6. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter

    2011-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... is demonstrated and the influence of disorder is discussed. The findings have a strong bearing on future nanophotonic devices....

  7. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter

    2010-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... is demonstrated and the influence of disorder is discussed. The findings have a strong bearing on future nanophotonic devices....

  8. Strong Electron–Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A 3 M 2 I 9 (A = Cs, Rb; M = Bi, Sb)

    Energy Technology Data Exchange (ETDEWEB)

    McCall, Kyle M.; Stoumpos, Constantinos C.; Kostina, Svetlana S.; Kanatzidis, Mercouri G.; Wessels, Bruce W.

    2017-04-26

    The optical and electronic properties of Bridgman grown single crystals of the wide-bandgap semiconducting defect halide perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) have been investigated. Intense Raman scattering was observed at room temperature for each compound, indicating high polarizability and strong electron–phonon coupling. Both low-temperature and room-temperature photoluminescence (PL) were measured for each compound. Cs3Sb2I9 and Rb3Sb2I9 have broad PL emission bands between 1.75 and 2.05 eV with peaks at 1.96 and 1.92 eV, respectively. The Cs3Bi2I9 PL spectra showed broad emission consisting of several overlapping bands in the 1.65–2.2 eV range. Evidence of strong electron–phonon coupling comparable to that of the alkali halides was observed in phonon broadening of the PL emission. Effective phonon energies obtained from temperature-dependent PL measurements were in agreement with the Raman peak energies. A model is proposed whereby electron–phonon interactions in Cs3Sb2I9, Rb3Sb2I9, and Cs3Bi2I9 induce small polarons, resulting in trapping of excitons by the lattice. The recombination of these self-trapped excitons is responsible for the broad PL emission. Rb3Bi2I9, Rb3Sb2I9, and Cs3Bi2I9 exhibit high resistivity and photoconductivity response under laser photoexcitation, indicating that these compounds possess potential as semiconductor hard radiation detector materials.

  9. Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes

    OpenAIRE

    Spataru, Catalin D.; Ismail-Beigi, Sohrab; Capaz, Rodrigo B.; Louie, Steven G.

    2005-01-01

    We present theoretical analysis and first-principles calculation of the radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic lifetime of the order of 10 ps is computed for the lowest optically active bright excitons. The intrinsic lifetime is however a rapid increasing function of the exciton momentum. Moreover, the electronic structure of the nanotubes dictates the existence of dark excitons nearby in energy to each bright exciton. Both effects strongly influence m...

  10. Bose-Einstein condensation and indirect excitons: a review.

    Science.gov (United States)

    Combescot, Monique; Combescot, Roland; Dubin, François

    2017-06-01

    We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath

  11. Bose-Einstein condensation and indirect excitons: a review

    Science.gov (United States)

    Combescot, Monique; Combescot, Roland; Dubin, François

    2017-06-01

    We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is ‘gray’. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath

  12. The confinement effect in spherical inhomogeneous quantum dots and stability of excitons

    Directory of Open Access Journals (Sweden)

    F. Benhaddou

    2017-06-01

    Full Text Available We investigate in this work the quantum confinement effect of exciton in spherical inhomogeneous quantum dots IQDs. The spherical core is enveloped by two shells. The inner shell is a semiconductor characterized by a small band-gap. The core and the outer shell are the same semiconductor characterized by a large band-gap. So there is a significant gap-offset creating a deep potential well where the excitons are localized and strongly confined. We have adopted the Ritz variational method to calculate numerically the excitonic ground state energy and its binding energy in the strong, moderate and low confinement regimes. The results show that the Ritz variational method is in good agreement with the perturbation method in strong confinement. There is a double confinement effect and dual control. The calculation checks the effective Rydberg R* at the asymptotic limit of bulk semiconductor when the thickness takes very large values. The excitonic binding energy increases, Thus giving the excitons a high stability even at ambient temperature. These nanosystems are promising in several applications: lighting, detection, biological labeling and quantum computing.

  13. The confinement effect in spherical inhomogeneous quantum dots and stability of excitons

    Science.gov (United States)

    Benhaddou, F.; Zorkani, I.; Jorio, A.

    2017-06-01

    We investigate in this work the quantum confinement effect of exciton in spherical inhomogeneous quantum dots IQDs. The spherical core is enveloped by two shells. The inner shell is a semiconductor characterized by a small band-gap. The core and the outer shell are the same semiconductor characterized by a large band-gap. So there is a significant gap-offset creating a deep potential well where the excitons are localized and strongly confined. We have adopted the Ritz variational method to calculate numerically the excitonic ground state energy and its binding energy in the strong, moderate and low confinement regimes. The results show that the Ritz variational method is in good agreement with the perturbation method in strong confinement. There is a double confinement effect and dual control. The calculation checks the effective Rydberg R* at the asymptotic limit of bulk semiconductor when the thickness takes very large values. The excitonic binding energy increases, Thus giving the excitons a high stability even at ambient temperature. These nanosystems are promising in several applications: lighting, detection, biological labeling and quantum computing.

  14. <strong>Active Fault Detection and Isolation for Hybrid Systems strong>

    DEFF Research Database (Denmark)

    Gholami, Mehdi; Schiøler, Henrik; Bak, Thomas

    2009-01-01

    An algorithm for active fault detection and isolation is proposed. In order to observe the failure hidden due to the normal operation of the controllers or the systems, an optimization problem based on minimization of test signal is used. The optimization based method imposes the normal and fault...... to the two tank bench mark example in presence of two faults....

  15. Exciton in type-II quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Sierra-Ortega, J; Escorcia, R A [Universidad del Magdalena, A. A. 731, Santa Marta (Colombia); Mikhailov, I D, E-mail: jsierraortega@gmail.co [Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

    2009-05-01

    We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantum dot and wetting layer induced by trapped exciton strongly depends on the quantum dot morphology and the strength of the magnetic field.

  16. Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides

    KAUST Repository

    Moody, Galan

    2016-03-14

    Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe2), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

  17. <strong>Active Fault Detection and Isolation for Hybrid Systems strong>

    DEFF Research Database (Denmark)

    Gholami, Mehdi; Schiøler, Henrik; Bak, Thomas;

    2009-01-01

    models predicted outputs such that their discrepancies are observable by passive fault diagnosis technique. Isolation of different faults is done by implementation a bank of Extended Kalman Filter (EKF) where the convergence criterion for EKF is confirmed by Genetic Algorithm (GA). The method is applied......An algorithm for active fault detection and isolation is proposed. In order to observe the failure hidden due to the normal operation of the controllers or the systems, an optimization problem based on minimization of test signal is used. The optimization based method imposes the normal and faulty...

  18. Diamagnetic excitons and exciton magnetopolaritons in semiconductors

    Science.gov (United States)

    Seisyan, R. P.

    2012-05-01

    Interband magneto-absorption in semiconductors is reviewed in the light of the diamagnetic exciton (DE) concept. Beginning with a proof of the exciton nature of oscillating-magnetoabsorption (the DE discovery), development of the DE concept is discussed, including definition of observation conditions, quasi-cubic approximation for hexagonal crystals, quantum-well effects in artificial structures, and comprehension of an important role of the DE polariton. The successful use of the concept application to a broad range of substances is reviewed, namely quasi-Landau magnetic spectroscopy of the ‘Rydberg’ exciton states in cubic semiconductors such as InP and GaAs and in hexagonal ones such as CdSe, the proof of exciton participation in the formation of optical spectra in narrow-gap semiconductors such as InSb, InAs, and, especially, PbTe, observation of DE spectra in semiconductor solid solutions like InGaAs. The most fundamental findings of the DE spectroscopy for various quantum systems are brought together, including the ‘Coulomb-well’ effect, fine structure of discrete oscillatory states in the InGaAs/GaAs multiple quantum wells, the magneto-optical observation of above-barrier exciton. Prospects of the DE physics in ultrahigh magnetic field are discussed, including technological creation of controllable low-dimensional objects with extreme oscillator strengths, formation of magneto-quantum exciton polymer, and even modelling of the hydrogen behaviour in the atmosphere of a neutron star.

  19. Exciton interference revealed by energy dependent exciton transfer rate for ring-structured molecular systems

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Yun-An, E-mail: yunan@gznc.edu.cn [Guizhou Provincial Key Laboratory of Computational Nanomaterial Science, Guizhou Education University, Guiyang, Guizhou 550018 (China)

    2016-01-14

    The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today’s nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored.

  20. Exciton ionization in multilayer transition-metal dichalcogenides

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm; Latini, Simone; Thygesen, Kristian Sommer;

    2016-01-01

    Photodetectors and solar cells based on materials with strongly bound excitons rely crucially on field-assisted exciton ionization. We study the ionization process in multilayer transition-metal dichalcogenides (TMDs) within the Mott-Wannier model incorporating fully the pronounced anisotropy......-principles material parameters, an analysis of several important TMDs reveals WSe2 and MoSe2 to be superior for applications relying on ionization of direct and indirect excitons, respectively....

  1. Fine structure of the exciton electroabsorption in semiconductor superlattices

    Science.gov (United States)

    Monozon, B. S.; Schmelcher, P.

    2017-02-01

    Wannier-Mott excitons in a semiconductor layered superlattice (SL) are investigated analytically for the case that the period of the superlattice is much smaller than the 2D exciton Bohr radius. Additionally we assume the presence of a longitudinal external static electric field directed parallel to the SL axis. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the electric field strength are presented in an explicit form. We focus on the fine structure of the ground quasi-2D exciton level formed by the series of closely spaced energy levels adjacent from the high frequencies. These levels are related to the adiabatically slow relative exciton longitudinal motion governed by the potential formed by the in-plane exciton state. It is shown that the external electric fields compress the fine structure energy levels, decrease the intensities of the corresponding optical peaks and increase the exciton binding energy. A possible experimental study of the fine structure of the exciton electroabsorption is discussed.

  2. Exciton laser rate equations

    Directory of Open Access Journals (Sweden)

    Garkavenko A. S.

    2011-08-01

    Full Text Available The rate equations of the exciton laser in the system of interacting excitons have been obtained and the inverted population conditions and generation have been derived. The possibility of creating radically new gamma-ray laser has been shown.

  3. Exciton correlations and input-output relations in non-equilibrium exciton superfluids

    Energy Technology Data Exchange (ETDEWEB)

    Ye, Jinwu, E-mail: jy306@ccs.msstate.edu [Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University, Beijing 100048 (China); Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Sun, Fadi; Yu, Yi-Xiang [Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China); Liu, Wuming [Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China)

    2013-02-15

    The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron-hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg-Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input-output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton-polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron-electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor {nu}{sub T}=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: Black-Right-Pointing-Pointer Establish the relations between photoluminescence and transport

  4. Incremental Cycle Detection, Topological Ordering, and Strong Component Maintenance

    CERN Document Server

    Haeupler, Bernhard; Mathew, Rogers; Sen, Siddhartha; Tarjan, Robert Endre

    2011-01-01

    We present two on-line algorithms for maintaining a topological order of a directed $n$-vertex acyclic graph as arcs are added, and detecting a cycle when one is created. Our first algorithm handles $m$ arc additions in $O(m^{3/2})$ time. For sparse graphs ($m/n = O(1)$), this bound improves the best previous bound by a logarithmic factor, and is tight to within a constant factor among algorithms satisfying a natural {\\em locality} property. Our second algorithm handles an arbitrary sequence of arc additions in $O(n^{5/2})$ time. For sufficiently dense graphs, this bound improves the best previous bound by a polynomial factor. Our bound may be far from tight: we show that the algorithm can take $\\Omega(n^2 2^{\\sqrt{2\\lg n}})$ time by relating its performance to a generalization of the $k$-levels problem of combinatorial geometry. A completely different algorithm running in $\\Theta(n^2 \\log n)$ time was given recently by Bender, Fineman, and Gilbert. We extend both of our algorithms to the maintenance of stron...

  5. Laser pulse induced multiple exciton kinetics in molecular ring structures

    Science.gov (United States)

    Hou, Xiao; Wang, Luxia

    2016-11-01

    Multiple excitons can be formed upon strong optical excitation of molecular aggregates and complexes. Based on a theoretical approach on exciton-exciton annihilation dynamics in supramolecular systems (May et al., 2014), exciton interaction kinetics in ring aggregates of two-level molecules are investigated. Excited by the sub-picosecond laser pulse, multiple excitons keep stable in the molecular ring shaped as a regular polygon. If the symmetry is destroyed by changing the dipole of a single molecule, the excitation of different molecules becomes not identical, and the changed dipole-dipole interaction initiates subsequent energy redistribution. Depending on the molecular distance and the dipole configuration, the kinetics undergo different types of processes, but all get stable within some hundreds of femtoseconds. The study of exciton kinetics will be helpful for further investigations of the efficiency of optical devices based on molecular aggregates.

  6. Coherent excitonic nonlinearity versus inhomogeneous broadening in single quantum wells

    DEFF Research Database (Denmark)

    Langbein, Wolfgang Werner; Borri, Paola; Hvam, Jørn Märcher;

    1998-01-01

    The coherent response of excitons in semiconductor nanostructures, as measured in four wave mixing (FWM) experiments, depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs-AlGaAs single quantum wells (SQW) of 4 nm to 25 nm well width. Two main mechanisms...

  7. Localized Excitons in Carbon Nanotubes.

    Science.gov (United States)

    Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-03-01

    It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.

  8. Exciton management in organic photovoltaic multidonor energy cascades.

    Science.gov (United States)

    Griffith, Olga L; Forrest, Stephen R

    2014-05-14

    Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

  9. Excitons in van der Waals heterostructures

    DEFF Research Database (Denmark)

    Latini, Simone; Olsen, Thomas; Thygesen, Kristian Sommer

    2015-01-01

    The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semiconductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions, a systematic investigation of the role of screening on tw...

  10. Excitonic dynamical Franz-Keldysh effect

    DEFF Research Database (Denmark)

    Nordstrøm, K.B.; Johnsen, Kristinn; Allen, S.J.

    1998-01-01

    The dynamical Franz-Keldysh effect is exposed by exploring near-band-gap absorption in the presence of intense THz electric fields. It bridges the gap between the de Franz-Keldysh effect and multiphoton absorption and competes with the THz ac Stark effect in shifting the energy of the excitonic...... resonance. A theoretical model which includes the strong THz field nonperturbatively via a nonequilibrium Green functions technique is able to describe the dynamical Franz-Keldysh effect in the presence of excitonic absorption....

  11. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.;

    1996-01-01

    The formation of spatially indirect excitons in superlattices with narrow minibands is theoretically and experimentally investigated. We identify the experimental conditions for the observation of interwell excitons and find a distinct excitonic state energetically located between the Is exciton ...

  12. Long-range exciton dissociation in organic solar cells

    National Research Council Canada - National Science Library

    Domenico Caruso; Alessandro Troisi

    2012-01-01

    It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs...

  13. An automated sawtooth detection algorithm for strongly varying plasma conditions and crash characteristics

    Science.gov (United States)

    Gude, A.; Maraschek, M.; Kardaun, O.; the ASDEX Upgrade Team

    2017-09-01

    A sawtooth crash algorithm that can automatically detect irregular sawteeth with strongly varying crash characteristics, including inverted crashes with central signal increase, has been developed. Such sawtooth behaviour is observed in ASDEX Upgrade with its tungsten wall, especially in phases with central ECRH. This application of ECRH for preventing impurity accumulation is envisaged also for ITER. The detection consists of three steps: a sensitive edge detection, a multichannel combination to increase detection performance, and a profile analysis that tests generic sawtooth crash features. The effect of detection parameters on the edge detection results has been investigated using synthetic signals and tested in an application to ASDEX Upgrade soft x-ray data.

  14. THE CHAOTIC DETECTION OF PERIODIC SHORT—IMPULSE SIGNALS UNDER STRONG NOISE BACKGROUND

    Institute of Scientific and Technical Information of China (English)

    LiYue; YangBaojun

    2002-01-01

    The periodic short-impulse signals under strong noise background are successfully detected with a special chaotic system invented by the authors.Simulation experiments show that the chaotic system is very sensitive to periodic shor-impulse signals submerged by strong noise background,and it can effectively restrain any zero-mean nose.The system has a stable working-detection limit of -83dB.

  15. THE CHAOTIC DETECTION OF PERIODIC SHORT-IMPULSE SIGNALS UNDER STRONG NOISE BACKGROUND

    Institute of Scientific and Technical Information of China (English)

    Li Yue; Yang Baojun

    2002-01-01

    The periodic short-impulse signals under strong noise background are successfully detected with a special chaotic system invented by the authors. Simulation experiments show that the chaotic system is very sensitive to periodic short-impulse signals submerged by strong noise background, and it can effectively restrain any zero-mean noise. The system has a stable working-detection limit of -83dB.

  16. Machine Learning Exciton Dynamics

    CERN Document Server

    Häse, Florian; Pyzer-Knapp, Edward; Aspuru-Guzik, Alán

    2015-01-01

    Obtaining the exciton dynamics of large photosynthetic complexes by using mixed quantum mechanics/molecular mechanics (QM/MM) is computationally demanding. We propose a machine learning technique, multi-layer perceptrons, as a tool to reduce the time required to compute excited state energies. With this approach we predict time-dependent density functional theory (TDDFT) excited state energies of bacteriochlorophylls in the Fenna-Matthews-Olson (FMO) complex. Additionally we compute spectral densities and exciton populations from the predictions. Different methods to determine multi-layer perceptron training sets are introduced, leading to several initial data selections. In addition, we compute spectral densities and exciton populations. Once multi-layer perceptrons are trained, predicting excited state energies was found to be significantly faster than the corresponding QM/MM calculations. We showed that multi-layer perceptrons can successfully reproduce the energies of QM/MM calculations to a high degree o...

  17. Optically induced rotation of a quantum dot exciton spin

    CERN Document Server

    Poem, E; Kodriano, Y; Benny, Y; Khatsevich, S; Avron, J E; Gershoni, D

    2011-01-01

    We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two non-degenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time-difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.

  18. Quasienergy Spectroscopy of Excitons

    DEFF Research Database (Denmark)

    Johnsen, Kristinn; Jauho, Antti-Pekka

    1999-01-01

    We theoretically study nonlinear optics of excitons under intense THz irradiation. In particular, the linear near-infrared absorption and resonantly enhanced nonlinear sideband generation are described. We predict a rich structure in the spectra which an be interpreted in terms of the quasienergy...

  19. Excitation of exciton states on a curved surface

    Science.gov (United States)

    Silotia, Poonam; Prasad, Vinod

    2016-05-01

    Excitonic transitions on the surface of a sphere have been studied in he presence of external static electric and laser fields. The spectrum and the various coupling matrix elements, (for n = 1 , 2 , 3), between few states of exciton have been evaluated in the absence and presence of excitonic Coulombic interaction with different values of dielectric constant. Variation of various physical quantities: energy eigenvalues, transition probability, orientational and alignment parameter, has been shown to have strong dependence on the laser field and static electric field.

  20. Control of Exciton Photon Coupling in Nano-structures

    Science.gov (United States)

    Liu, Xiaoze

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley

  1. Ghost Fano Resonance of Excitons in Twisted Bilayer Graphene

    Science.gov (United States)

    Liang, Yufeng

    2014-03-01

    Metallic systems are generally considered to be unable to harbor tightly bound excitons because of the strong screening effect as well as the absence of a finite band gap. Previously, exception has only been found in one-dimensional metallic carbon nanotubes due to the depressed screening effects and the symmetry gap. We explore the exciton spectra of twisted bilayer graphene (tBLG) and predict the existence of even more strongly bound exciton (with binding energy as large as 0.5eV) in this system despite of its higher dimensionality. Based on our results from first-principles simulations and effective model calculations, a mechanism known as the ghost Fano resonance is proposed for the bound exciton formation in metallic systems beyond the dimensonality-related argument. Our results shed light on engineering the e-h excitations in the few-layer van der Waals heterojunction. NSF Grant No. DMR-1207141.

  2. The CUSUM test for detecting structural changes in strong mixing processes

    CERN Document Server

    Azizzadeh, Fatemeh

    2012-01-01

    Strong mixing property holds for a broad class of linear and nonlinear time series models such as ARMA and GARCH models. In this article we study correlation structure of strong mixing sequences, and some asymptotic properties are presented. We also present a new method for detecting change point in correlation structure of strong mixing sequences, and present a nonparametric CUSUM test statistic for this. Asymptotic consistency of this test statistics is shown. This method is applied to simulated data of some linear and nonlinear models and power of the test is evaluated. For linear models, it is shown that this method have a better performance in compare to Berkes et al.(2009).

  3. Suppression of exciton-exciton annihilation in tungsten disulfide monolayers encapsulated by hexagonal boron nitrides

    Science.gov (United States)

    Hoshi, Yusuke; Kuroda, Takashi; Okada, Mitsuhiro; Moriya, Rai; Masubuchi, Satoru; Watanabe, Kenji; Taniguchi, Takashi; Kitaura, Ryo; Machida, Tomoki

    2017-06-01

    We investigates exciton-exciton annihilation (EEA) in tungsten disulfide (W S2) monolayers encapsulated by hexagonal boron nitride (hBN). It is revealed that decay signals observed by time-resolved photoluminescence (PL) are not strongly dependent on the exciton densities of hBN-encapsulated W S2 monolayers (W S2/hBN ) . In contrast, the sample without the bottom hBN layer (W S2/Si O2) exhibits a drastic decrease of decay time with increasing exciton density due to the appearance of a rapid PL decay component, signifying nonradiative EEA-mediated recombination. Furthermore, the EEA rate constant of W S2/hBN was determined as (6.3 ±1.7 ) ×10-3c m2s-1 , being about 2 orders of magnitude smaller than that of W S2/Si O2 . Thus, the observed EEA rate reduction played a key role in enhancing luminescence intensity at high exciton densities in the W S2 monolayer.

  4. Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons.

    Science.gov (United States)

    Anankine, Romain; Beian, Mussie; Dang, Suzanne; Alloing, Mathieu; Cambril, Edmond; Merghem, Kamel; Carbonell, Carmen Gomez; Lemaître, Aristide; Dubin, François

    2017-03-24

    We study spatially indirect excitons of GaAs quantum wells, confined in a 10  μm electrostatic trap. Below a critical temperature of about 1 K, we detect macroscopic spatial coherence and quantized vortices in the weak photoluminescence emitted from the trap. These quantum signatures are restricted to a narrow range of density, in a dilute regime. They manifest the formation of a four-component superfluid, made by a low population of optically bright excitons coherently coupled to a dominant fraction of optically dark excitons.

  5. Triplet exciton dissociation in singlet exciton fission photovoltaics

    Energy Technology Data Exchange (ETDEWEB)

    Jadhav, Priya J.; Mohanty, Aseema; Bulovic, Vladimir; Baldo, Marc A. [Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Brown, Patrick R. [Department of Physics, Massachusetts Institute of Technology, Cambridge, MA (United States); Thompson, Nicholas [Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Wunsch, Benjamin [School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA (United States); Yost, Shane R.; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G. [Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA (United States)

    2012-12-04

    Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  6. Energy and Information Transfer Via Coherent Exciton Wave Packets

    Science.gov (United States)

    Zang, Xiaoning

    associated excitations were dubbed twisted excitons. Twisted exciton packets can be manipulated as they travel down molecular chains, and this has applications in quantum information science as well. In each setting considered, exciton dynamics were initially studied using a simple tight-binding formalism. This misses the actual many-body interactions and multiple energy levels associated real systems. To remedy this, I adapted an existing time-domain Density Functional Theory code and applied it to study the dynamics of exciton wave packets on quasi-one-dimensional systems. This required the use of high-performance computing and the construction of a number of key auxiliary codes. Establishing the requisite methodology constituted a substantial part of the entire thesis. Surprisingly, this effort uncovered a computational issue associated with Rabi oscillations that had been incorrectly characterized in the literature. My research elucidated the actual problem and a solution was found. This new methodology was an integral part of the overall computational analysis. The thesis then takes up the a detailed consideration of the prospect for creating systems that support a strong measure of transport coherence. While physical implementations include molecular assemblies, solid-state superlattices, and even optical lattices, I decided to focus on assemblies of nanometer-sized silicon quantum dots. First principles computational analysis was used to quantify reorganization within individual dots and excitonic coupling between dots. Quantum dot functionalizations were identified that make it plausible to maintain a measure of excitonic coherence even at room temperatures. Attention was then turned to the use of covalently bonded bridge material to join quantum dots in a way that facilitates efficient exciton transfer. Both carbon and silicon structures were considered by considering the way in which subunits might be best brought together. This resulted in a set of design criteria

  7. Exciton center-of-mass localization and dielectric environment effect in monolayer WS2

    Science.gov (United States)

    Hichri, Aïda; Ben Amara, Imen; Ayari, Sabrine; Jaziri, Sihem

    2017-06-01

    The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. In this paper, we study the strong localization of exciton center-of-mass motion within random potential fluctuations caused by the monolayer defects. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping, and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

  8. Novel exciton systems in 2D TMD monolayers and heterobilayers

    Science.gov (United States)

    Yu, Hongyi

    In this talk, two exciton systems in transition metal dichalcogenides (TMDs) monolayer and heterobilayer will be discussed. In TMD monolayers, the strong e-h Coulomb exchange interaction splits the exciton and trion dispersions into two branches with zero and finite gap, respectively. Each branch is a center-of-mass wave vector dependent coherent superposition of the two valleys, which leads to a valley-orbit coupling and possibly a trion valley Hall effect. The exchange interaction also eliminates the linear polarization of the negative trion PL emission. In TMD heterobilayers with a type-II band alignment, the low energy exciton has an interlayer configuration with the e and h localized in opposite layers. Because of the inevitable twist or/and lattice mismatch between the two layers, the bright interlayer excitons are located at finite center-of-mass velocities with a six-fold degeneracy. The corresponding photon emission is elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity determined by the valley indices of the e and h. Some experimental results on the interlayer excitons in the WSe2-MoSe2 heterobilayers will also be presented. The interlayer exciton exhibits a long lifetime as well as a long depolarization time, which facilitate the observation of a PL polarization ring pattern due to the valley dependent exciton-exciton interaction induced expansion. The works were supported by the Research Grant Council of Hong Kong (HKU17305914P, HKU705513P), the Croucher Foundation, and the HKU OYRA and ROP.

  9. Excitons in asymmetric quantum wells

    Science.gov (United States)

    Grigoryev, P. S.; Kurdyubov, A. S.; Kuznetsova, M. S.; Ignatiev, I. V.; Efimov, Yu. P.; Eliseev, S. A.; Petrov, V. V.; Lovtcius, V. A.; Shapochkin, P. Yu.

    2016-09-01

    Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

  10. Time and direction of arrival detection and filtering for imaging in strongly scattering random media

    CERN Document Server

    Borcea, Liliana; Tsogka, Chrysoula

    2016-01-01

    We study detection and imaging of small reflectors in heavy clutter, using an array of transducers that emits and receives sound waves. Heavy clutter means that multiple scattering of the waves in the heterogeneous host medium is strong and overwhelms the arrivals from the small reflectors. Building on the adaptive time-frequency filter of [1], we propose a robust method for detecting the direction of arrival of the direct echoes from the small reflectors, and suppressing the unwanted clutter backscatter. This improves the resolution of imaging. We illustrate the performance of the method with realistic numerical simulations in a non-destructive testing setup.

  11. Detecting the amplitude mode of strongly interacting lattice bosons by Bragg scattering.

    Science.gov (United States)

    Bissbort, Ulf; Götze, Sören; Li, Yongqiang; Heinze, Jannes; Krauser, Jasper S; Weinberg, Malte; Becker, Christoph; Sengstock, Klaus; Hofstetter, Walter

    2011-05-20

    We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting condensate of ultracold atoms in an optical lattice. By the comparison of our experimental data with a spatially resolved, time-dependent bosonic Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime. A systematic shift of the sound and amplitude modes' resonance frequencies due to the finite Bragg beam intensity is observed.

  12. Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells

    Science.gov (United States)

    Fedichkin, F.; Andreakou, P.; Jouault, B.; Vladimirova, M.; Guillet, T.; Brimont, C.; Valvin, P.; Bretagnon, T.; Dussaigne, A.; Grandjean, N.; Lefebvre, P.

    2015-05-01

    We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μ m is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 μ m . The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

  13. Sensitive Detection of Individual Neutral Atoms in a Strong Coupling Cavity QED System

    Institute of Scientific and Technical Information of China (English)

    ZHANG Peng-Fei; ZHANG Yu-Chi; LI Gang; DU Jin-Jin; ZHANG Yan-Feng; GUO Yan-Qiang; WANG Jun-Min; ZHANG Tian-Cai; LI Wei-Dong

    2011-01-01

    We experimentally demonstrate real-time detection of individual cesium atoms by using a high-finesse optical micro-cavity in a strong coupling regime.A cloud of cesium atoms is trapped in a magneto-optical trap positioned at 5 mm above the micro-cavity center.The atoms fall down freely in gravitation after shutting off the magnetooptical trap and pass through the cavity.The cavity transmission is strongly affected by the atoms in the cavity, which enables the micro-cavity to sense the atoms individually.We detect the single atom transits either in the resonance or various detunings.The single atom vacuum-Rabi splitting is directly measured to be Ω = 2π × 23.9 MHz.The average duration of atom-cavity coupling of about 110μs is obtained according to the probability distribution of the atom transits.%@@ We experimentally demonstrate real-time detection of individual cesium atoms by using a high-finesse optical micro-cavity in a strong coupling regime.A cloud of cesium atoms is trapped in a magneto-optical trap positioned at 5mm above the micro-cavity center.The atoms fall down freely in gravitation after shutting off the magnetooptical trap and pass through the cavity.The cavity transmission is strongly affected by the atoms in the cavity, which enables the micro-cavity to sense the atoms individually.We detect the single atom transits either in the resonance or various detunings.The single atom vacuum-Rabi splitting is directly measured to be Ω=2π×23.9 MHz.The average duration of atom-cavity coupling of about 110μs is obtained according to the probability distribution of the atom transits.

  14. Chaotic system for the detection of periodic signals under the background of strong noise

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    We propose a method to study the chaotic system for the detection of periodic signals in the presence of strong background noise. The numerical experiments indicate that the chaotic system constructed from the modified Duffing-Holmes equation is sensitive to the weak periodic signal mixed with noise, and it has certain immunity to noise. The signal to noise ratio for the system can reach to about -91 dB.

  15. Excitonic complexes in GaN/(Al,Ga)N quantum dots

    Science.gov (United States)

    Elmaghraoui, D.; Triki, M.; Jaziri, S.; Muñoz-Matutano, G.; Leroux, M.; Martinez-Pastor, J.

    2017-03-01

    Here we report a theoretical investigation of excitonic complexes in polar GaN/(Al,Ga)N quantum dots (QDs). A sum rule between the binding energies of charged excitons is used to calculate the biexciton binding energy. The binding energies of excitonic complexes in GaN/AlN are shown to be strongly correlated to the QD size. Due to the large hole localization, the positively charged exciton energy is found to be always blueshifted compared to the exciton one. The negatively charged exciton and the biexciton energy can be blueshifted or redshifted according to the QD size. Increasing the size of GaN/AlN QDs makes the identification of charged excitons difficult, and the use of an Al0.5Ga0.5N barrier can be advantageous for clear identification. Our theoretical results for the binding energy of exciton complexes are also confronted with values deduced experimentally for InAs/GaAs QDs, confirming our theoretical prediction for charged excitonic complexes in GaN/(Al,Ga)N QDs. Finally, we realize that the trends of excitonic complexes in QDs are significantly related to competition between the local charge separation (whatever its origin) and the correlation effect. Following our findings, entangled photons pairs can be produced in QDs with careful control of their size in order to obtain zero exciton–biexciton energy separation.

  16. Exciton Polaritons in Microcavities New Frontiers

    CERN Document Server

    Sanvitto, Daniele

    2012-01-01

    In the past decade, there has been a burst of new and fascinating physics associated to the unique properties of two-dimensional exciton polaritons, their recent demonstration of condensation under non-equilibrium conditions and all the related quantum phenomena, which have stimulated extensive research work. This monograph summarizes the current state of the art of research on exciton polaritons in microcavities: their interactions, fast dynamics, spin-dependent phenomena, temporal and spatial coherence, condensation under non-equilibrium conditions, related collective quantum phenomena and most advanced applications. The monograph is written by the most active authors who have strongly contributed to the advances in this area. It is of great interests to both physicists approaching this subject for the first time, as well as a wide audience of experts in other disciplines who want to be updated on this fast moving field.

  17. Exciton Formation in Disordered Semiconductors

    DEFF Research Database (Denmark)

    Klochikhin, A.; Reznitsky, A.; Permogorov, S.

    1999-01-01

    Stationary luminescence spectra of disordered solid solutions can be accounted by the model of localized excitons. Detailed analysis of the long time decay kinetics of luminescence shows that exciton formation in these systems is in great extent due to the bimolecular reaction of separated carrie...

  18. Exciton Formation in Disordered Semiconductors

    DEFF Research Database (Denmark)

    Klochikhin, A.; Reznitsky, A.; Permogorov, S.

    1999-01-01

    Stationary luminescence spectra of disordered solid solutions can be accounted by the model of localized excitons. Detailed analysis of the long time decay kinetics of luminescence shows that exciton formation in these systems is in great extent due to the bimolecular reaction of separated carrier...

  19. Detection of mini-UAVs in the presence of strong topographic relief: a multisensor perspective

    Science.gov (United States)

    Böniger, Urs; Ott, Beat; Wellig, Peter; Aulenbacher, Uwe; Klare, Jens; Nussbaumer, Thomas; Leblebici, Yusuf

    2016-10-01

    Based on the steadily growing use of mini-UAVs for numerous civilian and military applications, mini-UAVs have been recognized as an increasing potential threat. Therefore, counter-UAV solutions addressing the peculiarities of this class of UAVs have recently received a significant amount of attention. Reliable detection, localization, identification and tracking represents a fundamental prerequisite for such counter-UAV systems. In this paper, we focus on the assessment of different sensor technologies and their ability to detect mini-UAVs in a representative rural Swiss environment. We conducted a field trial in August 2015, using different, primarily short range, experimental sensor systems from armasuisse and selected research partners. After an introduction into the challenges for UAV detection in regions with strong topographic relief, we will introduce the experimental setup and describe the key results from this joint experiment.

  20. Bose condensation of interwell excitons in double quantum wells

    CERN Document Server

    Larionov, A V; Ni, P A; Dubonos, S V; Hvam, I; Soerensen, K

    2002-01-01

    The luminescence of the interwell excitons in the GaAs/AlGaAs double quantum wells, containing large-scale fluctuations of the random potential in the heteroboundary planes, is studied. The properties of the excitons, wherein the excited electron and hole are spatially separated between the neighboring quantum wells by the density and temperature variation within the domain limits of the scale below one micron, are investigated. The interwell excitons by low pumping (below 50 mW) are strongly localized due to the small-scale fluctuations of the random potential. The localized excitons line grows by increase in the resonance excitation capacity through the threshold method. With the temperature growth this line disappears in the spectrum (T sub c <= 3.4 K). The above phenomenon is related to the Bose-Einstein condensation in the quasi-two-dimensional system of the interwell excitons. The critical values of the exciton density and temperature in the studied temperature range (1.5-3.4 K) grow according to the...

  1. Suppression of space broadening of exciton polariton transport by Bloch oscillation effect

    Science.gov (United States)

    Duan, Xudong; Zou, Bingsuo; Zhang, Yongyou

    2015-12-01

    We theoretically study the transport of exciton polaritons under different applied photon potentials. The relation between the photon potentials and the thickness of the cavity layer is first calculated by finite-element simulation. The theoretical analysis and numerical calculation indicate that the cavity photon potential is proportional to the thickness of the cavity layer with the coefficient being about 1.8 meV nm-1. Further, the periodic and linear photon potentials are considered to control the transport of the exciton polaritons in weak- and strong-field pump situations. In both situations the periodic potential cannot by itself effectively suppress the scatterings of the disorder potentials of the cavity photons and excitons and the nonlinear exciton-exciton interaction. When the linear potential is added to the cavity photons, the exciton polariton transport exhibits the Bloch oscillation behavior. Importantly, the polariton Bloch oscillation can strongly suppress the space broadening of the exciton polariton transport due to the disorder potentials and nonlinear exciton-exciton interaction, which is beneficial for designing the polariton circuits.

  2. Ultrafast exciton-exciton coherent transfer in molecular aggregates and its application to light-harvesting systems.

    Science.gov (United States)

    Hyeon-Deuk, Kim; Tanimura, Yoshitaka; Cho, Minheang

    2007-08-21

    Effects of the exciton-exciton coherence transfer (EECT) in strongly coupled molecular aggregates are investigated from the reduced time-evolution equation which we have developed to describe EECT. Starting with the nonlinear response function, we obtained explicit contributions from EECT to four-wave-mixing spectrum such as photon echo, taking into account double exciton states, static disorder, and heat-bath coupling represented by arbitrary spectral densities. By using the doorway-window picture and the projection operator technique, the transfer rates between two different electronic coherent states are obtained within a framework of cumulant expansion at high temperature. Applications of the present theory to strongly coupled B850 chlorophylls in the photosynthetic light harvesting system II (LH2) are discussed. It is shown that EECT is indispensable in properly describing ultrafast phenomena of strongly coupled molecular aggregates such as LH2 and that the EECT contribution to the two-dimensional optical spectroscopy is not negligible.

  3. Thermalization of Hot Free Excitons in ZnSe-Based Quantum Wells

    DEFF Research Database (Denmark)

    Hoffmann, J.; Umlauff, M.; Kalt, H.

    1997-01-01

    Thermalization of hot-exciton populations in ZnSe quantum wells occurs on a time scale of 100 ps. Strong exciton-phonon coupling in II-VI semiconductors leads to a direct access to the thermalization dynamics via time-resolved spectroscopy of phonon-assisted luminescence. The experimental spectra...

  4. Coherent dynamics of interwell excitons in GaAs/AlxGa1-xAs superlattices

    DEFF Research Database (Denmark)

    Mizeikis, V.; Birkedal, Dan; Langbein, Wolfgang Werner;

    1997-01-01

    Coherent exciton dynamics in a GaAs/AlxGa1-xAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One ...

  5. The Detection of a Population of Submillimeter-Bright, Strongly-Lensed Galaxies

    CERN Document Server

    Negrello, Mattia; De Zotti, G; Cooray, A; Verma, A; Bock, J; Frayer, D T; Gurwell, M A; Omont, A; Neri, R; Dannerbauer, H; Leeuw, L L; Barton, E; Cooke, J; Kim, S; da Cunha, E; Rodighiero, G; Cox, P; Bonfield, D G; Jarvis, M J; Serjeant, S; Ivison, R J; Dye, S; Aretxaga, I; Hughes, D H; Ibar, E; Bertoldi, F; Valtchanov, I; Eales, S; Dunne, L; Driver, S P; Auld, R; Buttiglione, S; Cava, A; Grady, C A; Clements, D L; Dariush, A; Fritz, J; Hill, D; Hornbeck, J B; Kelvin, L; Lagache, G; Lopez-Caniego, M; Gonzalez-Nuevo, J; Maddox, S; Pascale, E; Pohlen, M; Rigby, E E; Robotham, A; Simpson, C; Smith, D J B; Temi, P; Thompson, M A; Woodgate, B E; York, D G; Aguirre, J E; Beelen, A; Blain, A; Baker, A J; Birkinshaw, M; Blundell, R; Bradford, C M; Burgarella, D; Danese, L; Dunlop, J S; Fleuren, S; Glenn, J; Harris, A I; Kamenetzky, J; Lupu, R E; Maddalena, R J; Madore, B F; Maloney, P R; Matsuhara, H; Michalowski, M J; Murphy, E J; Naylor, B J; Nguyen, H; Popescu, C; Rawlings, S; Rigopoulou, D; Scott, D; Scott, K S; Seibert, M; Smail, I; Tuffs, R J; Vieira, J D; van der Werf, P P; Zmuidzinas, J; 10.1126/science.1193420

    2010-01-01

    Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty starforming galaxies. However the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

  6. Microcavity controlled coupling of excitonic qubits.

    Science.gov (United States)

    Albert, F; Sivalertporn, K; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W

    2013-01-01

    Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits--like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots--is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

  7. Stimulated emission of free excitons in Cd{sub 1-x}Mn{sub x}Te under nonresonant two-photon excitation

    Energy Technology Data Exchange (ETDEWEB)

    Jang, J.I. [Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States)], E-mail: joon-jang@northwestern.edu; Mani, S.; Ketterson, J.B. [Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Park, H.Y. [Department of Semiconductor Applications, Ulsan College, San 29 Mugeo Dong, Ulsan 680-749 (Korea, Republic of)], E-mail: hypark@mail.uc.ac.kr

    2008-08-25

    We report on free excitons coexisting with exciton magnetic polarons (EMPs) in bulk semimagnetic semiconductors of Cd{sub 1-x}Mn{sub x}Te for 0.04{<=}x{<=}0.36 at 2 K under nonresonant two-photon excitation. This two-photon excitation not only generates free excitons but also more efficiently creates EMPs compared with ordinary one-photon excitation. Stimulated emission from free excitons is demonstrated under strong two-photon excitation.

  8. Suppression of space broadening of exciton polariton beams by Bloch oscillation effects

    CERN Document Server

    Duan, Xudong; Zhang, Yongyou

    2015-01-01

    We theoretically study the transport of exciton polaritons under different applied photon potentials. The relation between the photon potentials and the thickness of the cavity layer is calculated by the finite element simulation. The theoretical analysis and numerical calculation indicate that the cavity photon potential is proportional to the thickness of the cavity layer with the coefficient being about $1.8$ meV/nm. Further, the periodic and linear photon potentials are considered to control the transport of the exciton polaritons in weak- and strong-field pump situations. In both situations the periodic potential cannot by itself effectively suppress the scatterings of the disorder potentials of the cavity photons and excitons and the nonlinear exciton-exciton interaction. When the linear potential is added to the cavity photons, the polariton transport exhibits the Bloch oscillation behavior. Importantly, the polariton Bloch oscillation can strongly suppress the space broadening due to the disorder pote...

  9. Exciton polarizability in semiconductor nanocrystals.

    Science.gov (United States)

    Wang, Feng; Shan, Jie; Islam, Mohammad A; Herman, Irving P; Bonn, Mischa; Heinz, Tony F

    2006-11-01

    The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.

  10. Hyperspherical theory of anisotropic exciton

    CERN Document Server

    Muljarov, E A; Tikhodeev, S G; Bulatov, A E; Birman, Joseph L; 10.1063/1.1286772

    2012-01-01

    A new approach to the theory of anisotropic exciton based on Fock transformation, i.e., on a stereographic projection of the momentum to the unit 4-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a basis of the perturbation theory. The binding energies, wave functions and oscillator strengths of elongated as well as flattened excitons are obtained numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributed between optically active and formerly inactive states, making the latter optically active. An approximate analytical solution of the anisotropic exciton problem taking into account the angular momentum conserving terms is obtained. This solution gives the binding energies of moderately anisotropic exciton with a good accuracy and provides a useful qualitative description of the energy level evolution.

  11. Online Sensor Fault Detection Based on an Improved Strong Tracking Filter

    Science.gov (United States)

    Wang, Lijuan; Wu, Lifeng; Guan, Yong; Wang, Guohui

    2015-01-01

    We propose a method for online sensor fault detection that is based on the evolving Strong Tracking Filter (STCKF). The cubature rule is used to estimate states to improve the accuracy of making estimates in a nonlinear case. A residual is the difference in value between an estimated value and the true value. A residual will be regarded as a signal that includes fault information. The threshold is set at a reasonable level, and will be compared with residuals to determine whether or not the sensor is faulty. The proposed method requires only a nominal plant model and uses STCKF to estimate the original state vector. The effectiveness of the algorithm is verified by simulation on a drum-boiler model. PMID:25690553

  12. Non-destructive detection of ions using atom-cavity collective strong coupling

    CERN Document Server

    Dutta, Sourav

    2016-01-01

    We present a technique, based on atoms coupled to an optical cavity, for non-destructive detection of trapped ions. We demonstrate the vacuum-Rabi splitting (VRS), arising due to the collective strong coupling of ultracold Rb atoms to a cavity, to change in presence of trapped Rb+ ions. The Rb+ ions are optically dark and the Rb atoms are prepared in a dark magneto-optical trap (MOT). The VRS is measured on an optically open transition of the initially dark Rb atoms. The measurement itself is fast, non-destructive and has sufficient fidelity to permit the measurement of atomic-state selective ion-atom collision rate. This demonstration illustrates a method based on atom-cavity coupling to measure two particle interactions generically and non-destructively.

  13. Strong continuous variable EPR-steering with a detection efficiency above 96%

    CERN Document Server

    Steinlechner, Sebastian; Eberle, Tobias; Schnabel, Roman

    2011-01-01

    In 1935, Einstein, Podolsky, and Rosen reported a gedanken experiment which became famous as the EPR-paradox. In the same year, Schr\\"odinger introduced the terms entanglement and steering in order to describe the underlying effect that a measurement on subsystem A of a certain class of entangled states may apparently allow for a remote steering of the measurement outcome at subsystem B, without the presence of a physical interaction between the subsystems. In this work we report on the observation of unprecedented strong EPR-steering in the gaussian regime, quantified by an EPR co-variance product of about 0.04 < 1, where 1 is the critical value. Together with a high detection efficiency of more than 96%, our result is an important milestone towards applications of gaussian entanglement distribution.

  14. Single-molecule spectroscopy unmasks the lowest exciton state of the B850 assembly in LH2 from Rps. acidophila.

    Science.gov (United States)

    Kunz, Ralf; Timpmann, Kõu; Southall, June; Cogdell, Richard J; Freiberg, Arvi; Köhler, Jürgen

    2014-05-01

    We have recorded fluorescence-excitation and emission spectra from single LH2 complexes from Rhodopseudomonas (Rps.) acidophila. Both types of spectra show strong temporal spectral fluctuations that can be visualized as spectral diffusion plots. Comparison of the excitation and emission spectra reveals that for most of the complexes the lowest exciton transition is not observable in the excitation spectra due to the cutoff of the detection filter characteristics. However, from the spectral diffusion plots we have the full spectral and temporal information at hand and can select those complexes for which the excitation spectra are complete. Correlating the red most spectral feature of the excitation spectrum with the blue most spectral feature of the emission spectrum allows an unambiguous assignment of the lowest exciton state. Hence, application of fluorescence-excitation and emission spectroscopy on the same individual LH2 complex allows us to decipher spectral subtleties that are usually hidden in traditional ensemble spectroscopy.

  15. Scaling laws of Rydberg excitons

    Science.gov (United States)

    Heckötter, J.; Freitag, M.; Fröhlich, D.; Aßmann, M.; Bayer, M.; Semina, M. A.; Glazov, M. M.

    2017-09-01

    Rydberg atoms have attracted considerable interest due to their huge interaction among each other and with external fields. They demonstrate characteristic scaling laws in dependence on the principal quantum number n for features such as the magnetic field for level crossing or the electric field of dissociation. Recently, the observation of excitons in highly excited states has allowed studying Rydberg physics in cuprous oxide crystals. Fundamentally different insights may be expected for Rydberg excitons, as the crystal environment and associated symmetry reduction compared to vacuum give not only optical access to many more states within an exciton multiplet but also extend the Hamiltonian for describing the exciton beyond the hydrogen model. Here we study experimentally and theoretically the scaling of several parameters of Rydberg excitons with n , for some of which we indeed find laws different from those of atoms. For others we find identical scaling laws with n , even though their origin may be distinctly different from the atomic case. At zero field the energy splitting of a particular multiplet n scales as n-3 due to crystal-specific terms in the Hamiltonian, e.g., from the valence band structure. From absorption spectra in magnetic field we find for the first crossing of levels with adjacent principal quantum numbers a Br∝n-4 dependence of the resonance field strength, Br, due to the dominant paramagnetic term unlike for atoms for which the diamagnetic contribution is decisive, resulting in a Br∝n-6 dependence. By contrast, the resonance electric field strength shows a scaling as Er∝n-5 as for Rydberg atoms. Also similar to atoms with the exception of hydrogen we observe anticrossings between states belonging to multiplets with different principal quantum numbers at these resonances. The energy splittings at the avoided crossings scale roughly as n-4, again due to crystal specific features in the exciton Hamiltonian. The data also allow us to

  16. Strongly lensed neutral hydrogen emission: detection predictions with current and future radio interferometers

    CERN Document Server

    Deane, R P; Heywood, I

    2015-01-01

    Strong gravitational lensing provides some of the deepest views of the Universe, enabling studies of high-redshift galaxies only possible with next-generation facilities without the lensing phenomenon. To date, 21 cm radio emission from neutral hydrogen has only been detected directly out to z~0.2, limited by the sensitivity and instantaneous bandwidth of current radio telescopes. We discuss how current and future radio interferometers such as the Square Kilometre Array (SKA) will detect lensed HI emission in individual galaxies at high redshift. Our calculations rely on a semi-analytic galaxy simulation with realistic HI disks (by size, density profile and rotation), in a cosmological context, combined with general relativistic ray tracing. Wide-field, blind HI surveys with the SKA are predicted to be efficient at discovering lensed HI systems, increasingly so at z > 2. This will be enabled by the combination of the magnification boosts, the steepness of the HI luminosity function at the high-mass end, and t...

  17. Strong emissive nanofibers of organogels for the detection of volatile acid vapors.

    Science.gov (United States)

    Xue, Pengchong; Sun, Jiabao; Yao, Boqi; Gong, Peng; Zhang, Zhenqi; Qian, Chong; Zhang, Yuan; Lu, Ran

    2015-03-16

    Two L-phenylalanine derivatives with 5,8-bis(2-(carbazol-3-yl)vinyl)quinoxaline (PCQ) and 5,8-bis[2-(carbazol-3-yl)]-2,3-dimethylquinoxaline (DCQ) as fluorophores were synthesized, and their photophysical properties were measured and compared. The two compounds were found to gelate some organic solvents and self-assemble into 1D nanofibers in gels. The wet gel of PCQ emitted a weak orange fluorescence, but the DCQ gel had a strong green one. This result can be due to the presence of two methyl groups and the nonplanar conformation of fluorophore in DCQ. The gel film of DCQ also showed significantly stronger fluorescence than that of PCQ. Thus, the wet gel and xerogel film of DCQ were selected to study their sensing properties to acids. The yellow wet gel of DCQ transformed into a brown sol upon the addition of 0.2 equiv trifluoroacetic acid (TFA), accompanied by emission quenching. The xerogel film of DCQ rapidly responded to volatile acids, such as TFA, HCl, and HOAc. The fluorescence of the xerogel film was gradually quenched with increased concentration of volatile acid vapors. The fibrous film exhibited low detection limits for volatile acid. The detection limits of the thin films for TFA, HCl, and HOAc reached 43, 122, and 950 ppb, respectively.

  18. NaV2O5 : An exotic exciton system

    NARCIS (Netherlands)

    Mostovoy, MV; Khomskii, DI; Knoester, J; Cho, K; Matsui, A

    2001-01-01

    We show that the phase transition which sodium vanadate undergoes at T-c = 34 K is driven by a charge ordering. The relevant effective Hamiltonian is of the Frenkel exciton type, with a very large bandwidth to molecular energy ratio. This causes strong non-Heitler-London effects and a temperature de

  19. NaV2O5 : An Exotic Exciton System

    NARCIS (Netherlands)

    Mostovoy, Maxim V.; Khomskii, Daniel I.; Knoester, Jasper

    2001-01-01

    We show that the phase transition which sodium vanadate undergoes at Tc = 34 K is driven by a charge ordering. The relevant effective Hamiltonian is of the Frenkel exciton type, with a very large bandwidth to molecular energy ratio. This causes strong non-Heitler-London effects and a temperature dep

  20. Nongeneric dispersion of excitons in the bulk of WSe2

    Science.gov (United States)

    Schuster, R.; Wan, Y.; Knupfer, M.; Büchner, B.

    2016-08-01

    We combine electron energy-loss spectroscopy (EELS) and density functional theory (DFT) calculations to study the dispersion and effective mass of excitons in the bulk of WSe2. Our EELS data suggest substantial deviations from the generic quadratic momentum dependence along the Γ K direction. From the DFT-derived Kohn-Sham states we deduce the EELS response without the inclusion of particle-hole attraction to study the possible role of the single-particle band structure on the exciton behavior. Based on this analysis we argue in favor of a strongly momentum dependent particle-hole interaction in WSe2 and other group-VI-transition-metal dichalcogenides.

  1. Optics of plasmon-exciton nanomaterials.

    Science.gov (United States)

    Sukharev, Maxim; Nitzan, Abraham

    2017-08-14

    This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued. © 2017 IOP Publishing Ltd.

  2. Reconfigurable exciton-plasmon interconversion for nanophotonic circuits

    Science.gov (United States)

    Lee, Hyun Seok; Luong, Dinh Hoa; Kim, Min Su; Jin, Youngjo; Kim, Hyun; Yun, Seokjoon; Lee, Young Hee

    2016-11-01

    The recent challenges for improving the operation speed of nanoelectronics have motivated research on manipulating light in on-chip integrated circuits. Hybrid plasmonic waveguides with low-dimensional semiconductors, including quantum dots and quantum wells, are a promising platform for realizing sub-diffraction limited optical components. Meanwhile, two-dimensional transition metal dichalcogenides (TMDs) have received broad interest in optoelectronics owing to tightly bound excitons at room temperature, strong light-matter and exciton-plasmon interactions, available top-down wafer-scale integration, and band-gap tunability. Here, we demonstrate principal functionalities for on-chip optical communications via reconfigurable exciton-plasmon interconversions in ~200-nm-diameter Ag-nanowires overlapping onto TMD transistors. By varying device configurations for each operation purpose, three active components for optical communications are realized: field-effect exciton transistors with a channel length of ~32 μm, field-effect exciton multiplexers transmitting multiple signals through a single NW and electrical detectors of propagating plasmons with a high On/Off ratio of~190. Our results illustrate the unique merits of two-dimensional semiconductors for constructing reconfigurable device architectures in integrated nanophotonic circuits.

  3. Large Range Manipulation of Exciton Species in Monolayer WS2

    CERN Document Server

    Wei, Ke; Yang, Hang; Cheng, Xiangai; Jiang, Tian

    2016-01-01

    Unconventional emissions from exciton and trion in monolayer WS2 are studied by photoexcitation. Excited by 532nm laser beam, the carrier species in the monolayer WS2 are affected by the excess electrons escaping from photoionization of donor impurity, the concentration of which varies with different locations of the sample. Simply increasing the excitation power at room temperature, the excess electron and thus the intensity ratio of excited trion and exciton can be continuously tuned over a large range from 0.1 to 7.7. Furthermore, this intensity ratio can also be manipulated by varying temperature. However, in this way the resonance energy of the exciton and trion show red-shifts with increasing temperature due to electron-phonon coupling. The binding energy of the trion is determined to be ~23meV and independent to temperature, indicating strong Coulomb interaction of carriers in such 2D materials.

  4. Non-Markovian Quantum Jumps in Excitonic Energy Transfer

    CERN Document Server

    Rebentrost, Patrick; Aspuru-Guzik, Alan

    2009-01-01

    We utilize the novel non-Markovian quantum jump (NMQJ) approach to stochastically simulate exciton dynamics derived from a time-convolutionless master equation. For relevant parameters and time scales, the time-dependent, oscillatory decoherence rates can have negative regions, a signature of non-Markovian behavior and of the revival of coherences. This can lead to non-Markovian population beatings for a dimer system at room temperature. We show that strong exciton-phonon coupling to low frequency modes can considerably modify transport properties. We observe increased exciton transport, which can be seen as an extension of recent environment-assisted quantum transport (ENAQT) concepts to the non-Markovian regime. Within the NMQJ method, the Fenna-Matthew-Olson protein is investigated as a prototype for larger photosynthetic complexes.

  5. Resonant Rayleigh scattering of exciton-polaritons in multiple quantum wells

    DEFF Research Database (Denmark)

    Malpuech, Guillaume; Kavokin, Alexey; Langbein, Wolfgang Werner;

    2000-01-01

    A theoretical concept of resonant Rayleigh scattering (RRS) of exciton-polaritons in multiple quantum wells (QWs) is presented. The optical coupling between excitons in different QWs can strongly affect the RRS dynamics, giving rise to characteristic temporal oscillations on a picosecond scale....... Bragg and anti-Bragg arranged QW structures with the same excitonic parameters are predicted to have drastically different RRS spectra. Experimental data on the RRS from multiple QWs show the predicted strong temporal oscillations at small scattering angles, which are well explained by the presented...

  6. Rayleigh surface wave interaction with the 2D exciton Bose-Einstein condensate

    Energy Technology Data Exchange (ETDEWEB)

    Boev, M. V.; Kovalev, V. M., E-mail: vadimkovalev@isp.nsc.ru [Russian Academy of Sciences, Institute of Semiconductor Physics, Siberian Branch (Russian Federation)

    2015-06-15

    We describe the interaction of a Rayleigh surface acoustic wave (SAW) traveling on the semiconductor substrate with the excitonic gas in a double quantum well located on the substrate surface. We study the SAW attenuation and its velocity renormalization due to the coupling to excitons. Both the deformation potential and piezoelectric mechanisms of the SAW-exciton interaction are considered. We focus on the frequency and excitonic density dependences of the SAW absorption coefficient and velocity renormalization at temperatures both above and well below the critical temperature of Bose-Einstein condensation of the excitonic gas. We demonstrate that the SAW attenuation and velocity renormalization are strongly different below and above the critical temperature.

  7. Light propagation in tunable exciton-polariton one-dimensional photonic crystals

    CERN Document Server

    Sedov, E S; Arakelian, S M; Kavokin, A V

    2016-01-01

    Simulations of propagation of light beams in specially designed multilayer semiconductor structures (one-dimensional photonic crystals) with embedded quantum wells reveal characteristic optical properties of resonant hyperbolic metamaterials. A strong dependence of the refraction angle and the optical beam spread on the exciton radiative lifetime is revealed. We demonstrate the strong negative refraction of light and the control of the group velocity of light by an external bias through its effect upon the exciton radiative properties.

  8. Excitonic pairing and insulating transition in two-dimensional semi-Dirac semimetals

    Science.gov (United States)

    Wang, Jing-Rong; Liu, Guo-Zhu; Zhang, Chang-Jin

    2017-02-01

    A sufficiently strong long-range Coulomb interaction can induce excitonic pairing in gapless Dirac semimetals, which generates a finite gap and drives the semimetal-insulator quantum phase transition. This phenomenon is in close analogy to dynamical chiral symmetry breaking in high-energy physics. In most realistic Dirac semimetals, including suspended graphene, the Coulomb interaction is too weak to open an excitonic gap. The Coulomb interaction plays a more important role at low energies in a two-dimensional semi-Dirac semimetal, in which the fermion spectrum is linear in one component of momenta and quadratic in the other, than a Dirac semimetal, and indeed leads to breakdown of Fermi liquid theory. We study dynamical excitonic gap generation in a two-dimensional semi-Dirac semimetal by solving the Dyson-Schwinger equation, and show that a moderately strong Coulomb interaction suffices to induce excitonic pairing. Additional short-range four-fermion coupling tends to promote excitonic pairing. Among the available semi-Dirac semimetals, we find that the TiO2/VO2 nanostructure provides a promising candidate for the realization of an excitonic insulator. We also apply the renormalization group method to analyze the strong coupling between the massless semi-Dirac fermions and the quantum critical fluctuation of the excitonic order parameter at the semimetal-insulator quantum critical point, and reveal non-Fermi liquid behaviors of semi-Dirac fermions.

  9. Seismic detection system for blocking the dangerous installations in case of strong earthquake occurrence

    Science.gov (United States)

    Ghica, Daniela; Corneliu Rau, Dan; Ionescu, Constantin; Grigore, Adrian

    2010-05-01

    During the last 70 years, four major earthquakes occurred in the Vrancea seismic area affected Romania territory: 10 November 1940 (Mw = 7.7, 160 km depth), 4 March 1977 (Mw = 7.5, 100 km depth), 30 August 1986 (Mw = 7.2, 140 km depth), 30 May 30 1990 (Mw = 6.9, 80 km depth). Romania is a European country with significant seismicity. So far, the 1977 event had the most catastrophic consequences: about 33,000 residences were totally destroyed or partially deteriorated, 1,571 people dies and another 11,300 were injured. Moreover, 61 natural-gas pipelines were damaged, causing destructive fires. The total losses were estimated at 3 mld. U.S. dollars. Recent studies clearly pointed out that in case of a strong earthquake occurrence in Vrancea region (Ms above 7), the biggest danger regarding the major cities comes from explosions and fires started immediately after the earthquake, and the most important factor of risk are the natural gas distribution networks. The damages are strongly amplified by the fact that, simultaneously, water and electric energy lines distributions are damaged too, making impossible the efficient firemen intervention, for localizing the fire sources. Presently, in Romania safe and efficient accepted solutions for improving the buildings securing, using antiseismic protection of the dangerous installations as natural-gas pipelines are not available. Therefore, we propose a seismic detection system based on a seismically actuated gas shut-off valve, which is automatically shut down in case of a seismic shock. The device is intended to be installed in the natural-gas supply line outside of buildings, as well at each user (group of users), inside of the buildings. The seismic detection system for blocking the dangerous installations in case of a strong earthquake occurrence was designed on the basis of 12 criteria enforced by the US regulations for seismic valves, aimed to eliminate the critical situations as fluids and under pressure gases leakage

  10. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.;

    1997-01-01

    The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier-Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by th...

  11. Exciton-plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles

    CERN Document Server

    Weijie, Zhao; Bo, Liu; Ivan, Verzhbitskiy; Shisheng, Li; Francesco, Giustiniano; Daichi, Kozawa; Ping, Loh Kian; Kazunari, Matsuda; Koichi, Okamoto; Rupert, Oulton F; Goki, Eda

    2016-01-01

    Hybrid systems of excitons strongly coupled to localized surface plasmons supported by metallic nanoparticles define a new approach to control light-matter interactions. Here, we report exciton-plasmon coupling in two-dimensional (2D) semiconductors, such as MoS2 and WS2, hybridized with silver nanoparticles. Prominent photoluminescence enhancement in monolayer MoS2 was observed with localized surface plasmon resonance (LSPR) tuned to the exciton resonance. By tuning the excitation energy, the contributions from near field enhancement and radiative emission rate enhancement via Purcell effect were resolved. Strong coherent dipole-dipole coupling between excitons and LSPR in resonant condition manifests as an electromagnetically induced transparency window in the extinction spectra of the localized surface plasmon. In this strong coupling regime a new quasi-particle, known as a plexciton, is expected to exhibit distinct properties, which exist in neither of the original particles. Our results demonstrate that ...

  12. Collective state of interwall excitons on GaAs/AlGaAs double quantum wells under pulse resonant excitation

    CERN Document Server

    Larionov, A V; Hvam, J; Soerensen, K

    2002-01-01

    The time evolution and kinetics of the photoluminescence (PL) spectra of the interwall excitons under the pulse resonant excitation of the interwall excitons are studied in the GaAs/AlGaAs binary quantum well. It is established, that the collective exciton phase originates with the time delay relative to the exciting pulse (several nanoseconds), which is conditioned by the density and temperature relaxation to the equilibrium values.The origination of the collective phase of the interwall excitons is accompanied by the strong narrowing of the corresponding photoluminescence line, the superlinear growth of its intensity and large time of change in the degree of the circular polarization.The collective exciton phase originates at the temperatures < 6 K and the interwall excitons densities 3 x 10 sup 1 sup 0 cm sup - sup 2

  13. Methods for Creation and Detection of Ultra-Strong Artificial Ionization in the Upper Atmosphere (Invited)

    Science.gov (United States)

    Bernhardt, P. A.; Siefring, C. L.; Briczinski, S. J.; Kendall, E. A.; Watkins, B. J.; Bristow, W. A.; Michell, R.

    2013-12-01

    The High Frequency Active Auroral Research Program (HAARP) transmitter in Alaska has been used to produce localized regions of artificial ionization at altitudes between 150 and 250 km. High power radio waves tuned near harmonics of the electron gyro frequency were discovered by Todd Pederson of the Air Force Research Laboratory to produce ionosonde traces that looked like artificial ionization layers below the natural F-region. The initial regions of artificial ionization (AI) were not stable but had moved down in altitude over a period of 15 minutes. Recently, artificial ionization has been produced by the 2nd, 3rd, 4th and 6th harmonics transmissions by the HAARP. In march 2013, the artificial ionization clouds were sustained for more the 5 hours using HAARP tuned to the 4 fce at the full power of 3.6 Mega-Watts with a twisted-beam antenna pattern. Frequency selection with narrow-band sweeps and antenna pattern shaping has been employed for optimal generation of AI. Recent research at HAARP has produced the longest lived and denser artificial ionization clouds using HF transmissions at the harmonics of the electron cyclotron frequency and ring-shaped radio beams tailored to prevent the descent of the clouds. Detection of artificial ionization employs (1) ionosonde echoes, (2) coherent backscatter from the Kodiak SuperDARN radar, (3) enhanced ion and plasma line echoes from the HAARP MUIR radar at 400 MHz, (4) high resolution optical image from ground sites, and (5) unique stimulated electromagnetic emissions, and (6) strong UHF and L-Band scintillation induced into trans-ionospheric signals from satellite radio beacons. Future HAARP experiments will determine the uses of long-sustained AI for enhanced HF communications.

  14. Persistent Scatterer Interferometry based detection of strong subsidence in Semarang, Indonesia

    Science.gov (United States)

    Cahyadi Kalia, Andre

    2016-04-01

    The City of Semarang (Indonesia) faces land subsidence since more than 100 years. The impact for the cities approximately 1.3 million inhabitants is severe: strong subsidence (up to several cm per year) affect the living environment, buildings and infrastructure. The main reasons for the subsidence is groundwater extraction, compaction of coastal sediments and construction load. In order to monitor the spatio-temporal variability of the subsidence phenomena the Persistent Scatterer Interferometry (PSI) is used. The presentation will show multiple PSI results and assess their characteristics with respect to PS density and coverage. The PSI analysis is based on SAR data stacks from ERS-1/-2 C-band data (1996-2000), ERS-1/-2 & Envisat-ASAR C-band data (2002-2006) and ALOS-Palsar L-band data (2006-2011). For the assessment of the PSI results thematic data (geological, hydrogeological maps) as well as orthorectified optical images (IKONOS 2005) are used. All three PSI results show an overall pattern of increasing subsidence towards the coastline where the subsurface is built up by unconsolidated coastal sediments. However, the PSI results based on C-band SAR data show a lower PS density ( 500 PS/km2 in urban areas) and PS coverage (no PSs in areas with rural land cover in the PSI results based on C-band) compared to the PSI result based on L-band SAR data. The main reason for this differences is the longer wavelength of the L-band (λ = 23.6 cm) compared to the C-band (λ = 5.6 cm) resulting in less temporal phase decorrelation through an increased penetration depth and higher capability to detect fast displacements.

  15. Coherent Exciton Dynamics in the Presence of Underdamped Vibrations.

    Science.gov (United States)

    Dijkstra, Arend G; Wang, Chen; Cao, Jianshu; Fleming, Graham R

    2015-02-19

    Recent ultrafast optical experiments show that excitons in large biological light-harvesting complexes are coupled to molecular vibration modes. These high-frequency vibrations will not only affect the optical response, but also drive the exciton transport. Here, using a model dimer system, the frequency of the underdamped vibration is shown to have a strong effect on the exciton dynamics such that quantum coherent oscillations in the system can be present even in the case of strong noise. Two mechanisms are identified to be responsible for the enhanced transport efficiency: critical damping due to the tunable effective strength of the coupling to the bath, and resonance coupling where the vibrational frequency coincides with the energy gap in the system. The interplay of these two mechanisms determines parameters responsible for the most efficient transport, and these optimal control parameters are comparable to those in realistic light-harvesting complexes. Interestingly, oscillations in the excitonic coherence at resonance are suppressed in comparison to the case of an off-resonant vibration.

  16. Magnetic exciton dispersion in praseodymium

    DEFF Research Database (Denmark)

    Rainford, B. D.; Houmann, Jens Christian Gylden

    1971-01-01

    Measurements of the dispersion of magnetic excitons have been made in a single crystal of praseodymium metal using inelastic neutron scattering. A preliminary analysis of the data yields the first detailed information about the exchange interactions and the crystal field splittings in the light...

  17. Exciton dynamics in molecular aggregates

    NARCIS (Netherlands)

    Augulis, R.; Pugžlys, A.; Loosdrecht, P.H.M. van; Pugzlys, A

    2006-01-01

    The fundamental aspects of exciton dynamics in double-wall cylindrical aggregates of cyanine dyes are studied by means of frequency resolved femtosecond pump-probe spectroscopy. The collective excitations of the aggregates, resulting from intermolecular dipole-dipole interactions have the characteri

  18. Exciton dynamics in molecular aggregates

    NARCIS (Netherlands)

    Augulis, R.; Pugžlys, A.; Loosdrecht, P.H.M. van; Pugzlys, A

    2006-01-01

    The fundamental aspects of exciton dynamics in double-wall cylindrical aggregates of cyanine dyes are studied by means of frequency resolved femtosecond pump-probe spectroscopy. The collective excitations of the aggregates, resulting from intermolecular dipole-dipole interactions have the

  19. Excitonic Bose-Einstein condensation in Ta2NiSe5 above room temperature

    Science.gov (United States)

    Seki, K.; Wakisaka, Y.; Kaneko, T.; Toriyama, T.; Konishi, T.; Sudayama, T.; Saini, N. L.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; Takagi, H.; Mizokawa, T.; Ohta, Y.

    2014-10-01

    We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on Ta2NiSe5 across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that Ta2NiSe5 falls in the Bose-Einstein condensation regime of the excitonic insulator state.

  20. Role of Exciton Screening in the 7/3 Fractional Quantum Hall Effect

    Science.gov (United States)

    Balram, Ajit C.; Wu, Ying-Hai; Sreejith, G. J.; Wójs, Arkadiusz; Jain, Jainendra K.

    2013-05-01

    The excitations of the 7/3 fractional Hall state, one of the most prominent states in the second Landau level, are not understood. We study the effect of screening by composite fermion excitons and find that it causes a strong renormalization at 7/3, thanks to a relatively small exciton gap and a relatively large residual interaction between composite fermions. The excitations of the 7/3 state are to be viewed as composite fermions dressed by a large exciton cloud. Their wide extent has implications for experiments as well as for analysis of finite system exact diagonalization studies.

  1. Rabi Oscillation of Exciton Dressed by Phonons In a Quantum Dot

    Institute of Scientific and Technical Information of China (English)

    李惠生; 朱卡的

    2003-01-01

    The effect of strong exciton-phonon interaction on the excitonic Rabi oscillations in a coherently driven quantum dot in a high-Q single mode cavity is investigated theoretically. We show that the Rabi oscillation of exciton dressed by phonons can persists with the Rabi frequency ge-λ/2 at absolute zero temperature, where g is the single-photon Rabi frequency and λ is the Huang-Rhys factor. The results also present that such coherent oscillations can be modified by manipulating the Rabi frequency of the driving field.

  2. Polarised two-photon excitation of quantum well excitons for manipulation of optically pumped terahertz lasers

    Energy Technology Data Exchange (ETDEWEB)

    Slavcheva, G., E-mail: gsk23@bath.ac.uk [Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ (United Kingdom); Kavokin, A.V., E-mail: A.Kavokin@soton.ac.uk [School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ (United Kingdom); Spin Optics Laboratory, St. Petersburg State University, 1, Ulyanovskaya 198504 (Russian Federation)

    2014-11-15

    Optical pumping of excited exciton states in a semiconductor quantum well embedded in a microcavity is a tool for realisation of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited (2p) and ground (1s) exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarisation of both pumping photons. Five-fold variation of the threshold power for terahertz lasing by switching from circular to co-linear pumping is predicted. We identify photon polarisation configurations for achieving maximum THz photon generation quantum efficiency.

  3. Theory of exciton dynamics in molecular aggregates in presence of polaronic effects

    Science.gov (United States)

    van Dijk, Leon; Spano, Frank C.; Bobbert, Peter A.

    2012-03-01

    We present a theory for the dynamics of excitons in molecular aggregates in presence of polaronic effects caused by a strong coupling to an intramolecular vibration. Transitions between exciton states are assumed to occur by weak coupling to low-energy acoustic phonons. The theory can describe measurements of the polarization anisotropy decay of helical aggregates of oligo(p-phenylene-vinylene) derivatives in solution. The fitted cut-off frequency of the acoustic phonons agrees well with an estimate of the Debye frequency of the solvent. The inclusion of polaronic effects leads for long times to a slower, but for short times to a faster exciton dynamics.

  4. Spatially indirect excitons in coupled quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Lai, Chih-Wei Eddy [Univ. of California, Berkeley, CA (United States)

    2004-03-01

    Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer)2 were

  5. Harvesting, Coupling, and Control of Single-Exciton Coherences in Photonic Waveguide Antennas.

    Science.gov (United States)

    Mermillod, Q; Jakubczyk, T; Delmonte, V; Delga, A; Peinke, E; Gérard, J-M; Claudon, J; Kasprzak, J

    2016-04-22

    We perform coherent nonlinear spectroscopy of individual excitons strongly confined in single InAs quantum dots (QDs). The retrieval of their intrinsically weak four-wave mixing (FWM) response is enabled by a one-dimensional dielectric waveguide antenna. Compared to a similar QD embedded in bulk media, the FWM detection sensitivity is enhanced by up to 4 orders of magnitude, over a broad operation bandwidth. Three-beam FWM is employed to investigate coherence and population dynamics within individual QD transitions. We retrieve their homogenous dephasing in a presence of low-frequency spectral wandering. Two-dimensional FWM reveals off-resonant Förster coupling between a pair of distinct QDs embedded in the antenna. We also detect a higher order QD nonlinearity (six-wave mixing) and use it to coherently control the FWM transient. Waveguide antennas enable us to conceive multicolor coherent manipulation schemes of individual emitters.

  6. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

    KAUST Repository

    Moody, Galan

    2015-09-18

    The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

  7. An image sensor capable of detecting nano-ampere transient signals with strong background illumination

    Energy Technology Data Exchange (ETDEWEB)

    Chu, D.D.; Dixon, D.A. [Sandia National Labs., Albuquerque, NM (United States); Thelen, D.C. Jr. [New Mexico Univ., Albuquerque, NM (United States). Microelectronics Research Center

    1995-10-01

    A readout detector integrated circuit (IC) has been developed which is capable of detecting nano-ampere photo-current signals of interest in a high (micro-ampere) background illumination or DC noise level (SNR=92dB). The readout detector sensor IC processes transient signals of interest from a separate photodiode array chip. Low noise signal conditioning, filtering, and signal thresholding implement smart sensor detection of only ``active pixels.`` This detector circuit can also be used to perform signal conditioning for other sensor applications that require detection of very small signals in a high background noise environment.

  8. Nonuniform Excitonic Charge Distribution Enhances Exciton-Phonon Coupling in ZnSe/CdSe Alloyed Quantum Dots.

    Science.gov (United States)

    Gong, Ke; Kelley, David F; Kelley, Anne Myers

    2017-02-02

    Zinc to cadmium cation exchange of ZnSe quantum dots has been used to produce a series of alloyed Zn1-xCdxSe quantum dots. As x increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens as x approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon peak from 251 cm(-1) in pure ZnSe to 210 cm(-1) in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons.

  9. Damping of Rabi oscillations in intensity-dependent photon echoes from exciton complexes in a CdTe/(Cd,Mg)Te single quantum well

    Science.gov (United States)

    Poltavtsev, S. V.; Reichelt, M.; Akimov, I. A.; Karczewski, G.; Wiater, M.; Wojtowicz, T.; Yakovlev, D. R.; Meier, T.; Bayer, M.

    2017-08-01

    We study Rabi oscillations detected in the coherent optical response from various exciton complexes in a 20-nm-thick CdTe/(Cd,Mg)Te quantum well using time-resolved photon echoes. In order to evaluate the role of exciton localization and inhomogeneous broadening we use selective excitation with spectrally narrow ps pulses. We demonstrate that the transient profile of the photon echo from the localized trion (X-) and the donor-bound exciton (D0X ) transitions strongly depends on the strength of the first pulse. It acquires a non-Gaussian shape and experiences significant advancement for pulse areas larger than π due to non-negligible inhomogeneity-induced dephasing of the oscillators during the optical excitation. Next, we observe that an increase of the area of either the first (excitation) or the second (rephasing) pulse leads to a significant damping of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex (D0X ). The measurements are analyzed using a theoretical model based on the optical Bloch equations which accounts for the inhomogeneity of optical transitions in order to reproduce the complex shape of the photon echo transients. In addition, the spreading of Rabi frequencies within the ensemble due to the spatial variation of the intensity of the focused Gaussian beams and excitation-induced dephasing are incorporated in our model, which is able to explain the fading and damping of Rabi oscillations. By analyzing the results of the simulation for X- and D0X complexes we are able to establish a correlation between the degree of localization and the transition dipole moments determined as μ (X- )=73 D and μ (D0X )=58 D.

  10. Coherent exciton-polariton devices

    Science.gov (United States)

    Fraser, Michael D.

    2017-09-01

    The Bose-Einstein condensate of exciton-polaritons has emerged as a unique, coherent system for the study of non-equilibrium, macroscopically coherent Bose gases, while the full confinement of this coherent state to a semiconductor chip has also generated considerable interest in developing novel applications employing the polariton condensate, possibly even at room temperature. Such devices include low-threshold lasers, precision inertial sensors, and circuits based on superfluidity with ultra-fast non-linear elements. While the demonstration and development of such devices are at an early stage, rapid progress is being made. In this review, an overview of the exciton-polariton condensate system and the established and emerging material systems and fabrication techniques are presented, followed by a critical, in-depth assessment of the ability of the coherent polariton system to deliver on its promise of devices offering either new functionality and/or room-temperature operation.

  11. A Thermally Activated Exciton-Exciton Collision Process in ZnO Microrods

    Institute of Scientific and Technical Information of China (English)

    ZHAO Dong-Xu; LIU Yi-Chun; SHEN De-Zhen; LU You-Ming; ZHANG Ji-Ying; FAN Xi-Wu

    2004-01-01

    @@ Room-temperature P-band emission induced by an exciton-exciton collision process was observed in ZnO microrods. Both temperature- and excitation-intensity-dependent photoluminescence (PL) measurements were conducted.

  12. One Dimensional Time-Dependent Tunnelling of Excitons

    Science.gov (United States)

    Kilcullen, Patrick; Salayka-Ladouceur, Logan; Malmgren, Kevin; Reid, Matthew; Shegelski, Mark R. A.

    2017-03-01

    We study the time-dependent tunnelling of excitons in one dimension using numerical integration based on the Crank-Nicholson method. A complete development of the time-dependent simulator is provided. External barriers studied include single and double delta barriers. We find that the appearance of transmission resonances depends strongly on the dielectric constant, relative effective masses, and initial spatial spread of the wavefunction. A discussion regarding applications to realistic systems is provided.

  13. Exciton Seebeck effect in molecular systems

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Yun-An, E-mail: yunan@nano.gznc.edu.cn [Guizhou Provincial Key Laboratory of Computational Nanomaterial Science, Guizhou Normal College, Guiyang, Guizhou 550018 (China); Cai, Shaohong [Guizhou Key Laboratory of Economic System Simulation, Guizhou University of Finance and Economics, Guiyang 550004 (China)

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  14. Exciton Seebeck effect in molecular systems.

    Science.gov (United States)

    Yan, Yun-An; Cai, Shaohong

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  15. Detection and characterisation of strong resistance to phosphine in Brazilian Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae).

    Science.gov (United States)

    Lorini, Irineu; Collins, Patrick J; Daglish, Gregory J; Nayak, Manoj K; Pavic, Hervoika

    2007-04-01

    As failure to control Rhyzopertha dominica (F.) with phosphine is a common problem in the grain-growing regions of Brazil, a study was undertaken to investigate the frequency, distribution and strength of phosphine resistance in R. dominica in Brazil. Nineteen samples of R. dominica were collected between 1991 and 2003 from central storages where phosphine fumigation had failed to control this species. Insects were cultured without selection until testing in 2005. Each sample was tested for resistance to phosphine on the basis of the response of adults to discriminating concentrations of phosphine (20 and 48 h exposures) and full dose-response assays (48 h exposure). Responses of the Brazilian R. dominica samples were compared with reference susceptible, weak-resistance and strong-resistance strains from Australia in parallel assays. All Brazilian population samples showed resistance to phosphine: five were diagnosed with weak resistance and 14 with strong resistance. Five samples showed levels of resistance similar to the reference strong-resistance strain. A representative highly resistant sample was characterised by exposing mixed-age cultures to a range of constant concentrations of phosphine for various exposure periods. Time to population extinction (TPE) and time to 99.9% suppression of population (LT(99.9)) values of this sample were generally similar to those of the reference strong-resistance strain. For example, at 0.1, 0.5 and 1.0 mg L(-1), LT(99.9) values for BR33 and the reference strong-resistance strain were respectively 21, 6.4 and 3.7 days and 17, 6.2 and 3.8 days. With both strains, doubling phosphine concentrations to 2 mg L(-1) resulted in increased LT(99.9) and TPE. High level and frequency of resistance in all population samples, some of which had been cultured without selection for up to 12 years, suggest little or no fitness deficit associated with phosphine resistance. The present research indicates that widespread phosphine resistance may

  16. Detection of the Helicobacter pylori dupA gene is strongly affected by the PCR design

    NARCIS (Netherlands)

    Abadi, Amin Talebi Bezmin; Loffeld, Ruud J L F; Constancia, Ashandra C; Wagenaar, Jaap A; Kusters, Johannes G

    2014-01-01

    The Helicobacter pylori virulence gene dupA is usually detected by PCR, but the primer binding sites used are highly variable. Our newly designed qPCR against a conserved region of dupA was positive in 64.2% of 394 clinical isolates while the positivity rate of the commonly used PCRs ranged from 29.

  17. Exciton-polariton dynamics in quantum dot-cavity system

    Energy Technology Data Exchange (ETDEWEB)

    Neto, Antonio F.; Lima, William J.; Villas-Boas, Jose M. [Universidade Federal de Uberlandia (UFU), MG (Brazil). Inst. de Fisica

    2012-07-01

    Full text: One of the basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. This imply in know all sources of decoherence and elaborate ways to avoid them. In recent work, A. Laucht et al. [1] presented detailed theoretical and experimental investigations of electrically tunable single quantum dot (QD) - photonic crystal (PhC) nanocavity systems operating in the strong coupling regime of the light matter interaction. Unlike previous studies, where the exciton-cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, they employ the quantum confined Stark-effect to electro-optically control the exciton-cavity detuning. The new built device enabled them to systematically probe the emission spectrum of the strongly coupled system as a function of external control parameters, as for example the incoherent excitation power density or the lattice temperature. Those studies reveal for the first time insights in dephasing mechanisms of 0D exciton polaritons [1]. In another study [2], using a similar device, they investigate the coupling between two different QDs with a single cavity mode. In both works, incoherent pumping was used, but for quantum information, coherent and controlled excitations are necessary. Here, we theoretically investigate the dynamics a single quantum dot inside a cavity under coherent pulse excitation and explore a wide range of parameters, as for example, the exciton-cavity detunings, the excitation power, the spontaneous decay, and pure dephasing. We use density matrix formalism in the Lindblad form, and we solve it numerically. Our results show that coherent excitation can be used to probe strong coupling between exciton and cavity mode by monitoring the exciton Rabi oscillation as function of the cavity detuning. This can give new insights for future experimental measurement focusing on quantum

  18. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system.

    Science.gov (United States)

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-05-13

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

  19. Ballistic spin transport in exciton gases

    Science.gov (United States)

    Kavokin, A. V.; Vladimirova, M.; Jouault, B.; Liew, T. C. H.; Leonard, J. R.; Butov, L. V.

    2013-11-01

    Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

  20. Quantum theory of exciton-photon coupling in photonic crystal slabs with embedded quantum wells

    CERN Document Server

    Gerace, D

    2007-01-01

    A theoretical description of radiation-matter coupling for semiconductor-based photonic crystal slabs is presented, in which quantum wells are embedded within the waveguide core layer. A full quantum theory is developed, by quantizing both the electromagnetic field with a spatial modulation of the refractive index and the exciton center of mass field in a periodic piecewise constant potential. The second-quantized hamiltonian of the interacting system is diagonalized with a generalized Hopfield method, thus yielding the complex dispersion of mixed exciton-photon modes including losses. The occurrence of both weak and strong coupling regimes is studied, and it is concluded that the new eigenstates of the system are described by quasi-particles called photonic crystal polaritons, which can occur in two situations: (i) below the light line, when a resonance between exciton and non-radiative photon levels occurs (guided polaritons), (ii) above the light line, provided the exciton-photon coupling is larger than th...

  1. Energy scaling for multi-exciton complexes in semiconductor quantum dots

    Science.gov (United States)

    Ipatov, Andrey; Gerchikov, Leonid; Christiano, Jordan

    2017-08-01

    The ground state properties of an multi-exciton (ME) complex localized in a nanoscale semiconductor quantum dot (QD) have been studied. The calculations have been performed using the envelope function approximation for electron and hole motion in the QD. The many-body quantum mechanical treatment of the electron-hole dynamics was done within the Density Functional Theory approach. The ground state energy dependencies upon QD radius, number of electron-hole pairs, QD dielectric function and effective masses of electron and holes have been analyzed. It is demonstrated that when multi-exciton complex is strongly localized within the QD, the physical properties of the system are determined by a single parameter, the ratio of QD and free exciton radii, and its binding energy is given by the function of this parameter multiplied by the binding energy of an isolated exciton in bulk semiconductor.

  2. In Situ Detection of Strong Langmuir Turbulence Processes in Solar Type III Radio Bursts

    Science.gov (United States)

    Golla, Thejappa; Macdowall, Robert J.; Bergamo, M.

    2012-01-01

    The high time resolution observations obtained by the WAVES experiment of the STEREO spacecraft in solar type III radio bursts show that Langmuir waves often occur as intense localized wave packets. These wave packets are characterized by short durations of only a few ms and peak intensities, which well exceed the supersonic modulational instability (MI) thresholds. These timescales and peak intensities satisfy the criterion of the solitons collapsed to spatial scales of a few hundred Debye lengths. The spectra of these wave packets consist of primary spectral peaks corresponding to beam-resonant Langmuir waves, two or more sidebands corresponding to down-shifted and up-shifted daughter Langmuir waves, and low frequency enhancements below a few hundred Hz corresponding to daughter ion sound waves. The frequencies and wave numbers of these spectral components satisfy the resonance conditions of the modulational instability (MI). Moreover, the tricoherences, computed using trispectral analysis techniques show that these spectral components are coupled to each other with a high degree of coherency as expected of the MI type of four wave interactions. The high intensities, short scale lengths, sideband spectral structures and low frequency spectral enhancements and, high levels of tricoherences amongst the spectral components of these wave packets provide unambiguous evidence for the supersonic MI and related strong turbulence processes in type III radio bursts. The implication of these observations include: (1) the MI and related strong turbulence processes often occur in type III source regions, (2) the strong turbulence processes probably play very important roles in beam stabilization as well as conversion of Langmuir waves into escaping radiation at the fundamental and second harmonic of the electron plasma frequency, fpe, and (3) the Langmuir collapse probably follows the route of MI in type III radio bursts.

  3. Resonantly excited exciton dynamics in two-dimensional MoSe2 monolayers

    Science.gov (United States)

    Scarpelli, L.; Masia, F.; Alexeev, E. M.; Withers, F.; Tartakovskii, A. I.; Novoselov, K. S.; Langbein, W.

    2017-07-01

    We report on the exciton and trion density dynamics in a single layer of MoSe2, resonantly excited and probed using three-pulse four-wave mixing (FWM), at temperatures from 300 K to 77 K. A multiexponential third-order response function for amplitude and phase of the heterodyne-detected FWM signal including four decay processes is used to model the data. We provide a consistent interpretation within the intrinsic band structure, not requiring the inclusion of extrinsic effects. We find an exciton radiative lifetime in the subpicosecond range consistent to what has been recently reported by Jakubczyk et al. [Nano Lett. 16, 5333 (2016), 10.1021/acs.nanolett.6b01060]. After the dominating radiative decay, the remaining exciton density, which has been scattered from the initially excited direct spin-allowed radiative state into dark states of different nature by exciton-phonon scattering or disorder scattering, shows a slower dynamics, covering 10-ps to 10-ns time scales. This includes direct spin-allowed transitions with larger in-plane momentum, as well as indirect and spin-forbidden exciton states. We find that exciton-exciton annihilation is not relevant in the observed dynamics, in variance from previous finding under nonresonant excitation. The trion density at 77 K reveals a decay of the order of 1 ps, similar to what is observed for the exciton. After few tens of picoseconds, the trion dynamics resembles the one of the exciton, indicating that trion ionization occurs on this time scale.

  4. Detection of Strong Short-Term Variability in NGC 6946 X-1

    CERN Document Server

    Rao, Fengyun; Kaaret, Philip

    2010-01-01

    Using two archival XMM-Newton observations, we identify strong X-ray flux variations in NGC 6946 X-1 indicating it is the most variable ultraluminous X-ray source (ULX) on mHz time scales known so far. The 1-10 keV lightcurve exhibits variability with a fractional rms amplitude of 60% integrated in the frequency range of 1-100 mHz. The power spectral density of the source shows a flat-topped spectrum that breaks at about 3 mHz with possible quasi-periodic oscillations (QPOs) near 8.5 mHz. Black hole binaries usually produce strong fast variability in the hard or intermediate state. The energy spectrum of NGC 6946 X-1 is dominated by two components, a 0.18~keV thermal disk and a power law with a photon index of ~2.2, which is consistent with the intermediate state. The characteristic time scales of the X-ray emission suggests that the ULX may contain a black hole with a mass on the order of 10^3.

  5. Suitable triggering algorithms for detecting strong ground motions using MEMS accelerometers

    Science.gov (United States)

    Jakka, Ravi Sankar; Garg, Siddharth

    2015-03-01

    With the recent development of digital Micro Electro Mechanical System (MEMS) sensors, the cost of monitoring and detecting seismic events in real time can be greatly reduced. Ability of MEMS accelerograph to record a seismic event depends upon the efficiency of triggering algorithm, apart from the sensor's sensitivity. There are several classic triggering algorithms developed to detect seismic events, ranging from basic amplitude threshold to more sophisticated pattern recognition. Algorithms based on STA/LTA are reported to be computationally efficient for real time monitoring. In this paper, we analyzed several STA/LTA algorithms to check their efficiency and suitability using data obtained from the Quake Catcher Network (network of MEMS accelerometer stations). We found that most of the STA/LTA algorithms are suitable for use with MEMS accelerometer data to accurately detect seismic events. However, the efficiency of any particular algorithm is found to be dependent on the parameter set used (i.e., window width of STA, LTA and threshold level).

  6. Strong critical current density enhancement in NiCu/NbN superconducting nanostripes for optical detection

    Science.gov (United States)

    Marrocco, N.; Pepe, G. P.; Capretti, A.; Parlato, L.; Pagliarulo, V.; Peluso, G.; Barone, A.; Cristiano, R.; Ejrnaes, M.; Casaburi, A.; Kashiwazaki, N.; Taino, T.; Myoren, H.; Sobolewski, Roman

    2010-08-01

    We present measurements of ferromagnet/superconductor (NiCu/NbN) and plain superconducting (NbN) nanostripes with the linewidth ranging from 150 to 300 nm. The NiCu (3 nm)/NbN (8 nm) bilayers, as compared to NbN (8 nm), showed a up to six times increase in their critical current density, reaching at 4.2 K the values of 5.5 MA/cm2 for a 150 nm wide nanostripe meander and 12.1 MA/cm2 for a 300 nm one. We also observed six-time sensitivity enhancement when the 150 nm wide NiCu/NbN nanostripe was used as an optical detector. The strong critical current enhancement is explained by the vortex pinning strength and density increase in NiCu/NbN bilayers and confirmed by approximately tenfold increase in the vortex polarizability factor.

  7. Plexciton Dynamics: Exciton-Plasmon Coupling in a J-Aggregate-Au Nanoshell Complex Provides a Mechanism for Nonlinearity

    Energy Technology Data Exchange (ETDEWEB)

    Fofang, Nche T. [Rice Univ., Houston, TX (United States); Grady, Nathaniel K. [Rice Univ., Houston, TX (United States); Fan, Zhiyuan [Ohio Univ., Athens, OH (United States); Govorov, Alexander [Ohio Univ., Athens, OH (United States); Halas, Naomi J. [Rice Univ., Houston, TX (United States)

    2011-03-18

    Coherently coupled plasmons and excitons give rise to new optical excitations- plexcitons - due to the strong coupling of these two oscillator systems. Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system. Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon.

  8. Triplet excitons as sensitive spin probes for structure analysis of extended defects in microcrystalline silicon

    Science.gov (United States)

    Meier, Christoph; Teutloff, Christian; Behrends, Jan; Bittl, Robert; Astakhov, Oleksandr; Lips, Klaus

    2016-07-01

    Electrically detected magnetic resonance (EDMR) spectroscopy is employed to study the influence of triplet excitons on the photocurrent in state-of-the-art microcrystalline silicon thin-film solar cells. These triplet excitons are used as sensitive spin probes for the investigation of their electronic and nuclear environment in this mixed-phase material. According to low-temperature EDMR results obtained from solar cells with different excitons reside at extended defects in the crystallites of microcrystalline silicon that give rise to shallow states in the silicon band gap. The excitons possess a rather delocalized wave function, couple to electron spins in conduction band tail states nearby, and take part in a spin-dependent recombination process. Our study shows that extended defects such as grain boundaries or stacking faults in the crystalline part of the material act as charge carrier traps that can influence the material conductivity.

  9. A method of detecting capture γ-rays in a strong pulsed radiation field

    Institute of Scientific and Technical Information of China (English)

    徐荣昆; 唐正元; 何锡钧

    2002-01-01

    We present a technology for diagnosing the D-T fusion process by detecting capture γ-rays. This technologyprovides an alternative route to diagnosing the D-T reaction process when a great deal of heavy Z materials surroundsthe D-T region. A very important aspect of this paper is to focus on the methods of shielding low-energy γ-rays whoseradiation intensity is 106 times higher than that of the capture γ-rays. Another aspect is about how to distinguishsignal from noise. The result of a 50/1 signal-to-noise ratio indicates that the designed double-magnetic spectrographis very successful for diagnosing the D-T fusion reaction process.

  10. Quasiparticle Gaps and Exciton Coulomb Energies in Si Nanoshells

    Energy Technology Data Exchange (ETDEWEB)

    Frey, K. [University of Illinois, Chicago; Idrobo Tapia, Juan C [ORNL; Tiago, Murilo L [ORNL; Reboredo, Fernando A [ORNL; Ogut, Serdar [University of Illinois, Chicago

    2009-01-01

    Quasiparticle gaps and exciton Coulomb energies of H-passivated spherical Si nanoshells are computed using rst principles SCF and GW methods. We nd that the quasiparticle gap of a nanoshell depends on both its inner radius R1 (weakly) and outer radius R2 (strongly). These dependences on R1 and R2 are mostly consistent with electrostatics of a metallic shell. We also nd that the unscreened Coulomb energy ECoul in Si nanoshells has a somewhat unexpected size dependence at xed outer radius R2: ECoul decreases as the nanoshell becomes more conning, contrary to what one would expect from quantum connement eects. We show that this is a consequence of an increase in the average electron-hole distance, giving rise to reduced exciton Coulomb energies in spite of the reduction in the conning nanoshell volume.

  11. Multiple exciton collection in a sensitized photovoltaic system.

    Science.gov (United States)

    Sambur, Justin B; Novet, Thomas; Parkinson, B A

    2010-10-01

    Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO(2) single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO(2) facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.

  12. Jointly Tuned Plasmonic–Excitonic Photovoltaics Using Nanoshells

    KAUST Repository

    Paz-Soldan, Daniel

    2013-04-10

    Recent advances in spectrally tuned, solution-processed plasmonic nanoparticles have provided unprecedented control over light\\'s propagation and absorption via engineering at the nanoscale. Simultaneous parallel progress in colloidal quantum dot photovoltaics offers the potential for low-cost, large-area solar power; however, these devices suffer from poor quantum efficiency in the more weakly absorbed infrared portion of the sun\\'s spectrum. Here, we report a plasmonic-excitonic solar cell that combines two classes of solution-processed infrared materials that we tune jointly. We show through experiment and theory that a plasmonic-excitonic design using gold nanoshells with optimized single particle scattering-to-absorption cross-section ratios leads to a strong enhancement in near-field absorption and a resultant 35% enhancement in photocurrent in the performance-limiting near-infrared spectral region. © 2013 American Chemical Society.

  13. Exciton-like molecules and interaction resonances in bilayer ultracold gases

    Science.gov (United States)

    Kanasz-Nagy, Marton; Demler, Eugene; Zarand, Gergely

    2016-05-01

    I will discuss how confinement effects in bilayer quantum gases can lead to interlayer molecular states, that appear both at positive and negative energies and even at layer separations many times larger than the interspecies scattering length. Similar to excitons in bilayer quantum wells, the lifetime of the molecules grows significantly with increasing layer separation, allowing for their detection in simple shaking experiments. Moreover, these molecular states also give rise to sharp interspecies Feshbach resonances, enabling one to control the interaction between the two species geometrically, simply by changing the layer separation. Rather counterintuitively, the species can be made strongly interacting, by separating them from each other. Support: Hungarian Research Fund OTKA Grant K105149, the Harvard-MIT CUA, the DARPA OLE program, the NSF Grant DMR-1308435, the AFOSR New Quantum Phases of Matter MURI on Ultracold Molecules, and the ARO-MURI on Atomtronics projects.

  14. Radiative recombination of excitons in amorphous semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Singh, Jai [School of Engineering and Logistics, Faculty Technology, B-41, Charles Darwin University, Darwin, NT 0909 (Australia)]. E-mail: jai.singh@cdu.edu.au

    2005-04-15

    A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments.

  15. Detectability of lupine seeds by ELISA and PCR may be strongly influenced by potential differences between cultivars.

    Science.gov (United States)

    Röder, Martin; Kleiner, Kornelia; Sachs, Andrea; Keil, Nicole; Holzhauser, Thomas

    2013-06-26

    Accurate methods for allergen detection are needed for the verification of allergen labeling and the avoidance of hidden allergens. But systematic data on the influence of different cultivars of allergenic crop species on their detectability in enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) are lacking. As one example, seeds of 14 different cultivars of lupine (Lupinus albus, Lupinus angustifolius, Lupinus luteus) were investigated for total protein according to a Kjeldahl method, and for their relative quantitative detectability in three commercial lupine-specific ELISA tests and four lupine-specific PCR methods. Total Kjeldahl nitrogen allowed an accurate quantification of total protein. Relative differences in quantitative response between cultivars of 390-5050% and 480-13,600% were observed between ELISA kits and PCR methods, respectively. Hence, quantitative results of selected ELISA and PCR methods may be strongly influenced by the examined lupine cultivar.

  16. Nationwide registry-based analysis of cancer clustering detects strong familial occurrence of Kaposi sarcoma.

    Science.gov (United States)

    Kaasinen, Eevi; Aavikko, Mervi; Vahteristo, Pia; Patama, Toni; Li, Yilong; Saarinen, Silva; Kilpivaara, Outi; Pitkänen, Esa; Knekt, Paul; Laaksonen, Maarit; Artama, Miia; Lehtonen, Rainer; Aaltonen, Lauri A; Pukkala, Eero

    2013-01-01

    Many cancer predisposition syndromes are rare or have incomplete penetrance, and traditional epidemiological tools are not well suited for their detection. Here we have used an approach that employs the entire population based data in the Finnish Cancer Registry (FCR) for analyzing familial aggregation of all types of cancer, in order to find evidence for previously unrecognized cancer susceptibility conditions. We performed a systematic clustering of 878,593 patients in FCR based on family name at birth, municipality of birth, and tumor type, diagnosed between years 1952 and 2011. We also estimated the familial occurrence of the tumor types using cluster score that reflects the proportion of patients belonging to the most significant clusters compared to all patients in Finland. The clustering effort identified 25,910 birth name-municipality based clusters representing 183 different tumor types characterized by topography and morphology. We produced information about familial occurrence of hundreds of tumor types, and many of the tumor types with high cluster score represented known cancer syndromes. Unexpectedly, Kaposi sarcoma (KS) also produced a very high score (cluster score 1.91, p-value <0.0001). We verified from population records that many of the KS patients forming the clusters were indeed close relatives, and identified one family with five affected individuals in two generations and several families with two first degree relatives. Our approach is unique in enabling systematic examination of a national epidemiological database to derive evidence of aberrant familial aggregation of all tumor types, both common and rare. It allowed effortless identification of families displaying features of both known as well as potentially novel cancer predisposition conditions, including striking familial aggregation of KS. Further work with high-throughput methods should elucidate the molecular basis of the potentially novel predisposition conditions found in this

  17. Nationwide registry-based analysis of cancer clustering detects strong familial occurrence of Kaposi sarcoma.

    Directory of Open Access Journals (Sweden)

    Eevi Kaasinen

    Full Text Available Many cancer predisposition syndromes are rare or have incomplete penetrance, and traditional epidemiological tools are not well suited for their detection. Here we have used an approach that employs the entire population based data in the Finnish Cancer Registry (FCR for analyzing familial aggregation of all types of cancer, in order to find evidence for previously unrecognized cancer susceptibility conditions. We performed a systematic clustering of 878,593 patients in FCR based on family name at birth, municipality of birth, and tumor type, diagnosed between years 1952 and 2011. We also estimated the familial occurrence of the tumor types using cluster score that reflects the proportion of patients belonging to the most significant clusters compared to all patients in Finland. The clustering effort identified 25,910 birth name-municipality based clusters representing 183 different tumor types characterized by topography and morphology. We produced information about familial occurrence of hundreds of tumor types, and many of the tumor types with high cluster score represented known cancer syndromes. Unexpectedly, Kaposi sarcoma (KS also produced a very high score (cluster score 1.91, p-value <0.0001. We verified from population records that many of the KS patients forming the clusters were indeed close relatives, and identified one family with five affected individuals in two generations and several families with two first degree relatives. Our approach is unique in enabling systematic examination of a national epidemiological database to derive evidence of aberrant familial aggregation of all tumor types, both common and rare. It allowed effortless identification of families displaying features of both known as well as potentially novel cancer predisposition conditions, including striking familial aggregation of KS. Further work with high-throughput methods should elucidate the molecular basis of the potentially novel predisposition conditions

  18. Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals.

    Science.gov (United States)

    Grivas, Christos; Li, Chunyong; Andreakou, Peristera; Wang, Pengfei; Ding, Ming; Brambilla, Gilberto; Manna, Liberato; Lagoudakis, Pavlos

    2013-01-01

    Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton-bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.

  19. Magnetic brightening and control of dark excitons in monolayer WSe2

    Science.gov (United States)

    Zhang, Xiao-Xiao; Cao, Ting; Lu, Zhengguang; Lin, Yu-Chuan; Zhang, Fan; Wang, Ying; Li, Zhiqiang; Hone, James C.; Robinson, Joshua A.; Smirnov, Dmitry; Louie, Steven G.; Heinz, Tony F.

    2017-09-01

    Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

  20. Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube-C60 Heterojunctions

    Energy Technology Data Exchange (ETDEWEB)

    Dowgiallo, Anne-Marie; Mistry, Kevin S.; Johnson, Justin C.; Reid, Obadiah G.; Blackburn, Jeffrey L.

    2016-05-19

    The efficiency of thin-film organic photovoltaic (OPV) devices relies heavily upon the transport of excitons to type-II heterojunction interfaces, where there is sufficient driving force for exciton dissociation and ultimately the formation of charge carriers. Semiconducting single-walled carbon nanotubes (SWCNTs) are strong near-infrared absorbers that form type-II heterojunctions with fullerenes such as C60. Although the efficiencies of SWCNT-fullerene OPV devices have climbed over the past few years, questions remain regarding the fundamental factors that currently limit their performance. In this study, we determine the exciton diffusion length in the C60 layer of SWCNT-C60 bilayer active layers using femtosecond transient absorption measurements. We demonstrate that hole transfer from photoexcited C60 molecules to SWCNTs can be tracked by the growth of narrow spectroscopic signatures of holes in the SWCNT 'reporter layer'. In bilayers with thick C60 layers, the SWCNT charge-related signatures display a slow rise over hundreds of picoseconds, reflecting exciton diffusion through the C60 layer to the interface. A model based on exciton diffusion with a Beer-Lambert excitation profile, as well as Monte Carlo simulations, gives the best fit to the data as a function of C60 layer thickness using an exciton diffusion length of approximately 5 nm.

  1. Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2

    Science.gov (United States)

    Sim, Sangwan; Lee, Doeon; Noh, Minji; Cha, Soonyoung; Soh, Chan Ho; Sung, Ji Ho; Jo, Moon-Ho; Choi, Hyunyong

    2016-11-01

    The optical Stark effect is a coherent light-matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states.

  2. Effect of carrier confinement on effective mass of excitons and estimation of ultralow disorder in Al x Ga1-x As/GaAs quantum wells by magneto-photoluminescence.

    Science.gov (United States)

    Haldar, S; Dixit, V K; Vashisht, Geetanjali; Khamari, Shailesh Kumar; Porwal, S; Sharma, T K; Oak, S M

    2017-07-07

    Effect of charge carrier confinement and ultra-low disorder acquainted in AlGaAs/GaAs multi-quantum well system is investigated via Magneto-photoluminescence spectroscopy. Significant increase of effective mass is observed for the confined exciton in narrow QWs. The foremost reason behind such an observation is due to the induced non-parabolicity in bands. Moreover, as the thickness of the QW are reduced, confined excitons in QW experience atomic irregularities at the hetero-junctions and their effects are prominent in the photoluminescence linewidth. Amount of photoluminescence line-broadening caused by the atomic irregularities at the hetero-junctions is correlated with average fluctuation (δ 1) in QW thickness. The estimated δ 1 for Al0.3Ga0.7As/GaAs QWs are found to be ±(0.14 - 1.6)× 'one monolayer thickness of GaAs layer'. Further, the strong perturbations due to magnetic field in a system helps in realizing optical properties of exciton in QWs, where magnetic field is used as a probe to detect ultralow defects in the QW. Additionally, the influence of magnetic field on the free and bound exciton luminescence is explained by a simple model. The proposed approach for measuring the interface and volume defects in an ultra-low disordered system by Magneto-PL spectroscopy technique will be highly beneficial in high mobility devices for advanced applications.

  3. Visualization of molecular excitons diffusion

    Science.gov (United States)

    Salamatova, Evgeniia; Kozlov, Oleg V.; Luponosov, Yuriy N.; Solodukhin, Alexander N.; Toropynina, Viktoria Y.; Ponomarenko, Sergei A.; Pshenichnikov, Maxim S.

    2016-09-01

    Small organic molecules of the push-pull architecture are rapidly gaining their status in the organic electronics applications. In densely packed molecular films, both intra- and intermolecular interactions play an essential role for the device performance. Here we study two different molecules, a highly symmetric star-shaped one and its newly synthesized single arm analogue, for their photophysical properties. Both chromophores were dissolved in a solid matrix at different concentrations to vary their separation and therefore intermolecular coupling. We show that in both molecules the population relaxation accelerates by more than a factor of 10 at shorter intermolecular distances due to self-quenching thereby reducing the exciton survival time. The transient anisotropy dynamics are also quite similar, with their substantial acceleration at shorter interchromophore distances due to exciton diffusion caused by the Förster-like resonance energy transfer. However, the anisotropy values are noticeably lower for the star-shaped molecule because of intramolecular mixing of different polarization states. Finally, a model is presented that accounts for the observed results.

  4. Growth temperature dependence of exciton lifetime in wurtzite InP nanowires grown on silicon substrates

    Science.gov (United States)

    Chauvin, N.; Hadj Alouane, M. H.; Anufriev, R.; Khmissi, H.; Naji, K.; Patriarche, G.; Bru-Chevallier, C.; Gendry, M.

    2012-01-01

    InP nanowires grown on silicon substrate are investigated using time-resolved spectroscopy. A strong modification of the exciton lifetime is observed (from 0.11 to 1.2 ns) when the growth temperature is increased from 340 °C to 460 °C. This strong dependence is not related to the density of zinc-blende insertions in the wurtzite nanowires or to the wurtzite exciton linewidth. The excitation power dependence of the lifetime and linewidth is investigated, and these results allow us to interpret the growth temperature dependence on the lifetime as a consequence of the reduction of the surface recombination velocity with the growth temperature.

  5. Bright, long-lived and coherent excitons in carbon nanotube quantum dots.

    Science.gov (United States)

    Hofmann, Matthias S; Glückert, Jan T; Noé, Jonathan; Bourjau, Christian; Dehmel, Raphael; Högele, Alexander

    2013-07-01

    Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances, promise long electron spin coherence times in a nuclear-spin free lattice for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions for optoelectronic applications. Excitons in semiconducting single-walled carbon nanotubes could facilitate the upconversion of spin, mechanical or hybrid spin-mechanical degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites, environmental dephasing and emission intermittence. Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

  6. Plasmon-exciton interaction and screening of exciton in ZnO-based thin film on bulk Pt as analyzed by spectroscopic ellipsometry

    Science.gov (United States)

    Darma, Yudi; Dimas Satrya, Christoforus; Marlina, Resti; Kurniawan, Robi; Herng, Tun Seng; Ding, Jun; Rusydi, Andrivo

    2017-01-01

    We study plasmon-exciton interaction in ZnO-based thin film on bulk Pt by using high resolution spectroscopic ellipsometry. ZnO films on quartz are used as reference. This study shows the strong electronic interactions between ZnO film and Pt by considering the significant suppression of exciton in ZnO film, in comparison to ZnO film on quartz. We found that plasmon in Pt are responsible to provide transferred electron for electronic blocking of exciton in ZnO film induce by spontaneous recombination from Pt. In the case of Cu doped ZnO film, we confirm screening effects on exciton and a localized interband transition for both systems (ZnO film on Pt and ZnO film on quartz). In Cu-doped ZnO film, electronic blocking of exciton by Pt plasmon is more pronounce rather than screening effect by interband transition. Our results show the importance of plasmon from substrate and doping to modify the optical properties of wide bandgap semiconductor.

  7. Ionization of excitons moving perpendicularly to magnetic field in GaAs/Al sub x Ga sub 1 sub - sub x As superlattice

    CERN Document Server

    Zakharchenya, B P

    2001-01-01

    By means of resonance Raman spectroscopy one investigated into the effect of magnetic field on spectrum of excitons associated with various minibands in superlattices. Intensity of Raman scattering in acoustic phonons with involvement of exciton ground state linked with the second miniband is detected to become weaker abruptly in weak magnetic fields in case the vector of its velocity is orthogonal to the external magnetic field. This phenomenon is explained by exciton ionization in electric field occurring within system of coordinates linked with exciton moving perpendicular to the external magnetic field

  8. Excitonic polaritons of zinc diarsenide single crystals

    Science.gov (United States)

    Syrbu, N. N.; Stamov, I. G.; Zalamai, V. V.; Dorogan, A.

    2017-02-01

    Excitonic polaritons of ZnAs2 single crystals had been investigated. Parameters of singlet excitons with D2bar(z) symmetry and orthoexcitons 2D1bar(y)+D2bar(x) had been determined. Spectral dependencies of ordinary and extraordinary dispersion of refractive index had been calculated using interferential reflection and transmittance spectra. It was shown, that A excitonic series were due to hole (V1) and electron (C1) bands. The values of effective masses of electrons (mc*=0.10 m0) and holes (mv1*=0.89 m0) had been estimated. It was revealed that the hole mass mv1* changes from 1.03 m0 to 0.55 m0 at temperature increasing from 10 K up to 230 K and that the electron mass mc* does not depend on temperature. The integral absorption A (eV cm-1) of the states n=1, 2 and 3 of D2bar(z) excitons depends on the An≈n-3 equality, which it is characteristic for S-type excitonic functions. Temperature dependences of the integral absorption of ground states for D2bar(z) and D2bar(D) excitons differ. The ground states of B and C excitons formed by V3 - C1 and V4 - C1 bands and its parameters had been determined.

  9. Instantaneous Rayleigh scattering from excitons localized in monolayer islands

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Leosson, Kristjan; Jensen, Jacob Riis;

    2000-01-01

    We show that the initial dynamics of Rayleigh scattering from excitons in quantum wells can be either instantaneous or delayed, depending on the exciton ensemble studied. For excitation of the entire exciton resonance, a finite rise time given by the inverse inhomogeneous broadening: of the exciton...... resonance is observed. Instead, when exciting only a subsystem of the exciton resonance, in our case excitons localized in quantum well regions of a specific monolayer thickness, the rise has an instantaneous component. This is due to the spatial nonuniformity of the initially excited exciton polarization...

  10. Growth of self-assembled (Ga)InAs/GaAs quantum dots and realization of high quality microcavities for experiments in the field of strong exciton photon coupling; Selbstorganisiertes Wachstum von (Ga)InAs/GaAs-Quantenpunkten und Entwicklung von Mikroresonatoren hoechster Guete fuer Experimente zur starken Exziton-Photon-Kopplung

    Energy Technology Data Exchange (ETDEWEB)

    Loeffler, Andreas

    2008-11-05

    cavity mode and the exciton could be observed. A Rabi-splitting of about 60 {mu}eV was measured for circular GaInAs dots with an indium content of 43 % and diameters between 20 and 25 nm. we could conclude an oscillator strength of approximately 40-50 for the enlarged quantum dot structures. In contrast to that, the slightly smaller dots with an indium content of 43 % only show an oscillator strength of about 15-20. Furthermore, doped microcavities were realized with regard to electrically driven devices. The investigated electrically driven mircocavities with embedded GaInAs quantum dots were operating in the weak coupling regime and showed a clear Purcell effect with a Purcell factor in resonance of about 10. Due to the use of enlarged GaInAs quantum dots, we were able to reach the strong coupling regime with a vacuum Rabi-splitting of 85 {mu}eV also for electrically driven micropillars. (orig.)

  11. Excitons in the Fractional Quantum Hall Effect

    Science.gov (United States)

    Laughlin, R. B.

    1984-09-01

    Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.

  12. Exciton-photon coupling in a ZnSe based microcavity fabricated using epitaxial liftoff

    CERN Document Server

    Curran, A; Morrod, J K; Prior, K A; Warburton, R J

    2007-01-01

    We report the observation of strong exciton-photon coupling in a ZnSe based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn$_{0.9}$Cd$_{0.1}$Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO$_2$/Ta$_2$O$_5$ mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of $23.6 meV$ at $20 K$; composite evolution of the lower and upper polaritons; and narrowing of the lower polariton linewidth near resonance. The heavy hole exciton oscillator strength per quantum well is also deduced to be $1.78 \\times 10^{13} cm^{-2}$.

  13. Optical spectrum of MoS2: many-body effects and diversity of exciton states.

    Science.gov (United States)

    Qiu, Diana Y; da Jornada, Felipe H; Louie, Steven G

    2013-11-22

    We present first-principles calculations of the optical response of monolayer molybdenum disulfide employing the GW-Bethe-Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. We show that monolayer MoS2 possesses a large and diverse number of strongly bound excitonic states with novel k-space characteristics that were not previously seen experimentally or theoretically. The absorption spectrum is shown to be dominated by excitonic states with a binding energy close to 1 eV and by strong electron-phonon broadening in the visible to ultraviolet range. Our results explain recent experimental measurements and resolve inconsistencies between previous GW-BSE calculations.

  14. HST hot Jupiter Transmission Spectral Survey: A detection of Na and strong optical absorption in HAT-P-1b

    CERN Document Server

    Nikolov, N; Pont, F; Burrows, A S; Fortney, J J; Ballester, G E; Evans, T M; Huitson, C M; Wakeford, H R; Wilson, P A; Aigrain, S; Deming, D; Gibson, N P; Henry, G W; Knutson, H; Etangs, A Lecavelier des; Showman, A P; Vidal-Madjar, A; Zahnle, K

    2013-01-01

    We present an optical to near-infrared transmission spectrum of the hot Jupiter HAT-P-1b, based on HST observations, covering the spectral regime from 0.29 to 1.027{\\mu}m with STIS, which is coupled with a recent WFC3 transit (1.087 to 1.687{\\mu}m). We derive refined physical parameters of the HAT-P-1 system, including an improved orbital ephemeris. The transmission spectrum shows a strong absorption signature shortward of 0.55{\\mu}m, with a strong blueward slope into the near-ultraviolet. We detect atmospheric sodium absorption at a 3.3{\\sigma} significance level, but find no evidence for the potassium feature. The red data implies a marginally flat spectrum with a tentative absorption enhancement at wavelength longer than ~0.85{\\mu}m. The STIS and WFC3 spectra differ significantly in absolute radius level (4.3 +/- 1.6 pressure scale heights), implying strong optical absorption in the atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be explained by stellar activity, as simulta- neous st...

  15. Optical nutation in the exciton range of spectrum

    Energy Technology Data Exchange (ETDEWEB)

    Khadzhi, P. I. [Academy of Sciences of Moldova, Institute of Applied Physics (Moldova, Republic of); Vasiliev, V. V., E-mail: vasscorp@mail.ru [Taras Shevchenko Transnistria State University (Moldova, Republic of)

    2013-08-15

    Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

  16. Role of delocalized exciton states of light-harvesting pigments in excitation energy transfer in natural photosynthesis

    Science.gov (United States)

    Fetisova, Zoya; Mauring, Koit; Taisova, Alexandra

    1995-02-01

    Photosynthesis is an extremely efficient converter of light into chemical energy, with an observed quantum yield for primary photochemistry approximately 90%. To achieve this the photosynthetic apparatus must be highly optimized, and some of the design principles that may be involved have been suggested. The role of delocalized exciton states of light-harvesting pigments in the energy transfer process has been considered by mathematical simulation of the light-harvesting process in model systems. Namely, it has been shown that aggregation of antenna pigments (allowing to consider each aggregate as a supermolecule) is biologically expedient, as an efficient strategy for light harvesting in photosynthesis. The question of whether this design principle is realized in a natural antenna has been examined for the 3D chlorosomal superantenna of green bacteria with the hole-burning spectroscopy. Spectral hole burning studies of intact cells of green bacteria Chlorobium phaeovibriodes. Chloroflexus aurantiacus and Chlorobium limicola have proven that the Qy- absorption system of antenna bacteriochlorophylls e or c (BChl e or BChl c) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 transition band of the lowest exciton state of BChl e and BChl c aggregates has been directly detected as the lowest energy inhomogeneously broadened band of the 1.8 K near-infrared excitation spectrum. These lowest energy bands have different spectral position of their maximums: approximately 739 nm in C.phaeovibriodes (BChl e band), approximately 752 nm in C.aurantiacus (BChl c band) and approximately 774 nm in C.limicola (BChl c band) cells. However, these bands display a number of fundamentally similar spectral features: (1) The magnitude of inhomogeneous broadening of these bands is 90 - 100 cm-1; (2) The width of each band is 2 - 3 times less than that of the monomeric BChl c (or BChl e) in vitro at 5 K; (3) Each band

  17. Charged-Exciton Complexes in Quantum Dots

    Institute of Scientific and Technical Information of China (English)

    XIE Wen-Fang

    2001-01-01

    It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.``

  18. Can disorder enhance incoherent exciton diffusion?

    CERN Document Server

    Lee, Elizabeth M Y; Willard, Adam P

    2015-01-01

    Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we present a general model, based upon F\\"orster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific diffusivity is broadened in a manner that results i...

  19. Exciton absorption in narrow armchair graphene nanoribbons

    Science.gov (United States)

    Monozon, B. S.; Schmelcher, P.

    2016-11-01

    We develop an analytical approach to the exciton optical absorption for narrow gap armchair graphene nanoribbons (AGNR). We focus on the regime of dominant size quantization in combination with the attractive electron-hole interaction. An adiabatic separation of slow and fast motions leads via the two-body Dirac equation to the isolated and coupled subband approximations. Discrete and continuous exciton states are in general coupled and form quasi-Rydberg series of purely discrete and resonance type character. The corresponding oscillator strengths and widths are derived. We show that the exciton peaks are blue-shifted, become broader and increase in magnitude upon narrowing the ribbon. At the edge of a subband the singularity related to the 1D density of states is transformed into finite absorption via the presence of the exciton. Our analytical results are in good agreement with those obtained by other methods including numerical approaches. Estimates of the expected experimental values are provided for realistic AGNR.

  20. Enormous excitonic effects in bulk, mono- and bi- layers of cuprous halides using many-body perturbation technique

    Science.gov (United States)

    Azhikodan, Dilna; Nautiyal, Tashi

    2017-10-01

    Cuprous halides (CuX with X = Cl, Br, I), intensely studied about four decades ago by experimentalists for excitons, are again drawing attention of researchers recently. Potential of cuprous halide systems for device applications has not yet been fully explored. We go beyond the one-particle picture to capture the two-particle physics (electron-hole interaction to form excitons). We have deployed the full tool kit of many-body perturbation technique, GW approximation + Bethe Salpeter equation, to unfurl the rich excitonic physics of the bulk as well as layers of CuX. The negative spin-orbit contribution at the valence band top in CuCl, compared to CuBr and CuI, is in good agreement with experiments. We note that CuX have exceptionally strong excitons, defying the linear fit (between the excitonic binding energy and band gap) encompassing many semiconductors. The mono- and bi- layers of cuprous halides are predicted to be rich in excitons, with exceptionally large binding energies and the resonance energies in UV/visible region. Hence this work projects CuX layers as good candidates for optoelectronic applications. With advancement of technology, we look forward to experimental realization of CuX layers and harnessing of their rich excitonic potential.

  1. Spin-exciton interaction and related micro-photoluminescence spectra of ZnSe:Mn DMS nanoribbon

    Science.gov (United States)

    Hou, Lipeng; Zhou, Weichang; Zou, Bingsuo; Zhang, Yu; Han, Junbo; Yang, Xinxin; Gong, Zhihong; Li, Jingbo; Xie, Sishen; Shi, Li-Jie

    2017-03-01

    For their spintronic applications the magnetic and optical properties of diluted magnetic semiconductors (DMS) have been studied widely. However, the exact relationships between the magnetic interactions and optical emission behaviors in DMS are not well understood yet due to their complicated microstructural and compositional characters from different growth and preparation techniques. Manganese (Mn) doped ZnSe nanoribbons with high quality were obtained by using the chemical vapor deposition (CVD) method. Successful Mn ion doping in a single ZnSe nanoribbon was identified by elemental energy-dispersive x-ray spectroscopy mapping and micro-photoluminescence (PL) mapping of intrinsic d-d optical transition at 580 nm, i.e. the transition of 4 T 1(4 G) → 6 A 1(6 s),. Besides the d-d transition PL peak at 580 nm, two other PL peaks related to Mn ion aggregates in the ZnSe lattice were detected at 664 nm and 530 nm, which were assigned to the d-d transitions from the Mn2+-Mn2+ pairs with ferromagnetic (FM) coupling and antiferromagnetic (AFM) coupling, respectively. Moreover, AFM pair formation goes along with strong coupling with acoustic phonon or structural defects. These arguments were supported by temperature-dependent PL spectra, power-dependent PL lifetimes, and first-principle calculations. Due to the ferromagnetic pair existence, an exciton magnetic polaron (EMP) is formed and emits at 460 nm. Defect existence favors the AFM pair, which also can account for its giant enhancement of spin-orbital coupling and the spin Hall effect observed in PRL 97, 126603(2006) and PRL 96, 196404(2006). These emission results of DMS reflect their relation to local sp-d hybridization, spin-spin magnetic coupling, exciton-spin or phonon interactions covering structural relaxations. This kind of material can be used to study the exciton-spin interaction and may find applications in spin-related photonic devices besides spintronics.

  2. Soliton physics with semiconductor exciton-polaritons in confined systems

    Science.gov (United States)

    Sich, Maksym; Skryabin, Dmitry V.; Krizhanovskii, Dmitry N.

    2016-10-01

    In the past decade, there has been a significant progress in the study of non-linear polariton phenomena in semiconductor microcavities. One of the key features of non-linear systems is the emergence of solitons. The complexity and the inherently strong nonlinearity of the polariton system made it a perfect sandpit for observing solitonic effects in half-light half-matter environment. This review focuses on the theory and the latest experimental elucidating physics as well as potential applications of conservative and dissipative solitons in exciton-polariton systems. xml:lang="fr"

  3. Excitonic nonlinearities in single-wall carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Nguyen, D.T.; Voisin, C.; Roussignol, P. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Roquelet, C.; Lauret, J.S. [Laboratoire de Photonique Quantique et Moleculaire de l' Ecole Normale Superieure de Cachan (France); Cassabois, G. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Laboratoire Charles Coulomb, UMR5221, Universite Montpellier 2, Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, Montpellier (France)

    2012-05-15

    Excitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the exciton-exciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single-photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient exciton-exciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral-hole burning experiments bring evidence of pure dephasing induced by exciton-exciton scattering (EES) in the k-space. This mechanism controls the exciton collision-induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  4. Symposium GC: Nanoscale Charge Transport in Excitonic Solar Cells

    Energy Technology Data Exchange (ETDEWEB)

    Bommisetty, Venkat [Univ. of South Dakota, Vermillion, SD (United States)

    2011-06-23

    This paper provides a summary only and table of contents of the sessions. Excitonic solar cells, including all-organic, hybrid organic-inorganic and dye-sensitized solar cells (DSSCs), offer strong potential for inexpensive and large-area solar energy conversion. Unlike traditional inorganic semiconductor solar cells, where all the charge generation and collection processes are well understood, these excitonic solar cells contain extremely disordered structures with complex interfaces which results in large variations in nanoscale electronic properties and has a strong influence on carrier generation, transport, dissociation and collection. Detailed understanding of these processes is important for fabrication of highly efficient solar cells. Efforts to improve efficiency are underway at a large number of research groups throughout the world focused on inorganic and organic semiconductors, photonics, photophysics, charge transport, nanoscience, ultrafast spectroscopy, photonics, semiconductor processing, device physics, device structures, interface structure etc. Rapid progress in this multidisciplinary area requires strong synergetic efforts among researchers from diverse backgrounds. Such effort can lead to novel methods for development of new materials with improved photon harvesting and interfacial treatments for improved carrier transport, process optimization to yield ordered nanoscale morphologies with well defined electronic structures.

  5. Excitons in C-60 studied by temperature-dependent optical second-harmonic generation

    NARCIS (Netherlands)

    Janner, A.M; Eder, R; Koopmans, B; Jonkman, H.T.; Sawatzky, G.A

    1995-01-01

    The electric-dipole-forbidden T-1(1g) excitonic state of solid C-60 at hw=1.81 eV can be probed with a second-harmonic generation (SHG) experiment. We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering

  6. Application of the theory of excitons to study of the positronium in matter. Optical transition during positronium formation in matter

    CERN Document Server

    Pietrow, M

    2016-01-01

    Considerable similarity between positronium atom and an exciton in a quantum dot is indicated. Following this, we apply the calculation regime from the theory of excitons to describe some aspects of formation of a positronium in matter. We consider the possibility of photonic deexcitation during Ps formation and show the way of calculation of its probability. The photonic transitions speculated here, if detected, allows improving experimental studies of solid matter with positron techniques.

  7. Plasmonic-exciton coupling in synthesized metal/semiconductor hybrid nanocomposites

    Energy Technology Data Exchange (ETDEWEB)

    Gadalla, A.; Hamad, D. A. [Physics Department, Assiut University, Assiut (Egypt); Mohamed, M. B. [National Institute of Laser Enhanced science (NIELS), Cairo University, Cairo (Egypt)

    2015-12-31

    A new method has been developed to grow plasmonic semiconductor nanocomposites of Au/CdSe and Ag/CdSe. Their chemical composition and crystal structure are determined by X-ray diffraction. The collective optical properties of the prepared semiconductor nanohybrid have been measured using spectrophotometer techniques and compared to those of the individual components. The electron transfer processes from CdSe to the gold are faster than that of the silver. Au/CdSe has a strong plasmonic-excitonic coupling, but Ag/CdSe has a weak plasmonic-excitonic coupling.

  8. Quantum kinetic exciton-LO-phonon interaction in CdSe

    DEFF Research Database (Denmark)

    Woggon, Ulrike; Gindele, Frank; Langbein, Wolfgang

    2000-01-01

    Oscillations with a period of similar to 150 fs are observed in the four-wave mixing (FWM) signal of bulk CdSe and interpreted in terms of non-Markovian exciton-LO-phonon scattering. The experiments show evidence of phonon quantum kinetics in semiconductors of strong polar coupling strength...... and high exciton binding energy. By comparison of the spectral and temporal response of the FWM signal in bulk CdSe and CdSe quantum dots, we demonstrate the influence of continuum states on the interference of electron-hole pair polarizations coupled via an LO phonon....

  9. Measuring Exciton Migration in Conjugated Polymer Films with Ultrafast Time Resolved Stimulated Emission Depletion Microscopy

    Science.gov (United States)

    Penwell, Samuel

    Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion (STED) microscopy. STED is typically used in biology with sparse well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated the extension of STED to conjugated polymer films and nanoparticles of MEH-PPV and CN-PPV, despite the presence of two photon absorption, by taking care to first understand the material's photophysical properties. We then further adapt this approach, by introducing a second ultrafast STED pulse at a variable delay. Excitons that migrate away from the initial subdiffraction excitation volume during the ps-ns time delay, are preferentially quenched by the second STED pulse, while those that remain in the initial volume survive. The resulting effect of the second STED pulse is modulated by the degree of migration over the ultrafast time delay, thus providing a new method to study exciton migration. Since this technique utilizes subdiffraction optical excitation and detection volumes with ultrafast time resolution, it provides a means of spatially and temporally resolving measurements of exciton migration on the native length and time scales. In this way, we will obtain a spatiotemporal map of exciton distributions and migration that will help to correlate the energetic landscape to film morphology at the nanoscale.

  10. Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

    Science.gov (United States)

    Graf, Arko; Held, Martin; Zakharko, Yuriy; Tropf, Laura; Gather, Malte C.; Zaumseil, Jana

    2017-09-01

    Exciton-polaritons are hybrid light-matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting (6,5) single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (>10 kA cm-2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ~104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material-device combination paves the way to carbon-based polariton emitters and possibly lasers.

  11. Nanoscale mapping of excitonic processes in single-layer MoS2 using tip-enhanced photoluminescence microscopy.

    Science.gov (United States)

    Su, Weitao; Kumar, Naresh; Mignuzzi, Sandro; Crain, Jason; Roy, Debdulal

    2016-05-19

    In two-dimensional (2D) semiconductors, photoluminescence originating from recombination processes involving neutral electron-hole pairs (excitons) and charged complexes (trions) is strongly affected by the localized charge transfer due to inhomogeneous interactions with the local environment and surface defects. Herein, we demonstrate the first nanoscale mapping of excitons and trions in single-layer MoS2 using the full spectral information obtained via tip-enhanced photoluminescence (TEPL) microscopy along with tip-enhanced Raman spectroscopy (TERS) imaging of a 2D flake. Finally, we show the mapping of the PL quenching centre in single-layer MoS2 with an unprecedented spatial resolution of 20 nm. In addition, our research shows that unlike in aperture-scanning near field microscopy, preferential exciton emission mapping at the nanoscale using TEPL and Raman mapping using TERS can be obtained simultaneously using this method that can be used to correlate the structural and excitonic properties.

  12. Recombination dynamics of type-II excitons in (Ga,In)As/GaAs/Ga(As,Sb) heterostructures

    Science.gov (United States)

    Gies, S.; Holz, B.; Fuchs, C.; Stolz, W.; Heimbrodt, W.

    2017-01-01

    (Ga,In)As/GaAs/Ga(As,Sb) multi-quantum well heterostructures have been investigated using continuous wave and time-resolved photoluminescence spectroscopy at various temperatures. A complex interplay was observed between the excitonic type-II transitions with electrons in the (Ga,In)As well and holes in the Ga(As,Sb) well and the type-I excitons in the (Ga,In)As and Ga(As,Sb) wells. The type-II luminescence exhibits a strongly non-exponential temporal behavior below a critical temperature of T c = 70 K. The transients were analyzed in the framework of a rate-equation model. It was found that the exciton relaxation and hopping in the localized states of the disordered ternary Ga(As,Sb) are the decisive processes to describe the dynamics of the type-II excitons correctly.

  13. GaN nanorods grown on Si (111 substrates and exciton localization

    Directory of Open Access Journals (Sweden)

    Holmes Mark

    2011-01-01

    Full Text Available Abstract We have investigated exciton localization in binary GaN nanorods using micro- and time-resolved photoluminescence measurements. The temperature dependence of the photoluminescence has been measured, and several phonon replicas have been observed at the lower energy side of the exciton bound to basal stacking faults (I1. By analyzing the Huang-Rhys parameters as a function of temperature, deduced from the phonon replica intensities, we have found that the excitons are strongly localized in the lower energy tails. The lifetimes of the I1 and I2 transitions were measured to be < 100 ps due to enhanced surface recombination. PACS: 78.47.+p, 78.55.-m, 78.55.Cr, 78.66.-w, 78.66.Fd

  14. Time-resolved photoluminescence study of excitonic relaxation in one-dimensional systems

    Science.gov (United States)

    Tanino, H.; Rühle, W. W.; Takahashi, K.

    1988-12-01

    Self-trapped exciton luminescence of quasi-one-dimensional (1D) halogen-bridged mixed-valence platinum complexes [Pt(II) (EA)4][Pt(IV)Cl2(EA)4] Cl4.4H2O (EA=ethylamine) and [Pt(II)(en)2] [Pt(IV)Cl2(en)2](ClO4)4 (en=1,2-diaminoethane) are studied by time-resolved photoluminescence experiments. The lifetimes of the luminescence of self-trapped exciton are exceptionally short, of the order of 100 psec. We interpret the short lifetime by a ``giant oscillator strength'' caused by a strong coupling between the electron and hole of the 1D charge transfer exciton and an extended polaronlike character of the 1D state. The lifetimes of the broad luminescence and of the resonant Raman lines during the barrier-free relaxation process are both faster than 7 psec.

  15. Coherent Exciton and Biexciton Nonlinearities in Semiconductor Nanostructures: Effects of Disorder

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Borri, Paola; Hvam, Jørn Märcher

    1999-01-01

    The coherent response of excitons in semiconductor nanostructures measured in four-wave mixing (FWM) depends strongly on the inhomogenous broadening of the exciton transition. We investigate InAs/Al0.3Ga0.7As single quantum wells (SQW) and AlxGa1-xAs mixed crystals. Additional to the usual phase......-exciton-states. Here, the third-order polarization at X-XX is not at the same resonance as the first-order polarization at 0-X. Consequently, the rephasing of the microscopic third-order polarization to the macroscopic FWM photon echo is blurred by the non-perfect correlation of X and XX energies, leading to a fast...

  16. Splitting between bright and dark excitons in transition metal dichalcogenide monolayers

    Science.gov (United States)

    Echeverry, J. P.; Urbaszek, B.; Amand, T.; Marie, X.; Gerber, I. C.

    2016-03-01

    The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors MoS2 and WSe2 are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe-Salpeter equation on top of G W wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.

  17. Excitonic polaritons of zinc diarsenide single crystals

    Energy Technology Data Exchange (ETDEWEB)

    Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of); Stamov, I.G. [T.G. Shevchenko State University of Pridnestrovie, Tiraspol, Republic of Moldova (Moldova, Republic of); Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of); Dorogan, A. [Technical University of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of)

    2017-02-01

    Excitonic polaritons of ZnAs{sub 2} single crystals had been investigated. Parameters of singlet excitons with Г{sub 2}¯(z) symmetry and orthoexcitons 2Г{sub 1}¯(y)+Г{sub 2}¯(x) had been determined. Spectral dependencies of ordinary and extraordinary dispersion of refractive index had been calculated using interferential reflection and transmittance spectra. It was shown, that A excitonic series were due to hole (V{sub 1}) and electron (C{sub 1}) bands. The values of effective masses of electrons (m{sub c}{sup *}=0.10 m{sub 0}) and holes (m{sub v1}{sup *}=0.89 m{sub 0}) had been estimated. It was revealed that the hole mass m{sub v1}{sup *} changes from 1.03 m{sub 0} to 0.55 m{sub 0} at temperature increasing from 10 K up to 230 K and that the electron mass m{sub c}{sup *} does not depend on temperature. The integral absorption A (eV cm{sup −1}) of the states n=1, 2 and 3 of Г{sub 2}¯(z) excitons depends on the A{sub n}≈n{sup −3} equality, which it is characteristic for S-type excitonic functions. Temperature dependences of the integral absorption of ground states for Г{sub 2}¯(z) and Г{sub 2}¯(Ñ…) excitons differ. The ground states of B and C excitons formed by V{sub 3} – C{sub 1} and V{sub 4} – C{sub 1} bands and its parameters had been determined.

  18. Exploring ultrafast dynamics of excitons and multiexcitons in "giant" nanocrystal quantum dots

    Science.gov (United States)

    Sampat, Siddharth

    In this work, we have performed extensive time resolved photoluminescence (PL) studies to further the understanding of charge dynamics in semiconductor nanocrystal quantum dots (QDs). Recent developments in QD synthesis have introduced a new set of QD known as "giant" quantum dots (gQDs) that consist of a CdSe core coated with up to 19 monolayers of a CdS shell. The thick shell layer is grown using a SILAR method resulting in a defect free, alloyed CdSe/CdS interface. This has been attributed to gQDs exhibiting excellent optical properties such as high excitonic quantum yield (QY), prolonged photostability and inhibition of flourescence intermittency ("blinking"), which is regularly observed in conventional QDs. In gQDs, however, owing to unique fabrication methods and material selection, the Auger process is strongly suppressed resulting in efficient radiative recombination of photogenerated excitons as well as high PL QY of charged excitonic and multiexcitonic species. We perform extensive single gQDs studies that establish the role played by gQD shell thickness and core size in governing their optical properties. It is found that both the core and shell dimensions can be tuned in order to achieve the smallest gQDs with the highest vii Auger suppression resulting in photostable dots with high QYs. Next, we perform a study of multiexcitonic species in gQDs that are encapsulated in an insulating SiO2shell. These silica-coated gQDs exhibit strong PL from charged excitons, biexcitons as well as triexcitons. This observation has led to an accurate description of excitonic and multiexcitonic behavior which is modeled using a statistical scaling approach. As a demonstration of the practical applicability of gQDs, energy transfer of excitons as well as multiexcitons to different substrates is studied. Finally, a back gated silicon nanomembrane FET device is discussed that exhibits a large photocurrent increase when sensitized with QDs.

  19. Enhancing detection sensitivity in gradient liquid chromatography via post-column refocusing and strong-solvent remobilization.

    Science.gov (United States)

    De Vos, Jelle; Desmet, Gert; Eeltink, Sebastiaan

    2016-07-15

    We developed earlier the post-column refocusing strategy for isocratic separations, which employs trapping target analytes after an analytical separation and additionally focusing them using a strong remobilization solvent prior to detection, and have now extended it to high-speed gradient LC. A gradient separation of antibiotics and its metabolites, applying a linear aqueous acetonitrile gradient from 2 to 65% (v/v) ACN containing 0.1% FA in 10min, performed on an analytical column was selected as an application. Eluted heart-cut fractions were directed from the analytical silica C18 column to a trap column packed with Hypercarb particles. The remobilization of the target analytes was performed in back-flush mode using solvent mixtures tuned to maximize the solvent strength by mixing isopropanol into the remobilization solvent containing acetonitrile. Additionally, a viscosity-calibration experiment showed that the viscosity difference between trapping and remobilization solvents should be smaller than 0.15mPa·s to prevent viscous fingering. To keep the viscosity difference below this limit, during the gradient separation performed on the analytical column, the composition of the remobilization solvent was changed in time. An empirical equation is provided that allows for the selection of the optimal remobilization-solvent composition. To maximize the signal enhancement, the loading time of target analytes on the trap column should be optimized. Peak dispersion was further minimized by applying a flow rate that corresponded to the optimal van-Deemter flow rate of the trap column (20μL/min). Finally, decreasing the diameter of the trap column from 1mm to 0.3mm led to a significant enhancement of the detection sensitivity with more than one order of magnitude. Using an optimized trap configuration and elution/remobilization conditions, a signal enhancement of a factor of 14 was achieved for sulfaguanidine (early-eluting compound in the gradient separation) and 7

  20. Numerical modeling of exciton-polariton Bose-Einstein condensate in a microcavity

    Science.gov (United States)

    Voronych, Oksana; Buraczewski, Adam; Matuszewski, Michał; Stobińska, Magdalena

    2017-06-01

    A novel, optimized numerical method of modeling of an exciton-polariton superfluid in a semiconductor microcavity was proposed. Exciton-polaritons are spin-carrying quasiparticles formed from photons strongly coupled to excitons. They possess unique properties, interesting from the point of view of fundamental research as well as numerous potential applications. However, their numerical modeling is challenging due to the structure of nonlinear differential equations describing their evolution. In this paper, we propose to solve the equations with a modified Runge-Kutta method of 4th order, further optimized for efficient computations. The algorithms were implemented in form of C++ programs fitted for parallel environments and utilizing vector instructions. The programs form the EPCGP suite which has been used for theoretical investigation of exciton-polaritons. Catalogue identifier: AFBQ_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFBQ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: BSD-3 No. of lines in distributed program, including test data, etc.: 2157 No. of bytes in distributed program, including test data, etc.: 498994 Distribution format: tar.gz Programming language: C++ with OpenMP extensions (main numerical program), Python (helper scripts). Computer: Modern PC (tested on AMD and Intel processors), HP BL2x220. Operating system: Unix/Linux and Windows. Has the code been vectorized or parallelized?: Yes (OpenMP) RAM: 200 MB for single run Classification: 7, 7.7. Nature of problem: An exciton-polariton superfluid is a novel, interesting physical system allowing investigation of high temperature Bose-Einstein condensation of exciton-polaritons-quasiparticles carrying spin. They have brought a lot of attention due to their unique properties and potential applications in polariton-based optoelectronic integrated circuits. This is an out-of-equilibrium quantum system confined

  1. Applications of Ultrafast Terahertz Pulses for Intra-ExcitonicSpectroscopy of Quasi-2D Electron-Hole Gases

    Energy Technology Data Exchange (ETDEWEB)

    Kaindl, Robert A.; Carnahan, Marc A.; Hagele, Daniel; Chemla, D.S.

    2006-09-02

    Excitons are of fundamental interest and of importance foropto-electronic applications of bulk and nano-structured semiconductors.This paper discusses the utilization of ultrafast terahertz (THz) pulsesfor the study of characteristic low-energy excitations of photoexcitedquasi 2D electron-hole (e-h) gases. Optical-pump THz-probe spectroscopyat 250-kHz repetition rate is employed to detect characteristic THzsignatures of excitons and unbound e-h pairs in GaAs quantum wells.Exciton and free-carrier densities are extracted from the data using atwo-component model. We report the detailed THz response and pairdensities for different photoexcitation energies resonant to heavy-holeexcitons, light-hole excitons, or the continuum of unbound pairs. Suchexperiments can provide quantitative insights into wavelength, time, andtemperature dependence of the low-energy response and composition ofoptically excited e-h gases in low-dimensionalsemiconductors.

  2. Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR

    Energy Technology Data Exchange (ETDEWEB)

    Smith, Albert A.; Corzilius, Björn; Haze, Olesya; Swager, Timothy M.; Griffin, Robert G., E-mail: rgg@mit.edu [Department of Chemistry and Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)

    2013-12-07

    We present electron paramagnetic resonance experiments for which solid effect dynamic nuclear polarization transitions were observed indirectly via polarization loss on the electron. This use of indirect observation allows characterization of the dynamic nuclear polarization (DNP) process close to the electron. Frequency profiles of the electron-detected solid effect obtained using trityl radical showed intense saturation of the electron at the usual solid effect condition, which involves a single electron and nucleus. However, higher order solid effect transitions involving two, three, or four nuclei were also observed with surprising intensity, although these transitions did not lead to bulk nuclear polarization—suggesting that higher order transitions are important primarily in the transfer of polarization to nuclei nearby the electron. Similar results were obtained for the SA-BDPA radical where strong electron-nuclear couplings produced splittings in the spectrum of the indirectly observed solid effect conditions. Observation of high order solid effect transitions supports recent studies of the solid effect, and suggests that a multi-spin solid effect mechanism may play a major role in polarization transfer via DNP.

  3. Observation of strongly forbidden solid effect dynamic nuclear polarization transitions via electron-electron double resonance detected NMR.

    Science.gov (United States)

    Smith, Albert A; Corzilius, Björn; Haze, Olesya; Swager, Timothy M; Griffin, Robert G

    2013-12-01

    We present electron paramagnetic resonance experiments for which solid effect dynamic nuclear polarization transitions were observed indirectly via polarization loss on the electron. This use of indirect observation allows characterization of the dynamic nuclear polarization (DNP) process close to the electron. Frequency profiles of the electron-detected solid effect obtained using trityl radical showed intense saturation of the electron at the usual solid effect condition, which involves a single electron and nucleus. However, higher order solid effect transitions involving two, three, or four nuclei were also observed with surprising intensity, although these transitions did not lead to bulk nuclear polarization--suggesting that higher order transitions are important primarily in the transfer of polarization to nuclei nearby the electron. Similar results were obtained for the SA-BDPA radical where strong electron-nuclear couplings produced splittings in the spectrum of the indirectly observed solid effect conditions. Observation of high order solid effect transitions supports recent studies of the solid effect, and suggests that a multi-spin solid effect mechanism may play a major role in polarization transfer via DNP.

  4. Excitonic and vibrational coherence in artificial photosynthetic systems studied by negative-time ultrafast laser spectroscopy.

    Science.gov (United States)

    Han, Dongjia; Xue, Bing; Du, Juan; Kobayashi, Takayoshi; Miyatake, Tomohiro; Tamiaki, Hitoshi; Xing, Xin; Yuan, Wei; Li, Yanyan; Leng, Yuxin

    2016-09-21

    Quantum coherences between excitonic states are believed to have a substantial impact on excitation energy transfer in photosynthetic systems. Here, the excitonic and vibrational coherence relaxation dynamics of artificially synthetic chlorosomes are studied by a sub 7 fs negative-time-delay laser spectroscopy at room temperature. The results provide direct evidence for the quantum coherence of the excitonic dephasing time of 23 ± 1 fs at physiologically relevant temperatures, which is significant in the initial step of energy transfer in chlorosome or chlorosome-like photosynthetic systems. Meanwhile, coherent molecular vibrations in the excited state are also detected without the effect of wave-packet motion in the ground state, which shows that the excited state wave-packet motion contributes greatly to the vibrational modes of ∼150 and ∼1340 cm(-1) in artificial chlorosome systems.

  5. Electron nuclear double resonance with donor-bound excitons in silicon

    CERN Document Server

    Franke, David P; Riemann, Helge; Abrosimov, Nikolai V; Becker, Peter; Pohl, Hans-Joachim; Itoh, Kohei M; Thewalt, Michael L W; Brandt, Martin S

    2016-01-01

    We measure electron nuclear double resonance (ENDOR) of phosphorus in isotopically purified $^{28}$Si by optical-electrical hybrid detection via the donor-bound exciton transitions. This allows for the control of both nuclear and electron spin of donors in samples, where the hyperfine splitting is not resolved in the optical spectrum and hence significantly relaxes the requirements on the experimental setup.

  6. Ultrafast exciton transfers in DNA and its nonlinear optical spectroscopy

    OpenAIRE

    Kim, Hyeon-Deuk; Tanimura, Yoshitaka; Cho, Minhaeng

    2008-01-01

    We have calculated the nonlinear response function of a DNA duplex helix including the contributions from the exciton population and coherence transfers by developing an appropriate exciton theory as well as by utilizing a projector operator technique. As a representative example of DNA double helices, the B-form (dA)10-(dT)10 is considered in detail. The Green functions of the exciton population and coherence transfer processes were obtained by developing the DNA exciton Hamiltonian. This en...

  7. Non-conservation of excitons in finite molecular chain

    Energy Technology Data Exchange (ETDEWEB)

    Tosic, Bratislav, E-mail: btosic@yahoo.co [Vojvodina Academy of Science and Arts, 21000 Novi Sad, Dunavska 37 (Serbia); Sajfert, Vjekoslav, E-mail: sajfertv@nadlanu.co [University of Novi Sad, Technical Faculty ' M. Pupin' , 23000 Zrenjanin, Djure Djakovica bb (Serbia); Maskovic, Ljiljana, E-mail: maskovicm@yahoo.co.u [Academy of Criminalistic and Police Studies, 11000 Belgrade, Zemun (Serbia); Bednar, Nikola, E-mail: bednar.nikola@gmail.co [University of Novi Sad, Faculty of Technical Sciences, 21000 Novi Sad, Trg Dositeja Obradovica 6 (Serbia)

    2010-11-15

    We have analyzed a linear molecular chain with exciton excitations when the number of excitons is not conserved. The dispersion law depends on two independent variables and it is surfaced in a 3D plot. The same conclusion is valid for the concentrations of excitons and exciton pairs. As it was expected, physical characteristics of the finite chain depend on spatial coordinates. All results are compared to the corresponding results of an infinite chain.

  8. Resonant Transfer of Excitons and Quantum Computation

    CERN Document Server

    Lovett, B; Nazir, A; Kothari, B; Briggs, A; Lovett, Brendon; Reina, John H.; Nazir, Ahsan; Kothari, Beeneet; Briggs, Andrew

    2003-01-01

    The excitation-energy transfer--the so-called Forster resonant energy transfer--plays a key role in light harvesting processes in photosynthetic organisms in nature. Here we give two methods for performing quantum logic operations by tailoring this interaction. The first implementation uses a coupled quantum dot molecule where the exciton-exciton interaction and the Forster coupling are controlled by means of the dot size, interdot separation, material composition, confinement potential and applied electric field to obtain high fidelity logic. The second proposes the use of biological systems for embodying qubits where, as a result of a stronger Forster interaction, extended exciton states are expected. These states are likely to be more immune to decoherence.

  9. Exciton-polariton trapping and potential landscape engineering

    Science.gov (United States)

    Schneider, C.; Winkler, K.; Fraser, M. D.; Kamp, M.; Yamamoto, Y.; Ostrovskaya, E. A.; Höfling, S.

    2017-01-01

    Exciton-polaritons in semiconductor microcavities have become a model system for the studies of dynamical Bose-Einstein condensation, macroscopic coherence, many-body effects, nonclassical states of light and matter, and possibly quantum phase transitions in a solid state. These low-mass bosonic quasiparticles can condense at comparatively high temperatures up to 300 K, and preserve the fundamental properties of the condensate, such as coherence in space and time domain, even when they are out of equilibrium with the environment. Although the presence of a confining potential is not strictly necessary in order to observe Bose-Einstein condensation, engineering of the polariton confinement is a key to controlling, shaping, and directing the flow of polaritons. Prototype polariton-based optoelectronic devices rely on ultrafast photon-like velocities and strong nonlinearities exhibited by polaritons, as well as on their tailored confinement. Nanotechnology provides several pathways to achieving polariton confinement, and the specific features and advantages of different methods are discussed in this review. Being hybrid exciton-photon quasiparticles, polaritons can be trapped via their excitonic as well as photonic component, which leads to a wide choice of highly complementary trapping techniques. Here, we highlight the almost free choice of the confinement strengths and trapping geometries that provide powerful means for control and manipulation of the polariton systems both in the semi-classical and quantum regimes. Furthermore, the possibilities to observe effects of the polariton blockade, Mott insulator physics, and population of higher-order energy bands in sophisticated lattice potentials are discussed. Observation of such effects could lead to realization of novel polaritonic non-classical light sources and quantum simulators.

  10. Resonance effects of excitons and electrons. Basics and applications

    Energy Technology Data Exchange (ETDEWEB)

    Geru, Ion [Moldovan Academy of Sciences, Chisinau (Moldova, Republic of). Inst. of Chemistry; Suter, Dieter [Technische Univ. Dortmund (Germany). Fakultaet Physik

    2013-08-01

    Represents the first book on non-traditional resonance effects of excitons in semiconductors. Explains resonance phenomena of excitons and electrons in solids. Presents the Knight shift at the Bose-Einstein condensation of excitons. This book presents the various types of resonance effects on excitons, biexcitons and the local electronic centers (LEC) in solids, such as paramagnetic and paraelectric resonances on excitons, exciton acoustic resonance at intra- and interband transitions, radio-optical double resonance on excitons, hole-nuclear double resonance on localized biexcitons, ENDOR and acoustic ENDOR on LEC. The criteria for the generation of coherent photons, phonons and magnons by excitons are explained. The interactions of excitons and biexcitons with paramagnetic centers and nuclear spins, the indirect interaction between the PC through a field of excitons as well as the quasienergy spectrum of excitons and spin systems are discussed. It is proved that the interaction of paramagnetic centers with excitons increases the spin relaxation rate of paramagnetic centers in comparison with the case of their interaction with free carriers. The giant magneto-optical effects in semi-magnetic semiconductors are theoretically interpreted. In recent years, a new perspective has been added to these systems and their interactions: they can be used for storing and processing information in the form of quantum bits (qubits), the building blocks of quantum computers. The basics of this emerging technology are explained and examples of demonstration-type quantum computers based on localized spins in solids are discussed.

  11. Spin coherence and electromagnetically induced transparency via exciton correlations.

    Science.gov (United States)

    Phillips, Mark; Wang, Hailin

    2002-10-28

    We report experimental studies on exciton spin coherence induced via Coulomb correlations between excitons with opposite spins, including correlations associated with unbound as well as bound exciton pairs. Electromagnetically induced transparency resulting from the spin coherence is demonstrated in the transient optical response in GaAs quantum wells.

  12. Arsenic-bound excitons in diamond

    Science.gov (United States)

    Barjon, J.; Jomard, F.; Morata, S.

    2014-01-01

    A set of new excitonic recombinations is observed in arsenic-implanted diamond. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, an ionization energy of 0.41 eV is deduced for arsenic in diamond, which shows that arsenic is a shallower donor than phosphorus (0.6 eV), in agreement with theory.

  13. Heat pumping with optically driven excitons

    CERN Document Server

    Gauger, Erik M

    2010-01-01

    We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible.

  14. Switching exciton pulses through conical intersections

    CERN Document Server

    Leonhardt, K; Rost, J -M

    2013-01-01

    Exciton pulses transport excitation and entanglement adiabatically through Rydberg aggregates, assemblies of highly excited light atoms, which are set into directed motion by resonant dipole-dipole interaction. Here, we demonstrate the coherent splitting of such pulses as well as the spatial segregation of electronic excitation and atomic motion. Both mechanisms exploit local non-adiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses.

  15. Light propagation in tunable exciton-polariton one-dimensional photonic crystals

    OpenAIRE

    Sedov, E. S.; Cherotchenko, E. D.; Arakelian, S.M.; Kavokin, A. V.

    2016-01-01

    Simulations of propagation of light beams in specially designed multilayer semiconductor structures (one-dimensional photonic crystals) with embedded quantum wells reveal characteristic optical properties of resonant hyperbolic metamaterials. A strong dependence of the refraction angle and the optical beam spread on the exciton radiative lifetime is revealed. We demonstrate the strong negative refraction of light and the control of the group velocity of light by an external bias through its e...

  16. FRET-based nanoscale point-to-point and broadcast communications with multi-exciton transmission and channel routing.

    Science.gov (United States)

    Kuscu, Murat; Akan, Ozgur B

    2014-09-01

    Nanoscale communication based on Förster Resonance Energy Transfer (FRET) enables nanoscale single molecular devices to communicate with each other utilizing excitons generated on fluorescent molecules as information carriers. Based on the point-to-point single-exciton FRET-based nanocommunication model, we investigate the multiple-exciton case for point-to-point and broadcast communications following an information theoretical approach and conducting simulations through Monte Carlo approach. We demonstrate that the multi-exciton transmission significantly improves the channel reliability and the range of the communication up to tens of nanometers for immobile nanonodes providing high data transmission rates. Furthermore, our analyses indicate that multi-exciton transmission enables broadcasting of information from a transmitter nanonode to many receiver nanonodes pointing out the potential of FRET-based communication to extend over nanonetworks. In this study, we also propose electrically and chemically controllable routing mechanisms exploiting the strong dependence of FRET rate on spectral and spatial characteristics of fluorescent molecules. We show that the proposed routing mechanisms enable the remote control of information flow in FRET-based nanonetworks. The high transmission rates obtained by multi-exciton scheme for point-to-point and broadcast communications, as well as the routing opportunities make FRET-based communication promising for future molecular computers.

  17. Hereditary tyrosinemia type 1: Strong association with haplotype 6 in French Canadians permits simple carrier detection and prenatal diagnosis

    Energy Technology Data Exchange (ETDEWEB)

    Demers, S.I.; Phaneuf, D.; Tanguay, R.M. (Centre de Recherche du CHUL, Quebec (Canada))

    1994-08-01

    Hereditary tyrosinemia type 1 (HT1), a severe inborn error of tyrosine catabolism, is caused by deficiency of the terminal enzyme, fumarylacetoacetate hydrolase (FAH). The highest reported frequency of HT1 is in the French Canadian population, especially in the Saguenay-Lac-St-Jean region. Using human FAH cDNA probes, the authors have identified 10 haplotypes with TaqI, KpnI, RsaI, BglII, and MspI RFLPs in 118 normal chromosomes from the French Canadian population. Interestingly, in 29 HT1 children, a prevalent haplotype, haplotype 6, was found to be strongly associated with the disease, at a frequency of 90% of alleles, as compared with [approximately] 18% in 35 control individuals. This increased to 96% in the 24 patients originating from Saguenay-Lac-St-Jean. These results suggest that one or only a few prevailing mutations are responsible for most of the HT1 cases in Saguenay-Lac-St-Jean. Since most patients were found to be homozygous for a specific haplotype in this population, FAH RFLPs have permitted simple carrier detection in nine different informative HT1 families, with a confidence level of 99.9%. Heterozygosity rate values obtained from 52 carriers indicated that [approximately] 88% of families at risk from Saguenay-Lac-St-Jean are fully or partially informative. Prenatal diagnosis was also achieved in an American family. Analysis of 24 HT1 patients from nine countries gave a frequency of [approximately] 52% for haplotype 6, suggesting a relatively high association, worldwide, of HT1 with this haplotype. 31 refs., 1 fig., 3 tabs.

  18. Hereditary tyrosinemia type I: strong association with haplotype 6 in French Canadians permits simple carrier detection and prenatal diagnosis.

    Science.gov (United States)

    Demers, S I; Phaneuf, D; Tanguay, R M

    1994-08-01

    Hereditary tyrosinemia type 1 (HT1), a severe inborn error of tyrosine catabolism, is caused by deficiency of the terminal enzyme, fumarylacetoacetate hydrolase (FAH). The highest reported frequency of HT1 is in the French Canadian population, especially in the Saguenay-Lac-St-Jean region. Using human FAH cDNA probes, we have identified 10 haplotypes with TaqI, KpnI, RsaI, BglII, and MspI RFLPs in 118 normal chromosomes from the French Canadian population. Interestingly, in 29 HT1 children, a prevalent haplotype, haplotype 6, was found to be strongly associated with the disease, at a frequency of 90% of alleles, as compared with approximately 18% in 35 control individuals. This increased to 96% in the 24 patients originating from Saguenay-Lac-St-Jean. These results suggest that one or only a few prevailing mutations are responsible for most of the HT1 cases in Saguenay-Lac-St-Jean. Since most patients were found to be homozygous for a specific haplotype in this population, FAH RFLPs have permitted simple carrier detection in nine different informative HT1 families, with a confidence level of 99.9%. Heterozygosity rate values obtained from 52 carriers indicated that approximately 88% of families at risk from Saguenay-Lac-St-Jean are fully or partially informative. Prenatal diagnosis was also achieved in an American family. Analysis of 24 HT1 patients from nine countries gave a frequency of approximately 52% for haplotype 6, suggesting a relatively high association, worldwide, of HT1 with this haplotype.

  19. Exciton Absorption Spectra by Linear Response Methods: Application to Conjugated Polymers.

    Science.gov (United States)

    Mosquera, Martín A; Jackson, Nicholas E; Fauvell, Thomas J; Kelley, Matthew S; Chen, Lin X; Schatz, George C; Ratner, Mark A

    2017-03-15

    The theoretical description of the time-evolution of excitons requires, as an initial step, the calculation of their spectra, which has been inaccessible to most users due to the high computational scaling of conventional algorithms and accuracy issues caused by common density functionals. Previously (J. Chem. Phys. 2016, 144, 204105), we developed a simple method that resolves these issues. Our scheme is based on a two-step calculation in which a linear-response TDDFT calculation is used to generate orbitals perturbed by the excitonic state, and then a second linear-response TDDFT calculation is used to determine the spectrum of excitations relative to the excitonic state. Herein, we apply this theory to study near-infrared absorption spectra of excitons in oligomers of the ubiquitous conjugated polymers poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7). For P3HT and MEH-PPV oligomers, the calculated intense absorption bands converge at the longest wavelengths for 10 monomer units, and show strong consistency with experimental measurements. The calculations confirm that the exciton spectral features in MEH-PPV overlap with those of the bipolaron formation. In addition, our calculations identify the exciton absorption bands in transient absorption spectra measured by our group for oligomers (1, 2, and 3 units) of PTB7. For all of the cases studied, we report the dominant orbital excitations contributing to the optically active excited state-excited state transitions, and suggest a simple rule to identify absorption peaks at the longest wavelengths. We suggest our methodology could be considered for further developments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.

  20. Charge Transfer Fluorescence and 34 nm Exciton Diffusion Length in Polymers with Electron Acceptor End Traps.

    Science.gov (United States)

    Zaikowski, Lori; Mauro, Gina; Bird, Matthew; Karten, Brianne; Asaoka, Sadayuki; Wu, Qin; Cook, Andrew R; Miller, John R

    2015-06-18

    Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17-127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence, and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps, the trap depths are 0.06 (p-xylene), 0.13 (THF), and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization, and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ∼50% of the excitons, and that the exciton diffusion length is LD = 34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. The efficiency of exciton capture depends on chain length but not on trap depth, solvent polarity, or which trap group is present.

  1. Biexciton formation and exciton coherent coupling in layered GaSe

    Energy Technology Data Exchange (ETDEWEB)

    Dey, P.; Paul, J.; Stevens, C. E.; Glikin, N.; Karaiskaj, D., E-mail: karaiskaj@usf.edu [Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620 (United States); Moody, G. [National Institute of Standards and Technology, 325 Broadway, Boulder, Colarado 80305 (United States); Kovalyuk, Z. D.; Kudrynskyi, Z. R. [Chernivtsi Department, Frantsevich Institute of Material Sciences Problems, The National Academy of Sciences of Ukraine, 5, Iryna Vilde St., 58001 Chernivtsi (Ukraine); Romero, A. H. [Physics Department, West Virginia University, Morgantown, West Virginia 26506-6315 (United States); Cantarero, A. [Materials Science Institute, University of Valencia, P.O. Box 2205, 46071 Valencia (Spain); Hilton, D. J. [Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294 (United States)

    2015-06-07

    Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ∼2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with “ab initio” theoretical calculations of the phonon spectra, indicate strong interaction with the A{sub 1}{sup ′} phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.

  2. Bright and dark excitons in semiconductor carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Tretiak, Sergei [Los Alamos National Laboratory

    2008-01-01

    We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low

  3. Magnetic field effects on the Rabi splitting and radiative decay rates of the exciton-polariton states in a semiconductor microcavity

    Science.gov (United States)

    Fenniche, H.; Jaziri, S.; Bennaceur, R.

    1998-12-01

    We study theoretically a particular type of semiconductor microcavity formed by a quantum well embedded inside it and the distributed Bragg reflectors presenting a gradual structure. We apply to this structure a static magnetic field along the growth direction. In the strong coupling regime between the confined exciton and cavity modes, we evaluate the polariton Rabi splitting corresponding to the two lowest lying exciton states: HH1-CB1 and HH2-CB2 as a function of the applied magnetic field. In high magnetic field and for distinct reflectivities, we find that the Rabi splitting magnitude of the HH2-CB2 exciton is close to the fundamental one (HH1-CB1). In the presence of the magnetic field, the polariton Rabi splitting can be obtained even in low reflectivity. The dispersion polariton radiative decay rates related to the two lowest lying exciton states: HH1-CB1 and HH2-CB2 are calculated for different magnetic field values. At k //=0 and in the weak coupling regime, the polariton radiative decay rates are evaluated for both the HH1-CB1 and HH2-CB2 excitons. We show that for the fundamental excitonic state, the magnetic field value which determines the transition from the weak to the strong coupling regime is different from the HH2-CB2 exciton state.

  4. Free-exciton states in crystalline GaTe

    Science.gov (United States)

    Wan, J. Z.; Brebner, J. L.; Leonelli, R.

    1995-12-01

    Polarized properties of both the singlet and triplet ground exciton states in the photoluminescence and transmission spectra of crystalline GaTe are explained based on the possible symmetry properties of the energy band edge of GaTe. Some experimental results about excited exciton states in GaTe are presented and discussed. The energy positions of exciton series in GaTe follow the three-dimensional direct allowed Wannier exciton formula just as in the the other III-VI layered compounds of GaSe and InSe. The nonthermalized, ``hot'' nature of excitons inside GaTe under higher optical excitation intensities is also discussed.

  5. Electrical Control of Excitons in Semiconductor Nanostructures

    DEFF Research Database (Denmark)

    Kirsanské, Gabija

    The scope of this thesis covers investigation of the exciton Mott transition in coupled quantum wells, fabrication of photonic-crystal structures with embedded self-assembled quantum dots, and tuning of their properties by means of an external electric field. In the first part of the thesis the f...

  6. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    2007-01-01

    We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with opposite charges and a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum of their relative motion is well described...

  7. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum is well described by a one-dimensional effective Hamiltonian...

  8. Properties of Excitons Bound to Ionized Donors

    DEFF Research Database (Denmark)

    Skettrup, Torben; Suffczynski, M.; Gorzkowski, W.

    1971-01-01

    Binding energies, interparticle distances, oscillator strengths, and exchange corrections are calculated for the three-particle complex corresponding to an exciton bound to an ionized donor. The results are given as functions of the mass ratio of the electron and hole. Binding of the complex is o...

  9. Exciton Transfer Integrals Between Polymer Chains

    CERN Document Server

    Barford, W

    2007-01-01

    The line-dipole approximation for the evaluation of the exciton transfer integral, $J$, between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wavefunction, it is shown analytically that $J \\sim L$ when the chain lengths are smaller than the separation between them, or $J\\sim L^{-1}$ when the chain lengths are larger than their separation, where $L$ is the polymer length. Scaling relations are also obtained numerically for the more realistic standing-wave approximation for the exciton center-of-mass wavefunction, where it is found that for chain lengths larger than their separation $J \\sim L^{-1.8}$ or $J \\sim L^{-2}$, for parallel or collinear chains, respectively. These results have important implications for the photo-physics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the F\\"orster transfer rate for exciton migration decreases with chain lengths l...

  10. Polaronic Effects of an Exciton in a Cylindrical Quantum Wire

    Institute of Scientific and Technical Information of China (English)

    WANG Rui-Qiang; XIE Hong-Jing; GUO Kang-Xian; YU You-Bin; DENG Yong-Qing

    2005-01-01

    The effects of exciton-optical phonon interaction on the binding energy and the total and reduced effective masses of an exciton in a cylindrical quantum wire have been investigated. We adopt a perturbative-PLL [T.D. Lee,F. Low, and D. Pines, Phys. Rev. B90 (1953) 297] technique to construct an effective Hamiltonian and then use a variational solution to deal with the exciton-phonon system. The interactions of exciton with the longitudinal-optical phonon and the surface-optical phonon have been taken into consideration. The numerical calculations for GaAs show that the influences of phonon modes on the exciton in a quasi-one-dimensional quantum wire are considerable and should not be neglected. Moreover the numerical results for heavy- and light-hole exciton are obtained, which show that the polaronic effects on two types of excitons are very different but both depend heavily on the sizes of the wire.

  11. Theory of quantum light emission from a strongly-coupled single quantum dot photonic-crystal cavity system.

    Science.gov (United States)

    Hughes, Stephen; Yao, P

    2009-03-02

    We present a rigorous medium-dependent theory for describing the quantum field emitted and detected from a single quantum dot exciton, strongly coupled to a planar photonic crystal nanocavity, from which the exact spectrum is derived. By using simple mode decomposition techniques, this exact spectrum is subsequently reduced to two separate user-friendly forms, in terms of the leaky cavity mode emission and the radiation mode emission. On application to study exciton-cavity coupling in the strong coupling regime, besides a pronounced modification of the usual vacuum Rabi spectral doublet, we predict several new effects associated with the leaky cavity mode emission, including the appearance of an off-resonance cavity mode and a loss-induced on-resonance spectral triplet. The cavity mode emission is shown to completely dominate the emitted spectrum, even for large cavity-exciton detunings, whereby the usual cavity-QED formulas developed for radiation-mode emission drastically fail. These predictions are in qualitative agreement with several "mystery observations" reported in recent experiments, and apply to a wide range of semiconductor cavities.

  12. Direct measurement of exciton valley coherence in monolayer WSe2

    KAUST Repository

    Hao, Kai

    2016-02-29

    In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time—a crucial quantity for valley pseudospin manipulation—is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron–hole pairs) in monolayer WSe2 (refs ,). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron–hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

  13. The nonlinear optical rectification of a confined exciton in a quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Xie Wenfang, E-mail: xiewf@gzhu.edu.c [School of Physics and Electronic Engineering, Guangzhou University, Guangzhou 510006 (China)

    2011-05-15

    An exciton in a disc-like quantum dot (QD) with the parabolic confinement, under applied electric field, is studied within the framework of the effective-mass approximation. The nonlinear optical rectification between the ground and the first-excited states has been examined through the computed energies and wave functions in details for the excitons. The results show that the optical rectification susceptibility obtained in a disc-like QD reach the magnitude of 10{sup -2} m/V, which is 3-4 orders of magnitude higher than in one-dimensional QDs. It is found that the second-order nonlinear optical properties of exciton states in a QD are strongly affected by the confinement strength and the electric field. - Research highlights: {yields} The magnitude of the nonlinear optical rectification of the excitons confined in a disc-like quantum dot may reach 10{sup -2} m/V. It is much higher than that of the other low-dimensional semiconductors, e.g., quantum wells, and one-dimensional semiparabolic quantum dots. {yields} The nonlinear optical rectification of the excitons confined in a disc-like quantum dot is strongly dependent on the confinement frequency. In order to obtain the larger optical rectification coefficients in quantum dots, we can change the confinement frequency. {yields} The calculated results also reveal that an applied electric field has a great influence on the nonlinear optical rectification susceptibility. In order to obtain the larger optical rectification coefficients in quantum dots we can induce the electric field.

  14. Energy transfer efficiency in the chromophore network strongly coupled to a vibrational mode.

    Science.gov (United States)

    Mourokh, Lev G; Nori, Franco

    2015-11-01

    Using methods from condensed matter and statistical physics, we examine the transport of excitons through the photosynthetic complex from a receiving antenna to a reaction center. Writing the equations of motion for the exciton creation-annihilation operators, we are able to describe the exciton dynamics, even in the regime when the reorganization energy is of the order of the intrasystem couplings. We determine the exciton transfer efficiency in the presence of a quenching field and protein environment. While the majority of the protein vibrational modes are treated as a heat bath, we address the situation when specific modes are strongly coupled to excitons and examine the effects of these modes on the energy transfer efficiency in the steady-state regime. Using the structural parameters of the Fenna-Matthews-Olson complex, we find that, for vibrational frequencies below 16 meV, the exciton transfer is drastically suppressed. We attribute this effect to the formation of a "mixed exciton-vibrational mode" where the exciton is transferred back and forth between the two pigments with the absorption or emission of vibrational quanta, instead of proceeding to the reaction center. The same effect suppresses the quantum beating at the vibrational frequency of 25 meV. We also show that the efficiency of the energy transfer can be enhanced when the vibrational mode strongly couples to the third pigment only, instead of coupling to the entire system.

  15. Vibronic enhancement of excitation energy transport: Interplay between local and non-local exciton-phonon interactions

    Science.gov (United States)

    Lee, Myeong H.; Troisi, Alessandro

    2017-02-01

    It has been reported in recent years that vibronic resonance between vibrational energy of the intramolecular nuclear mode and excitation-energy difference is crucial to enhance excitation energy transport in light harvesting proteins. Here we investigate how vibronic enhancement induced by vibronic resonance is influenced by the details of local and non-local exciton-phonon interactions. We study a heterodimer model with parameters relevant to the light-harvesting proteins with the surrogate Hamiltonian quantum dynamics method in a vibronic basis. In addition, the impact of field-driven excitation on the efficiency of population transfer is compared with the instantaneous excitation, and the effect of multi-mode vibronic coupling is presented in comparison with the coupling to a single effective vibrational mode. We find that vibronic enhancement of site population transfer is strongly suppressed with the increase of non-local exciton-phonon interaction and increasing the number of strongly coupled high-frequency vibrational modes leads to a further decrease in vibronic enhancement. Our results indicate that vibronic enhancement is present but may be much smaller than previously thought and therefore care needs to be taken when interpreting its role in excitation energy transport. Our results also suggest that non-local exciton-phonon coupling, which is related to the fluctuation of the excitonic coupling, may be as important as local exciton-phonon coupling and should be included in any quantum dynamics model.

  16. Internal field induced exciton binding energy and the optical transition in a strained Mg based II–VI quantum well

    Energy Technology Data Exchange (ETDEWEB)

    Elangovan, P. [Department of Physics, Maamallan Institute of Technology, Chennai 602105 (India); John Peter, A., E-mail: a.john.peter@gmail.com [Department of Physics, Government Arts College, Melur 625 106. Madurai (India); Kyoo Yoo, Chang [Center for Environmental Studies/Green Energy Center, Deptartment of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of)

    2013-11-15

    Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1−x}Mg{sub x}O strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn{sub 1−x}Mg{sub x}O well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: • Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1−x}Mg{sub x}O strained quantum well is investigated. • The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. • The oscillator strength and the exciton lifetime for radiative recombination are computed. • The internal field induced transition energy of strained ZnO/Zn{sub 1−x}Mg{sub x}O well is investigated. • The results show that the nonlinear optical properties strongly depend on Mg composition.

  17. Excitonic effects in third-harmonic generation: The case of carbon nanotubes and nanoribbons

    Science.gov (United States)

    Attaccalite, C.; Cannuccia, E.; Grüning, M.

    2017-03-01

    Linear and nonlinear optical properties of low-dimensional nanostructures have attracted great interest from the scientific community as tools to probe the strong confinement of electrons and for possible applications in optoelectronic devices. In particular it has been shown that the linear optical response of carbon nanotubes [F. Wang et al., Science 308, 838 (2005), 10.1126/science.1110265] and graphene nanoribbons [Nat. Commun. 5 4253 (2014), 10.1038/ncomms5253] is dominated by bounded electron-hole pairs, excitons. The role of excitons in linear response has been widely studied, but still, little is known about their effect on nonlinear susceptibilities. Using a recently developed methodology [Phys. Rev. B 88, 235113 (2013), 10.1103/PhysRevB.88.235113] based on well-established ab initio many-body perturbation theory approaches, we find that quasiparticle shifts and excitonic effects significantly modify the third-harmonic generation in carbon nanotubes and graphene nanoribbons. For both systems the net effect of many-body effects is to reduce the intensity of the main peak in the independent-particle spectrum and redistribute the spectral weight among several excitonic resonances.

  18. Excitonic recombinations in h-BN: From bulk to exfoliated layers

    Science.gov (United States)

    Pierret, A.; Loayza, J.; Berini, B.; Betz, A.; Plaçais, B.; Ducastelle, F.; Barjon, J.; Loiseau, A.

    2014-01-01

    Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First, the link between the presence of structural defects and the recombination intensity of trapped excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replicas of exciton recombinations. Second, the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations, indicating extremely compact excitons in h-BN.

  19. Dissociating excitons photogenerated in semiconducting carbon nanotubes at polymeric photovoltaic heterojunction interfaces.

    Science.gov (United States)

    Bindl, Dominick J; Safron, Nathaniel S; Arnold, Michael S

    2010-10-26

    Semiconducting single-walled carbon nanotubes (s-SWCNTs) have strong near-infrared and visible absorptivity and exceptional charge transport characteristics, rendering them highly attractive semiconductor absorbers for photovoltaic and photodetector technologies. However, these applications are limited by a poor understanding of how photogenerated charges, which are bound as excitons in s-SWCNTs, can be dissociated in large-area solid-state devices. Here, we measure the dissociation of excitons in s-SWCNT thin films that form planar heterojunction interfaces with polymeric photovoltaic materials using an exciton dissociation-sensitive photocapacitor measurement technique that is advantageously insensitive to optically induced thermal photoconductive effects. We find that fullerene and polythiophene derivatives induce exciton dissociation, resulting in electron and hole transfer, respectively, away from optically excited s-SWCNTs. Significantly weaker or no charge transfer is observed using wider gap polymers due to insufficient energy offsets. These results are expected to critically guide the development of thin film s-SWCNT-based photosensitive devices.

  20. Microscopic origins of the surface exciton photoluminescence peak in ZnO nanostructures

    Science.gov (United States)

    Biswas, Mahua; Jung, Yun Suk; Kim, Hong Koo; Kumar, Kumarappan; Hughes, Gregory J.; Newcomb, S.; Henry, Martin O.; McGlynn, Enda

    2011-06-01

    We report photoluminescence (PL) studies of the surface exciton peak in ZnO nanostructures at ~3.367 eV aimed at elucidation of the nature and origin of the emission and its relationship to the nanostructure morphology. PL spectra in conjunction with localized voltage application in high vacuum and different gas atmospheres show a consistent variation (and recovery), allowing an association of the PL to a bound excitonic transition at the ZnO surface, which is modified by an adsorbate. PL studies of samples treated by plasma and of samples exposed to UV light under high vacuum conditions, both well-known processes for desorption of surface adsorbed oxygen, show no consistent effects on the surface exciton peak indicating the lack of involvement of oxygen species. X-ray photoelectron spectroscopy data strongly suggest involvement of adsorbed OH species. X-ray diffraction, scanning, and transmission electron microscopy data are presented also, and the relationship of the surface exciton peak to the nanostructure morphology is discussed.

  1. Extreme sensitivity of circular dichroism to long-range excitonic couplings in helical supramolecular assemblies.

    Science.gov (United States)

    van Dijk, Leon; Bobbert, Peter A; Spano, Frank C

    2010-01-21

    Circular dichroism (CD) spectroscopy is an ideal tool for studying the self-assembly of helical supramolecular assemblies since it is very sensitive to extended excitonic couplings between chiral chromophores. We show that the CD spectrum retains its high sensitivity to long-range interactions even in the presence of extreme disorder and strong interaction with vibrations when excitations are mainly localized on individual molecules. We derive a universal expression for the first moment of the CD spectrum of helical assemblies in terms of a modulated sum over excitonic couplings, which is independent of the strength of the energetic disorder, the spatial correlation of the disorder, and the strength of the interaction with vibrations. This demonstrates that excitonic couplings can be directly extracted from experimental CD spectra without having information about the energetic disorder and vibrational interactions. We apply our results to helical assemblies of functionalized chiral oligo(p-phenylenevinylene) molecules and show that existing theoretical values for the excitonic couplings should be adapted in order to obtain agreement with the experimental CD spectrum.

  2. Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments

    Science.gov (United States)

    Jadczak, J.; Kutrowska-Girzycka, J.; Kapuściński, P.; Huang, Y. S.; Wójs, A.; Bryja, L.

    2017-09-01

    We report on detailed temperature dependent (T = 7–295 K) optical spectroscopy studies of WSe2, WS2, MoSe2 and MoS2 monolayers exfoliated onto the same SiO2/Si substrate. In the high energy region of absorption type (reflectivity contrast—RC) and emission (photo-luminescence—PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe2 and WS2 we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS2, we observe an additional line (L 0), which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L 0 and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L 0 resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe2 and WS2 as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7–295 K the total PL intensity decreases moderately in WSe2 and WS2, strongly in MoS2 and dramatically in MoSe2.

  3. Quantum Transport Detected by Strong Proximity Interaction at a Graphene-WS2 van der Waals Interface.

    Science.gov (United States)

    O'Farrell, E C T; Avsar, A; Tan, J Y; Eda, G; Özyilmaz, B

    2015-09-09

    Magnetotransport measurements demonstrate that graphene in a van der Waals heterostructure is a sensitive probe of quantum transport in an adjacent WS2 layer via strong Coulomb interactions. We observe a large low-field magnetoresistance (≫ e(2)/h) and a -ln T temperature dependence of the resistance. In-plane magnetic field resistance indicates the origin is orbital and nonclassical. We demonstrate a strong electron-hole asymmetry in the mobility and coherence length of graphene demonstrating the presence of localized Coulomb interactions with ionized donors in the WS2 substrate, which ultimately leads to screening as the Fermi level of graphene is tuned toward the conduction band of WS2. This leads us to conclude that graphene couples to quantum localization processes in WS2 via the Coulomb interaction and results in the observed signatures of quantum transport. Our results show that theoretical descriptions of the van der Waals interface should not ignore localized strong correlations.

  4. Indirect excitons in (111) GaAs double quantum wells

    Science.gov (United States)

    Hubert, C.; Biermann, K.; Hernández-Mínguez, A.; Santos, P. V.

    2017-08-01

    We study the dynamics of indirect (or dipolar) excitons (interwell IXs) in GaAs (111) double quantum wells (DQWs) subjected to a transverse electric field. In comparison with single (111) QWs, these DQWs can store, for a comparable applied fields and optical excitation density, a density of interwell IXs much larger than in SQWs, thus leading to stronger interwell IX- IX repulsive interactions. We show by means of spatially-resolved optical spectroscopy that interwell IXs in (111) DWQs can be transported over distances exceeding 60 μm. From the spectral dependence of the interwell IX spatial distribution profiles, we show that the long transport distances are due to drift forces arising from the strong interwell IX- IX interactions.

  5. Room temperature organic exciton-polariton flow exploiting high-speed, high-Q propagating modes

    CERN Document Server

    Lerario, Giovanni; Cannavale, Alessandro; Mangione, Federica; Gambino, Salvatore; Dominici, Lorenzo; De Giorgi, Milena; Gigli, Giuseppe; Sanvitto, Daniele

    2015-01-01

    Exciton-polaritons, bosonic quasi-particles formed by the interaction of light and matter, have shown a plethora of exciting phenomena that have been chiefly restricted to inorganic semiconductors and low temperature operation. Only recently, polariton condensation and non-linear effects have been demonstrated with polymers and organic molecules, making these systems suited for a realistic new generation of all-optical devices. However, polariton propagation in the plane of the device, essential for on-chip integration, is still limited by the very strong dissipation inherent to present organic microcavities. Here, we demonstrate strong-coupling of organic excitons with a Bloch surface wave (Q $\\approx$ 3000) which sustains polariton propagation for distances longer than 300 $\\mu$m and polariton lifetimes of about 1 ps, a record value in organic devices. The group velocity of the polariton mode is found to be $\\approx$ 50% the speed of light, about two order of magnitude higher than in any planar microcavity.

  6. Semiconductor-metal nanoparticle molecules: hybrid excitons and the nonlinear fano effect.

    Science.gov (United States)

    Zhang, Wei; Govorov, Alexander O; Bryant, Garnett W

    2006-10-06

    Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

  7. Linear and nonlinear optics of hybrid plasmon-exciton nanomaterials in the presence of overlapping resonances

    CERN Document Server

    Sukharev, Maxim; Pachter, Ruth

    2015-01-01

    We consider a hybrid plasmon-exciton system comprised of a resonant molecular subsystem and three Au wires supporting a dipole mode which can be coupled to a dark mode in controllable fashion by variation of a symmetry parameter. The physics of such a system under strong coupling conditions is examined in detail. It is shown that if two wires supporting the dark mode are covered with molecular layers the system exhibits four resonant modes for a strong coupling regime due to asymmetry and lifted degeneracy of the molecular state in this case, while upon having molecular aggregates covering the top wire with dipolar mode, three resonant modes appear. Pump-probe simulations are performed to scrutinize the quantum dynamics and find possible ways to control plasmon-exciton materials. It is demonstrated that one can design hybrid nanomaterials with highly pronounced Fano-type resonances when excited by femtosecond lasers.

  8. Multiple Exciton Generation in Colloidal Nanocrystals

    Directory of Open Access Journals (Sweden)

    Charles Smith

    2013-12-01

    Full Text Available In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG, can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

  9. Microcavity controlled coupling of excitonic qubits

    CERN Document Server

    Albert, F; Kasprzak, J; Strauß, M; Schneider, C; Höfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W

    2012-01-01

    Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. The most relevant mechanism of coherent coupling of distant qubits is coupling via the electromagnetic field. Here, we demonstrate the controlled coherent coupling of spatially separated excitonic qubits via the photon mode of a solid state microresonator. This is revealed by two-dimensional spectroscopy of the sample's coherent response, a sensitive and selective probe of the coherent coupling. The experimental results are quantitatively described by a rigorous theory of the cavity mediated coupling within a cluster of quantum dots excitons. Having demonstrated this mechanism, it can be used in extended coupling channels - sculptured, for instance, in photonic crystal cavities - to enable a long-range, non-local wiring up of individual emitters in solids.

  10. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    CERN Document Server

    Olšina, Jan; Wang, Chen; Cao, Jianshu

    2014-01-01

    Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined by the non-canonical thermal distribution resulting from the entanglement with the phonon bath. The decay of the excited state population to the common ground state establishes a non-equilibrium steady-state flux driven by the sunlight, and it defines a time window to observe the transition from dynamical to static coherence. For the parameters relevant to photosynthetic systems, the exciton dynamics initiated by the sunlight exhibits a non-negligible amount of dynamical coherence (quantum beats) on the sub-picosecond timescale; however, ...

  11. Excitonic and photonic processes in materials

    CERN Document Server

    Williams, Richard

    2015-01-01

    This book is expected to present state-of-the-art understanding of a selection of excitonic and photonic processes in useful materials from semiconductors to insulators to metal/insulator nanocomposites, both inorganic and organic.  Among the featured applications are components of solar cells, detectors, light-emitting devices, scintillators, and materials with novel optical properties.  Excitonic properties are particularly important in organic photovoltaics and light emitting devices, as also in questions of the ultimate resolution and efficiency of new-generation scintillators for medical diagnostics,  border security, and nuclear nonproliferation.  Novel photonic and optoelectronic applications benefit from new material combinations and structures to be discussed.

  12. Resonance Raman scattering and excitonic spectra in TlInS{sub 2} crystals

    Energy Technology Data Exchange (ETDEWEB)

    Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of); Stamov, I.G. [T.G. Shevchenko State University of Pridnestrovie, 25 Oktyabrya Street 107, 3300 Tiraspol, Republic of Moldova (Moldova, Republic of); Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Ursaki, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of); Dorogan, V. [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of)

    2015-04-15

    The excitons ground and excited states for E∥a and E∥b polarizations in absorption and reflection spectra of TlInS{sub 2} crystals were detected. The fundamental parameters of excitons and bands were determined at k=0. The resonance Raman spectra were investigated in the region of excitons transitions. The resonance Raman scattering spectra with participation of optical phonons that are active at the center of Brillouin zone were identified. The Raman scattering in Y(YX)Z and Y(ZX)Z geometries at 10 K with excitation by He–Ne laser was researched. Energies of phonons with A{sub g} and B{sub g} symmetries were determined. It was shown that the number of modes at 10 K was two times lower than expected according to theoretical calculations. - Highlights: • The resonance Raman scattering in geometry Y(YX)Z and Y(ZX)Z at 10 K was investigated. • Energies of phonons with A{sub g} and B{sub g} symmetries were determined. • The experimental and theoretical calculations completely conform if crystals are described by symmetry group D{sub 4h}{sup 15}. • The main parameters of excitons and bands were determined. • The model of electron transitions in k=0 was suggested.

  13. Spin-polarized exciton quantum beating in hybrid organic-inorganic perovskites

    Science.gov (United States)

    Odenthal, Patrick; Talmadge, William; Gundlach, Nathan; Wang, Ruizhi; Zhang, Chuang; Sun, Dali; Yu, Zhi-Gang; Valy Vardeny, Z.; Li, Yan S.

    2017-09-01

    Hybrid organic-inorganic perovskites have emerged as a new class of semiconductors that exhibit excellent performance as active layers in photovoltaic solar cells. These compounds are also highly promising materials for the field of spintronics due to their large and tunable spin-orbit coupling, spin-dependent optical selection rules, and their predicted electrically tunable Rashba spin splitting. Here we demonstrate the optical orientation of excitons and optical detection of spin-polarized exciton quantum beating in polycrystalline films of the hybrid perovskite CH3NH3PbClxI3-x. Time-resolved Faraday rotation measurement in zero magnetic field reveals unexpectedly long spin lifetimes exceeding 1 ns at 4 K, despite the large spin-orbit couplings of the heavy lead and iodine atoms. The quantum beating of exciton states in transverse magnetic fields shows two distinct frequencies, corresponding to two g-factors of 2.63 and -0.33, which we assign to electrons and holes, respectively. These results provide a basic picture of the exciton states in hybrid perovskites, and suggest they hold potential for spintronic applications.

  14. Ultracold Gas of Excitons in Traps

    Science.gov (United States)

    2012-06-08

    Thomas,, G. Grosso,, M. Remeika,, A. T. Hammack,, A. D. Meyertholen,, M. M. Fogler ,, L.V. Butov,, M. Hanson,, A. C. Gossard. Trapping Indirect...M. Fogler , L. V. Butov, A. V. Kavokin, K. L. Campman, A. A. High, A. C. Gossard. Spontaneous coherence in a cold exciton gas, Nature, (03 2012...56:19 7 A. High, A. Thomas, G. Grosso, M. Remeika, A. Hammack, A. Meyertholen, M. Fogler , L. Butov, M. Hanson, A. Gossard. Trapping Indirect

  15. Multiple Exciton Generation in Colloidal Silicon Nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.

    2007-01-01

    Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap {identical_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

  16. Transport Theory for Metals with Excitonic Instabilities

    OpenAIRE

    Breitkreiz, Maxim

    2015-01-01

    Metals with excitonic instabilities are multiband systems with significant electron-electron interaction. The electronic transport in such systems is affected by collective fluctuations of the electrons, leading to anomalous features in the measured transport coefficients. Many of these anomalies have not been well understood because the transport mechanisms in these systems tend to be rather complex. The complexity arises, on the one hand, from the multiband nature and, on the other, fro...

  17. Semiconductor-metal nanoparticle molecules: hybrid excitons and non-linear Fano effect

    OpenAIRE

    Zhang, Wei; Govorov, Alexander O.; Bryant, Garnett W.

    2006-01-01

    Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. The resultant superstructure may provide new physical properties not encountered in homogeneous systems. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low inciden...

  18. Singlet exciton fission in nanostructured organic solar cells.

    Science.gov (United States)

    Jadhav, Priya J; Mohanty, Aseema; Sussman, Jason; Lee, Jiye; Baldo, Marc A

    2011-04-13

    Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.

  19. Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials

    KAUST Repository

    Tizei, Luiz H. G.

    2015-03-01

    Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.

  20. Chiral topological excitons in the monolayer transition metal dichalcogenides

    Science.gov (United States)

    Gong, Z. R.; Luo, W. Z.; Jiang, Z. F.; Fu, H. C.

    2017-02-01

    We theoretically investigate the chiral topological excitons emerging in the monolayer transition metal dichalcogenides, where a bulk energy gap of valley excitons is opened up by a position dependent external magnetic field. We find two emerging chiral topological nontrivial excitons states, which exactly connects to the bulk topological properties, i.e., Chern number = 2. The dependence of the spectrum of the chiral topological excitons on the width of the magnetic field domain wall as well as the magnetic filed strength is numerically revealed. The chiral topological valley excitons are not only important to the excitonic transport due to prevention of the backscattering, but also give rise to the quantum coherent control in the optoelectronic applications.

  1. Excitonic Stark effect in MoS2 monolayers

    Science.gov (United States)

    Scharf, Benedikt; Frank, Tobias; Gmitra, Martin; Fabian, Jaroslav; Žutić, Igor; Perebeinos, Vasili

    2016-12-01

    We theoretically investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a quadratic Stark shift, of the order of a few meV for fields of 10 V/μ m , in the linear absorption spectra. The spectral weight of the main exciton peaks decreases by a few percent with an increasing electric field due to the exciton field ionization into free carriers as reflected in the exciton wave functions. Subpicosecond exciton decay lifetimes at fields of a few tens of V/μ m could be utilized in solar energy harvesting and photodetection. We find simple scaling relations of the exciton binding, radius, and oscillator strength with the dielectric environment and an electric field, which provides a path to engineering the MoS2 electro-optical response.

  2. Topological Excitonic Superfluids in Three Dimensions

    Science.gov (United States)

    Gilbert, Matthew; Hankiewicz, Ewelina; Kim, Youngseok

    2013-03-01

    We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current. Army Research Office (ARO) under contract number W911NF-09-1-0347, the Office of Naval Research (ONR) under contract number N0014-11-1-0728, and the Air Force Office of Scientific Research (AFOSR) under contract number FA9550-10-1-0459, DFG Grant HA 5893

  3. Quantum fluctuations in semiconductor quantum dots and their contributions to the self-energy functions of exciton states

    Science.gov (United States)

    Mutygullina, A. A.; Khamadeev, M. A.; Blum, D. O.; Shirdelhavar, A. H.

    2017-06-01

    Influence of quantum fluctuations in a system consisting of a quantum dot and the reservoir of acoustic phonons on processes in which the quantum dot takes part is investigated. Under some conditions this influence is shown to be very strong. We find a contribution from the quantum fluctuations to the self-energy function of the exciton coupled to the quantum dot.

  4. Compensation of Dipolar-Exciton Spin Splitting in Magnetic Field

    OpenAIRE

    Gorbunov, A. V.; Timofeev, V. B.

    2012-01-01

    Magnetoluminescence of spatially indirect dipolar excitons collected in 25 nm GaAs/AlGaAs single quantum well within a lateral potential trap has been studied in perpendicular magnetic field in Faraday geometry. The paramagnetic spin splitting of the luminescence line of the heavy-hole excitons in the trap centre is completely compensated at magnetic fields below critical value, around 2 Tesla. The effect of spin-splitting compensation is caused by the exchange interaction in dense exciton Bo...

  5. Determination of Surface Exciton Energies by Velocity Resolved Atomic Desorption

    Energy Technology Data Exchange (ETDEWEB)

    Hess, Wayne P.; Joly, Alan G.; Beck, Kenneth M.; Sushko, Petr V.; Shluger, Alexander L.

    2004-08-20

    We have developed a new method for determining surface exciton band energies in alkali halides based on velocity-resolved atomic desorption (VRAD). Using this new method, we predict the surface exciton energies for K1, KBr, KC1, and NaC1 within +0.15 eV. Our data, combined with the available EELS data for alkali fluorides, demonstrate a universal linear correlation with the inverse inter-atomic distance in these materials. The results suggest that surface excitons exist in all alkali halides and their excitation energies can be predicted from the known bulk exciton energies and the obtained correlation plot.

  6. Exciton migration and quenching in poly(propylene imine) dendrimers

    Science.gov (United States)

    Minevičiūtė, I.; Gulbinas, V.; Franckevičius, M.; Vaišnoras, R.; Marcos, M.; Serrano, J. L.

    2009-05-01

    Exciton migration between chromophore groups of the poly(propylene imine) dendrimer in chloroform solution and in solid state has been investigated by means of the time-resolved fluorescence measurements. Fluorescence decay kinetics, dynamic band shift and the depolarization rate have been analyzed. Exciton migration in a single dendrimer was found to be slow in comparison with temperature-dependent chromophore reorientation time of 150-600 ps. In a solid state chromophore groups form collective excitonic states responsible for the dendrimer film fluorescence. Exciton migration and localization to the lowest energy sites within the distributed density of states take place on a subnanosecond-nanosecond time scale.

  7. Revealing and Characterizing Dark Excitons Through Coherent Multidimensional Spectroscopy

    CERN Document Server

    Tollerud, Jonathan O; Davis, Jeffrey A

    2016-01-01

    Dark excitons are of fundamental importance in a broad range of contexts, but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify different types of dark excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms and coupling strengths. The observations of coherent coupling between bright and dark excitons hint at a role for a multi-step process by which excitons in the barrier can relax into the quantum wells.

  8. Nonlinear Raman Shift Induced by Exciton-to-Trion Transformation in Suspended Trilayer MoS2

    CERN Document Server

    Taghinejad, Hossein; Tarasov, Alexey; Tsai, Meng-Yen; Hosseinnia, Amir H; Campbell, Philip M; Eftekhar, Ali A; Vogel, Eric M; Adibi, Ali

    2015-01-01

    Layered two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS2) have recently attracted remarkable attention because of their unique physical properties. Here, we use photoluminescence (PL) and Raman spectroscopy to study the formation of the so- called trions in a synthesized freestanding trilayer MoS2. A trion is a charged quasi-particle formed by adding one electron or hole to a neutral exciton (a bound electron-hole pair). We demonstrate accurate control over the transformation of excitons to trions by tuning the power of the optical pump (laser). Increasing the power of the excitation laser beyond a certain threshold (~ 4 mW) allows modulation of trion-to-exciton PL intensity ratio as well as the spectral linewidth of both trions and excitons. Via a systematic and complementary Raman analysis we disclose a strong coupling between laser induced exciton-to-trion transformation and the characteristic phononic vibrations of MoS2. The onset of such an optical transformation corresponds to the ...

  9. Density-dependent dynamical coexistence of excitons and free carriers in the organolead perovskite C H3N H3Pb I3

    Science.gov (United States)

    Wang, Wei; Li, Yu; Wang, Xiangyuan; Lv, Yanping; Wang, Shufeng; Wang, Kai; Shi, Yantao; Xiao, Lixin; Chen, Zhijian; Gong, Qihuang

    2016-10-01

    The high efficiency of perovskite solar cells benefits from the high density of photoinduced free carriers. We studied how exciton and free carriers, as the two major photoproducts, coexist inside the C H3N H3Pb I3 perovskite. A new density-resolved spectroscopic method was developed for this purpose. The density-dependent coexistence of excitons and free carriers over a wide density range was experimentally observed. The quantitative analysis on the density-resolved spectra revealed a moderate exciton binding energy of 24 ± 2 meV. The results effectively proved that the strong ionic polarization inside the perovskite has a negligible contribution to exciton formation. The spectra also efficiently uncovered the effective mass of electron-hole pairs. Our spectroscopic method and the results profoundly enrich the understanding of the photophysics in perovskite materials for photovoltaic applications.

  10. Coherent control of ground state excitons in the nonlinear regime within an ensemble of self-assembled InAs quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Moldaschl, Thomas; Mueller, Thomas; Golka, Sebastian; Parz, Wolfgang; Strasser, Gottfried; Unterrainer, Karl [Photonics Institute and Center for Micro- and Nanostructures, Vienna University of Technology (Austria)

    2009-04-15

    In this work femtosecond spectral hole burning spectroscopy is used to resonantly excite ground state excitons in an ensemble of self-assembled InAs/GaAs quantum dots with a strong pump pulse. Two fundamental coherent nonlinear effects are observed with the aid of the intrinsic time- and frequency resolution of the setup: The low temperature Rabi oscillation of the two-level system associated with the excitonic ground state transition and the observation of two-photon absorption in the surrounding GaAs crystal matrix. The emergence of the latter effect also infers the existence of charged excitons in the nominally undoped QD sample, backed up by the observation of additional spectral holes next to the excitonic transitions. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  11. Exciton dynamics in GaAs/(Al,Ga)As core-shell nanowires with shell quantum dots

    Science.gov (United States)

    Corfdir, Pierre; Küpers, Hanno; Lewis, Ryan B.; Flissikowski, Timur; Grahn, Holger T.; Geelhaar, Lutz; Brandt, Oliver

    2016-10-01

    We study the dynamics of excitons in GaAs/(Al,Ga)As core-shell nanowires by continuous-wave and time-resolved photoluminescence and photoluminescence excitation spectroscopy. Strong Al segregation in the shell of the nanowires leads to the formation of Ga-rich inclusions acting as quantum dots. At 10 K, intense light emission associated with these shell quantum dots is observed. The average radiative lifetime of excitons confined in the shell quantum dots is 1.7 ns. We show that excitons may tunnel toward adjacent shell quantum dots and nonradiative point defects. We investigate the changes in the dynamics of charge carriers in the shell with increasing temperature, with particular emphasis on the transfer of carriers from the shell to the core of the nanowires. We finally discuss the implications of carrier localization in the (Al,Ga)As shell for fundamental studies and optoelectronic applications based on core-shell III-As nanowires.

  12. Estimating Excitonic Effects in the Absorption Spectra of Solids: Problems and Insight from a Guided Iteration Scheme

    Science.gov (United States)

    Rigamonti, Santiago; Botti, Silvana; Veniard, Valérie; Draxl, Claudia; Reining, Lucia; Sottile, Francesco

    2015-04-01

    A major obstacle for computing optical spectra of solids is the lack of reliable approximations for capturing excitonic effects within time-dependent density functional theory. We show that the accurate prediction of strongly bound electron-hole pairs within this framework using simple approximations is still a challenge and that available promising results have to be revisited. Deriving a set of analytical formulas we analyze and explain the difficulties. We deduce an alternative approximation from an iterative scheme guided by previously available knowledge, significantly improving the description of exciton binding energies. Finally, we show how one can "read" exciton binding energies from spectra determined in the random phase approximation, without any further calculation.

  13. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    Science.gov (United States)

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-08-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

  14. Intermediate type excitons in Schottky barriers of A3B6 layer semiconductors and UV photodetectors

    Science.gov (United States)

    Alekperov, O. Z.; Guseinov, N. M.; Nadjafov, A. I.

    2006-09-01

    Photoelectric and photovoltaic spectra of Schottky barrier (SB) structures of InSe, GaSe and GaS layered semiconductors (LS) are investigated at quantum energies from the band edge excitons of corresponding materials up to 6.5eV. Spectral dependences of photoconductivity (PC) of photo resistors and barrier structures are strongly different at the quantum energies corresponding to the intermediate type excitons (ITE) observed in these semiconductors. It was suggested that high UV photoconductivity of A3B6 LS is due to existence of high mobility light carriers in the depth of the band structure. It is shown that SB of semitransparent Au-InSe is high sensitive photo detector in UV region of spectra.

  15. Optical properties of localized excitons in semiconductor nanostructures

    DEFF Research Database (Denmark)

    Leosson, Kristjan; Hvam, Jørn Märcher; Langbein, Wolfgang Werner;

    2002-01-01

    Denne afhandling beskriver optiske undersøgelser af lokaliserede excitoner i III-V halvleder nanostrukturer. Det drejer sig især om tredimensional lokalisering af excitoner i to typer af selvorganiserede systemer, nemlig kvantebrønde med fluktuerende lagtykkelse og såkaldte selv-dannede kvantepun...

  16. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jørn Märcher

    1998-01-01

    The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering...

  17. Molecular weight dependence of exciton diffusion in poly(3-hexylthiophene)

    DEFF Research Database (Denmark)

    Masri, Zarifi; Ruseckas, Arvydas; Emelianova, Evguenia V.

    2013-01-01

    A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial π–π aggregates of polymer chromophores and about 100...

  18. Coherent secondary emission from resonantly excited two-exciton states

    DEFF Research Database (Denmark)

    Birkedal, Dan

    2000-01-01

    to the nonlinear susceptibility. The method exploits that emission from two-exciton coherences can occur in non-specular directions, with the recoil momentum taken up by an exciton left behind in the sample. Using ultrafast spectral interferometry we demonstrate the presence of this new coherent component...

  19. Bose Condensation of Interwell Excitons in Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Timofeev, V. B.; Ni, P. A.

    2002-01-01

    in the domain. With a rise in temperature, this line disappears from the spectrum (Tc 3.4 K). The observed phenomenon is attributed to Bose–Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature...

  20. Magnetic excitons in singlet-ground-state ferromagnets

    DEFF Research Database (Denmark)

    Birgeneau, R.J.; Als-Nielsen, Jens Aage; Bucher, E.

    1971-01-01

    The authors report measurements of the dispersion of singlet-triplet magnetic excitons as a function of temperature in the singlet-ground-state ferromagnets fcc Pr and Pr3Tl. Well-defined excitons are observed in both the ferromagnetic and paramagnetic regions, but with energies which are nearly...

  1. Bose Condensation of Interwell Excitons in Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Timofeev, V. B.; Ni, P. A.

    2002-01-01

    The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n–i–n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring...

  2. Weakly disordered two-dimensional Frenkel excitons

    Science.gov (United States)

    Boukahil, A.; Zettili, Nouredine

    2004-03-01

    We report the results of studies of the optical properties of weakly disordered two- dimensional Frenkel excitons in the Coherent Potential Approximation (CPA). An approximate complex Green's function for a square lattice with nearest neighbor interactions is used in the self-consistent equation to determine the coherent potential. It is shown that the Density of States is very much affected by the logarithmic singularities in the Green's function. Our CPA results are in excellent agreement with previous investigations by Schreiber and Toyozawa using the Monte Carlo simulation.

  3. Excitons in conjugated polymers from first principles

    Science.gov (United States)

    van der Horst, J.-W.; Bobbert, P. A.; Pasveer, W. F.; Michels, M. A. J.; Brocks, G.; Kelly, P. J.

    2002-08-01

    By a combination of ab-initio computational techniques, based on density-functional theory, GW theory, and the Bethe-Salpeter equation, we study the opto-electronic properties of several conjugated polymers and in particular the properties of excitons. We study three different situations: (I) an isolated polymer chain, (II) a chain embedded in a dielectric medium, and (III) a polymer crystal. Surprisingly, the results obtained for situation (II) generally agree best with experiment. We discuss possible reasons for this rule and an interesting exception.

  4. Directing energy transport in organic photovoltaic cells using interfacial exciton gates.

    Science.gov (United States)

    Menke, S Matthew; Mullenbach, Tyler K; Holmes, Russell J

    2015-04-28

    Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor-acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit.

  5. Superfluid phase transition in two-dimensional excitonic systems

    Energy Technology Data Exchange (ETDEWEB)

    Apinyan, V.; Kopeć, T.K., E-mail: kopec@int.pan.wroc.pl

    2014-03-01

    We study the superfluid phase transition in the two-dimensional (2D) excitonic system. Employing the extended Falicov–Kimball model (EFKM) and considering the local quantum correlations in the system composed of conduction band electrons and valence band holes we demonstrate the existence of the excitonic insulator (EI) state in the system. We show that at very low temperatures, the particle phase stiffness in the pure-2D excitonic system, governed by the non-local cross correlations, is responsible for the vortex–antivortex binding phase-field state, known as the Berezinskii–Kosterlitz–Thouless (BKT) superfluid state. We demonstrate that the existence of excitonic insulator phase is a necessary prerequisite, leading to quasi-long-range order in the 2D excitonic system.

  6. Activated singlet exciton fission in a semiconducting polymer.

    Science.gov (United States)

    Musser, Andrew J; Al-Hashimi, Mohammed; Maiuri, Margherita; Brida, Daniele; Heeney, Martin; Cerullo, Giulio; Friend, Richard H; Clark, Jenny

    2013-08-28

    Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.

  7. How to Draw Energy Level Diagrams in Excitonic Solar Cells.

    Science.gov (United States)

    Zhu, X-Y

    2014-07-03

    Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells.

  8. Excitons in quantum-dot quantum-well nanoparticles

    Institute of Scientific and Technical Information of China (English)

    史俊杰

    2002-01-01

    A variational calculation is presented for the ground-state properties of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles. The relationship between the exciton states and structure parameters of QDQW nanoparticles is investigated, in which both the heavy-hole and the light-hole exciton states are considered. The results show that the confinement energies of the electron and hole states and the exciton binding energies depend sensitively on the well width and core radius of the QDQW structure. A detailed comparison between the heavy-hole and light-hole exciton states is given. Excellent agreement is found between experimental results and our calculated 1se-1sh transition energies.

  9. Excitons in conjugated polymers: wavefunctions, symmetries, and quantum numbers.

    Science.gov (United States)

    Barford, William; Paiboonvorachat, Nattapong

    2008-10-28

    We introduce a mapping from configuration interaction singles wavefunctions, expressed as linear combinations of particle-hole excitations between Hartree-Fock molecular orbitals, to real-space exciton wavefunctions, expressed as linear combinations of particle-hole excitations between localized Wannier functions. The exciton wavefunction is a two-dimensional amplitude for the exciton center-of-mass coordinate, R, and the electron-hole separation (or relative coordinate), r, having an exact analogy to one-dimensional hydrogenlike wavefunctions. We describe the excitons by their appropriate quantum numbers, namely, the principle quantum number, n, associated with r and the center-of-mass pseudomomentum quantum number, j, associated with R. In addition, for models with particle-hole symmetry, such as the Pariser-Parr-Pople model, we emphasize the connection between particle-hole symmetry and particle-hole parity. The method is applied to the study of excitons in trans-polyacetylene and poly(para-phenylene).

  10. Exciton-Phonon Dynamics with Long-Range Interaction

    CERN Document Server

    Laskin, Nick

    2011-01-01

    Exciton-phonon dynamics on a 1D lattice with long-range exciton-exciton interaction have been introduced and elaborated. Long-range interaction leads to a nonlocal integral term in the motion equation of the exciton subsystem if we go from discrete to continuous space. In some particular cases for power-law interaction, the integral term can be expressed through a fractional order spatial derivative. A system of two coupled equations has been obtained, one is a fractional differential equation for the exciton subsystem, the other is a standard differential equation for the phonon subsystem. These two equations present a new fundamental framework to study nonlinear dynamics with long-range interaction. New approaches to model the impact of long-range interaction on nonlinear dynamics are: fractional generalization of Zakharov system, Hilbert-Zakharov system, Hilbert-Ginzburg-Landau equation and nonlinear Hilbert-Schrodinger equation. Nonlinear fractional Schrodinger equation and fractional Ginzburg-Landau equa...

  11. On the detection of strong emissions of methane in the Arctic using spectral measurements from IASI and GOSAT

    Science.gov (United States)

    Bourakkadi, Zakia; Payan, Sébastien; Bureau, Jérôme

    2014-05-01

    Boreal ecosystems store significant quantities of organic carbon (C) for thousands of years. Most are presently sequestered in permafrost. In recent years, several studies highlighted that climate warming and thawing of permafrost in the Arctic acts on the mobilization of old stored carbon (c) and contribute to a sustained release of methane (CH4) to the atmosphere [1],[2],[3]. Because methane is an important greenhouse gas, it is necessary to estimate his sources and sinks in the Arctic. The objective of this study is to evaluate and quantify methane strong emission in this region of the globe using spectral measurements from two satellite instruments: IASI-MetOp and TANSO-FTS. The Infrared Atmospheric Sounding Interferometer (IASI) is a Fourier transform spectrometer coupled with an integrated imaging system that observes and measures infrared radiation emitted by the Earth and the atmosphere in the spectral range 645-2760cm-1, which covered the methane ?4 and ?3 absorption band[4]. IASI provides global Earth's coverage twice a day and delivers about 1 300 000 spectra per day[5]. The second instrument which we will use in this study is the Thermal And Near-infrared Sensor for Carbon Observation-Fourier Transform Spectrometer (TANSO-FTS), it has a wide TIR band (5,5-14,3μm)which contain the methane ?4absorption band.TANSO-FTS completes one revolution in about 100 minutes and it comes back to the same location in 3 days period. Over these 3 days, FTS takes 56 000 measurements covering the entire globe[6]. To have a good estimation of methane emission above the Arctic, we must exploit a vast amount of spectral information from IASI and TANSO-FTS. But it is well known that performing line-by-line radiative transfer model is a time-consuming process. So, if we need to exploit large data we have to look fast method. In this work, we will use a simple approach based on the Singular Value Decomposition (SVD) to identify spectra over large source of methane. A more

  12. Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy.

    Science.gov (United States)

    March, Samuel A; Clegg, Charlotte; Riley, Drew B; Webber, Daniel; Hill, Ian G; Hall, Kimberley C

    2016-12-15

    Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2-55 meV) reported in recent years.

  13. Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy

    Science.gov (United States)

    March, Samuel A.; Clegg, Charlotte; Riley, Drew B.; Webber, Daniel; Hill, Ian G.; Hall, Kimberley C.

    2016-12-01

    Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2–55 meV) reported in recent years.

  14. Room-temperature exciton-polaritons with two-dimensional WS2

    Science.gov (United States)

    Flatten, L. C.; He, Z.; Coles, D. M.; Trichet, A. A. P.; Powell, A. W.; Taylor, R. A.; Warner, J. H.; Smith, J. M.

    2016-09-01

    Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensation and superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS2 coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of ħΩRabi = 70 meV, exceeding the exciton linewidth. Our data are well described by a transfer matrix model appropriate for the large linewidth regime. This work provides a platform towards observing strongly correlated polariton phenomena in compact photonic devices for ambient temperature applications.

  15. Exciton Dynamics in Semiconducting Carbon Nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Graham, Matt [University of California, Berkeley; Chmeliov, Javgenij [Vilnius University, Lithuania; Ma, Yingzhong [ORNL; Shinohara, Nori [Nagoya University, Japan; Green, Alexander A. [Northwestern University, Evanston; Hersam, Mark C. [Northwestern University, Evanston; Valkunas, Leonas [Vilnius University, Lithuania; Fleming, Graham [University of California, Berkeley

    2010-01-01

    We report femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E22) is excited than that measured by exciting its first transition-allowed state (E11). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E22 excitation, whereas the corresponding amplitude scales linearly with the square root of the E11 excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E22 and E11 excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.

  16. Exciton Correlations in Intramolecular Singlet Fission.

    Science.gov (United States)

    Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Appavoo, Kannatassen; Steigerwald, Michael L; Campos, Luis M; Sfeir, Matthew Y

    2016-06-15

    We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases.

  17. PbSe Nanocrystal Excitonic Solar Cells

    KAUST Repository

    Choi, Joshua J.

    2009-11-11

    We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that Is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date. © 2009 American Chemical Society.

  18. Exciton States and Linear Optical Spectra of Semiconducting Carbon Nanotubes under Uniaxial Strain

    Institute of Scientific and Technical Information of China (English)

    YU Gui-Li; JIA Yong-Lei

    2009-01-01

    Considering the exciton effect,the linear optical spectra of semiconducting single-walled carbon nanotubes (SWNTs) under uniaxial strain are theoretically studied by using the standard formulae of Orr and Ward [Mol.Phys.20(1971)513].It is found that due to the wrapping effect existing in the semiconducting zigzag tubes,the excitation energies of the linear optical spectra show two different kinds of variations with increasing uniaxial strain,among which one decreases such as tube (11,0),and the other increases firstly and then decreases such as tube (10,0).These variations of the linear optical spectra are consistent with the changes of the exciton binding energies or the (quasi)continuum edge of these SWNTs calculated in our previous work,which can be used as a supplemented tool to detect the deformation degree of an SWNT under uniaxiai strain.

  19. 强噪声背景下的微弱信号检测新方法%New Method of Weak Signal Detection with Strong Noise Background

    Institute of Scientific and Technical Information of China (English)

    韦艳冰; 陆绮荣; 黄彬; 高冬美

    2011-01-01

    A new method for weak signal detection with strong noise background is proposed. A scheme which combines hardware with software for realization of this method is designed. In this method, the signals are detected and processed using the classical instrument magnifying technique and single-chip microcomputer control. By means of virtual instrumentation simulation and display, this design provides effective methods to detect μA-level weak signal in pyroelectric infrared detector. Through the experiment of artificial low-frequency weak-signal detection, the system fully reveals its practicality and superiority in the field of weak signal detection.%提出新的强噪声背景下的微弱信号检测方法,设计一种硬件与软件相结合的实现方案.采用经典的仪表放大技术和单片机控制技术对数据进行检测和处理,并通过虚拟仪器技术仿真和显示,为解决热释电红外探测器中μA级微弱信号的检测提供了十分有效的方法.最后通过对模拟低频微弱信号的检测实验,充分显示该系统在微弱信号检测方面的实用性和优越性.

  20. Strong decoherence

    CERN Document Server

    Gell-Mann, Murray; Gell-Mann, Murray; Hartle, James B

    1997-01-01

    We introduce a condition for the strong decoherence of a set of alternative histories of a closed quantum-mechanical system such as the universe. The condition applies, for a pure initial state, to sets of homogeneous histories that are chains of projections, generally branch-dependent. Strong decoherence implies the consistency of probability sum rules but not every set of consistent or even medium decoherent histories is strongly decoherent. Two conditions characterize a strongly decoherent set of histories: (1) At any time the operators that effectively commute with generalized records of history up to that moment provide the pool from which --- with suitable adjustment for elapsed time --- the chains of projections extending history to the future may be drawn. (2) Under the adjustment process, generalized record operators acting on the initial state of the universe are approximately unchanged. This expresses the permanence of generalized records. The strong decoherence conditions (1) and (2) guarantee wha...

  1. Theory of exciton transfer and diffusion in conjugated polymers

    Energy Technology Data Exchange (ETDEWEB)

    Barford, William, E-mail: william.barford@chem.ox.ac.uk [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); Tozer, Oliver Robert [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); University College, University of Oxford, Oxford OX1 4BH (United Kingdom)

    2014-10-28

    We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral

  2. Excitons, polarons, and laser action in poly(p-phenylene vinylene) films

    Science.gov (United States)

    Österbacka, R.; Wohlgenannt, M.; Shkunov, M.; Chinn, D.; Vardeny, Z. V.

    2003-05-01

    We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)-E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogeneration consists of two contributions; the intersystem crossing and, at higher energies, singlet fission. From the onset of the singlet fission process at ESF=2ET, we could estimate the energy of the lowest-lying triplet exciton, 1 3Bu, at 1.55 eV, with a singlet-triplet splitting as large as 0.9 eV. From photoinduced absorption spectroscopy we measured the triplet-triplet transition, T→T*, to be 1.45 eV. The quantum efficiency spectrum for polaron photogeneration also consists of two contributions: one extrinsic and the other intrinsic. The latter shows a monotonically increasing function of energy with an energy onset at 3.3 eV. The intrinsic photogeneration process is analyzed with a model of free-electron-hole pair photogeneration, which separate more effectively at higher energy. The carrier generation quantum yield at 3.65 eV is estimated to be 0.5%. The quantum efficiency for photoluminescence, on the other hand, shows one single step-function process, with an

  3. Strong Force

    CERN Document Server

    Without the strong force, there could be no life. The carbon in living matter is synthesised in stars via the strong force. Lighter atomic nuclei become bound together in a process called nuclear fusion. A minor change in this interaction would make life impossible. As its name suggests, the strong force is the most powerful of the 4 forces, yet its sphere of influence is limited to within the atomic nucleus. Indeed it is the strong force that holds together the quarks inside the positively charged protons. Without this glue, the quarks would fly apart repulsed by electromagnetism. In fact, it is impossible to separate 2 quarks : so much energy is needed, that a second pair of quarks is produced. Text for the interactive: Can you pull apart the quarks inside a proton?

  4. Formation and condensation of excitonic bound states in the generalized Falicov-Kimball model

    Science.gov (United States)

    Farkašovský, Pavol

    2017-01-01

    The density-matrix-renormalization-group method and the Hartree-Fock approximation with the charge-density-wave instability are used to study a formation and condensation of excitonic bound states in the generalized Falicov-Kimball model. In particular, we examine effects of various factors, such as the f -electron hopping, the local and nonlocal hybridizations, as well as the increasing dimension of the system on the excitonic momentum distribution N (q ) and especially on the number of zero-momentum excitons N0=N (q =0 ) in the condensate. It is found that the negative values of the f -electron hopping integrals tf support the formation of a zero-momentum condensate, whereas the positive values of tf have the fully opposite effect. The opposite effects on the formation of the condensate exhibit also the local and nonlocal hybridizations. The first one strongly supports the formation of the condensate, whereas the second one destroys it completely. Moreover, it was shown that the zero-momentum condensate remains robust with increasing dimension of the system.

  5. Binding energy of exciton in a nanowire superlattice in magnetic and electric fields

    Energy Technology Data Exchange (ETDEWEB)

    Galvan-Moya, J E; Gutierrez, W [Escuela de Fisica, Universidad Industrial de Santander, Bucaramanga, Colombia A.A. 678 (Colombia); Moscoso, C, E-mail: edogalvan@gmail.co [Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia A.A. 5997 (Colombia)

    2010-02-01

    We study the binding energy of excitons in a cylindrical GaAs/Ga{sub 1-x}Al{sub x}As nanowire superlattice, embedded in Ga{sub 1-y}Al{sub y}As matrix, in the presence of magnetic and electric fields applied parallel to the growth direction. We express the exciton trial function as a product of one-particle wave functions of the electron and the hole with variationally determined envelope function, which describes the exciton intrinsic properties and depends only on the electron-hole separation. By using a functional derivative technique, we derive a differential equation for this envelope function, which we solve numerically. By varying the wire radius, interwell barrier width and well sizes we obtain binding energies ranging in character from one for strongly coupled superlattice to that for a system of stack of isolated disks. The behaviour of the binding energies and the charge distributions as functions of the interwell coupling, well sizes, and the external fields is consistently described with our simple formalism.

  6. Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures.

    Science.gov (United States)

    Najmaei, Sina; Mlayah, Adnen; Arbouet, Arnaud; Girard, Christian; Léotin, Jean; Lou, Jun

    2014-12-23

    We report on the fabrication of monolayer MoS2-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic-excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS2 excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS2 monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS2 monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS2 temperature increase is nearly four times larger than in the MoS2 reference temperatures. This study shines light on the plasmonic-excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion.

  7. Room temperature Tamm-Plasmon Exciton-Polaritons with a WSe2 monolayer

    CERN Document Server

    Lundt, Nils; Cherotchenko, Evgeniia; Iff, Oliver; Nalitov, Anton V; Klaas, Martin; Betzold, Simon; Dietrich, Christof P; Kavokin, Alexey V; Höfling, Sven; Schneider, Christian

    2016-01-01

    Solid state cavity quantum electrodynamics is a rapidly advancing field which explores the frontiers of light-matter coupling. Plasmonic approaches are of particular interest in this field, since they carry the potential to squeeze optical modes to spaces significantly below the diffraction limit1,2, enhancing light-matter coupling. They further serve as an architecture to design ultra-fast, non-linear integrated circuits with smallest footprints3. Transition metal dichalcogenides are ideally suited as the active material in such circuits as they interact strongly with light at the ultimate monolayer limit4. Here, we implement a Tamm-plasmon-polariton structure, and study the coupling to a monolayer of WSe2, hosting highly stable excitons5. Exciton-Polariton formation at room temperature is manifested in the characteristic energy-momentum dispersion relation studied in photoluminescence, featuring an anti-crossing between the exciton and photon modes with a Rabi-splitting of 23.5 meV. Creating polaritonic qua...

  8. Spin-flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers

    Science.gov (United States)

    Slobodeniuk, A. O.; Basko, D. M.

    2016-09-01

    We perform a theoretical study of radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers. This decay necessarily involves an electronic spin flip. The intrinsic decay mechanism due to interband spin-flip dipole moment perpendicular to the monolayer plane, gives a rate about 100-1000 times smaller than that of bright excitons. However, we find that this mechanism also introduces an energy splitting due to a local field effect, and the whole oscillator strength is contained in the higher-energy component, while the lowest-energy state remains dark and needs an extrinsic spin-flip mechanism for the decay. Rashba effect due to a perpendicular electric field or a dielectric substrate, gives a negligible radiative decay rate (about 107 times slower than that of bright excitons). Spin flip due to Zeeman effect in a sufficiently strong in-plane magnetic field can give a decay rate comparable to that due to the intrinsic interband spin-flip dipole.

  9. Exciton Transfer in Carbon Nanotube Aggregates for Energy Harvesting Applications

    Science.gov (United States)

    Davoody, Amirhossein; Karimi, Farhad; Knezevic, Irena

    Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct and exchange Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. The exciton transfer rate increases monotonically with increasing temperature. Support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0008712.

  10. Ultrafast dynamics of exciton fission in polycrystalline pentacene.

    Science.gov (United States)

    Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H

    2011-08-10

    We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. © 2011 American Chemical Society

  11. Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

    KAUST Repository

    Andernach, Rolf

    2015-07-22

    We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple timescales and investigated the mechanism of triplet exciton formation. During sensitization, single exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and find that 60% of the complex triplet excitons are transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and up-conversion layers.

  12. Exciton condensation and its influence on the specific heat

    Energy Technology Data Exchange (ETDEWEB)

    Wachter, P., E-mail: wachter@solid.phys.ethz.ch [Laboratorium fuer Festkoerperphysik, ETH Zuerich, 8093 Zuerich (Switzerland); Bucher, B. [HSR Hochschule fuer Technik, 8640 Rapperswil (Switzerland)

    2013-01-01

    In rare earth compounds with localized 4f states the observation of bound 4f-hole-5d-electron states, excitons, is questionable. On the other hand the same compounds exhibit p-d excitons, which are derived from itinerant bands. In rare earth compounds, which exhibit intermediate valence, 4f-5d hybridization produces a narrow, several 10 meV wide 4f band. Now 4f-5d excitons are possible and have been observed in TmSe{sub 0.45}Te{sub 0.55} and similar compositions. The special band structure of these materials permits an enormous amount of excitons ( Almost-Equal-To 10{sup 21} cm{sup -3}), which condense in a first order transition at low temperatures and high pressure. This static and immense concentration of excitons dominates the heat conductivity and the thermal diffusivity and even exhibits the phenomenon of superfluidity in a solid. The measured specific heat shows that phonons couple to these excitons forming exciton polarons.

  13. Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier

    Energy Technology Data Exchange (ETDEWEB)

    LYO,SUNGKWUN K.

    1999-12-06

    We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.

  14. Excitonic effects in the nonlinear optical response of a Si(111) surface

    Energy Technology Data Exchange (ETDEWEB)

    Stamova, Maria; Rebentrost, Frank [Max-Planck-Institut fuer Quantenoptik, 85748 Garching (Germany)

    2010-08-15

    We discuss methods to calculate the linear and nonlinear optical spectra for cyclic cluster models of an ideal Si(111) surface. The cluster approach offers the possibility to implement the excitonic effects due to the Coulomb interaction between electron and hole in a relatively straight-forward way. In order to appproximate a situation resembling a surface we use clusters with several hundreds of Si atoms. The electronic structure is obtained from a tight-binding parametrization of the hamiltonian. A time-dependent density operator formalism is used to calculate the response functions S({tau}) and S({tau}{sub 1},{tau}{sub 2}) for the optical polarization, which also directly describe the response to ultrashort pulses. Their Fourier transforms are the frequency-dependent optical susceptibilities {chi}{sup (1)}(-{omega};{omega}) and {chi}{sup (2)} (-{omega}{sub 1}-{omega}{sub 2};{omega}{sub 1},{omega}{sub 2}) for second-harmonic ({omega}{sub 1} ={omega}{sub 2}) or sum-frequency generation from surfaces. The excitonic Coulomb interaction is treated in the time-dependent Hartree-Fock approximation, leading to large sets of differential equations that are integrated explicitly. The results on the linear susceptibility are in accord with earlier findings on the excitonic origin of the relative intensities of the E{sub 1} and E{sub 2} peaks near 3.4 and 4.3 eV. We present new results on excitonic effects in the nonlinear spectra and investigate in particular the surface-related peaks near 2{Dirac_h}{omega}= 1.3-1.5 and 2.4 eV that govern the strong enhancement observed in SHG of clean silicon surfaces. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  15. Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

    CERN Document Server

    Yu, Yiling; Xu, Chao; Barrette, Andy; Gundogdu, Kenan; Cao, Linyou

    2015-01-01

    We quantitatively evaluate the exciton-exciton annihilation (EEA) and its effect on light emission properties in monolayer TMDC materials, including WS2, MoS2, and WSe2. The EEA rate is found to be 0.3 cm2/s and 0.1 cm2/s for suspended WS2 and MoS2 monolayers, respectively, and subject to the influence from substrates, being 0.1 cm2/s and 0.05 cm2/s for the supported WS2 and MoS2 on sapphire substrates. It can substantially affect the luminescence efficiency of suspended monolayers even at an exciton concentration as low as 109 cm-2, but plays a milder role for supported monolayers due to the effect of the substrate. However, regardless the presence of substrates or not, the lasing threshold of the monolayer is always predominantly determined by the EEA, which is estimated to be 12-18 MW/cm2 if using 532 nm as the pumping wavelength.

  16. Hot exciton cooling and multiple exciton generation in PbSe quantum dots.

    Science.gov (United States)

    Kumar, Manoj; Vezzoli, Stefano; Wang, Zilong; Chaudhary, Varun; Ramanujan, Raju V; Gurzadyan, Gagik G; Bruno, Annalisa; Soci, Cesare

    2016-11-16

    Multiple exciton generation (MEG) is a promising process to improve the power conversion efficiency of solar cells. PbSe quantum dots (QDs) have shown reasonably high MEG quantum yield (QY), although the photon energy threshold for this process is still under debate. One of the reasons for this inconsistency is the complicated competition of MEG and hot exciton cooling, especially at higher excited states. Here, we investigate MEG QY and the origin of the photon energy threshold for MEG in PbSe QDs of three different sizes by studying the transient absorption (TA) spectra, both at the band gap (near infrared, NIR) and far from the band gap energy (visible range). The comparison of visible TA spectra and dynamics for different pump wavelengths, below, around and above the MEG threshold, provides evidence of the role of the Σ transition in slowing down the exciton cooling process that can help MEG to take over the phonon relaxation process. The universality of this behavior is confirmed by studying QDs of three different sizes. Moreover, our results suggest that MEG QY can be determined by pump-probe experiments probed above the band gap.

  17. Excitonic fine structure and binding energies of excitonic complexes in single InAs quantum dashes

    Science.gov (United States)

    Mrowiński, P.; Zieliński, M.; Świderski, M.; Misiewicz, J.; Somers, A.; Reithmaier, J. P.; Höfling, S.; Sek, G.

    2016-09-01

    The fundamental electronic and optical properties of elongated InAs nanostructures embedded in quaternary InGaAlAs barrier are investigated by means of high-resolution optical spectroscopy and many-body atomistic tight-binding theory. These wire-like shaped, self-assembled nanostructures are known as quantum dashes and are typically formed during the molecular beam epitaxial growth on InP substrates. In this paper, we study properties of excitonic complexes confined in quantum dashes emitting in a broad spectral range from below 1.2 to 1.55 μm. We find peculiar trends for the biexciton and negative trion binding energies, with pronounced trion binding in smaller size quantum dashes. These experimental findings are then compared and qualitatively explained by atomistic theory. The theoretical analysis shows a fundamental role of correlation effects for the absolute values of excitonic binding energies. Eventually, we determine the bright exciton fine structure splitting (FSS), where both the experiment and theory predict a broad distribution of the splitting varying from below 50 to almost 180 μeV. We identify several key factors determining the FSS values in such nanostructures, including quantum dash size variation and composition fluctuations.

  18. Room-temperature exciton-polaritons with two-dimensional WS$_2$

    CERN Document Server

    Flatten, Lucas C; Coles, David M; Trichet, Aurelien A P; Powell, Alex W; Taylor, Robert A; Warner, Jamie H; Smith, Jason M

    2016-01-01

    Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensationand superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS$_2$ coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of $\\hbar \\Omega_{\\rm{Rabi}} = 70$ meV, exceeding the e...

  19. Exciton optical transitions in a hexagonal boron nitride single crystal

    Energy Technology Data Exchange (ETDEWEB)

    Museur, L. [Laboratoire de Physique des Lasers - LPL, CNRS UMR 7538, Institut Galilee, Universite Paris 13, 93430 Villetaneuse (France); Brasse, G.; Maine, S.; Ducastelle, F.; Loiseau, A. [ONERA - Laboratoire d' Etude des Microstructures - LEM, ONERA-CNRS, UMR 104, BP 72, 92322 Chatillon Cedex (France); Pierret, A. [ONERA - Laboratoire d' Etude des Microstructures - LEM, ONERA-CNRS, UMR 104, BP 72, 92322 Chatillon Cedex (France); CEA-CNRS, Institut Neel/CNRS, Universite J. Fourier, CEA/INAC/SP2M, 17 rue des Martyrs, 38 054 Grenoble Cedex 9 (France); Attal-Tretout, B. [ONERA - Departement Mesures Physiques - DMPh, 27 Chemin de la Huniere, 91761 Palaiseau Cedex (France); Barjon, J. [GEMaC, Universite de Versailles St Quentin, CNRS Bellevue, 1 Place Aristide Briand, 92195 Meudon Cedex (France); Watanabe, K.; Taniguchi, T. [National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044 (Japan); Kanaev, A. [Laboratoire des Sciences des Procedes et des Materiaux - LSPM, CNRS UPR 3407, Universite Paris 13, 93430 Villetaneuse (France)

    2011-06-15

    Near band gap photoluminescence (PL) of a hexagonal boron nitride single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by trapped-exciton optical transitions, while the photoluminescence excitation (PLE) spectra show features assigned to free excitons. Complementary photoconductivity and PLE measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  20. Excitonic AND Logic Gates on DNA Brick Nanobreadboards

    Science.gov (United States)

    2015-01-01

    A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems. PMID:25839049

  1. Excitonic AND Logic Gates on DNA Brick Nanobreadboards.

    Science.gov (United States)

    Cannon, Brittany L; Kellis, Donald L; Davis, Paul H; Lee, Jeunghoon; Kuang, Wan; Hughes, William L; Graugnard, Elton; Yurke, Bernard; Knowlton, William B

    2015-03-18

    A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems.

  2. Ordered Dissipative Structures in Exciton Systems in Semiconductor Quantum Wells

    Directory of Open Access Journals (Sweden)

    Andrey A. Chernyuk

    2006-02-01

    Full Text Available A phenomenological theory of exciton condensation in conditions of inhomogeneous excitation is proposed. The theory is applied to the study of the development of an exciton luminescence ring and the ring fragmentation at macroscopical distances from the central excitation spot in coupled quantum wells. The transition between the fragmented and the continuous ring is considered. With assumption of a defect in the structure, a possibility of a localized island of the condensed phase in a fixed position is shown. Exciton density distribution is also analyzed in the case of two spatially separated spots of the laser excitation.

  3. Generalized master equation for modular exciton density transfer

    CERN Document Server

    Jang, Seogjoo; Fleming, Graham; Whaley, K Birgitta

    2013-01-01

    A generalized master equation (GME) governing quantum evolution of modular exciton density (MED) is derived for large scale light harvesting systems composed of weakly interacting modules of multiple chromophores. The GME-MED offers a practical framework to incorporate real time coherent quantum dynamics calculations at small length scales into dynamics over large length scales, without assumptions of time scale separation or specific forms of intra-module quantum dynamics. A test of the GME-MED for four sites of the Fenna-Matthews-Olson complex demonstrates how coherent dynamics of excitonic populations over many coupled chromophores can be accurately described by transitions between subgroups (modules) of delocalized excitons.

  4. Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

    KAUST Repository

    Choi, Joshua J.

    2010-05-12

    Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices. © 2010 American Chemical Society.

  5. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    Energy Technology Data Exchange (ETDEWEB)

    Repp, J. [Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universität München, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (Germany); Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München (Germany); Schinner, G. J.; Schubert, E. [Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (Germany); Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München (Germany); Rai, A. K.; Wieck, A. D. [Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum (Germany); Reuter, D. [Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum (Germany); Department Physik, Universität Paderborn, 33098 Paderborn (Germany); Wurstbauer, U.; Holleitner, A. W. [Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universität München, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (Germany); and others

    2014-12-15

    We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.

  6. Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes.

    Science.gov (United States)

    Hofmann, Matthias S; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J; Högele, Alexander

    2016-05-11

    We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes.

  7. Evidence of Excitonic Optical Tamm States using Molecular Materials

    CERN Document Server

    Núñez-Sánchez, S; Murshidy, M M; Abdel-Hady, A G; Serry, M Y; Adawi, A M; Rarity, J G; Oulton, R; Barnes, W L

    2015-01-01

    We report the first experimental observation of an Excitonic Optical Tamm State supported at the interface between a periodic multilayer dielectric structure and an organic dye-doped polymer layer. The existence of such states is enabled by the metal-like optical properties of the excitonic layer based on aggregated dye molecules. Experimentally determined dispersion curves, together with simulated data, including field profiles, allow us to identify the nature of these new modes. Our results demonstrate the potential of organic excitonic materials as a powerful means to control light at the nanoscale, offering the prospect of a new alternative type of nanophotonics based on molecular materials.

  8. Predicting polarizabilities and lifetimes of excitons on conjugated polymer chains

    Science.gov (United States)

    van der Horst, J.-W.; Bobbert, P. A.; de Jong, P. H. L.; Michels, M. A. J.; Siebbeles, L. D. A.; Warman, J. M.; Gelinck, G. H.; Brocks, G.

    2001-02-01

    The properties of excitons on three different conjugated polymers in solution are investigated both experimentally and theoretically. The theoretical description of the excitons is obtained by solving the electron-hole Bethe-Salpeter equation (BSE) for the polymers, starting from a calculation within density-functional theory. The calculated radiative lifetimes and polarizabilities of the excitons are compared with experimental results from time-resolved fluorescence decay and flash-photolysis microwave conductivity measurements. The quantitative agreement demonstrates the predictive power of the theoretical approach.

  9. Bona-fide, strong-variable galactic Luminous Blue Variable stars are fast rotators: detection of a high rotational velocity in HR Carinae

    CERN Document Server

    Groh, Jose H; Hillier, D John; Barba, Rodolfo; Fernandez-Lajus, Eduardo; Gamen, Roberto C; Moises, Alessandro; Solivella, Gladys; Teodoro, Mairan

    2009-01-01

    We report optical observations of the Luminous Blue Variable (LBV) HR Carinae which show that the star has reached a visual minimum phase in 2009. More importantly, we detected absorptions due to Si IV 4088-4116 Angstroms. To match their observed line profiles from 2009 May, a high rotational velocity of vrot=150 +- 20 km/s is needed (assuming an inclination angle of 30 degrees), implying that HR Car rotates at ~0.88 +- 0.2 of its critical velocity for break-up (vcrit). Our results suggest that fast rotation is typical in all strong-variable, bona-fide galactic LBVs, which present S Dor-type variability. Strong-variable LBVs are located in a well-defined region of the HR diagram during visual minimum (the "LBV minimum instability strip"). We suggest this region corresponds to where vcrit is reached. To the left of this strip, a forbidden zone with vrot/vcrit>1 is present, explaining why no LBVs are detected in this zone. Since dormant/ex LBVs like P Cygni and HD 168625 have low vrot, we propose that LBVs can ...

  10. Exciton and multi-exciton dynamics in CdSe/Cd1-xZnxS quantum dots

    Science.gov (United States)

    Righetto, Marcello; Minotto, Alessandro; Bozio, Renato

    2016-04-01

    The outstanding optical properties of Semiconductor Quantum Dots (QDs) have attracted much interest for over two decades. The development of synthetic methods for the production of core-shell QDs has opened the way to attaining almost ideal emitting properties. Their implementation in opto-electronic devices, such as light emitting diodes (LEDs) and lasers, requires a full understanding of the fine details of their photophysics. The exciton dynamics of core and coreshell QDs was extensively studied by means of pump and probe (P and P) and transient photoluminescence (TRPL) spectroscopies. Nevertheless, the wealth of possible exciton and multi-exciton decay mechanisms, operating on comparable time-scales, results in complex signals. In this work, the exciton dynamics of a complete CdSe/Cd1-xZnxS series is investigated, with a focus on exciton trapping processes. Insights into the energy distribution of exciton traps are unveiled by wavelength resolve QY measurements. Multicolor P and P measurements give a deeper insight into the dynamics of exciton trapping and Auger recombinations. An inversion method is proposed as a powerful tool for separating different contribution in complex P and P transients. The outcomes of this work clarify the role of core/shell interfaces and surfaces in modulating the optical properties and suggest possible routes for their improvement.

  11. Hot exciton dissociation in polymer solar cells.

    Science.gov (United States)

    Grancini, G; Maiuri, M; Fazzi, D; Petrozza, A; Egelhaaf, H-J; Brida, D; Cerullo, G; Lanzani, G

    2013-01-01

    The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC(60)BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

  12. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    CERN Document Server

    Kenny, Elise P

    2015-01-01

    Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. We also caution ...

  13. Multiple excitons and the electron phonon bottleneck in semiconductor quantum dots: An ab initio perspective

    Science.gov (United States)

    Prezhdo, Oleg V.

    2008-07-01

    The article presents the current perspective on the nature of photoexcited states in semiconductor quantum dots (QDs). The focus is on multiple excitons and photo-induced electron-phonon dynamics in PbSe and CdSe QDs, and the advocated view is rooted in the results of ab initio studies in both energy and time domains. As a new type of material, semiconductor QDs represent the borderline between chemistry and physics, exhibiting both molecular and bulk-like properties. Similar to atoms and molecules, the electronic spectra of QD show discrete bands. Just as bulk semiconductors, QDs comprise multiple copies of the elementary unit cell, and are characterized by valence and conduction bands. The electron-phonon coupling in QDs is weaker than in molecules, but stronger than in bulk semiconductors. Unlike either material, the QD properties can be tuned continuously by changing QD size and shape. The molecular and bulk points of view often lead to contradicting conclusions. For example, the molecular view suggests that the excitations in QDs should exhibit strong electron-correlation (excitonic) effects, and that the electron-phonon relaxation should be slow due to the discrete nature of the optical bands and the mismatch of the electronic energy gaps with vibrational frequencies. In contrast, a finite-size limit of bulk properties indicates that the kinetic energy of quantum confinement should be significantly greater than excitonic effects and that the electron-phonon relaxation inside the quasi-continuous bands should be efficient. Such qualitative differences have generated heated discussions in the literature. The great potential of QDs for a variety of applications, including photovoltaics, spintronics, lasers, light-emitting diodes, and field-effect transistors makes it crutual to settle the debates. By synthesizing different viewpoints and presenting a unified atomistic picture of the excited state processes, our ab initio analysis clarifies the controversies

  14. Metal-Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton-Plasmon Coupling.

    Science.gov (United States)

    Strelow, Christian; Theuerholz, T Sverre; Schmidtke, Christian; Richter, Marten; Merkl, Jan-Philip; Kloust, Hauke; Ye, Ziliang; Weller, Horst; Heinz, Tony F; Knorr, Andreas; Lange, Holger

    2016-08-10

    Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of of the two constituents, due to the exciton-plasmon interaction. Here, we report on a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization. The system allows us to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton-plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. We experimentally observe a strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling. We develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, we also consider possible quantum optical effects. We find a good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects.

  15. Exciton delocalization and transport in photosystem I of cyanobacteria Synechococcus elongates: simulation study of coherent two-dimensional optical signals.

    Science.gov (United States)

    Abramavicius, Darius; Mukamel, Shaul

    2009-04-30

    Electronic excitations and the optical properties of the photosynthetic complex PSI are analyzed using an effective exciton model developed by Vaitekonis et al. [Photosynth. Res. 2005, 86, 185]. States of the reaction center, the linker states, the highly delocalized antenna states and the red states are identified and assigned in absorption and circular dichroism spectra by taking into account the spectral distribution of density of exciton states, exciton delocalization length, and participation ratio in the reaction center. Signatures of exciton cooperative dynamics in nonchiral and chirality-induced two-dimensional (2D) photon-echo signals are identified. Nonchiral signals show resonances associated with the red, the reaction center, and the bulk antenna states as well as transport between them. Spectrally overlapping contributions of the linker and the delocalized antenna states are clearly resolved in the chirality-induced signals. Strong correlations are observed between the delocalized antenna states, the linker states, and the RC states. The active space of the complex covering the RC, the linker, and the delocalized antenna states is common to PSI complexes in bacteria and plants.

  16. High-resolution study of the yellow excitons in Cu2O subject to an electric field

    Science.gov (United States)

    Heckötter, J.; Freitag, M.; Fröhlich, D.; Aßmann, M.; Bayer, M.; Semina, M. A.; Glazov, M. M.

    2017-01-01

    We have used high-resolution transmission spectroscopy to study the exciton level spectrum in Cu2O subject to a longitudinal external electric field, i.e., in the geometry where the transmitted light is propagating along the field direction. Different experimental configurations given by the field orientation relative to the crystal and the light polarization have been explored. We focus on the range of small principal quantum numbers n ≤7 . The number of exciton states belonging to a particular principal quantum number increases with n , leading to an enhanced complexity of the spectra. Still, in particular, for n =3 ,...,5 , a spectral separation of the different lines is feasible and identification as well as assignment of the dominant state character are possible. We find a strong dependence of the spectra on the chosen light propagation direction and polarization configuration, reflecting the inadequacy of the hydrogen model for describing the excitons. With increasing the field excitonic states with different parity become mixed, leading to optical activation of states that are dark in zero field. As compared with atoms, due to the reduced Rydberg energy states with different n can be brought into resonance in the accessible electric field strength range. When this occurs, we observe mostly crossing of levels within the experimental accuracy showing that the electron and hole motion remains regular. The observed features are well described by detailed calculations accounting for the spin-orbit coupling, the cubic anisotropy effects, and the symmetry-imposed optical selection rules.

  17. Exciton mechanisms and modeling of the ionoluminescence in silica

    Science.gov (United States)

    Bachiller-Perea, D.; Jiménez-Rey, D.; Muñoz-Martín, A.; Agulló-López, F.

    2016-03-01

    A theoretical model is presented in order to discuss detailed kinetic data describing the evolution of the two main ionoluminescence bands at 650 nm (1.9 eV) and 460 nm (2.7 eV) in silica as a function of the irradiation fluence at room temperature. The model is based on the generation of self-trapped excitons (STEs), their hopping migration through the silica network and their recombination at non-bridging oxygen hole and Type II oxygen-deficient centers to produce the red and blue emission bands, respectively. For heavy ions, which have a high electronic stopping power, the two emission yields experience a fast initial growth with fluence up to a maximum value and then decrease at a fairly comparable rate. The fluence for such a maximum strongly increases on decreasing ion mass and stopping power and, finally, for H and He it is not observed for any of the investigated energy and fluence range. This kinetic behavior is explained in terms of the strong structural distortions (compaction) induced by the heavy-ion irradiations. In particular, it is proposed that these strong structural distortions cause a significant decrease in the STE migration length and, consequently, in the recombination rates at the two active recombination centers. The model offers a good quantitative accordance with detailed infrared spectroscopy reporting on the changes in the frequency ω 4 of a first-order vibrational mode in the SiO2 network as a function of irradiation fluence.

  18. Spitzer UltRa Faint SUrvey Program (SURFS UP). II. IRAC-Detected Lyman-Break Galaxies at 6 < z < 10 Behind Strong-Lensing Clusters

    CERN Document Server

    Huang, Kuang-Han; Lemaux, Brian C; Ryan,, R E; Hoag, Austin; Castellano, Marco; AmorÍn, Ricardo; Fontana, Adriano; Brammer, Gabriel B; Cain, Benjamin; Lubin, L M; Merlin, Emiliano; Schmidt, Kasper B; Schrabback, Tim; Treu, Tommaso; Gonzalez, Anthony H; Von Der Linden, Anja; Knight, Robert I

    2015-01-01

    We study the stellar population properties of the IRAC-detected $6 \\lesssim z \\lesssim 10$ galaxy candidates from the Spitzer UltRa Faint SUrvey Program (SURFS UP). Using the Lyman Break selection technique, we find a total of 16 new galaxy candidates at $6 \\lesssim z \\lesssim 10$ with $S/N \\geq 3$ in at least one of the IRAC $3.6\\mu$m and $4.5\\mu$m bands. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of $\\sim 1.2$--$5.5$. We find that the IRAC-detected $6 \\lesssim z \\lesssim 10$ sample is likely not a homogeneous galaxy population: some are relatively massive (stellar mass as high as $4 \\times 10^9\\,M_{\\odot}$) and evolved (age $\\lesssim 500$ Myr) galaxies, while others are less massive ($M_{\\text{stellar}}\\sim 10^8\\,M_{\\odot}$) and very young ($\\sim 10$ Myr) galaxies with strong nebular emission lines that boost their rest-frame optical fluxes. We identify two Ly$\\alpha$ emitters in our sample from the Keck DEIMOS...

  19. Directional Control of Plasmon-Exciton interaction with Plexcitonic Crystals

    Science.gov (United States)

    Balci, Sinan; Karademir, Ertugrul; Kocabas, Coskun; Aydinli, Atilla

    2015-03-01

    Plexcitons are strongly coupled plasmon excitons modes. In this work, we developed a platform, consisting of one and two dimensional corrugated surface patterns coated with a thin metal film and a dye solution. This system shows a controlled coupling action based on the excitation direction of SPP modes. Our scheme is based on the control of wavelengths of the forbidden SPP modes. Three kinds of patterns have been tested; a one dimensional uniform, a triangular, and a square lattice type crystals. For all three cases, lowest wavelength of the band gap is observed in Γ to M direction. For triangular and square lattice cases, band gap center oscillates between two finite values for every 60° and 90°s, respectively. We utilized this behavior to control SPP and J-aggregate coupling. We observe directional dependence of Rabi splitting energy varying between 0 meV and 60 meV . Square lattice gives the ability to tune a larger band gap, whereas triangular lattice gives higher number of symmetry points. Simulations show that, an 80 nm deep triangular lattice with 280 nm periodicity can result in omnidirectional decoupling of plexcitons. TUBITAK, Grants 110T790, 110T589, and 112T091.

  20. Vibrational exciton-mediated quantum state transfert: a simple model

    CERN Document Server

    Pouthier, Vincent J C

    2012-01-01

    A communication protocol is proposed in which quantum state transfer is mediated by a vibrational exciton. We consider two distant molecular groups grafted on the sides of a lattice. These groups behave as two quantum computers where the information in encoded and received. The lattice plays the role of a communication channel along which the exciton propagates and interacts with a phonon bath. Special attention is paid for describing the system involving an exciton dressed by a single phonon mode. The Hamiltonian is thus solved exactly so that the relevance of the perturbation theory is checked. Within the nonadiabatic weak-coupling limit, it is shown that the system supports three quasi-degenerate states that define the relevant paths followed by the exciton to tunnel between the computers. When the model parameters are judiciously chosen, constructive interferences take place between these paths. Phonon-induced decoherence is minimized and a high-fidelity quantum state transfer occurs over a broad temperat...

  1. Fate of the excitonic insulator in the presence of phonons

    Science.gov (United States)

    Zenker, B.; Fehske, H.; Beck, H.

    2014-11-01

    The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on Ta2NiSe5 ,1 T -TiSe2, and TmSe0.45Te0.55 , being candidates for realizing the excitonic-insulator state, suggest, however, that the underlying lattice plays a significant role. Employing the Kadanoff-Baym approach we address this issue theoretically. We show that owing to the electron-phonon coupling a static lattice distortion may arise at the excitonic instability. Most importantly such a distortion will destroy the acoustic phase mode being present if the electron-hole pairing and condensation is exclusively driven by the Coulomb interaction. The absence of off-diagonal long-range order, when lattice degrees of freedom are involved, challenges that excitons in these materials form a superfluid condensate of Bose particles or Cooper pairs composed of electrons and holes.

  2. Neutral and positively charged excitons in narrow quantum ring

    Energy Technology Data Exchange (ETDEWEB)

    Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D. [Escuela de Física, Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

    2014-05-15

    We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.

  3. Synthesis of zinc oxide microrods and nano-fibers with dominant exciton emission at room temperature

    Energy Technology Data Exchange (ETDEWEB)

    Ramos-Brito, F., E-mail: fro_brito@yahoo.com.m [Laboratorio de Materiales Optoelectronicos del Centro de Ciencias de Sinaloa, Ave. de las Americas 2771 Col. Villa Universidad 80010, Culiacan, Sinaloa (Mexico); Alejo-Armenta, C. [Laboratorio de Materiales Optoelectronicos del Centro de Ciencias de Sinaloa, Ave. de las Americas 2771 Col. Villa Universidad 80010, Culiacan, Sinaloa (Mexico); Garcia-Hipolito, M. [Departamento de Materiales Metalicos y Ceramicos, Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, AP 70-360, Coyoacan 04510, DF (Mexico); Camarillo, E.; Hernandez A, J. [Instituto de Fisica, Universidad Nacional Autonoma de Mexico, AP 20-364, Alvaro Obregon 01000, DF (Mexico); Falcony, C. [Departamento de Fisica, CINVESTAV-IPN, AP 14-740, 07000, DF (Mexico); Murrieta S, H. [Instituto de Fisica, Universidad Nacional Autonoma de Mexico, AP 20-364, Alvaro Obregon 01000, DF (Mexico)

    2011-05-15

    Employing a simple chemical synthesis method, hexagonal-shaped zinc oxide microrods and zinc oxide nano-fibers were deposited on pyrex-glass and aluminum substrates, respectively. Both kinds of deposits showed zincite crystalline phase with lattice parameters: a=3.2498 A and c=5.2066 A. Microrods showed very uniform wide and large sizes of around 1 and 10 {mu}m, respectively. Both deposits were homogeneous over all substrate surfaces. Microrods and nano-fibers resulted with good optical quality and with preferential crystalline growth in [1 0 1 0]and [0 0 0 1]directions. The principal optical characteristics for both microrods and nano-fibers were: a) room-temperature photo and cathodo-luminescent spectra with strong exciton emission centered around 390 nm and with FWHMs around 125 and 160 meV, respectively, b) poor photo and cathode-luminescent emissions in the visible region of the electromagnetic spectrum, c) energy band gap of 3.32 eV, d) good emission efficiency supported by the not-required high energy densities to obtain strong exciton emission and e) good ZnO stoichiometry endorsed by photoluminescent results. These characteristics make of these microrods and nano-fibers good for potential photonic applications. - Research highlights: {yields} Microrods and nano-fibers resulted with good optical quality and with preferential crystalline growth in [1 0 1 0]and [0 0 0 1]directions. {yields} Microrods and nano-fibers resulted with good emission efficiency supported by the not-required high energy densities to obtain strong exciton emission. {yields} The wet chemical method is appropriated for deposition of microrods and nano-fibers with the desired optical properties for its possible application in photonics.

  4. Exciton-Dependent Pre-formation Probability of Composite Particles

    Institute of Scientific and Technical Information of China (English)

    ZHANG Jing-Shang; WANG Ji-Min; DUAN Jun-Feng

    2007-01-01

    In Iwamoto-Harada model the whole phase space is full of fermions. When the momentum distributions of the exciton states are taken into account, the pre-formation probability of light composite particles could be improved,and the exciton state-dependent pre-formation probability has been proposed. The calculated results indicate that the consideration of the momentum distribution enhances the pre-formation probability of [1,m] configuration, and suppresses that of [l > 1, m] configurations seriously.

  5. One dimensional models of excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia Decebal; Duclos, P.; Pedersen, Thomas Garm

    Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable.......Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable....

  6. Tuning of Exciton States in a Magnetic Quantum Ring

    OpenAIRE

    Ghazaryan, Areg; Manaselyan, Aram; Chakraborty, Tapash

    2014-01-01

    We have studied the exciton states in a CdTe quantum ring in an external magnetic field containing a single magnetic impurity. We have used the multiband approximation which includes the heavy hole - light hole coupling effects. The electron-hole spin interactions and the s, p-d interactions between the electron, hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin...

  7. Quantum confinement of excitons in wurtzite InP nanowires

    Science.gov (United States)

    Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.

    2015-05-01

    Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.

  8. Control of Exciton Dynamics in Nanodots for Quantum Operations

    Science.gov (United States)

    Chen, Pochung; Piermarocchi, C.; Sham, L. J.

    2001-08-01

    We present a theory to further a new perspective of proactive control of exciton dynamics in the quantum limit. Circularly polarized optical pulses in a semiconductor nanodot are used to control the dynamics of two interacting excitons of opposite polarizations. Shaping of femtosecond laser pulses keeps the quantum operation within the decoherence time. Computation of the fidelity of the operations and application to the complete solution of a minimal quantum computing algorithm demonstrate in theory the feasibility of quantum control.

  9. Coherence and Optical Emission from Bilayer Exciton Condensates

    Directory of Open Access Journals (Sweden)

    D. W. Snoke

    2011-01-01

    Full Text Available Experiments aimed at demonstrating Bose-Einstein condensation of excitons in two types of experiments with bilayer structures (coupled quantum wells are reviewed, with an emphasis on the basic effects. Bose-Einstein condensation implies the existence of a macroscopic coherence, also known as off-diagonal long-range order, and proposed tests and past claims for coherence in these excitonic systems are discussed.

  10. Excitons, biexcitons, and phonons in ultrathin CdSe/ZnSe quantum structures

    DEFF Research Database (Denmark)

    Gindele, F.; Woggon, Ulrike; Langbein, Wolfgang Werner

    1999-01-01

    The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments....... The nanostructures consist of a coherently strained Zn1-xCdxSe/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase...

  11. Strong Fluorescent Smart Organogel as a Dual Sensing Material for Volatile Acid and Organic Amine Vapors.

    Science.gov (United States)

    Xue, Pengchong; Yao, Boqi; Wang, Panpan; Gong, Peng; Zhang, Zhenqi; Lu, Ran

    2015-11-23

    An L-phenylalanine derivative (C12PhBPCP) consisting of a strong emission fluorophore with benzoxazole and cyano groups is designed and synthesized to realize dual responses to volatile acid and organic amine vapors. The photophysical properties and self-assembly of the said derivative in the gel phase are also studied. C12PhBPCP can gelate organic solvents and self-assemble into 1 D nanofibers in the gels. UV/Vis absorption spectral results show H-aggregate formation during gelation, which indicates strong exciton coupling between fluorophores. Both wet gel and xerogel emit strong green fluorescence because the cyano group suppresses fluorescence quenching in the self-assemblies. Moreover, the xerogel film with strong green fluorescence can be used as a dual chemosensor for quantitative detection of volatile acid and organic amine vapors with fast response times and low detection limits owing to its large surface area and amplified fluorescence quenching. The detection limits are 796 ppt and 25 ppb for gaseous aniline and trifluoroacetic acid (TFA), respectively.

  12. Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

    Energy Technology Data Exchange (ETDEWEB)

    Kovalev, V. M.; Chaplik, A. V., E-mail: chaplik@isp.nsc.ru [Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics, Siberian Branch (Russian Federation)

    2016-03-15

    A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas drag by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.

  13. Phase coherence and spectral functions in the two-dimensional excitonic systems

    Energy Technology Data Exchange (ETDEWEB)

    Apinyan, V., E-mail: V.Apinyan@int.pan.wroc.pl; Kopeć, T.K.

    2015-09-15

    The nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose–Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered.

  14. Strong-LAMP: A LAMP Assay for Strongyloides spp. Detection in Stool and Urine Samples. Towards the Diagnosis of Human Strongyloidiasis Starting from a Rodent Model

    Science.gov (United States)

    Gandasegui, Javier; Bajo Santos, Cristina; López-Abán, Julio; Saugar, José María; Rodríguez, Esperanza; Vicente, Belén; Muro, Antonio

    2016-01-01

    Background Strongyloides stercoralis, the chief causative agent of human strongyloidiasis, is a nematode globally distributed but mainly endemic in tropical and subtropical regions. Chronic infection is often clinically asymptomatic but it can result in severe hyperinfection syndrome or disseminated strongyloidiasis in immunocompromised patients. There is a great diversity of techniques used in diagnosing the disease, but definitive diagnosis is accomplished by parasitological examination of stool samples for morphological identification of parasite. Until now, no molecular method has been tested in urine samples as an alternative to stool samples for diagnosing strongyloidiasis. This study aimed to evaluate the use of a new molecular LAMP assay in a well-established Wistar rat experimental infection model using both stool and, for the first time, urine samples. The LAMP assay was also clinically evaluated in patients´ stool samples. Methodology/Principal Findings Stool and urine samples were obtained daily during a 28-day period from rats infected subcutaneously with different infective third-stage larvae doses of S. venezuelensis. The dynamics of parasite infection was determined by daily counting the number of eggs per gram of feces from day 1 to 28 post-infection. A set of primers for LAMP assay based on a DNA partial sequence in the 18S rRNA gene from S. venezuelensis was designed. The set up LAMP assay (namely, Strong-LAMP) allowed the sensitive detection of S. venezuelensis DNA in both stool and urine samples obtained from each infection group of rats and was also effective in S. stercoralis DNA amplification in patients´ stool samples with previously confirmed strongyloidiasis by parasitological and real-time PCR tests. Conclusions/Significance Our Strong-LAMP assay is an useful molecular tool in research of a strongyloidiasis experimental infection model in both stool and urine samples. After further validation, the Strong-LAMP could also be potentially

  15. Exciton coherence and energy transport in the light-harvesting dimers of allophycocyanin.

    Science.gov (United States)

    Womick, Jordan M; Moran, Andrew M

    2009-12-03

    Femtosecond transient grating and photon echo spectroscopies with a sub-20 fs time resolution are applied to allophycocyanin (APC), a protein located at the base of the phycobilisome antenna of cyanobacteria. Coupling between pairs of phycocyanobilin pigments with nondegenerate energy levels gives rise to the four-level exciton electronic structure of APC. Spectroscopic signals obtained in multiple experiments (e.g., linear absorption, fluorescence, transient grating, 2D Fourier transform photon echo) are used to constrain the parameters of a Frenkel exciton Hamiltonian. Comparison between experiment and theory yields a robust microscopic understanding of the electronic and nuclear relaxation dynamics. In agreement with previous work, transient absorption anisotropy establishes that internal conversion between the exciton states of the dimer occurs with time constants of 35, 220, and 280 fs. The sub-100 fs dynamics are decomposed into three distinct relaxation processes: electronic population transfer, intramolecular vibrational energy redistribution, and the dephasing of electronic and nuclear coherences. Model calculations show that the sub-100 fs red-shift in the transient absorption signal spectrum reflects interference between stimulated emission (ESE) and excited state absorption (ESA) signal components. It is also established that the pigment fluctuations in the dimer are not well-correlated, although further experiments will be required to precisely quantify the amount of correlation. The findings of this paper suggest that the light harvesting function of APC is enhanced by nondegeneracy of the pigments comprising the dimer and strong vibronic coupling of intramolecular modes on the phycocyanobilins. We find that the exciton states are 96% localized to the individual molecular sites within a particular dimer. Localization of the transition densities, in turn, is suggested to promote significant vibronic coupling which serves to both broaden the absorption

  16. B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD -57° 3509

    Science.gov (United States)

    Przybilla, N.; Fossati, L.; Hubrig, S.; Nieva, M.-F.; Järvinen, S. P.; Castro, N.; Schöller, M.; Ilyin, I.; Butler, K.; Schneider, F. R. N.; Oskinova, L. M.; Morel, T.; Langer, N.; de Koter, A.; BOB Collaboration

    2016-03-01

    Aims: We report the detection of a magnetic field in the helium-strong star CPD -57° 3509 (B2 IV), a member of the Galactic open cluster NGC 3293, and characterise the star's atmospheric and fundamental parameters. Methods: Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. Results: We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD -57° 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in Hα is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23 750 ± 250 K and 4.05 ± 0.10, respectively, and a surface helium fraction of 0.28 ± 0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD -57° 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its

  17. B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD-57{\\deg} 3509

    CERN Document Server

    Przybilla, N; Hubrig, S; Nieva, M -F; Järvinen, S P; Castro, N; Schöller, M; Ilyin, I; Butler, K; Schneider, F R N; Oskinova, L M; Morel, T; Langer, N; de Koter, A

    2016-01-01

    We report the detection of a magnetic field in the helium-strong star CPD-57 3509 (B2 IV), a member of the Galactic open cluster NGC3293, and characterise the star's atmospheric and fundamental parameters. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD-57 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centr...

  18. Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

    Science.gov (United States)

    Bhunia, Amit; Bansal, Kanika; Henini, Mohamed; Alshammari, Marzook S.; Datta, Shouvik

    2016-10-01

    Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of "negative activation energy" after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

  19. Theoretical study of excitonic complexes in semiconductors quantum wells; Estudo teorico de complexos excitonicos em pocos quanticos de semicondutores

    Energy Technology Data Exchange (ETDEWEB)

    Dacal, Luis Carlos Ogando

    2001-08-01

    A physical system where indistinguishable particles interact with each other creates the possibility of studying correlation and exchange effect. The simplest system is that one with only two indistinguishable particles. In condensed matter physics, these complexes are represented by charged excitons, donors and acceptors. In quantum wells, the valence band is not parabolic, therefore, the negatively charged excitons and donors are theoretically described in a simpler way. Despite the fact that the stability of charged excitons (trions) is known since the late 50s, the first experimental observation occurred only at the early 90s in quantum well samples, where their binding energies are one order of magnitude larger due to the one dimensional carriers confinement. After this, these complexes became the subject of an intense research because the intrinsic screening of electrical interactions in semiconductor materials allows that magnetic fields that are usual in laboratories have strong effects on the trion binding energy. Another rich possibility is the study of trions as an intermediate state between the neutral exciton and the Fermi edge singularity when the excess of doping carriers is increased. In this thesis, we present a theoretical study of charged excitons and negatively charged donors in GaAs/Al{sub 0.3}Ga{sub 0.7}As quantum wells considering the effects of external electric and magnetic fields. We use a simple, accurate and physically clear method to describe these systems in contrast with the few and complex treatments s available in the literature. Our results show that the QW interface defects have an important role in the trion dynamics. This is in agreement with some experimental works, but it disagrees with other ones. (author)

  20. Coherent Interaction and Relaxation Processes of Zero-Dimensional Excitons in CdSe/Zn1-xCdxSe Quantum Structures

    DEFF Research Database (Denmark)

    Gindele, F.; Woggon, Ulrike; Langbein, Wolfgang

    1999-01-01

    Biexciton formation and phonon-assisted relaxation in monolayer CdSe/ZnSe structures is studied by photoluminescence (PL) and time-integrated four-wave mixing (TIFWM). The biexciton binding energy is enhanced by a factor of 5 compared to the bulk value. The exciton dynamics is strongly modified due...

  1. SPITZER ULTRA FAINT SURVEY PROGRAM (SURFS UP). II. IRAC-DETECTED LYMAN-BREAK GALAXIES AT 6 ≲ z ≲ 10 BEHIND STRONG-LENSING CLUSTERS

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Kuang-Han; Bradač, Maruša; Hoag, Austin; Cain, Benjamin; Lubin, L. M.; Knight, Robert I. [University of California Davis, 1 Shields Avenue, Davis, CA 95616 (United States); Lemaux, Brian C. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Ryan, R. E. Jr.; Brammer, Gabriel B. [Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, F-13388 Marseille (France); Castellano, Marco; Amorin, Ricardo; Fontana, Adriano; Merlin, Emiliano [INAF—Osservatorio Astronomico di Roma Via Frascati 33, I-00040 Monte Porzio Catone (Italy); Schmidt, Kasper B. [Department of Physics, University of California, Santa Barbara, CA 93106 (United States); Schrabback, Tim [Argelander-Institut für Astronomie, Auf Dem Hügel 71, D-53121 Bonn (Germany); Treu, Tommaso [Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095 (United States); Gonzalez, Anthony H. [Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611 (United States); Linden, Anja von der, E-mail: khhuang@ucdavis.edu, E-mail: astrokuang@gmail.com [Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305 (United States)

    2016-01-20

    We study the stellar population properties of the IRAC-detected 6 ≲ z ≲ 10 galaxy candidates from the Spitzer UltRa Faint SUrvey Program. Using the Lyman Break selection technique, we find a total of 17 galaxy candidates at 6 ≲ z ≲ 10 from Hubble Space Telescope images (including the full-depth images from the Hubble Frontier Fields program for MACS 1149 and MACS 0717) that have detections at signal-to-noise ratios  ≥ 3 in at least one of the IRAC 3.6 and 4.5 μm channels. According to the best mass models available for the surveyed galaxy clusters, these IRAC-detected galaxy candidates are magnified by factors of ∼1.2–5.5. Due to the magnification of the foreground galaxy clusters, the rest-frame UV absolute magnitudes M{sub 1600} are between −21.2 and −18.9 mag, while their intrinsic stellar masses are between 2 × 10{sup 8}M{sub ⊙} and 2.9 × 10{sup 9}M{sub ⊙}. We identify two Lyα emitters in our sample from the Keck DEIMOS spectra, one at z{sub Lyα} = 6.76 (in RXJ 1347) and one at z{sub Lyα} = 6.32 (in MACS 0454). We find that 4 out of 17 z ≳ 6 galaxy candidates are favored by z ≲ 1 solutions when IRAC fluxes are included in photometric redshift fitting. We also show that IRAC [3.6]–[4.5] color, when combined with photometric redshift, can be used to identify galaxies which likely have strong nebular emission lines or obscured active galactic nucleus contributions within certain redshift windows.

  2. Stark shift and electric-field-induced dissociation of excitons in monolayer MoS2 and hBN/MoS2 heterostructures

    DEFF Research Database (Denmark)

    Haastrup, Sten; Latini, Simone; Bolotin, Kirill;

    2016-01-01

    Efficient conversion of photons into electrical current in two-dimensional semiconductors requires, as a first step, the dissociation of the strongly bound excitons into free electrons and holes. Here we calculate the dissociation rates and energy shift of excitons in monolayer MoS2 as a function...... and screened electron-hole interaction defining the hydrogenic Hamiltonian are computed from first principles. For field strengths above 0.1 V/nm the dissociation lifetime is shorter than 1 ps, which is below the lifetime associated with competing decay mechanisms. Interestingly, encapsulation of the MoS2...

  3. Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.

    Science.gov (United States)

    Werschler, Florian; Hinz, Christopher; Froning, Florian; Gumbsheimer, Pascal; Haase, Johannes; Negele, Carla; de Roo, Tjaard; Mecking, Stefan; Leitenstorfer, Alfred; Seletskiy, Denis V

    2016-09-14

    The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

  4. The influence of the interband Coulomb interaction and the f-electron hopping on excitonic correlations in the extended Falicov-Kimball model

    Science.gov (United States)

    Farkašovský, Pavol

    2015-05-01

    The density matrix renormalization group (DMRG) method is used to examine the effects of the interband Coulomb interaction U between f and d electrons as well as the f-electron hopping on the stability of the excitonic phase in the one-dimensional Falicov-Kimball model (FKM) with local hybridization V. It is found that the interband Coulomb interaction significantly enhances the excitonic Pdf= average and that this effect is especially strong in the limit of small hybridization, where the interacting Pdf(U) excitonic value is enhanced several hundred times in comparison to its non-interacting Pdf(U=0) value. The further increase in P df is observed due to the non-zero f-electron hopping, but these changes are considerable only if the d and f bands have opposite parity. In addition, the examination of the interplay between the excitonic effects and the charge density wave (CDW) instability showed that the CDW and excitonic phase coexist up to relatively large values of local hybridization (V ∼ 0.13) . The ground-state phase diagram of the model in the V\\text-U plane is discussed.

  5. Exciton-phonon interaction in Al0.4Ga0.6N/Al0.53Ga0.47N multiple quantum wells

    Science.gov (United States)

    Liu, Ya-Li; Jin, Peng; Liu, Gui-Peng; Wang, Wei-Ying; Qi, Zhi-Qiang; Chen, Chang-Qing; Wang, Zhan-Guo

    2016-08-01

    The exciton-phonon interaction in Al0.4Ga0.6N/Al0.53Ga0.47N multiple quantum wells (MQWs) is studied by deep-ultraviolet time-integrated and time-resolved photoluminescence (PL). Up to four longitudinal-optical (LO) phonon replicas of exciton recombination are observed, indicating the strong coupling of excitons with LO phonons in the MQWs. Moreover, the exciton-phonon coupling strength in the MQWs is quantified by the Huang-Rhys factor, and it keeps almost constant in a temperature range from 10 K to 120 K. This result can be explained in terms of effects of fluctuations in the well thickness in the MQWs and the temperature on the exciton-phonon interaction. Project supported by the National Basic Research Program of China (Grant No. 2012CB619306), the Beijing Science and Technology Project, China (Grant No. Z151100003315024), and the National Natural Science Foundation of China (Grant No. 61404132).

  6. Impurity trapped excitons under high hydrostatic pressure

    Science.gov (United States)

    Grinberg, Marek

    2013-09-01

    Paper summarizes the results on pressure effect on energies of the 4fn → 4fn and 4fn-15d1 → 4fn transitions as well as influence of pressure on anomalous luminescence in Lnα+ doped oxides and fluorides. A model of impurity trapped exciton (ITE) was developed. Two types of ITE were considered. The first where a hole is localized at the Lnα+ ion (creation of Ln(α+1)+) and an electron is attracted by Coulomb potential at Rydberg-like states and the second where an electron captured at the Lnα+ ion (creation of Ln(α-1)+) and a hole is attracted by Coulomb potential at Rydberg-like states. Paper presents detailed analysis of nonlinear changes of energy of anomalous luminescence of BaxSr1-xF2:Eu2+ (x > 0.3) and LiBaF3:Eu2+, and relate them to ITE-4f65d1 states mixing.

  7. Photosynthetic light harvesting: excitons and coherence.

    Science.gov (United States)

    Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C; Scholes, Gregory D

    2014-03-06

    Photosynthesis begins with light harvesting, where specialized pigment-protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

  8. The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

    Institute of Scientific and Technical Information of China (English)

    Hu Xiao-Long; Zhang Jiang-Yong; Shang Jing-Zhi; Liu Wen-Jie; Zhang Bao-Ping

    2010-01-01

    This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements. With increasing cap layer thickness, the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon, described by Huang-Rhys factor, increases remarkably due to an enhancement of the internal electric field. With increasing excitation intensity, the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases. These results reveal that there is a large built-in electric field in the well layer and the exciton-LO-phonon coupling is strongly affected by the thickness of the cap layer.

  9. Vertical electric field induced suppression of fine structure splitting of excited state excitons in a single GaAs/AlGaAs island quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Ghali, Mohsen [Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577 (Japan); Laboratory of Nanophotonics, Physics Department, Faculty of Science, Kafrelsheikh University, 33516 Kafrelsheikh (Egypt); Ohno, Yuzo; Ohno, Hideo [Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577 (Japan)

    2015-09-21

    We report experimentally on fine structure splitting (FSS) of various excitonic transitions in single GaAs island quantum dots, formed by a monolayer thickness fluctuation in the narrow GaAs/AlGaAs quantum well, and embedded in an n-i-Schottky diode device. By applying a forward vertical electric field (F) between the top metallic contact and the sample substrate, we observed an in-plane polarization rotation of both the ground and the excited state excitons with increasing the electric field. The polarization rotations were accompanied with a strong decrease in the FSS of the ground as well as the excited state excitons with the field, until the FSS vanished as F approached 30 kV/cm.

  10. Transition from excitonic to plasma emission from localized electron-hole pair states in GaAs-AlGaAs quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Manzke, G.; Richter, F.; Semkat, D.; Burau, G.K.G.; Kieseling, F.; Stolz, H. [Institute of Physics, University of Rostock, 18051 Rostock (Germany)

    2011-04-15

    We present a theoretical analysis of the emission of localized excitons in GaAs-AlGaAs quantum wells, which shows a strong nonlinear behavior with increasing excitation. Considering the influence of dynamical screening both on the one-particle properties of carriers and on the whole spectrum of electron-hole pair states, we are able to explain the nonlinearity as a transition of the emission from excitonic to electron-hole pair states in the continuum (electron-hole plasma). Moreover, our theoretical approach based on the quasi-particle approximation for the carriers states and quantum kinetic effects in the screening describes the observed changes of the shift of the exciton energy from higher to lower energies at a temperature of T = 10 K (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  11. <strong>Mini-project>

    DEFF Research Database (Denmark)

    Katajainen, Jyrki

    2008-01-01

    In this project the goal is to develop the safe * family of containers for the CPH STL. The containers to be developed should be safer and more reliable than any of the existing implementations. A special focus should be put on strong exception safety since none of the existing prototypes available...... at the CPH STL can give this guarantee for all operations. In spite of the safety requirements, the strict running-time requirements specified in the C++ standard, and additional requirements specified in the CPH STL design documents, must be fulfilled....

  12. Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.

    Science.gov (United States)

    Scholes, Gregory D

    2015-12-24

    Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1⟩ ≈ (1)[TT] and quintet spin |Q⟩ ≈ (5)[TT]. It is shown that triplet pair states can be separated by a triplet-triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[T···T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton-exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination).

  13. Photon echo study of excitons and excitonic complexes in self-assembled quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Ikezawa, Michio [Institute of Physics, University of Tsukuba, Tsukuba 305-8571 (Japan)]. E-mail: mikezawa@sakura.cc.tsukuba.ac.jp; Nair, Selvakumar [Centre for Nanotechnology, University of Toronto, Toronto M5S 3E3 (Canada); Suto, Fumitaka [Institute of Physics, University of Tsukuba, Tsukuba 305-8571 (Japan); Masumoto, Yasuaki [Institute of Physics, University of Tsukuba, Tsukuba 305-8571 (Japan); Uchiyama, Chikako [Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11, Takeda, Kofu, Yamanashi 400-8511 (Japan); Aihara, Masaki [Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama-cho, Ikoma, Nara 630-0101 (Japan); Ruda, Harry [Centre for Nanotechnology, University of Toronto, Toronto M5S 3E3 (Canada)

    2007-01-15

    The authors have studied the excitons and excitonic complexes in two kinds of self-assembled quantum dots (QDs) using photon echo measurements. In GaAs strain-induced quantum dots (SIQDs), a pronounced biexcitonic beat with a period of 1 ps is observed. The biexciton binding energy in SIQDs is obtained from the beat period, and its magnetic field dependence is investigated. It is found that the biexciton binding energy is remarkably increased by the lateral confinement and they are almost independent of the applied magnetic field up to 8 T. A theoretical calculation of the biexciton binding energy in SIQDs is presented to explain the observed magnetic field dependence. In charge-tunable InP QDs, the photon echo signal shows dramatic changes depending on the electric bias. The decay profile of the echo intensity is not a single exponential but Gaussian-like function, which indicates non-Markovian nature of the dephasing process in this system. Theoretical calculation is done assuming tunneling induced dephasing mechanism, and it reproduces the experimental results quite well.

  14. Optically detected magnetic resonance (ODMR) studies of fullerene-doped pi-conjugated polymers

    Science.gov (United States)

    Shinar, Joseph; Lane, P. A.

    1995-12-01

    The X-band photoluminescence (PL) and PL-detected magnetic resonance (PLDMR) of C60- and C70-doped pristine 2,5-dihexoxy poly(p-phenylene vinylene) (DHOPPV), poly(3-dodecyl thiophene) (P3DT), and 2,5-dibutoxy poly(p-phenylene ethynylene) (DBOPPE) are described and discussed. While light (approximately 0.1 mol.%) doping of pristine DHOPPV sharply weakens the PL, it strongly enhances the PL-enhancing polaron and triplet exciton resonances, which are very weak in the undoped film. The polaron PLDMR is attributed to magnetic resonance enhancement of nonradiative trapped polaron pair recombination, which reduces the polaron population and consequently the rate at which they nonradiatively quench singlet excitons. Its behavior in 0.1 mol.% C60:DHOPPV is discussed in relation to the potential effects of C60-counterions, including the photogeneration of trapped polaron pairs at the expense of singlet excitons, and the fission of singlet excitons to such pairs. The emergence of the triplet resonance is discussed in relation to intersystem crossing from the singlet to the triplet manifold induced by the C60 dopant. At higher doping levels, the polaron and triplet exciton resonances weaken with increasing C60 content. The reduced polaron resonance is discussed in relation to the smaller relative change in the polaron population at the field for resonance, and the potential effects of the dissociation of the polaron pairs. The behavior of C70-doped pristine DHOPPV at up to 1 mol.% C70 is qualitatively similar to that of C60: DHOPPV, but the polaron resonance in P3DT and DBOPPE is less sensitive to C60. The triplet resonance is almost completely suppressed even in lightly doped P3DT and DBOPPE, which is attributed to quenching by the injected polarons.

  15. Energy transfer efficiency in the FMO complex strongly coupled to a vibronic mode

    CERN Document Server

    Mourokh, Lev

    2014-01-01

    Using methods of condensed matter and statistical physics, we examine the transport of excitons through the Fenna-Matthews-Olson (FMO) complex from a receiving antenna to a reaction center. Writing the equations of motion for the exciton creation/annihilation operators, we are able to describe the exciton dynamics, even in the regime when the reorganization energy is of the order of the intra-system couplings. In particular, we obtain the well-known quantum oscillations of the site populations. We determine the exciton transfer efficiency in the presence of a quenching field and protein environment. While the majority of the protein vibronic modes are treated as a heat bath, we address the situation when specific modes are strongly coupled to excitons and examine the effects of these modes on the quantum oscillations and the energy transfer efficiency. We find that, for the vibronic frequencies below 16 meV, the exciton transfer is drastically suppressed. We attribute this effect to the formation of "polaroni...

  16. Effect of polaron diffusion on exciton-polaron quenching in disordered organic semiconductors

    Science.gov (United States)

    Coehoorn, R.; Zhang, L.; Bobbert, P. A.; van Eersel, H.

    2017-04-01

    Exciton-polaron quenching (EPQ) is a major efficiency loss process in organic optoelectronic devices, in particular at high excitation densities. Within commonly used models, the rate is assumed to be given by the product of the exciton density, the polaron density, and a constant EPQ rate coefficient, which is proportional to the polaron diffusion coefficient and an EPQ capture radius. In this work, we study the effects of polaron diffusion on the EPQ rate in energetically disordered materials with a Gaussian density of states using kinetic Monte Carlo simulations, and show that the effective rate coefficient can depend strongly on the polaron concentration and on the electric field. We furthermore find that under realistic conditions, the effective value of the capture radius can exceed the expected value of ˜1 nm by up to two orders of magnitude. To a first approximation, the simulation results can be understood from macroscopic diffusion theory, adapted at finite electric fields to include the observed "polaron wind" effect. However, for strongly disordered systems we find distinct deviations from that theory, related to the very small time and spatial scales involved in the capture process.

  17. Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells

    Science.gov (United States)

    Kraus, Hannes; Heiber, Michael C.; Väth, Stefan; Kern, Julia; Deibel, Carsten; Sperlich, Andreas; Dyakonov, Vladimir

    2016-07-01

    A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.

  18. Controlling excitons. Concepts for phosphorescent organic LEDs at high brightness

    Energy Technology Data Exchange (ETDEWEB)

    Reineke, Sebastian

    2009-11-15

    This work focusses on the high brightness performance of phosphorescent organic light-emitting diodes (OLEDs). The use of phosphorescent emitter molecules in OLEDs is essential to realize internal electron-photon conversion efficiencies of 100 %. However, due to their molecular nature, the excited triplet states have orders of magnitude longer time constants compared to their fluorescent counterparts which, in turn, strongly increases the probability of bimolecular annihilation. As a consequence, the efficiencies of phosphorescent OLEDs decline at high brightness - an effect known as efficiency roll-off, for which it has been shown to be dominated by triplet-triplet annihilation (TTA). In this work, TTA of the archetype phosphorescent emitter Ir(ppy){sub 3} is investigated in time-resolved photoluminescence experiments. For the widely used mixed system CBP:Ir(ppy){sub 3}, host-guest TTA - an additional unwanted TTA channel - is experimentally observed at high excitation levels. By using matrix materials with higher triplet energies, this effect is efficiently suppressed, however further studies show that the efficiency roll-off of Ir(ppy)3 is much more pronounced than predicted by a model based on Foerster-type energy transfer, which marks the intrinsic limit for TTA. These results suggest that the emitter molecules show a strong tendency to form aggregates in the mixed film as the origin for enhanced TTA. Transmission electron microscopy images of Ir(ppy){sub 3} doped mixed films give direct proof of emitter aggregates. Based on these results, two concepts are developed that improve the high brightness performance of OLEDs. In a first approach, thin intrinsic matrix interlayers are incorporated in the emission layer leading to a one-dimensional exciton confinement that suppresses exciton migration and, consequently, TTA. The second concept reduces the efficiency roll-off by using an emitter molecule with slightly different chemical structure, i.e. Ir(ppy){sub 2

  19. Exciton effective mass enhancement in coupled quantum wells in electric and magnetic fields

    Science.gov (United States)

    Wilkes, J.; Muljarov, E. A.

    2016-02-01

    We present a calculation of exciton states in semiconductor coupled quantum wells in the presence of electric and magnetic fields applied perpendicular to the QW plane. The exciton Schrödinger equation is solved in real space in three-dimensions to obtain the Landau levels of both direct and indirect excitons. Calculation of the exciton energy levels and oscillator strengths enables mapping of the electric and magnetic field dependence of the exciton absorption spectrum. For the ground state of the system, we evaluate the Bohr radius, optical lifetime, binding energy and dipole moment. The exciton mass renormalization due to the magnetic field is calculated using a perturbative approach. We predict a non-monotonous dependence of the exciton ground state effective mass on magnetic field. Such a trend is explained in a classical picture, in terms of the ground state tending from an indirect to a direct exciton with increasing magnetic field.

  20. Single Molecule Photobleaching Probes the Exciton Wavefunction in a Multichromophoric System

    NARCIS (Netherlands)

    Hernando, J.; Hoogenboom, J.P.; Dijk, van E.M.H.P.; Garcia-Lopez, J.J.; Crego Calama, M.; Reinhoudt, D.N.; Hulst, van N.F.; Garcia-Parajo, M.F.

    2004-01-01

    The exciton wave function of a trichromophoric system is investigated by means of single molecule spectroscopy at room temperature. Individual trimers exhibit superradiance and loss of vibronic structure in emission spectrum, features proving exciton delocalization. We identify two distinct photodeg

  1. Systematic study of exciton diffusion length in organic semiconductors by six experimental methods

    NARCIS (Netherlands)

    Lin, Jason D. A.; Mikhnenko, Oleksandr V.; Chen, Jingrun; Masri, Zarifi; Ruseckas, Arvydas; Mikhailovsky, Alexander; Raab, Reilly P.; Liu, Jianhua; Blom, Paul W. M.; Loi, Maria Antonietta; Garcia-Cervera, Carlos J.; Samuel, Ifor D. W.; Thuc-Quyen Nguyen, [No Value

    Six experimental methods have been used to investigate the exciton diffusion length in materials with systematic chemical modifications. We find that exciton diffusion length correlates with molecular ordering. We discuss situations in which certain experimental techniques are more appropriate.

  2. Relaxation of localized excitons in CdSe/ZnSe heterostructures containing quantum islands of different sizes

    Energy Technology Data Exchange (ETDEWEB)

    Wachter, S.; Dal Don, B.; Schmidt, M.; Baldauf, M.; Dinger, A.; Kurtz, E.; Klingshirn, C.; Kalt, H. [Karlsruhe Univ. (T.H.) (Germany). Inst. fuer Angewandte Physik

    2001-03-08

    We report on localization dynamics of excitons in ensembles of self-organized CdSe islands embedded in ZnSe. The experimental methods employed are temperature dependent, spatially-resolved photoluminescence ({mu}-PL), spatially-integrated PL (macro-PL), as well as time-resolved PL (TRPL). We see the often observed non-monotonous shift of the PL maximum with temperature which we can explain by a redistribution of the excitons amongst the islands. The measured shift is compared with the exact shift of the bandgap deduced from {mu}-PL measurements and is found to depend strongly on island size and distribution. These transport processes are recovered in the temporal evolution of the PL. The decay time of the spectrally integrated PL reaches its maximum at exactly the same temperature at which the redshift of the macro-PL turns into a blueshift. (orig.)

  3. Intermediate type excitons in Schottky barriers of A{sup 3}B{sup 6} layer semiconductors and UV photodetectors

    Energy Technology Data Exchange (ETDEWEB)

    Alekperov, O.Z.; Guseinov, N.M.; Nadjafov, A.I. [Insitute of Physics of National Academy of Sinces of Azerbaijan, H. Javid av. 33, 1133 Baku (Azerbaijan)

    2006-09-15

    Photoelectric and photovoltaic spectra of Schottky barrier (SB) structures of InSe, GaSe and GaS layered semiconductors (LS) are investigated at quantum energies from the band edge excitons of corresponding materials up to 6.5eV. Spectral dependences of photoconductivity (PC) of photo resistors and barrier structures are strongly different at the quantum energies corresponding to the intermediate type excitons (ITE) observed in these semiconductors. It was suggested that high UV photoconductivity of A{sup 3}B{sup 6} LS is due to existence of high mobility light carriers in the depth of the band structure. It is shown that SB of semitransparent Au-InSe is high sensitive photo detector in UV region of spectra. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  4. Exciton coupling induces vibronic hyperchromism in light-harvesting complexes

    CERN Document Server

    Schulze, Jan; Kühn, Oliver; Pullerits, Tõnu

    2013-01-01

    The recently suggested possibility that weak vibronic transitions can be excitonically enhanced in light-harvesting complexes is studied in detail. A vibronic exciton dimer model which includes ground state vibrations is investigated using multi-configuration time-dependent Hartree method with a parameter set typical to photosynthetic light-harvesting complexes. Absorption spectra are discussed in dependence on the Coulomb coupling, the detuning of site energies, and the number of vibrational mode. Calculations of the fluorescence spectra show that the spectral densities obtained from the low temperature fluorescence line narrowing measurements of light-harvesting systems need to be corrected for the exciton effects. For the J-aggregate configuration, as in most of the light-harvesting complexes, the true spectral density has larger amplitude than what is obtained from the measurement.

  5. Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics

    Science.gov (United States)

    Scully, Shawn Ryan

    Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of

  6. Controlled Spin Transport in Planar Systems Through Topological Exciton

    CERN Document Server

    Abhinav, Kumar

    2015-01-01

    It is shown that a charge-neutral spin-1 exciton, possibly realizable only in planar systems like graphene and topological insulators, can be effectively used for controlled spin transport in such media. The effect of quantum and thermal fluctuations yield a parametric excitation threshold for its realization. This planar exciton differs from the conventional ones, as it owes its existence to the topological Chern-Simons (CS) term. The parity and time-reversal violating CS term can arise from quantum effects in systems with parity-breaking mass-gap. The spinning exciton naturally couples to magnetic field, leading to the possibility of controlled spin transport. Being neutral, it is immune to a host of effect, which afflicts spin transport through charged fermions.

  7. Exciton dephasing and biexciton binding in CdSe/ZnSe islands

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Tranitz, H.-P.; Preis, H;

    1999-01-01

    The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5-10 ps...

  8. Strong excitonic effects in CuAlO2 delafossite transparent conductive oxides

    DEFF Research Database (Denmark)

    Laskowski, Robert; Christensen, Niels Egede; Blaha, Peter;

    2009-01-01

    The imaginary part of the dielectric function of CuAlO2 has been calculated including the electron-hole correlation effects within Bethe-Salpeter formalism (BSE). In the initial step of the BSE solver the band structure was calculated within density-functional theory plus an orbital field (LDA...

  9. One-dimensional models of excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia Decebal; Duclos, Pierre; Pedersen, Thomas Garm

    2004-01-01

    Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one-dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable.......Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one-dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable....

  10. Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.

    Science.gov (United States)

    Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H

    2013-06-18

    Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons

  11. Realization of an all optical exciton-polariton router

    Energy Technology Data Exchange (ETDEWEB)

    Marsault, Félix; Nguyen, Hai Son; Tanese, Dimitrii; Lemaître, Aristide; Galopin, Elisabeth; Sagnes, Isabelle; Amo, Alberto [Laboratoire de Photonique et de Nanostructures, LPN/CNRS, Route de Nozay, 91460 Marcoussis (France); Bloch, Jacqueline, E-mail: jacqueline.bloch@lpn.cnrs.fr [Laboratoire de Photonique et de Nanostructures, LPN/CNRS, Route de Nozay, 91460 Marcoussis (France); Physics Department, Ecole Polytechnique, F-91128 Palaiseau Cedex (France)

    2015-11-16

    We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.

  12. Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells

    KAUST Repository

    Burkhard, George F.

    2009-12-09

    We investigate the internal quantum efficiencies (IQEs) of high efficiency poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells and find them to be lower at wavelengths where the PCBM absorbs. Because the exciton diffusion length in PCBM is too small, excitons generated in PCBM decay before reaching the donor-acceptor interface. This result has implications for most state of the art organic solar cells, since all of the most efficient devices use fullerenes as electron acceptors. © 2009 American Chemical Society.

  13. Explicit Correlated Exciton-Vibrational Dynamics of the FMO Complex

    CERN Document Server

    Schulze, Jan

    2015-01-01

    The coupled exciton-vibrational dynamics of a 3-site FMO model is investigated using the numerically exact multilayer multiconfiguration time-dependent Hartree approach. Thereby the vibrational mode specific coupling to local electronic transitions is adapted from a discretized experimental spectral density. The solution of the resulting time-dependent Schr\\"odinger equation including three electronic and 450 vibrational degrees of freedom is analyzed in terms of excitonic populations and coherences. Emphasis is put onto the role of specific ranges of vibrational frequencies. It is observed that modes between 160 and 300 cm$^{-1}$ are responsible for the subpicosecond population and coherence decay.

  14. Bimolecular Recombination Kinetics of an Exciton-Trion Gas

    Science.gov (United States)

    2015-07-01

    The second and third reactions are catalytic agents for the loss of excitons. This implies that the constants R and S will appear only in the exciton...change the electron density. 3 Note that Reaction 3 in Table 2 is catalytic as defined above, because it changes the electron and e−trion...from the PL laser, which is fixed. Table 2 Reactions that change νe Reaction Rate Constant 1. νe + νh → ϕ A 2. νe + τh → Ex P 3

  15. Environment-assisted quantum walks in excitonic energy transport

    Science.gov (United States)

    Mohseni, Masoud; Rebentrost, Patrick; Lloyd, Seth; Aspuru-Guzik, Alan

    2010-03-01

    Long-lived quantum coherence has recently been observed experimentally via ultrafast nonlinear spectroscopy in excitonic energy transfer within light-harvesting photosynthetic complexes, conjugated polymers, and marine alga even at room temperature. Here, we demonstrate that directed quantum walks lead to an enhancement of energy transfer efficiency in such systems. We introduce two complementary theoretical approaches, based on a Green's function method and energy transfer susceptibilities, to partition open quantum dynamics. We quantify the role of fundamental physical processes involved in energy transport. In particular, we examine the contributions of classical hopping, coherent excitonic Hamiltonian, and phonon-induced decoherence effects for pure dephasing, Markovian, and non-Markovian limits.

  16. Enhanced energy transport in genetically engineered excitonic networks

    Science.gov (United States)

    Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C.; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F.; Lloyd, Seth; Belcher, Angela M.

    2016-02-01

    One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.

  17. Defect Structure of Localized Excitons in a WSe2 Monolayer

    KAUST Repository

    Zhang, Shuai

    2017-07-26

    The atomic and electronic structure of intrinsic defects in a WSe2 monolayer grown on graphite was revealed by low temperature scanning tunneling microscopy and spectroscopy. Instead of chalcogen vacancies that prevail in other transition metal dichalcogenide materials, intrinsic defects in WSe2 arise surprisingly from single tungsten vacancies, leading to the hole (p-type) doping. Furthermore, we found these defects to dominate the excitonic emission of the WSe2 monolayer at low temperature. Our work provided the first atomic-scale understanding of defect excitons and paved the way toward deciphering the defect structure of single quantum emitters previously discovered in the WSe2 monolayer.

  18. Hybridized exciton-polariton resonances in core-shell nanoparticles

    CERN Document Server

    Gentile, Martin J

    2016-01-01

    The goal of nanophotonics is to control and manipulate light at length scales below the diffraction limit. Typically nanostructured metals are used for this purpose, light being confined by exploiting the surface plasmon-polaritons such structures support. Recently excitonic (molecular) materials have been identified as an alternative candidate material for nanophotonics. Here we use theoretical modelling to explore how hybridisation of surface exciton-polaritons can be achieved through appropriate nanostructuring. We focus on the extent to which the frequency of the hybridised modes can be shifted with respect to the underlying material resonances.

  19. Dynamics of Photogenerated Polaron-Excitons in Organic Semiconductors

    Science.gov (United States)

    Junior, Luiz A. Ribeiro; Neto, Pedro H. Oliveira; da Cunha, Wiliam F.; Silva, Geraldo M. e.

    In this work we performed numerical simulations of one π-conjugated polymer chain subjected to photogeneration. Within the SSH model modified to include the Brazoviskii-Kirova symmetry breaking term, we investigate the dynamics of photoexcitations to address the generation mechanism of polaron-excitons using the unrestricted Hartree-Fock approximation. It was obtained that after the photoexcitation the system relaxes spontaneously into a polaron-exciton in a transient state in a range of 200 fs. Our results also show that charged polarons are generated directly after this transient state.

  20. Nonperturbative theory of exciton-phonon resonances in semiconductor absorption

    Science.gov (United States)

    Hannewald, K.; Bobbert, P. A.

    2005-09-01

    We develop a theory of exciton-phonon sidebands in the absorption spectra of semiconductors. The theory does not rely on an ad hoc exciton-phonon picture, but is based on a more fundamental electron-phonon Hamiltonian, thus avoiding a priori assumptions about excited-state properties. We derive a nonperturbative compact solution that can be looked upon as the semiconductor version of the textbook absorption formula for a two-level system coupled to phonons. Accompanied by an illustrative numerical example, the importance and usefulness of our approach with respect to practical applications for semiconductors is demonstrated.

  1. Binding Energy of Excitons in a Quantum Ring

    Institute of Scientific and Technical Information of China (English)

    XIE Wen-Fang

    2008-01-01

    The binding energy of excitons confined to a quantum ring under the influence of perpendicular homogeneous magnetic field is calculated as a function of the ring radius. Calculations are made by using the method of exact diagonalization within the effective-mass approximation. The feature of binding energy of the ground state as a function of the ring radius for several values of the magnetic field has been revealed. The interesting feature of our study is that, in a quantum ring, the geometric structure of excitons may reveal transition.

  2. Refinements in the use of equivalent latitude for assimilating sporadic inhomogeneous stratospheric tracer observations, 1: Detecting transport of Pinatubo aerosol across a strong vortex edge

    Directory of Open Access Journals (Sweden)

    P. Good

    2004-01-01

    Full Text Available The use of PV equivalent latitude for assimilating stratospheric tracer observations is discussed - with particular regard to the errors in the equivalent latitude coordinate, and to the assimilation of sparse data. Some example measurements are assimilated: they sample the stratosphere sporadically and inhomogeneously. The aim was to obtain precise information about the isentropic tracer distribution and evolution as a function of equivalent latitude. Precision is important, if transport across barriers like the vortex edge are to be detected directly. The main challenges addressed are the errors in modelled equivalent latitude, and the non-ideal observational sampling. The methods presented allow first some assessment of equivalent latitude errors and a picture of how good or poor the observational coverage is. This information determines choices in the approach for estimating as precisely as possible the true equivalent latitude distribution of the tracer, in periods of good and poor observational coverage. This is in practice an optimisation process, since better understanding of the equivalent latitude distribution of the tracer feeds back into a clearer picture of the errors in the modelled equivalent latitude coordinate. Error estimates constrain the reliability of using equivalent latitude to make statements like 'this observation samples air poleward of the vortex edge' or that of more general model-measurement comparisons. The approach is demonstrated for ground-based lidar soundings of the Mount Pinatubo aerosol cloud, focusing on the 1991-92 arctic vortex edge between 475-520K. Equivalent latitude is estimated at the observation times and locations from Eulerian model tracers initialised with PV and forced by UK Meteorological Office analyses. With the model formulation chosen, it is shown that tracer transport of a few days resulted in an error distribution that was much closer to Gaussian form, although the mean error was not

  3. Exciton Dynamics and Many Body Interactions in Layered Semiconducting Materials Revealed with Non-linear Coherent Spectroscopy

    Science.gov (United States)

    Dey, Prasenjit

    Atomically thin, semiconducting transition metal dichalogenides (TMDs), a special class of layered semiconductors, that can be shaped as a perfect two dimensional material, have garnered a lot of attention owing to their fascinating electronic properties which are achievable at the extreme nanoscale. In contrast to graphene, the most celebrated two-dimensional (2D) material thus far; TMDs exhibit a direct band gap in the monolayer regime. The presence of a non-zero bandgap along with the broken inversion symmetry in the monolayer limit brands semiconducting TMDs as the perfect candidate for future optoelectronic and valleytronics-based device application. These remarkable discoveries demand exploration of different materials that possess similar properties alike TMDs. Recently, III-VI layered semiconducting materials (example: InSe, GaSe etc.) have also emerged as potential materials for optical device based applications as, similar to TMDs, they can be shaped into a perfect two-dimensional form as well as possess a sizable band gap in their nano-regime. The perfect 2D character in layered materials cause enhancement of strong Coulomb interaction. As a result, excitons, a coulomb bound quasiparticle made of electron-hole pair, dominate the optical properties near the bandgap. The basis of development for future optoelectronic-based devices requires accurate characterization of the essential properties of excitons. Two fundamental parameters that characterize the quantum dynamics of excitons are: a) the dephasing rate, gamma, which represents the coherence loss due to the interaction of the excitons with their environment (for example- phonons, impurities, other excitons, etc.) and b) excited state population decay rate arising from radiative and non-radiative relaxation processes. The dephasing rate is representative of the time scale over which excitons can be coherently manipulated, therefore accurately probing the source of exciton decoherence is crucial for

  4. Dynamical co-existence of excitons and free carriers in perovskite probed by density-resolved fluorescent spectroscopic method

    CERN Document Server

    Wang, Wei; Wang, Xiangyuan; Lv, Yanping; Wang, Shufeng; Wang, Kai; Shi, Yantao; Xiao, Lixin; Chen, Zhijian; Gong, Qihuang

    2016-01-01

    Using transient fluorescent spectra at time-zero, we develop a density-resolved fluorescent spectroscopic method for investigating photoproducts in CH3NH3PbI3 perovskite and related photophysics. The density dependent dynamical co-existence of excitons and free carriers over a wide density range is experimentally observed for the first time. The exciton binding energy (EB) and the effective mass of electron-hole pair can be estimated based on such co-existence. No ionic polarization is found contributing to photophysical behavior. It also solves the conflict between the large experimentally measured EB and the small predicted values. The spectroscopic method also helps to detect the true free carrier density under continuous illumination without the interference of ionic conductivity. Our methods and results profoundly enrich the study and understanding of the photophysics in perovskite materials for photovoltaic applications.

  5. Spectral signatures of x((5)) processes in four-wave mixing of homogeneously broadened excitons

    DEFF Research Database (Denmark)

    Langbein, W.; Meier, T.; Koch, S.W.;

    2001-01-01

    The influence of fifth-order coherences on the spectrally resolved four-wave mixing response of predominantly homogeneously broadened quasi-two-dimensional excitons is studied. Fifth-order signatures are discussed as a function of spectral position and excitation polarization. An exciton-biexcito...... of one- and two-exciton resonances up to the fifth order in the optical field.......The influence of fifth-order coherences on the spectrally resolved four-wave mixing response of predominantly homogeneously broadened quasi-two-dimensional excitons is studied. Fifth-order signatures are discussed as a function of spectral position and excitation polarization. An exciton...

  6. Quantum Statistical Properties of the Exciton in a Leaky Quasi-Mode Cavity

    Institute of Scientific and Technical Information of China (English)

    YU Zhao-Xian; JIAO Zhi-Yong

    2002-01-01

    We have studied quantum statistical properties of the exciton in a leaky quasi-mode cavity. It is shown that when the exciton is initially in a squeezed coherent state whereas cavity initially in a vacuum state, there is energy exchange between the exciton and cavity. Both the exciton and cavity may exhibit sub-Poissonian distribution and exist quadrature squeezing. Calculation shows that correlation between the exciton and cavity is classical, which implies that there is not the violation of the Cauchy-Schwartz inequality.

  7. Intrinsic Delocalization during the Decay of Excitons in Polymeric Solar Cells

    Directory of Open Access Journals (Sweden)

    Weikang Chen

    2016-11-01

    Full Text Available In bulk heterojunction polymer solar cells, external photoexcitation results in localized excitons in the polymer chain. After hot exciton formation and subsequent relaxation, the dipole moment drives the electron to partially transfer to extended orbitals from the original localized ones, leading to self-delocalization. Based on the dynamic fluorescence spectra, the delocalization of excitons is revealed to be an intrinsic property dominated by exciton decay, acting as a bridge for the exciton to diffuse in the polymeric solar cell. The modification of the dipole moment enhances the efficiency of polymer solar cells.

  8. Exciton-Phonon Scattering in CdSe/ZnSe Quantum Dots

    Institute of Scientific and Technical Information of China (English)

    张立功; 申德振; 范希武; 吕少哲

    2002-01-01

    A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantum dots with a ZnCdSe quantum well. We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantum dots and two-dimensional exciton in the CdSe wetting layer. The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantum dots in comparison with that in the two-dimensional wetting layer, which indicates the decrease of exciton and optical phonon coupling in the CdSe quantum dots.

  9. Transient demonstration of exciton behaviours in solid state cathodoluminescence under different driving voltage

    Institute of Scientific and Technical Information of China (English)

    Zhang Fu-Jun; Zhao Su-Ling; Xu Zheng; Huang Jin-Zhao; Xu Xu-Rong

    2007-01-01

    In the solid state cathodoluminescence (SSCL), organic materials were excited by hot electrons accelerated in silicon oxide (SiO2) layer under alternating current (AC). In this paper exciton behaviours were analysed by using transient spectra under different driving voltages. The threshold voltages of SSCL and exciton ionization were obtained from the transient spectra. The recombination radiation occurred when the driving voltage went beyond the threshold voltage of exciton ionization. Prom the transient spectrum of two kinds of luminescence (exciton emission and recombination radiation), it was demonstrated that recombination radiation should benefit from the exciton ionization.

  10. Temperature-dependent exciton recombination in asymmetrical ZnCdSe/ZnSe double quantum wells

    CERN Document Server

    Yu Guang You; Zhang, J Y; Zheng, Z H; Yang, B J; Zhao Xiao Wei; Shen De Zhen; Kong Xiang Gui

    1999-01-01

    Temperature-dependent exciton recombination in asymmetrical ZnCdSe/ZnSe double quantum wells is studied by recording photoluminescence spectra and photoluminescence decay spectra. The exciton tunnelling from the wide well to the narrow well and the thermal dissociation of excitons are two factors that influence the exciton recombination in this structure. In the narrow well, both of the two processes decrease the emission intensity, whereas, in the wide well, these two processes have contrary influences on the exciton density. The change of the emission intensity depends on which is the stronger one. (author)

  11. Dark-bright exciton spin-flip rates of quantum dots determined by a modified local density of optical states

    DEFF Research Database (Denmark)

    Lodahl, Peter; Johansen, Jeppe; Julsgaard, Brian;

    2009-01-01

    This work investigates the influence of dark excitons on the radiative dynamics of semiconductor quantum dots (QDs). Dark excitons have total angular momentum of 2 and contribute to the fine structure of the exciton ground state. As opposed to bright excitons that have total angular momentum 1...

  12. Excitonic Phase Transition in the Extended Three-Dimensional Falicov-Kimball Model

    Science.gov (United States)

    Apinyan, V.; Kopeć, T. K.

    2014-07-01

    We study the excitonic phase transition in a system of the conduction band electrons and valence band holes described by the three-dimensional (3D) extended Falicov-Kimball (EFKM) model with the tunable Coulomb interaction between both species. By lowering the temperature, the electron-hole system may become unstable with respect to the formation of the excitons, i.e, electron-hole pairs at temperature , exhibiting a gap in the particle excitation spectrum. To this end we implement the functional integral formulation of the EFKM, where the Coulomb interaction term is expressed in terms of U(1) phase variables conjugate to the local particle number, providing a useful representation of strongly correlated system. The effective action formalism allows us to formulate a problem in the phase-only action in the form of the quantum rotor model and to obtain analytical formulas for the critical lines and other quantities of physical interest like charge gap, chemical potential and the correlation length.

  13. Alternative approaches to electronic damage by ion-beam irradiation: Exciton models

    Energy Technology Data Exchange (ETDEWEB)

    Agullo-Lopez, F.; Munoz-Martin, A.; Zucchiatti, A. [Centro de Micro-Analisis de Materiales, Universidad Autonoma de Madrid, 28049, Madrid (Spain); Climent-Font, A. [Centro de Micro-Analisis de Materiales, Universidad Autonoma de Madrid, 28049, Madrid (Spain); Departamento de Fisica Aplicada, Universidad Autonoma de Madrid, 28049, Madrid (Spain)

    2016-11-15

    The paper briefly describes the main features of the damage produced by swift heavy ion (SHI) irradiation. After a short revision of the widely used thermal spike concept, it focuses on cumulative mechanisms of track formation which are alternative to those based on lattice melting (thermal spike models). These cumulative mechanisms rely on the production of point defects around the ion trajectory, and their accumulation up to a final lattice collapse or amorphization. As to the formation of point defects, the paper considers those mechanisms relying on direct local conversion of the excitation energy into atomic displacements (exciton models). A particular attention is given to processes based on the non-radiative recombination of excitons that have become self-trapped as a consequence of a strong electron-phonon interaction (STEs). These mechanisms, although operative under purely ionizing radiation in some dielectric materials, have been rarely invoked, so far, to discuss SHI damage. They are discussed in this paper together with relevant examples to materials such as Cu{sub 3}N, alkali halides, SiO{sub 2}, and LiNbO{sub 3}. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  14. Absorption and emission in quantum dots: Fermi surface effects of Anderson excitons

    Science.gov (United States)

    Helmes, R. W.; Sindel, M.; Borda, L.; von Delft, J.

    2005-09-01

    Recent experiments measuring the emission of exciton recombination in a self-organized single quantum dot (QD) have revealed that different effects occur when the wetting layer surrounding the QD becomes filled with electrons because the resulting Fermi sea can hybridize with the local electron levels on the dot. Motivated by these experiments, we study an extended Anderson model, which describes a local conduction band level coupled to a Fermi sea, but also includes a local valence band level. We are interested, in particular, in how many-body correlations resulting from the presence of the Fermi sea affect the absorption and emission spectra. Using Wilson’s numerical renormalization group method, we calculate the zero-temperature absorption (emission) spectrum of a QD, which starts from (ends up in) a strongly correlated Kondo ground state. We predict two features: First, we find that the spectrum shows a power-law divergence close to the threshold, with an exponent that can be understood by analogy to the well-known x-ray edge absorption problem. Second, the threshold energy ω0 —below which no photon is absorbed (above which no photon is emitted)—shows a marked, monotonic shift as a function of the exciton binding energy Uexc .

  15. High-energy side-peak emission of exciton-polariton condensates in high density regime.

    Science.gov (United States)

    Horikiri, Tomoyuki; Yamaguchi, Makoto; Kamide, Kenji; Matsuo, Yasuhiro; Byrnes, Tim; Ishida, Natsuko; Löffler, Andreas; Höfling, Sven; Shikano, Yutaka; Ogawa, Tetsuo; Forchel, Alfred; Yamamoto, Yoshihisa

    2016-05-19

    In a standard semiconductor laser, electrons and holes recombine via stimulated emission to emit coherent light, in a process that is far from thermal equilibrium. Exciton-polariton condensates-sharing the same basic device structure as a semiconductor laser, consisting of quantum wells coupled to a microcavity-have been investigated primarily at densities far below the Mott density for signatures of Bose-Einstein condensation. At high densities approaching the Mott density, exciton-polariton condensates are generally thought to revert to a standard semiconductor laser, with the loss of strong coupling. Here, we report the observation of a photoluminescence sideband at high densities that cannot be accounted for by conventional semiconductor lasing. This also differs from an upper-polariton peak by the observation of the excitation power dependence in the peak-energy separation. Our interpretation as a persistent coherent electron-hole-photon coupling captures several features of this sideband, although a complete understanding of the experimental data is lacking. A full understanding of the observations should lead to a development in non-equilibrium many-body physics.

  16. Complete study of excitonic fine-structure splitting in GaN/AlN quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Hoenig, Gerald; Winkelnkemper, Momme; Schliwa, Andrei; Hoffmann, Axel; Bimberg, Dieter [Institut fuer Festkoerperphysik, Technische Universitaet Berlin (Germany); Kindel, Christian [Institut fuer Festkoerperphysik, Technische Universitaet Berlin (Germany); Research Center for Advanced Science and Technology, University of Tokyo (Japan); Kako, Satoshi [Institute for Nano Quantum Information Electronics, University of Tokyo (Japan); Kawano, Takeshi; Oishi, Hiroaki [Research Center for Advanced Science and Technology, University of Tokyo (Japan); Arakawa, Yasuhiko [Research Center for Advanced Science and Technology, University of Tokyo (Japan); Institute for Nano Quantum Information Electronics, University of Tokyo (Japan); Institute of Industrial Science, University of Tokyo, Komaba, Meguro, Tokyo (Japan)

    2010-07-01

    A detailed understanding of the excitonic fine structure in quantum dots (QDs) is indispensable for their use in quantum cryptography devices. While the fine structure in As-based QDs has been studied extensively, there is a lack of such investigations for N-based QDs, which might operate at room temperature. We present the first complete study of excitonic fine-structure splitting (FSS) in GaN/AlN QDs. Our experimental studies reveal a huge FSS of up to 7 meV with a strong dependence on the emission energy inverse to that in As-based QDs. Our theoretical studies, performed with a configuration-interaction method based on realistic 8-band-k.p Hartree-Fock states, confirm the experimental results and identify the origin of FSS as lattice strain induced. Based on our results it is possible to induce a strain gradient (by micro mechanic techniques or structuring methods), which will reduce the FSS to zero for the emission of entangled photon pairs.

  17. Tunneling at νT = 1 in a bilayer quantum Hall exciton condensate

    Science.gov (United States)

    Nandi, D.; Khaire, T.; Finck, A. D. K.; Eisenstein, J. P.; Pfeiffer, L. N.; West, K. W.

    2014-03-01

    Closely-spaced bilayer quantum Hall systems at total filling factor νT = 1 exhibit spontaneous interlayer phase coherence. This phase coherence, which is tantamount to excitonic Bose condensation, is most dramatically revealed via interlayer tunneling measurements.In the condensed phase the tunneling current-voltage (IV) characteristic of this semiconductor system strongly resembles the dc Josephson effect observed in superconducting tunnel junctions. Here we report on a detailed study of this phenomenon. We find the maximum, or critical tunneling current Ic to be a well-defined global property of the macroscopic tunnel junction, insensitive to external circuit elements and the precise contact configuration used to observe it. Interestingly, the temperature dependence of Ic displays an unexpected scaling behavior. At the lowest temperatures the slope of the ``supercurrent'' branch of the tunneling IV curve, while extremely large, remains finite. Careful measurements in this regime suggest that dissipative processes arising from in-plane exciton transport limit the maximum tunneling conductance. Finally, comparisons of the experimentally observed IV with recent theoretical predictions will be discussed.

  18. Excitonic Doppler-Rabi Oscillations in a Moving Organic Slab

    Institute of Scientific and Technical Information of China (English)

    2005-01-01

    It is theoretically shown that excitonic Doppler-Rabi oscillations can occur in an organic slab moving along the axis of a high-Q cavity. Due to the √N enhancement of the vacuum Rabi frequency, this effect can be more easily observed than that in a moving two-level atom.

  19. Excitonic insulator transition in the conjugated polymer polyacene

    NARCIS (Netherlands)

    Rice, MJ; Gartstein, YN

    2004-01-01

    According to molecular orbital theory, the symmetrically positioned one-dimensional (I-D) conduction and valence bands of polyacene touch at the X point. Clearly, the exciton binding energy of this semimetal exceeds the band gap so that polyacene should be a textbook case of a semimetal undergoing a

  20. Optical absorption of charged excitons in semiconducting carbon nanotubes

    DEFF Research Database (Denmark)

    Rønnow, Troels Frimodt; Pedersen, Thomas Garm; Cornean, Horia

    2012-01-01

    In this article we examine the absorption coefficient of charged excitons in carbon nanotubes. We investigate the temperature and damping dependence of the absorption spectra. We show that the trion peak in the spectrum is asymmetric for temperatures greater than approximately 1 K whereas the abs...

  1. Exploration of exciton delocalization in organic crystalline thin films

    Science.gov (United States)

    Hua, Kim; Manning, Lane; Rawat, Naveen; Ainsworth, Victoria; Furis, Madalina

    The electronic properties of organic semiconductors play a crucial role in designing new materials for specific applications. Our group recently found evidence for a rotation of molecular planes in phthalocyanines that is responsible for the disappearance of a delocalized exciton in these systems for T >150K.................()().......1 In this study, we attempt to tune the exciton delocalization of small organic molecules using strain effects and alloying different molecules in the same family. The exciton behavior is monitored using time- and polarization resolved photolumniscence (PL) spectroscopy as a function of temperature. Specifically, organic crystalline thin films of octabutoxy phthalocyanine (H2OBPc), octyloxy phthalocyanines and H-bonded semiconductors such as the quinacridone and indigo derivatives are deposited on flexible substrates (i.e. Kapton and PEN) using an in-house developed pen-writing method.........2 that results in crystalline films with macroscopic long range order. The room temperature PL studies show redshift and changes in polarization upon bending of the film. Crystalline thin films of alloyed H2OBPc and octabutoxy naphthalocyanine with ratios ranging from 1:1 to 100:1 fabricated on both sapphire and flexible substrates are also explored using the same PL spectroscopy to elucidate the behaviors of delocalized excitons. .1N. Rawat, et al., J Phys Chem Lett 6, 1834 (2015). 2R. L. Headrick, et al., Applied Physics Letters 92, 063302 (2008). NSF DMR-1056589, NSF DMR-1062966.

  2. Continuum contribution to excitonic four-wave mixing

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Vadim, Lyssenko

    1996-01-01

    Summary form only given. We present an experimental and theoretical investigation of ultrafast transient four-wave mixing (TFWM) of GaAs-AlGaAs quantum wells for simultaneous excitation of exciton and continuum states. Recent TFWM experiments on semiconductors have shown unexpected results when...

  3. Excitons in van der Waals Heterostructures: A theoretical study

    DEFF Research Database (Denmark)

    Latini, Simone

    to experimental results. Ultimately this thesis puts forth a first-principles methodology that allows us to address scientific questions that are beyond the capability of existing state of the art techniques and enables 2D materials researcher to predict and design dielectric, electronic and excitonic properties...

  4. Timeresolved Speckle Analysis: Probing the Coherence of Excitonic Secondary Emission

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Hvam, Jørn Märcher; Zimmermann, R.

    1998-01-01

    A new technique to analyze the time-dependent coherence of light emitted in a non-specular direction is presented. We demonstrate that the coherence degree of the emission can be deduced from the intensity fluctuations over the emission directions (speckles). The secondary emission of excitons...

  5. Hamiltonian multiplex interaction based on excitons effect in semiconductor QCs

    Directory of Open Access Journals (Sweden)

    Arezu Jahanshir

    2014-11-01

    Full Text Available The subject of modern technology has been the focus of extensive theoretical investigations in semiconducting nanostructures which we know as quantum dots (QCs. The possibility of monitoring and controlling the properties of QCs attracted considerable attention to these objects, as an important basic system in future technology. So, the quantum-mechanical effects play a significant role in the description of the formation mechanism QCs, determination of mass spectrum, binding energy and other characteristics. Based on QFT and by using oscillator representation method (ORM and operator product expansion technique developed in QFT, we study the properties of electron-hole QDs, determine mass spectrum and peruse spin-spin interactions in exciton system and its multiple pair systems. This method has applications to calculate the binding energy of exciton system in ground and excited states with semi-nuclear structure in semiconductor QCs or cold atomic few-body systems and develop the general calculation’s theory of few-body systems based on the Coulomb interaction between particles by forming excitonic pairs in semiconductor QCs. We investigate the binding energy of exciton bound states. It is shown that fermion particles have a very small mass, and after bonding together by dynamically force, constituent particles become massive, which is analogous to what happens in QCD.

  6. Combined effects of local and nonlocal hybridization on formation and condensation of excitons in the extended Falicov-Kimball model

    Science.gov (United States)

    Farkašovský, Pavol

    2017-04-01

    We study the combined effects of local and nonlocal hybridization on the formation and condensation of the excitonic bound states in the extended Falicov-Kimball model by the density-matrix-renormalization-group (DMRG) method. Analysing the resultant behaviours of the excitonic momentum distribution N(q) we found, that unlike the local hybridization V, which supports the formation of the q=0 momentum condensate, the nonlocal hybridization Vn supports the formation of the q = π momentum condensate. The combined effect of local and nonlocal hybridization further enhances the excitonic correlations in q=0 as well as q = π state, especially for V and Vn values from the charge-density-wave (CDW) region. Strong effects of local and nonlocal hybridization are observed also for other ground-state quantities of the model such as the f-electron density, or the density of unbound d-electrons, which are generally enhanced with increasing V and Vn. The same calculations performed for nonzero values of f-level energy Ef revealed that this model can yield a reasonable explanation for the pressure-induced resistivity anomaly observed experimentally in TmSe0.45Te0.55 compound.

  7. Coherent optical writing and reading of the exciton spin state in single quantum dots

    CERN Document Server

    Benny, Y; Kodriano, Y; Poem, E; Presman, R; Galushko, D; Petroff, P M; Gershoni, D

    2010-01-01

    We demonstrate a one to one correspondence between the polarization state of a light pulse tuned to excitonic resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one to one mapping between any point on the Poincare sphere of the light polarization to a point on the Bloch sphere of the exciton spin.

  8. Influence of lattice vibrations on luminescence and transfer of excitons in WS2 monolayer semiconductors

    Science.gov (United States)

    Wang, X. H.; Su, Z. C.; Ning, J. Q.; Wang, M. Z.; Xu, S. J.; Han, S.; Jia, F.; Zhu, D. L.; Lu, Y. M.

    2016-11-01

    Monolayers of transition metal dichalcogenides (TMDs) have been recently demonstrated to be a new family of direct bandgap semiconductors exhibiting extraordinary excitonic effects and high-efficiency luminescence. Here we present a micro-photoluminescence (PL) study on temperature dependent luminescence of excitons from an exfoliated WS2 monolayer. It is found that lattice vibrations (i.e. phonons) have a profound influence on the excitonic luminescence of the WS2 monolayer in several aspects including the spectral peak shift, lineshape broadening, transfer, and even formation entropy of excitons. Our study not only leads to the determination of the fundamental excitonic bandgap: {{E}\\text{g}}=2.061~ eV at T=0 \\text{K} , but also reveals that 120 K is a ‘turning’ temperature for the competition and formation entropy of free excitons and defect-bound excitons in the studied 2D WS2 crystals.

  9. Exciton transport in thin-film cyanine dye J-aggregates

    CERN Document Server

    Valleau, Stéphanie; Yung, Man-Hong; Aspuru-Guzik, Alán

    2012-01-01

    We present a theoretical model for the study of exciton dynamics in J-aggregated monolayers of fluorescent dyes. The excitonic evolution is described by a Monte-Carlo wave function approach which allows for a unified description of the quantum (ballistic) and classical (diffusive) propagation of an exciton on a lattice in different parameter regimes. The transition between the ballistic and diffusive regime is controlled by static and dynamic disorder. As an example, the model is applied to three cyanine dye J-aggregates: TC, TDBC, and U3. Each of the molecule-specific structure and excitation parameters are estimated using time-dependent density functional theory. The exciton diffusion coefficients are calculated and analyzed for different degrees of film disorder and are correlated to the physical properties and the structural arrangement of molecules in the aggregates. Further, exciton transport is anisotropic and dependent on the initial exciton energy. The upper-bound estimation of the exciton diffusion ...

  10. Quantum many-body simulation using monolayer exciton-polaritons in coupled-cavities.

    Science.gov (United States)

    Wang, Hai-Xiao; Zhan, Alan; Xu, Yadong; Chen, Huanyang; You, Wen-Long; Majumdar, Arka; Jiang, JianHua

    2017-08-30

    Quantum simulation is a promising approach to understand complex strongly correlated many-body systems using relatively simple and tractable systems. Photon-based quantum simulators have great advantages due to the possibility of direct measurements of multi-particle correlations and ease of simulating non-equilibrium physics. However, interparticle interaction in existing photonic systems is often too weak limiting the potential of quantum simulation. Here we propose an approach to enhance the interparticle interaction using exciton-polaritons in MoS$_2$ monolayer quantum-dots embedded in 2D photonic crystal microcavities. Realistic calculation yields optimal repulsive interaction in the range of $1$-$10$~meV --- more than an order of magnitude greater than the state-of-art value. Such strong repulsive interaction is found to emerge neither in the photon-blockade regime for small quantum dot nor in the polariton-blockade regime for large quantum dot, but in the crossover between the two regimes with a moderate quantum-dot radius around 20~nm. The optimal repulsive interaction is found to be largest in MoS$_2$ among commonly used optoelectronic materials. Quantum simulation of strongly correlated many-body systems in a finite chain of coupled cavities and its experimental signature are studied via exact diagonalization of the many-body Hamiltonian. A method to simulate 1D superlattices for interacting exciton-polariton gases in serially coupled cavities is also proposed. Realistic considerations on experimental realizations reveal advantages of transition metal dichalcogenide monolayer quantum-dots over conventional semiconductor quantum-emitters. © 2017 IOP Publishing Ltd.

  11. Non-polar In x Ga1-x N/GaN quantum dots: impact of dot size and shape anisotropies on excitonic and biexcitonic properties

    Science.gov (United States)

    Kanta Patra, Saroj; Schulz, Stefan

    2017-01-01

    In this work, we present a theoretical analysis of the built-in potential, the excitonic and biexcitonic properties of non-polar InGaN/GaN quantum dots by means of self-consistent Hartree calculations using \\mathbf{k}\\centerdot \\mathbf{p} theory. Special attention is paid to the impact of dot size and shape anisotropies on the results. Our calculations reveal that even though non-polar InGaN/GaN quantum dots exhibit strongly reduced built-in fields when compared to c-plane dots, the excitonic and biexcitonic properties are significantly affected by these residual fields. Furthermore, changes in the built-in field when the geometrical dot features are modified, result in an unusual variation of the exciton binding energy. All these findings highlight that the dot geometry significantly affects electronic and optical properties of non-polar InGaN/GaN systems. This is further supported by comparing our theoretical data with experimental literature results. Here, we analyze also trends in exciton and biexciton binding energies and discuss the potential use of non-polar InGaN/GaN dots for entangled photon emission via the time reordering scheme.

  12. Optical absorption and refraction index change of a confined exciton in a spherical quantum dot nanostructure

    Science.gov (United States)

    Mathan Kumar, K.; John Peter, A.; Lee, C. W.

    2011-12-01

    Electronic energies of an exciton confined in a strained Zn1- x Cd x Se/ZnSe quantum dot have been computed as a function of dot radius with various Cd content. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption coefficients and the refractive index changes between the ground state ( L = 0) and the first excited state ( L = 1) are investigated. It is found that the optical properties in the strained ZnCdSe/ZnSe quantum dot are strongly affected by the confinement potentials and the dot radii. The intensity of the total absorption spectra increases for the transition between higher levels. The obtained optical nonlinearity brings out the fact that it should be considered in calculating the optical properties in low dimensional semiconductors especially in quantum dots.

  13. Exponential Tails Near the Band Edges of a One-Dimensional Exciton System

    Science.gov (United States)

    Avgin, I.; Boukahil, A.; Zettili, N.; Huber, D. L.

    2003-03-01

    We report the results of studies of the tails near the band edges of a one-dimensional Frenkel exciton system in the Coherent Potential Approximation (CPA). A Gaussian distribution of the transition frequencies with rms width σ (0.1 <= σ <= 2.0) is used. We found that the tails obey two different exponential power laws depending on the value of σ. In the weak disorder limit 0.1 <= σ <= 0.5, the tails of the density of states and the absorption line shape behave like expk|E|^3/2/σ^2 and in the strong disorder limit 0.5 <= σ <= 2.0 the tails behave like exp|E|^2/2σ^2\\. Our CPA results are in excellent agreement with our simulation data for the density of states over the entire range 0.1 <= σ <= 2.0, and with previous investigations for weak disorder.

  14. Excitons in topological Kondo insulators: Theory of thermodynamic and transport anomalies in SmB_{6}.

    Science.gov (United States)

    Knolle, Johannes; Cooper, Nigel R

    2017-03-03

    Kondo insulating materials lie outside the usual dichotomy of weakly versus correlated-band versus Mott-insulators. They are metallic at high temperatures but resemble band insulators at low temperatures because of the opening of an interaction-induced band gap. The first discovered Kondo insulator (KI) SmB_{6} has been predicted to form a topological KI (TKI). However, since its discovery thermodynamic and transport anomalies have been observed that have defied a theoretical explanation. Enigmatic signatures of collective modes inside the charge gap are seen in specific heat, thermal transport, and quantum oscillation experiments in strong magnetic fields. Here, we show that TKIs are susceptible to the formation of excitons and magnetoexcitons. These charge neutral composite particles can account for long-standing anomalies in SmB_{6}.

  15. Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites

    KAUST Repository

    Yin, Jun

    2017-01-20

    We theoretically characterize the unusual white-light emission properties of two-dimensional (2D) hybrid organic inorganic perovskites with an APbX(4) structure (where A is a bidentate organic cation and X = Cl, Br). In addition to band structure calculations including corrections due to spin orbit couplings and electron hole interactions, a computationally intensive molecular cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level. Upon adding or removing an electron from the neutral systems, we find that strongly localized small polarons form in the 2D clusters. The polaron charge density is distributed over just lattice sites, which is consistent with the calculated large polaron binding energies, on the order of similar to 0.4-1.2 eV.

  16. Triplet excitons in natural photosynthetic and artificial light harvesting systems: Measurement and modeling

    Science.gov (United States)

    Hartzler, Daniel Allen

    Under full sunlight, unprotected (Bacterio)Chlorophyll ((B)Chl) molecules photodegrade in a matter of minutes. This is the result of the generation of highly reactive singlet oxygen (1O2) by energy transfer from the (B)Chl triplet state (3(B)Chl) to the oxygen ground state. Natural photosynthetic systems must protect themselves from 1O2, typically done by positioning carotenoids within a few angstroms of each (B)Chl molecule to quench 3(B)Chl states. Using phosphorescence spectroscopy and computational modeling, we investigated alternative, carotenoid independent, mechanisms which nature may employ to prevent 1O2 sensitization by lowering the energy of 3(B)Chl below that of 1O2. The two proposed triplet lowering mechanisms investigated were: triplet state lowering by strong pigment-pigment interactions (i.e. triplet exciton formation) and triplet state lowering by pigment-protein interactions. Possible natural examples employing these mechanisms are two structures found in green sulfur bacteria: the chlorosome (an antenna containing ~100000 coupled BChl c, d, or e molecules with unexpectedly high photostability) and the Fenna-Matthews-Olson (FMO) complex (an auxiliary antenna containing eight seemingly unprotected BChl a molecules). Measurements performed on linear aggregates of the dye perylene diimide (PDI) show that triplet exciton formation does reduce the triplet state energy. However, direct measurement of triplet state energies for the chlorosome and FMO complex proved experimentally difficult, thus an alternative approach was used to calculate these energies using empirical and excitonic models. Since the use of excitonic modeling requires knowledge of both the pigment site energies and the pigment-pigment interactions (i.e. couplings), work was performed to catalog the monomeric singlet and triplet state energies of all known natural (B)Chl pigments by direct measurement or computational modeling and to characterize the triplet-triplet (T-T) coupling in

  17. Two-dimensional spectroscopy of molecular excitons in a model dimer system

    Science.gov (United States)

    Halpin, Alexei

    The physics of molecular excitons has been the subject of many recent studies using electronic two-dimensional photon-echo spectroscopy (2DPE), particularly in the context of light harvesting in photosynthesis. Since the spectra for multichromophoric aggregates are congested, particularly so at room temperature, we present a study of a model dimer comprised of identical chromophores with a well defined electronic coupling strength, to provide clear signatures for coherences between vibronic excitons in 2D spectra. We begin by describing the design of a broadband passively phase-stabilized interferometer for collection of 2D spectra, which also allows for the investigation of state preparation in 2D spectroscopy by using shaped excitation pulses. In experiments on the model dimer we observe strong oscillating off-diagonal features in the 2D spectra which are present only before the onset of dephasing, which occurs in less than 100 fs due to strong system-bath coupling. This is in contrast with the parent dye, where low amplitude oscillations associated with Raman active vibrations persist for several ps following excitation. The results of this comparative study indicate that the signals observed earlier in photosynthetic proteins likely reflect vibrational motion in isolated pigments, and not delocalized quantum coherence. While long-lived vibrational coherences are of questionable biological relevance at face value, we conclude with a discussion on initial findings using coherently controlled 2D spectroscopy, where we observe long-lived signatures associated to vibronic coherences at room temperature. These results point to new directions of study using multidimensional spectroscopy to unravel the role of coherence in excitation energy transfer in molecular aggregates in an experimentally direct fashion.

  18. Communication: Exciton analysis in time-dependent density functional theory: How functionals shape excited-state characters.

    Science.gov (United States)

    Mewes, Stefanie A; Plasser, Felix; Dreuw, Andreas

    2015-11-01

    Excited-state descriptors based on the one-particle transition density matrix referring to the exciton picture have been implemented for time-dependent density functional theory. State characters such as local, extended ππ(∗), Rydberg, or charge transfer can be intuitively classified by simple comparison of these descriptors. Strong effects of the choice of the exchange-correlation kernel on the physical nature of excited states can be found and decomposed in detail leading to a new perspective on functional performance and the design of new functionals.

  19. GPS weak signal detection method when strong and weak signals coexist%一种强弱信号并存时的GPS弱信号检测方法

    Institute of Scientific and Technical Information of China (English)

    秦红磊; 孙兴丽; 丛丽; 薛瑞

    2013-01-01

    To detect weak GPS satellite signal,generally,the method is used of increase the Pre-detection integration time (PIT),and each satellite uses the same PIT.In the actual environment,in many cases the strong signal and weak signal coexist,when the same PIT is used,the efficiency of the GPS signal to be detected is reduced.This paper proposed a variable threshold method for the weak GPS signal detection,i.e.first strong signals were detected using the shorter PIT,and then using longer PIT detect weak signals.By analysis detection probability,false alarm probability,leakage alarm probability and signal testing have proved that this method can greatly improve the GPS signal detection efficiency of the coexistence of the strong and weak signal.%对于微弱GPS卫星信号的检测,一般采用增加预检测积分时间(PIT)的方法,每颗卫星采用相同的PIT.在实际环境中,很多情况是强弱信号并存的,采用相同的PIT会降低GPS信号检测的效率.为此,提出了一种可变门限的微弱GPS信号检测方法,即先采用较短的PIT对强信号进行检测,再采用较长的PIT对弱信号进行检测.通过对检测概率、虚警概率和漏警概率的分析和实际信号的测试验证,证明该方法可大大提高在强弱信号并存时GPS信号的检测效率.

  20. Evaluation of defects in cuprous oxide through exciton luminescence imaging

    Energy Technology Data Exchange (ETDEWEB)

    Frazer, Laszlo, E-mail: jl@laszlofrazer.com [Department of Physics, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Lenferink, Erik J. [Department of Physics, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Chang, Kelvin B. [Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Poeppelmeier, Kenneth R. [Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (United States); Stern, Nathaniel P. [Department of Physics, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Ketterson, John B. [Department of Physics, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Department of Electrical Engineering and Computer Science, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States)

    2015-03-15

    The various decay mechanisms of excitons in cuprous oxide (Cu{sub 2}O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton luminescence from cuprous oxide crystals grown via the floating zone method showing that the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation of oxygen and copper vacancies, which leads to a negative correlation between luminescence of unlike vacancies. - Highlights: • We use luminescence to observe defects in high quality cuprous oxide crystals. • Strain is reduced by annealing. • Annealing causes annihilation of oxygen and copper vacancies.

  1. Collective oscillations in spatially modulated exciton-polariton condensate arrays

    Science.gov (United States)

    Tikhomirov, Andrey A.; Kanakov, Oleg I.; Altshuler, Boris L.; Ivanchenko, Mikhail V.

    2015-02-01

    We study collective dynamics of interacting centers of exciton-polariton condensation in presence of spatial inhomogeneity, as modeled by diatomic active oscillator lattices. The mode formalism is developed and employed to derive existence and stability criteria of plane wave solutions. It is demonstrated that k0 = 0 wave number mode with the binary elementary cell on a diatomic lattice possesses superior existence and stability properties. Decreasing net on-site losses (balance of dissipation and pumping) or conservative nonlinearity favors multistability of modes, while increasing frequency mismatch between adjacent oscillators detriments it. On the other hand, spatial inhomogeneity may recover stability of modes at high nonlinearities. Entering the region where all single-mode solutions are unstable we discover subsequent transitions between localized quasiperiodic, chaotic and global chaotic dynamics in the mode space, as nonlinearity increases. Importantly, the last transition evokes the loss of synchronization. These effects may determine lasing dynamics of interacting exciton-polariton condensation centers.

  2. Charge transfer excitons in C60-dimers and polymers

    CERN Document Server

    Harigaya, K

    1996-01-01

    Charge-transfer (CT) exciton effects are investigated for the optical absorption spectra of crosslinked C60 systems by using the intermediate exciton theory. We consider the C60-dimers, and the two (and three) molecule systems of the C60-polymers. We use a tight-binding model with long-range Coulomb interactions among electrons, and the model is treated by the Hartree-Fock approximation followed by the single-excitation configuration interaction method. We discuss the variations in the optical spectra by changing the conjugation parameter between molecules. We find that the total CT-component increases in smaller conjugations, and saturates at the intermediate conjugations. It decreases in the large conjugations. We also find that the CT-components of the doped systems are smaller than those of the neutral systems, indicating that the electron-hole distance becomes shorter in the doped C60-polymers.

  3. Excitonic Absorption of Semiconductor Nanorings under Terahertz Fields

    Institute of Scientific and Technical Information of China (English)

    ZHANG Tong-Yi; ZHAO Wei; ZHU Shao-Lan

    2005-01-01

    @@ The optical absorption of GaAs nanorings (NRs) under a dc electric field and a terahertz (THz) ac electric field applied in the plane containing the NRs is investigated theoretically. The NRs may enclose some magnetic flux in the presence of a magnetic field perpendicular to the NRs plane. Numerical calculation shows that the excitonic effects are essential to correctly describe the optical absorption in NRs. The applied lateral THz electric field, as well as the dc field leads to reduction, broadening and splitting of the exciton peak. In contrast to the presence of a dc field, significant optical absorption peak arises below the zero-field bandgap in the presence of a THz electric field at a certain frequency. The optical absorption spectrum depends evidently on the frequency and amplitude of the applied THz field and on the magnetic flux threading the NRs. This promises potential applications of NRs for magneto-optical and THz electro-optical sensing.

  4. Exciton and Biexciton Binding Energies in Rectangular Quantum Dots

    Institute of Scientific and Technical Information of China (English)

    LIU Yong-Hui; KONG Xiao-Jun

    2005-01-01

    @@ In the effective mass approximation, using the variational technology and a method of expanding the wavefunctions of exciton in terms of the eigenfunctions of the noninteracting electron-hole system, we calculate the exciton and biexciton ground state binding energies for rectangular quantum dots (QDs). In the calculation, a three-dimensional Fourier expansion of Coulomb potential is used to remove the numerical difficulty with the 1/r singularity, and it considerably reduces the computational effort. Our results agree fairly well with the previous results. It is found that the binding energies are highly correlated to the size of QDs. The quantum confinement effect of spherical QDs about biexciton is obviously larger than that of rectangular QDs when the well width is narrower than 2.0aB.

  5. Interaction of excitons with optical phonons in layer crystals

    Science.gov (United States)

    Nitsovich, Bohdan M.; Zenkova, C. Y.; Kramar, N. K.

    2002-02-01

    The investigation is concerned with layer crystals of the GaSe, InSe, GaTe, MoS2-type and other inorganic semiconductors, whose phonon spectrum has a great number of peculiarities, among them the availability of low-energy optical phonons. In this case the dispersion of these phonons can be essential and vary in character. The mass operator of the exciton-phonon system and the light absorption coefficient for different dispersion laws of optical phonons have been calculated. The influence of the sign of the phonon 'effective mass' on the exciton absorption band of layer crystals, which causes the opposite in sign dynamics of the absorption maximum shift, and the change of the absorption curve asymmetry have been determined.

  6. Chemical potential and compressibility of quantum Hall bilayer excitons,.

    Energy Technology Data Exchange (ETDEWEB)

    Skinner, Brian

    2016-02-25

    I consider a system of two parallel quantum Hall layers with total filling factor 0 or 1. When the distance between the layers is small enough, electrons and holes in opposite layers can form inter-layer excitons, which have a finite effective mass and interact via a dipole-dipole potential. I present results for the chemical potential u of the resulting bosonic system as a function of the exciton concentration n and the interlayer separation d. I show that both u and the interlayer capacitance have an unusual nonmonotonic dependence on d, owing to the interplay between an increasing dipole moment and an increasing effective mass with increasing d. Finally, I discuss the transition between the superfluid and Wigner crystal phases, which is shown to occur at d x n-1/10. Results are derived first via simple intuitive arguments, and then verified with more careful analytic derivations and numeric calculations.

  7. Diffusive Propagation of Exciton-Polaritons through Thin Crystal Slabs.

    Science.gov (United States)

    Zaitsev, D A; Il'ynskaya, N D; Koudinov, A V; Poletaev, N K; Nikitina, E V; Egorov, A Yu; Kavokin, A V; Seisyan, R P

    2015-06-19

    If light beam propagates through matter containing point impurity centers, the amount of energy absorbed by the media is expected to be either independent of the impurity concentration N or proportional to N, corresponding to the intrinsic absorption or impurity absorption, respectively. Comparative studies of the resonant transmission of light in the vicinity of exciton resonances measured for 15 few-micron GaAs crystal slabs with different values of N, reveal a surprising tendency. While N spans almost five decimal orders of magnitude, the normalized spectrally-integrated absorption of light scales with the impurity concentration as N(1/6). We show analytically that this dependence is a signature of the diffusive mechanism of propagation of exciton-polaritons in a semiconductor.

  8. Exciton localization in solution-processed organolead trihalide perovskites

    Science.gov (United States)

    He, Haiping; Yu, Qianqian; Li, Hui; Li, Jing; Si, Junjie; Jin, Yizheng; Wang, Nana; Wang, Jianpu; He, Jingwen; Wang, Xinke; Zhang, Yan; Ye, Zhizhen

    2016-03-01

    Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium-lead-halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

  9. Exciton lifetime measurements on single silicon quantum dots.

    Science.gov (United States)

    Sangghaleh, Fatemeh; Bruhn, Benjamin; Schmidt, Torsten; Linnros, Jan

    2013-06-01

    We measured the exciton lifetime of single silicon quantum dots, fabricated by electron beam lithography, reactive ion etching and oxidation. The observed photoluminescence decays are of mono-exponential character with a large variation (5-45 μs) from dot to dot, even for the same emission energy. We show that this lifetime variation may be the origin of the heavily debated non-exponential (stretched) decays typically observed for ensemble measurements.

  10. Exciton-mediated photothermal cooling in GaAs membranes

    CERN Document Server

    Xuereb, André; Naesby, Andreas; Polzik, Eugene S; Hammerer, Klemens

    2012-01-01

    Cooling of the mechanical motion of a GaAs nano-membrane using the photothermal effect mediated by excitons was recently demonstrated by some of us [K. Usami, et al., Nature Phys. 8, 168 (2012)] and provides a clear example of the use of thermal forces to cool down mechanical motion. Here, we report on a single-free-parameter theoretical model to explain the results of this experiment which matches the experimental data remarkably well.

  11. Lindblad theory of dynamical decoherence of quantum-dot excitons

    OpenAIRE

    Eastham, P. R.; Spracklen, A O; Keeling, Jonathan Mark James

    2013-01-01

    We use the Bloch-Redfield-Wangsness theory to calculate the effects of acoustic phonons in coherent control experiments where quantum-dot excitons are driven by shaped laser pulses. This theory yields a generalized Lindblad equation for the density operator of the dot, with time-dependent damping and decoherence due to phonon transitions between the instantaneous dressed states. It captures similar physics to the form recently applied to Rabi oscillation experiments [Ramsay et al., Phys. Rev....

  12. Exciton spin dynamics in ZnO epilayers

    Energy Technology Data Exchange (ETDEWEB)

    Lagarde, D.; Lombez, L.; Balocchi, A.; Renucci, P.; Carrere, H.; Amand, T.; Marie, X. [Laboratoire de Nanophysique, Magnetisme et Optoelectronique, INSA, 135 avenue de Rangueil, 31077 Toulouse Cedex 4 (France); Mei, Z.X.; Du, X.L.; Xue, Q.K. [Institute of Physics, Chinese Academy of Sciences and National Center for Nano-Science and Technology, Beijing 100080 (China)

    2007-07-01

    We used time-resolved optical orientation experiments to study the low temperature spin dynamics of a ZnO epilayer. The sample shows a circular polarisation of the donor-bound exciton of 11% with a decay time of 275 ps. A very narrow spectral dependence of the initial polarisation and a rapid decrease of the polarisation decay time with temperature are also observed. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  13. 强信号覆盖下弱信号检测算法性能分析%Performance Analysis of Detection Algorithm for Weak Signals Covered by Strong Signal

    Institute of Scientific and Technical Information of China (English)

    瞿孟虹; 何晓霜; 游凌

    2013-01-01

    Detection of weak signals covered by strong signal is an important problem in many fields including spectrum surveillance, threat assessment, jamming detection. In order to improve the detection algorithm and detection performance of weak signals covered by strong signal, the property of detection algorithm is studied, and the quantitative analysis also done on the unkown parameters, including signal noise ratio of strong signal, power ratio between strong and weak signals, timing error. Then by optimizing critical parameters of the algorithm, a modified algorithm is proposed, which could effecitvely suppress false alarming and raise the detection probility of week signals. Simulation indicates that the analyis is correct and the modified algorithm is feasible and effective.%  强信号覆盖下,弱信号存在性问题一直是频谱监视、威胁评估、干扰检测领域的重要研究内容。为了有针对性地改进基于峰度特性的弱信号检测算法,提高弱信号检测性能。通过定量分析强信号信噪比、强弱信号功率比、定时偏差对检测算法性能的影响,优化影响算法性能的关键参数实现途径,给出一种既能有效抑制虚警又能提高弱信号检测概率的修正算法。计算机仿真结果验证了分析结论的正确性及修正算法的有效性。

  14. 1H NMR detection of immobilized water molecules within a strong distal hydrogen-bonding network of substrate-bound human heme oxygenase-1.

    Science.gov (United States)

    Syvitski, Ray T; Li, Yiming; Auclair, Karine; Ortiz De Montellano, Paul R; La Mar, Gerd N

    2002-12-04

    Solution 1H NMR is used to probe the environments of the donor protons of eight strong hydrogen bonds on the distal side of the heme substrate in the cyanide-inhibited, substrate-bound complex of human heme oxygenase, hHO. It is demonstrated that significant magnetization transfer from the bulk water signal to the eight labile protons does not result from chemical exchange, but from direct nuclear Overhauser effect due to the dipolar interaction of these labile protons with "ordered" water molecules. The enzyme labile proton to water proton distances are estimated at approximately 3 A. It is proposed that the role of the strong hydrogen-bonding network is to immobilize numerous water molecules which both stabilize the activated hydroperoxy species and funnel protons to the active site.

  15. Role of strain on the coherent properties of GaAs excitons and biexcitons

    Science.gov (United States)

    Wilmer, Brian L.; Webber, Daniel; Ashley, Joseph M.; Hall, Kimberley C.; Bristow, Alan D.

    2016-08-01

    Polarization-dependent two-dimensional Fourier-transform spectroscopy (2DFTS) is performed on excitons in strained bulk GaAs layers, probing the coherent response for differing amounts of strain. Uniaxial tensile strain lifts the degeneracy of heavy-hole (HH) and light-hole (LH) valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the HH/LH exciton peak splitting, induces an asymmetry in the off-diagonal interaction coherences, increases the difference in the HH and LH exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound LH, HH, and mixed biexcitons.

  16. Role of Strain on the Coherent Properties of GaAs Excitons and Biexcitons

    CERN Document Server

    Wilmer, Brian L; Ashley, Joseph M; Hall, Kimberley C; Bristow, Alan D

    2016-01-01

    Polarization-dependent two-dimensional Fourier-transform spectroscopy (2DFTS) is performed on excitons in strained bulk GaAs layers probing the coherent response for differing amounts of strain. Biaxial tensile strain lifts the degeneracy of heavy-hole (HH) and light-hole (LH) valence states, leading to an observed splitting of the associated excitons at low temperature. Increasing the strain increases the magnitude of the HH/LH exciton peak splitting, induces an asymmetry in the off-diagonal coherences, increases the difference in the HH and LH exciton homogenous linewidths, and increases the inhomogeneous broadening of both exciton species. All results arise from strain-induced variations in the local electronic environment, which is not uniform along the growth direction of the thin layers. For cross-linear polarized excitation, wherein excitonic signals give way to biexcitonic signals, the high-strain sample shows evidence of bound LH, HH, and mixed biexcitons.

  17. The mechanism of long-range exciton diffusion in a nematically organized porphyrin layer.

    Science.gov (United States)

    Huijser, Annemarie; Savenije, Tom J; Meskers, Stefan C J; Vermeulen, Martien J W; Siebbeles, Laurens D A

    2008-09-17

    The exciton diffusion length in a nematically organized meso-tetra(4-n-butylphenyl)porphyrin (TnBuPP) layer was found to exceed 40 nm at a temperature of 90 K and to be equal to 22 +/- 3 nm at 300 K. The exciton diffusion coefficient decreases from > or = 3.1 x 10(-6) m(2)/s at 90 K to (2.5 +/- 0.5) x 10(-7) m(2)/s at 300 K. This thermal deactivation is attributed to exciton motion via a band mechanism. The motion of an exciton is not limited by polaronic effects; that is, the deformation of the atomic lattice around the exciton. The absence of polaronic self-trapping implies that the exciton diffusion coefficient can be enhanced by improvement of structural order and rigidity of the material.

  18. Theory for electric dipole superconductivity with an application for bilayer excitons.

    Science.gov (United States)

    Jiang, Qing-Dong; Bao, Zhi-qiang; Sun, Qing-Feng; Xie, X C

    2015-07-08

    Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we derive the London-type and Ginzburg-Landau-type equations for the electric dipole superconductors. By using these equations, we discover the Meissner-type effect and the electric dipole current Josephson effect. These effects can provide direct evidence for the formation of the exciton superfluid state in bilayer systems and pave new ways to drive an electric dipole current.

  19. Excitons in conjugated polymers: Do we need a paradigma change?

    Energy Technology Data Exchange (ETDEWEB)

    Beenken, Wichard J.D. [Department of Theoretical Physics I, Ilmenau University of Thechnology (Germany)

    2009-12-15

    We have previously shown that both, polymer conformation and dynamics are crucial for the exciton transport in conjugated polymers. Thereby we found that the usual Foerster-type hopping transfer model - even if one applies the line-dipole approximation - falls short in one crucial aspect: the nature of the sites the excitons are transferred between is still unclear. We found that the simple model of spectroscopic units defined as segments of the polymer chains separated by structural defects breaking the {pi}-conjugation is only justified for chemical defects like hydrogenated double bonds, or extreme gauche (90 ) torsions between the monomers. Both defects are far too rare in a well-prepared conjugated polymer to explain the mean spectroscopic-unit length of typically 6-7 monomers. Meanwhile, also the concept of dynamical formation of the spectroscopic units, we had previously suggested, has also failed. Thus the question of a paradigma change concerning the exciton transport in conjugated polymers appears on the agenda. (Abstract Copyright [2009], Wiley Periodicals, Inc.)

  20. Photoluminescence of localized excitons in InGan quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Usov, S. O., E-mail: S.Usov@mail.ioffe.ru; Tsatsul' nikov, A. F.; Lundin, V. V.; Sakharov, A. V.; Zavarin, E. E.; Ledentsov, N. N. [Russian Academy of Sciences, Ioffe Physicotechnical Institute (Russian Federation)

    2008-02-15

    Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev's model and that, in the active region of the structures under study, the dispersion {sigma} of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy E{sub U}, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that {sigma} and E{sub U}, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.

  1. Electronic absorption of Frenkel excitons in topologically disordered systems

    Science.gov (United States)

    Schweizer, Kenneth S.

    1986-10-01

    A self-consistent effective medium theory of the electronic absorption spectra of tightly bound dipolar excitons in simple fluids is developed within the adiabatic picture. The theoretical approach is based on the isomorphism between the path-integral formulation of quantum theory and classical statistical mechanics and is an extension of previous work [D. Chandler, K. S. Schweizer, and P. G. Wolynes, Phys. Rev. Lett. 49, 1100 (1982)]. The consequences of fluid structural disorder on resonant excitation transfer and the statistical fluctuations of single molecule energy levels are simultaneously treated. Detailed numerical calculations are performed to establish the dependence of the absorption spectrum on fluid density, short range order, and the relative magnitude of the resonant transfer vs the single site disorder. The density dependence of the spectral features are found to be a sensitive function of fluid structure and the relative strength of the localizing vs the delocalizing interactions. By comparing the liquid state results with the corresponding crystalline solid behavior, the consequences of topological disorder on the exciton spectrum are identified. The relevance of the theoretical predictions to spectroscopic probes of exciton delocalization in molecular liquids and glasses is discussed.

  2. The nature of singlet excitons in oligoacene molecular crystals

    KAUST Repository

    Yamagata, H.

    2011-01-01

    A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32cm-1, far smaller than the measured value of 631cm-1 and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601cm-1 and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport. © 2011 American Institute of Physics.

  3. Temperature dependence of excitonic transition in ZnSe/ZnCdSe quantum wells

    Institute of Scientific and Technical Information of China (English)

    GUO Zi-zheng; LIANG Xi-xia; BAN Shi-liang

    2005-01-01

    A theoretical calculation for the temperature dependence of the excitonic transition in ZnSe/ZnCdSe quantum wells is performed. The exciton binding energy is calculated with a variational technique by considering the temperature-dependence parameters. Our results show that the exciton binding energy reduces linearly with temperature increasing. We find that the strain due to lattice mismatch and differential thermal expansion decreases with the temperature increasing.

  4. Collective Behavior of a Spin-Aligned Gas of Interwell Excitons in Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Bayer, M.; Hvam, Jørn Märcher;

    2005-01-01

    The kinetics of a spin-aligned gas of interwell excitons in GaAs/AlGaAs double quantum wells (n–i–n heterostructure) is studied. The temperature dependence of the spin relaxation time for excitons, in which a photoexcited electron and hole are spatially separated between two adjacent quantum wells...... is associated with indirect evidence of the coherence of the collective phase of interwell excitons at temperatures below the critical value....

  5. Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites

    Science.gov (United States)

    Abdel-Baki, K.; Boitier, F.; Diab, H.; Lanty, G.; Jemli, K.; Lédée, F.; Garrot, D.; Deleporte, E.; Lauret, J. S.

    2016-02-01

    Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites has emerged for few years. When they are arranged in two dimensions, these materials can be considered as hybrid quantum wells. One consequence of the unique structure of 2D hybrid organic perovskites is a huge exciton binding energy that can be tailored through chemical engineering. We present experimental investigations of the exciton non-linearities by means of femtosecond pump-probe spectroscopy. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ˜100 ps. Moreover, an ultrafast intraband relaxation (energy.

  6. Dephasing in the quasi-two-dimensional exciton-biexciton system

    DEFF Research Database (Denmark)

    Langbein, Wolfgang Werner; Hvam, Jørn Märcher

    2000-01-01

    The polarization decay in the exciton-biexciton system of a homogeneously broadened single quantum well is studied by transient four-wave mixing. All three decay rates in the exciton-biexciton three-level system are deduced. The relation between the rates unravels correlations between scattering...... processes of excitons and biexcitons. Density and temperature dependences show that the involved processes are mainly radiative decay and phonon scattering. The radiative decay rate of the biexcitons is found to be comparable to the one of the excitons, and the involved spontaneous photon emissions from...

  7. Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells

    KAUST Repository

    Dimitrov, Stoichko

    2016-01-13

    The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

  8. On the binding energies of excitons in polar quantum well structures in a weak electric field

    Institute of Scientific and Technical Information of China (English)

    Wu Yun-Feng; Liang Xi-Xia; K. K. Bajaj

    2005-01-01

    The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for Ⅲ-Ⅴ and Ⅱ-Ⅵ compound semiconductor quantum well structures have been numerically computed.The results for GaAs/AlGaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton-phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.

  9. Efficient Exciton Diffusion and Resonance-Energy Transfer in Multi-Layered Organic Epitaxial Nanofibers

    DEFF Research Database (Denmark)

    Tavares, Luciana; Cadelano, Michele; Quochi, Francesco

    2015-01-01

    ) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multi-layered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T), serving as exciton donor and acceptor material, respectively. The high probability for RET processes...... is confirmed by Quantum Chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P...

  10. Direct observation of free-exciton thermalization in quantum-well structures

    DEFF Research Database (Denmark)

    Umlauff, M.; Hoffmann, J.; Kalt, H.

    1998-01-01

    We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum stales with picosecond laser pulses. The subs......We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum stales with picosecond laser pulses...

  11. Exciton binding energy in GaAsBiN spherical quantum dot heterostructures

    Science.gov (United States)

    Das, Subhasis; Dhar, S.

    2017-03-01

    The ground state exciton binding energies (EBE) of heavy hole excitons in GaAs1-x-yBixNy - GaAs spherical quantum dots (QD) are calculated using a variational approach under 1s hydrogenic wavefunctions within the framework of effective mass approximation. Both the nitrogen and the bismuth content in the material are found to affect the binding energy, in particular for larger nitrogen content and lower dot radii. Calculations also show that the ground state exciton binding energies of heavy holes increase more at smaller dot sizes as compared to that for the light hole excitons.

  12. Effects of Electric and Magnetic Fields on Pure Dephasing of Exciton Qubits

    Institute of Scientific and Technical Information of China (English)

    LIU Yun-Fei; XIAO Jing-Lin

    2009-01-01

    In a two-dimensional quantum dot (QD) with parabolic confinement potential, we investigate pure dephasing due to deformation potential exciton-bulk longitudinal acoustic phonons (LAP) interaction for exciton qubits under the influence of external static electric and magnetic fields by adopting the full quantum-mechanical method of Kunihiro Kojima and Akihisa Tomita. The wave function is found and the dependence of the pure dephusing factor on the confinement length of the QD and time and temperature is discussed. We find the external electric and magnetic fields have important effects on pure dephasing of exciton qubits because exciton-LAP interaction increases, leading to more pure dephasing.

  13. Spatial mapping of exciton lifetimes in single ZnO nanowires

    Directory of Open Access Journals (Sweden)

    J. S. Reparaz

    2013-07-01

    Full Text Available We investigate the spatial dependence of the exciton lifetimes in single ZnO nanowires. We have found that the free exciton and bound exciton lifetimes exhibit a maximum at the center of nanowires, while they decrease by 30% towards the tips. This dependence is explained by considering the cavity-like properties of the nanowires in combination with the Purcell effect. We show that the lifetime of the bound-excitons scales with the localization energy to the power of 3/2, which validates the model of Rashba and Gurgenishvili at the nanoscale.

  14. Excitonic optical nonlinearities and transport in the layered compound semiconductor GaSe

    DEFF Research Database (Denmark)

    Mizeikis, V.; Vadim, Lyssenko; Østergaard, John Erland;

    1995-01-01

    Dephasing and transient grating experiments in the direct excitonic absorption region of GaSe at low temperatures show that a fast relaxation within the one-dimensionally disordered excitonic band results in band filling being the dominant mechanism of the optical nonlinearity. Correspondingly, we...... observe a blueshift of the nonlinear signal with excitation density. The temperature dependence of the exciton diffusion constant measured in directions parallel to the GaSe layer planes indicates that temperature-independent scattering (trapping) and scattering by acoustic phonons determine the exciton...

  15. Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells

    Directory of Open Access Journals (Sweden)

    Stoichko D. Dimitrov

    2016-01-01

    Full Text Available The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

  16. Anomalous Photoluminescence of Weakly Confined Excitons including Radiative Correction in Nano-to-Bulk Crossover Regime

    Science.gov (United States)

    Matsuda, Takuya; Yokoshi, Nobuhiko; Ishihara, Hajime

    2015-06-01

    We develop a theoretical formalism to calculate photoluminescence (PL) spectrum of weakly confined excitons incorporating the microscopic nonlocal optical response. The nonlocality is caused by the center-of-mass (c. m.) motion of exciton and becomes remarkable in nano-to-bulk crossover regime. The theory successfully explains the characteristics of recently observed peculiar PL spectra in high quality CuCl films [5], wherein the signals appear at the exciton states with the very large radiative corrections not only for the lowest level but also for the higher ones including non-dipole types of excitons.

  17. On the kinetics and thermodynamics of excitons at the surface of semiconductor nanocrystals: Are there surface excitons?

    Energy Technology Data Exchange (ETDEWEB)

    Kambhampati, Patanjali, E-mail: pat.kambhampati@mcgill.ca

    2015-01-13

    Highlights: • The surface of semiconductor nanocrystals is one of their defining features. • The kinetics of surface trapping can be monitored by pump/probe spectroscopy. • The thermodynamics of surface trapping is revealed by photoluminescence spectroscopy. • We produce the first microscopic picture of how excitons are coupled to the surface. • We discuss the possibility of surface excitons in nanocrystals. - Abstract: The surface of semiconductor nanocrystals is one of their defining features by virtue of their nanometer size. Yet the surface is presently among the most poorly understood aspects of nanocrystal science. This perspective provides an overview of spectroscopic work that has revealed the first insights into the nature of the surface, focusing upon CdSe nanocrystals. We focus on two aspects of surface processes in nanocrystals: the kinetics of surface trapping and the thermodynamics of core/surface equilibria. We describe femtosecond pump/probe spectroscopic experiments which reveal the signatures of carrier trapping at the surface. We also describe temperature dependent steady-state photoluminescence experiments which reveal new aspects of the surface. This work suggest that the surface emission is largely driven by homogeneous broadening via phonon progressions. The implications are that the surface electronic state bears similarity to the quantized excitonic core of the nanocrystal.

  18. B fields in OB stars (BOB): Detection of a strong magnetic field in the O9.7 V star HD54879

    CERN Document Server

    Castro, N; Hubrig, S; Simón-Díaz, S; Schöller, M; Ilyin, I; Carrol, T A; Langer, N; Morel, T; Schneider, F R N; Przybilla, N; Herrero, A; de Koter, A; Oskinova, L M; Reisenegger, A; Sana, H

    2015-01-01

    The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code FASTWIND results in an effective temperature and a surface gravity of 33000$\\pm1000$ K and 4.0$\\pm0.1$ dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible ...

  19. Exciton optical absorption coefficients and refractive index changes in a strained InAs/GaAs quantum wire: The effect of the magnetic field

    Energy Technology Data Exchange (ETDEWEB)

    Arunachalam, N. [Department of Physics, Government Higher Secondary School, Saruguvalayapatti, Melur 625109, Madurai (India); John Peter, A., E-mail: a.john.peter@gmail.com [Center for Environmental Studies/Green Energy Center, Department of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do 446-701 (Korea, Republic of); Yoo, Chang Kyoo [Center for Environmental Studies/Green Energy Center, Department of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do 446-701 (Korea, Republic of)

    2012-06-15

    Magnetic field induced exciton binding energy is investigated in a strained InAs/GaAs quantum wire within the framework of single band effective mass approximation. The strain contribution to the potential is determined through deformation potentials. The interband emission energy of strained InAs/GaAs wire is investigated in the influence of magnetic field with the various structural parameters. Magnetic field induced photoionization cross section of the exciton is studied. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of magnetic field are analyzed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the magnetic field. The occurred blueshift of the resonant peak due to the magnetic field will give the information about the variation of two energy levels in the quantum well wire. The optical absorption coefficients and the refractive index changes are strongly dependent on the incident optical intensity and the magnetic field. - Highlights: Black-Right-Pointing-Pointer Magnetic field induced exciton binding energy is investigated in a strained InAs/GaAs quantum wire. Black-Right-Pointing-Pointer The strain contribution to the potential is determined through deformation potentials. Black-Right-Pointing-Pointer The interband emission energy of strained InAs/GaAs wire is investigated with the various structural parameters. Black-Right-Pointing-Pointer Magnetic field induced photoionization cross section of the exciton is studied. Black-Right-Pointing-Pointer The optical properties strongly depend on the incident optical intensity and the magnetic field.

  20. Excitonic energy transfer in polymer wrapped carbon nanotubes in gradually grown nanoassemblies.

    Science.gov (United States)

    Karachevtsev, Victor A; Plokhotnichenko, Alexander M; Glamazda, Alexander Yu; Leontiev, Victor S; Levitsky, Igor A

    2014-06-14

    We investigate the exciton energy transfer (ET) in nanoassemblies (nanotube based aggregates) formed by polymer wrapped single-walled carbon nanotubes (SWNTs) using photoluminescence (PL) spectroscopy and simulation. The distinctive feature of this study is the gradual growth of such nanostructures in aqueous medium induced by increasing the concentration of porphyrin molecules stitching nanotube-polymer complexes in densely packed assemblies. Experimental dependencies of PL intensity on the porphyrin concentration for different types of semiconducting SWNTs demonstrate step-like behavior controlled by the amount of bound nanotubes and are in good agreement with the simulating model. The simulation algorithm determines the criterion of the aggregate formation depending on the number of porphyrin molecules per tube and the cascade exciton energy transfer between neighboring semiconducting nanotubes of different chiralities. Aggregates of small sizes (up to six-eight individual tubes) contain mostly semiconducting species, while aggregates of a larger size (up to several tens of tubes) incorporate metallic SWNTs, inducing strong PL quenching. From the fitting procedure, an ET rate of 0.6 × 10(10) s(-1) has been determined which is consistent with the center to center distance (∼2.3 nm) between adjacent tubes separated by polymer and porphyrin molecules. The threshold of the dimer formation corresponds to one porphyrin molecule per ∼20 nm of tube lengths that was supported by molecular dynamics simulation. These findings provide insight into the ET mechanism in SWNT nanoassemblies of variable sizes, which can be gradually controlled by the external factor (the concentration of porphyrin molecules).