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Sample records for scanning molecular exciton

  1. Excitons in tubular molecular aggregates

    NARCIS (Netherlands)

    Didraga, C; Knoester, J

    2004-01-01

    We present a brief overview of recent work on the optical properties of molecular aggregates with a tubular (cylindrical) shape. The exciton states responsible for these properties can be distinguished with regard to a transverse wave number, which directly relates to optical selection rules and

  2. 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

  3. Exciton dynamics in perturbed vibronic molecular aggregates

    Directory of Open Access Journals (Sweden)

    C. Brüning

    2016-07-01

    Full Text Available A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states.

  4. 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...... times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient......, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions....

  5. 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.

  6. Ultrafast exciton dynamics at molecular surfaces

    Science.gov (United States)

    Monahan, Nicholas R.

    Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge

  7. Exciton-vibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

    Science.gov (United States)

    Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.

    2015-03-01

    The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be

  8. 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.

  9. Increasing applicability of slow light in molecular aggregate nanofilms with two-exciton dynamics.

    Science.gov (United States)

    Díaz, E; Martínez-Calzada, G C; Cabrera-Granado, E; Calderón, O G

    2016-06-01

    We study the slow-light performance in the presence of exciton-exciton interaction in films of linear molecular aggregates at the nanometer scale. In particular, we consider a four-level model to describe the creation/annihilation of two-exciton states that are relevant for high-intensity fields. Numerical simulations show delays comparable to those obtained for longer propagation distances in other media. Two-exciton dynamics could lead to larger fractional delays, even in presence of disorder, in comparison to the two-level approximation. We conclude that slow-light performance is a robust phenomenon in these systems under the increasing complexity of the two-exciton dynamics.

  10. Localized diabatization applied to excitons in molecular crystals

    Science.gov (United States)

    Jin, Zuxin; Subotnik, Joseph E.

    2017-06-01

    Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localized diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. These methods should be very useful for studying energy transfer through solids with ab initio calculations.

  11. Wannier-Mott Excitons in Nanoscale Molecular Ices

    Science.gov (United States)

    Chen, Y.-J.; Muñoz Caro, G. M.; Aparicio, S.; Jiménez-Escobar, A.; Lasne, J.; Rosu-Finsen, A.; McCoustra, M. R. S.; Cassidy, A. M.; Field, D.

    2017-10-01

    The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8 eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4 ×107 V m-1 , are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.

  12. Strong exciton-photon coupling in organic single crystal microcavity with high molecular orientation

    Energy Technology Data Exchange (ETDEWEB)

    Goto, Kaname [Department of Electronics, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan); Yamashita, Kenichi, E-mail: yamasita@kit.ac.jp [Faculty of Electrical Engineering and Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan); Yanagi, Hisao [Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan); Yamao, Takeshi; Hotta, Shu [Faculty of Materials Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)

    2016-08-08

    Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ∼100 meV even in the “half-vertical cavity surface emitting lasing” microcavity structure.

  13. Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

    DEFF Research Database (Denmark)

    Chang, J.F.; Clark, J.; Zhao, N.

    2006-01-01

    here a detailed study of interchain interaction effects on both charged polarons as well as neutral excitons in highly crystalline, high-mobility poly-3-hexylthiophene (P3HT) as a function of molecular weight. We find experimental evidence for reduced exciton bandwidth and increased polaron...

  14. Real-time observation of ultrafast Rabi oscillations between excitons and plasmons in metal/molecular aggregate hybrid nanostructures

    Directory of Open Access Journals (Sweden)

    Cerullo G.

    2013-03-01

    Full Text Available We demonstrate ultrafast coherent manipulation of the normal mode splitting in metal/molecular-aggregate nanostructures by real-time observation of Rabi oscillations between excitons and surface-plasmon-polaritons. Oscillations in exciton density on a 10-fs timescale control the Rabi splitting.

  15. Multiple exciton generation in quantum dots versus singlet fission in molecular chromophores for solar photon conversion.

    Science.gov (United States)

    Beard, Matthew C; Johnson, Justin C; Luther, Joseph M; Nozik, Arthur J

    2015-06-28

    Both multiple exciton generation (MEG) in semiconductor nanocrystals and singlet fission (SF) in molecular chromophores have the potential to greatly increase the power conversion efficiency of solar cells for the production of solar electricity (photovoltaics) and solar fuels (artificial photosynthesis) when used in solar photoconverters. MEG creates two or more excitons per absorbed photon, and SF produces two triplet states from a single singlet state. In both cases, multiple charge carriers from a single absorbed photon can be extracted from the cell and used to create higher power conversion efficiencies for a photovoltaic cell or a cell that produces solar fuels, like hydrogen from water splitting or reduced carbon fuels from carbon dioxide and water (analogous to biological photosynthesis). The similarities and differences in the mechanisms and photoconversion cell architectures between MEG and SF are discussed. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

  16. Dynamic coherence in excitonic molecular complexes under various excitation conditions

    Science.gov (United States)

    Chenu, Aurélia; Malý, Pavel; Mančal, Tomáš

    2014-08-01

    We investigate the relevance of dynamic quantum coherence in the energy transfer efficiency of molecular aggregates. We derive the time evolution of the density matrix for an open quantum system excited by light or by a neighboring antenna. Unlike in the classical case, the quantum description does not allow for a formal decomposition of the dynamics into sudden jumps in an observable quantity - an expectation value. Rather, there is a natural finite time-scale associated with the excitation process. We propose a simple experiment to test the influence of this time scale on the yield of photosynthesis. We demonstrate, using typical parameters of the Fenna-Matthews-Olson (FMO) complex and a typical energy transfer rate from the chlorosome baseplate, that dynamic coherences are averaged out in the complex even when the FMO model is completely free of all dissipation and dephasing.

  17. Dynamic coherence in excitonic molecular complexes under various excitation conditions

    Energy Technology Data Exchange (ETDEWEB)

    Chenu, Aurélia; Malý, Pavel; Mančal, Tomáš, E-mail: mancal@karlov.mff.cuni.cz

    2014-08-17

    Highlights: • Dynamic coherence does not improve energy transfer efficiency in natural conditions. • Photo-induced quantum jumps are discussed in classical context. • Natural time scale of a light excitation event is identified. • Coherence in FMO complex averages out under excitation by neighboring antenna. • This result is valid even in absence of dissipation. - Abstract: We investigate the relevance of dynamic quantum coherence in the energy transfer efficiency of molecular aggregates. We derive the time evolution of the density matrix for an open quantum system excited by light or by a neighboring antenna. Unlike in the classical case, the quantum description does not allow for a formal decomposition of the dynamics into sudden jumps in an observable quantity – an expectation value. Rather, there is a natural finite time-scale associated with the excitation process. We propose a simple experiment to test the influence of this time scale on the yield of photosynthesis. We demonstrate, using typical parameters of the Fenna–Matthews–Olson (FMO) complex and a typical energy transfer rate from the chlorosome baseplate, that dynamic coherences are averaged out in the complex even when the FMO model is completely free of all dissipation and dephasing.

  18. Molecular structure and exciton dynamics in organic conjugated polymers

    Science.gov (United States)

    Thomas, Alan K.

    , quenchable, isolated singlet excitations. The structure of J aggregates which leads to isolated excitations, and the role which inter-chain contact sites play in triplet formation from these singlet excitations is revealed. New structure-function relationships were uncovered in poly (3-alkyl-thienylenevinylene) (P3ATV) derivatives using resonance Raman and photocurrent spectroscopies. Time-dependent spectroscopic theory was used to interpret experimental Raman and absorption spectra that revealed the presence of structural polymorphs. These polymorphs provide an explanation of the spectroscopic evidence without presumption of a deactivating dark state in this unusually non-fluorescence material. Photovoltaic devices constructed from blends of poly (2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and PCBM blends were examined using Raman and photocurrent imaging techniques. These techniques were used to identify different packing states in blended thin films and correlate photocurrent production with local order. Intensity modulated spectroscopic techniques (IMPS) were then used to locate regions of non-geminate charge recombination at interfaces between amorphous and crystalline regions in working devices. Next, P3HT/PCBM OPV devices were exposed to ionizing radiation in a vacuum chamber. These devices were characterized before and after exposure, using standardized solar cell tests, Raman imaging, wide-field IMPS, and IMVS spectroscopies. An analysis of the spectroscopic data determined that the donor polymer is highly resistant to radiation damage, and that the degradation of device performance is due to an effect (cross-linking or degradation) within aggregates of the acceptor. This dissertation concludes with an interpretation of the significance of the findings contained herein to organic electronics, followed by a brief outlook for future work in these fields. Potential theories to describe and predict molecular interactions for organic polymers in

  19. Molecular dynamics simulations of a silver atom in water: evidence for a dipolar excitonic state.

    Science.gov (United States)

    Spezia, Riccardo; Nicolas, Cédric; Boutin, Anne; Vuilleumier, Rodolphe

    2003-11-14

    The properties of a silver atom in bulk water were studied for the first time by molecular dynamics simulations using two complementary mixed quantum-classical approaches. The first one consists of treating by quantum mechanics one electron only, which interacts with a classical silver cation and solvent through one-electron pseudopotentials. The second one is Car-Parrinello molecular dynamics that treats all the valence electrons quantum-mechanically. Very good agreement is obtained between these two methods, and the calculated absorption spectrum of the solvated silver atom agrees very well with experimental data. Both simulations reveal that the silver atom is in the critical region for the appearance of a dipolar excitonic state and exhibits a dipole moment of approximately 2 D with large fluctuations of +/-1 D. The structure of the solvation shell is also analyzed.

  20. A New Efficient Method for Calculation of Frenkel Exciton Parameters in Molecular Aggregates

    CERN Document Server

    Plötz, Per-Arno; Kühn, Oliver

    2013-01-01

    The Frenkel exciton Hamiltonian is at the heart of many simulations of excitation energy transfer in molecular aggregates. It separates the aggregate into Coulomb-coupled monomers. Here it is shown that the respective parameters, i.e. monomeric excitation energies and Coulomb couplings between transition densities, can be efficiently calculated using time-dependent tight-binding-based density functional theory (TD-DFTB). Specifically, Coulomb couplings are expressed in terms of self-consistently determined Mulliken transition charges. The determination of the sign of the coupling requires an additional super-molecule calculation. The approach is applied to two dimer systems. First, formaldehyde oxime for which a detailed comparison with standard DFT using the B3LYP and the PBE functionals is provided. Second, the Coulomb coupling is explored in dependence on the intermolecular coordinates for a perylene bisimide dimer. This provides structural evidence for the previously observed biphasic aggregation behavior...

  1. Room-Temperature Micron-Scale Exciton Migration in a Stabilized Emissive Molecular Aggregate.

    Science.gov (United States)

    Caram, Justin R; Doria, Sandra; Eisele, Dörthe M; Freyria, Francesca S; Sinclair, Timothy S; Rebentrost, Patrick; Lloyd, Seth; Bawendi, Moungi G

    2016-11-09

    We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton-exciton annihilation signatures under weak excitation flux. Fitting to a one-dimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm2/s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm2/s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport.

  2. Optical spectroscopy and system–bath interactions in molecular aggregates with full configuration interaction Frenkel exciton model

    Energy Technology Data Exchange (ETDEWEB)

    Seibt, Joachim; Sláma, Vladislav; Mančal, Tomáš, E-mail: mancal@karlov.mff.cuni.cz

    2016-12-20

    Highlights: • Standard Frenkel exciton model is extended to include inter-band coupling. • It is formally linked with configuration interaction method of quantum chemistry. • Spectral shifts due to inter-band coupling are found in molecular aggregates. • Effects of peak amplitude redistribution in two-dimensional spectra are found. - Abstract: Standard application of the Frenkel exciton model neglects resonance coupling between collective molecular aggregate states with different number of excitations. These inter-band coupling terms are, however, of the same magnitude as the intra-band coupling between singly excited states. We systematically derive the Frenkel exciton model from quantum chemical considerations, and identify it as a variant of the configuration interaction method. We discuss all non-negligible couplings between collective aggregate states, and provide compact formulae for their calculation. We calculate absorption spectra of molecular aggregate of carotenoids and identify significant band shifts as a result of inter-band coupling. The presence of inter-band coupling terms requires renormalization of the system–bath coupling with respect to standard formulation, but renormalization effects are found to be weak. We present detailed discussion of molecular dimer and calculate its time-resolved two-dimensional Fourier transformed spectra to find weak but noticeable effects of peak amplitude redistribution due to inter-band coupling.

  3. Accelerating FRET between Near-Infrared Emitting Quantum Dots Using a Molecular J-Aggregate as an Exciton Bridge.

    Science.gov (United States)

    Wang, Chen; Weiss, Emily A

    2017-09-13

    Fast energy transfer (EnT) among quantum dots (QDs) with near-infrared (NIR) emission is essential for fully exploiting their light harvesting and photon downconversion (multiexciton generation) abilities. This paper demonstrates a relayed EnT mechanism that accelerates the migration of NIR excitons between PbS QDs by a factor of 20 from that of one-step EnT through a polyelectrolyte and even a factor of ∼2 from that of one-step EnT between QDs in direct contact, by employing a J-aggregate (J-agg) of a cyanine dye as an exciton bridge. The donor QDs, acceptor QDs, and J-agg are electrostatically assembled into a sandwich structure with layer-by-layer deposition. Estimates of EnT rate and yield from transient and steady-state absorption and photoluminescence spectroscopies show that the rate-limiting step in the relay is EnT from the donor QD to the J-agg, while EnT from the J-agg to the acceptor QD occurs in J-agg with more intermolecular order. This work demonstrates the viability of relayed EnT through a molecular bridge as a strategy for accelerating long-distance exciton migration in assemblies of QDs, in particular in the near-infrared.

  4. Scaling Relations and Optimization of Excitonic Energy Transfer Rates between One-Dimensional Molecular Aggregates

    NARCIS (Netherlands)

    Chuang, Chern; Knoester, Jasper; Cao, Jianshu

    2014-01-01

    We theoretically study the distance, chain length, and temperature dependence of the electronic couplings as well as the excitonic energy transfer rates between one-dimensional (1D) chromophore aggregates. In addition to the well-known geometry dependent factor that leads to the deviation from

  5. Effects of inter-molecular charge-transfer excitons on the external quantum efficiency of zinc-porphyrin/C{sub 60} heterojunction photovoltaic cells

    Energy Technology Data Exchange (ETDEWEB)

    Ryuzaki, Sou; Kai, Toshihiro; Onoe, Jun [Department of Nuclear Engineering, Tokyo Institute of Technology, Tokyo 152-8550 (Japan); Toda, Yasunori; Adachi, Satoru, E-mail: jonoe@nr.titech.ac.jp [Department of Applied Physics, Hokkaido University, Sapporo, Hokkaido 060-8628 (Japan)

    2011-04-13

    We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C{sub 60}/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons.

  6. The relationship between the electric field induced dissociation of charge transfer (CT) excitons and the photocurrent in novel hybrid small molecular/polymeric solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Inal, Sahika; Neher, Dieter [Universitaet Potsdam (Germany). Institut fuer Physik und Astronomie; Sellinger, Alan [Institute of Materials Research and Engineering, Singapore (China)

    2010-07-01

    Complete dissociation of coulombically bound interfacial states is an ultimate step accounting for photovoltaic performance. Recent work has proposed that the emission of CT-exciton, i.e. an exciplex, is a competing process to the generation of free charges. Here, we investigated the photophysical processes in a bulk heterojunction system using a soluble poly(p-phenylenevinylene) donor and a novel small molecular electron acceptor based on Vinazene (2-vinyl-4,5-dicyanoimidazole). Recent work has shown that this blend exhibits a featureless emission, prominent at long wavelengths of the spectrum, which was attributed to a CT-exciton. We monitored the field induced dissociation of these CT-excitons by means of steady state and time resolved PL spectroscopy. Shortened decay times and reduced PL emission in blend film evidence the dissociation of the emissive intermolecular pair by the external electric field. Analyzing the dependence of the photocurrent and external quantum efficiency on the external field, the fate of the separated exciplex pairs is tackled. It is suggested that the formation of free carriers involves channels other than CT-excitons in such blends.

  7. Machine learning exciton dynamics

    Energy Technology Data Exchange (ETDEWEB)

    Häse, Florian [Harvard Univ., Cambridge, MA (United States); Technische Univ. Munchen, Garching (Germany). Dept. Physik; Valleau, Stéphanie [Harvard Univ., Cambridge, MA (United States); Pyzer-Knapp, Edward [Harvard Univ., Cambridge, MA (United States); Aspuru-Guzik, Alán [Harvard Univ., Cambridge, MA (United States)

    2016-04-01

    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 of accuracy with prediction errors contained within 0.01 eV (0.5%). Spectral densities and exciton dynamics are also in agreement with the TDDFT results. The acceleration and accurate prediction of dynamics strongly encourage the combination of machine learning techniques with ab initio methods.

  8. Manifestations of Vibronic Coupling Effects in Molecular Spectroscopy: from the Quenching of Excitonic Energy Splittings to the Clements Bands of SO2

    Science.gov (United States)

    Koppel, Horst

    2013-06-01

    We investigate the excitation of vibrational modes and its impact on the excitonic energy splittings in doubly hydrogen-bonded molecular dimers. The experimental analysis, performed in collaboration by S. Leutwyler and coworkers (Univ. Bern), is based on high-resolution resonant two-photon ionization spectroscopy. The potential energy surfaces underlying the theoretical investigation are obtained at the RICC2/aug-cc-pVTZ level and are used for the dynamical analysis in the framework of a well-established vibronic coupling approach. The vertical electronic Davydov splitting of the S_1 and S_2 excited states exceeds the observed excitonic splitting by a factor of 10--40. This discrepancy can be understood by considering the quenching of the excitonic splitting by the excitation of vibrational modes in the electronic transition. Two different approaches have been employed and found to reconcile theory and experiment. The analysis of the vibronic structure of the S_2 ← S_0 excitation spectrum focusses on the ortho-cyanophenol dimer as a representative example. Most of the observed spectral features can be reproduced by the calculations, although some deviations remain. In the second part, new results on the UV absorption spectrum of SO_2 will be presented. This is complementary to the excitonic systems in that higher vibrational energies are involved and a conical intersection is accessible to the nuclear motion. Using the concept of regularized diabatic states in combination with high-accuracy MRCI potential energy surfaces, semi-quantitative agreement with the complex experimental (low-resolution) spectrum has been achieved for the first time. P. Ottiger, S. Leutwyler and H. Köppel, J. Chem. Phys. 136, 174308 (2012). S. Kopec, P. Ottiger, S. Leutwyler and H. Köppel, J. Chem. Phys. 137, 184312 (2012). H. Köppel and B. Schubert, Mol. Phys. 104, 1069 (2006). C. Leveque, A. Komainda, R. Taieb and H. Köppel, J. Chem. Phys. 138, 044320 (2013).

  9. Excitonic emission of colloidal nano-crystals embedded in Molecular Beam Epitaxy grown ZnSe

    Energy Technology Data Exchange (ETDEWEB)

    Rashad, M; Pawlis, A; Schikora, D; Lischka, K [Department Physik, Universitaet Paderborn, Paderborn (Germany); Schoeps, O; Artemyev, M V; Woggon, U, E-mail: mahmed@mail.uni-paderborn.d [Technische Universitaet Berlin, Berlin (Germany)

    2010-09-01

    We combine ZnSe layers grown by molecular beam epitaxy with colloidal CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell nano-crystals (NCs) to achieve monolithic NC-semiconductor heterostructures. The NCs are prepared in solution and deposited by spray-coating on ZnSe buffer layers and subsequently overgrown by ZnSe. We find a blue shift of the photoluminescence of core/shell dots when they are overgrown by ZnSe. Rapid thermal annealing is used to improve the interface region between the NCs and the ZnSe matrix. The effect of different annealing temperatures on the optical properties of CdSe core, CdSe/ZnSe and CdSe/ZnS core/shell NCs overgrown with a cap layer of ZnSe is investigated. After annealing at 673 K the photoluminescence of these samples is red-shifted as compared to unprocessed samples. All photoluminescence results are explained by a model calculation with the following assumption about the 3D confining potential of the NCs: (i) the shell of core/shell NCs dissolves during ZnSe overgrowth, (ii) after overgrowth NCs are separated from the ZnSe matrix by an interface barrier, (iii) the height of this barrier is significantly reduced by annealing. For all three types of NCs we find an excellent quantitative agreement between the experimental and calculated NC transition energies. The absence of the barrier after annealing is further demonstrated by low temperature photoluminescence data of annealed samples which show enhanced diffusion of electron-hole pairs from ZnSe into the NCs.

  10. 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)

  11. Control of excitons in a bent bunch of molecular aggregates by dipole-dipole interaction with quantum dots

    Science.gov (United States)

    Zabolotskii, A. A.

    2017-10-01

    The nonlocal dipole-dipole interaction is studied between excitations in chromophores forming a bunch or a tube of J-aggregates and closely spaced quantum dots (QDs). Equations describing the evolution of exciton pulses in a quasi-one-dimensional medium are derived taking into account the interaction with the transition resonant to nanoparticles. It is shown that the efficient controllable resonance energy transfer can occur in the system between QDs and an exciton pulse. The efficiency of this process significantly increases if the bunch of aggregates is deformed to bend nanoparticles round. It is shown that the interaction of permanent dipole moments of QDs and chromophores leads to the formation of a potential barrier or a well. It is found that the combined influence of these factors can be used to efficiently control the dynamics of pulses in aggregates.

  12. 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.

  13. Nonlocal excitonic-mechanical interaction in a nanosystem

    Science.gov (United States)

    Zabolotskii, A. A.

    2016-11-01

    The dynamics of a nanoparticle during its dipole interaction with an excitonic excitation in an extended quasi-one-dimensional polarizable medium is investigated. Bundles of J-aggregates of dye molecules are considered as an example of the latter. The nonlocal excitonic-mechanical interaction between the field of an amplifying or absorbing nanoparticle and excitons in a bundle has been simulated numerically. It has been found that the interaction between the field of the induced nanoparticle dipole and the fields of the molecular dipoles in an aggregate can lead to a change in the particle trajectory and excitonic pulse shape. The possibility of controlling the nanoparticle by excitonic pulses and the reverse effect of the nanoparticle field on the dynamics of excitons due to the nonlocal excitonic-mechanical interaction has been demonstrated.

  14. Direct Imaging of Frenkel Exciton Transport by Ultrafast Microscopy.

    Science.gov (United States)

    Zhu, Tong; Wan, Yan; Huang, Libai

    2017-07-18

    Long-range transport of Frenkel excitons is crucial for achieving efficient molecular-based solar energy harvesting. Understanding of exciton transport mechanisms is important for designing materials for solar energy applications. One major bottleneck in unraveling of exciton transport mechanisms is the lack of direct measurements to provide information in both spatial and temporal domains, imposed by the combination of fast energy transfer (typically ≤1 ps) and short exciton diffusion lengths (typically ≤100 nm). This challenge requires developing experimental tools to directly characterize excitation energy transport, and thus facilitate the elucidation of mechanisms. To address this challenge, we have employed ultrafast transient absorption microscopy (TAM) as a means to directly image exciton transport with ∼200 fs time resolution and ∼50 nm spatial precision. By mapping population in spatial and temporal domains, such approach has unraveled otherwise obscured information and provided important parameters for testing exciton transport models. In this Account, we discuss the recent progress in imaging Frenkel exciton migration in molecular crystals and aggregates by ultrafast microscopy. First, we establish the validity of the TAM methods by imaging singlet and triplet exciton transport in a series of polyacene single crystals that undergo singlet fission. A new singlet-mediated triplet transport pathway has been revealed by TAM, resulting from the equilibrium between triplet and singlet exciton populations. Such enhancement of triplet exciton transport enables triplet excitons to migrate as singlet excitons and leads to orders of magnitude faster apparent triplet exciton diffusion rate in the picosecond and nanosecond time scales, favorable for solar cell applications. Next we discuss how information obtained by ultrafast microscopy can evaluate coherent effects in exciton transport. We use tubular molecular aggregates that could support large exciton

  15. Exciton Resonances in Novel Silicon Carbide Polymers

    Science.gov (United States)

    Burggraf, Larry; Duan, Xiaofeng

    2015-05-01

    A revolutionary technology transformation from electronics to excitionics for faster signal processing and computing will be advantaged by coherent exciton transfer at room temperature. The key feature required of exciton components for this technology is efficient and coherent transfer of long-lived excitons. We report theoretical investigations of optical properties of SiC materials having potential for high-temperature excitonics. Using Car-Parinello simulated annealing and DFT we identified low-energy SiC molecular structures. The closo-Si12C12 isomer, the most stable 12-12 isomer below 1100 C, has potential to make self-assembled chains and 2-D nanostructures to construct exciton components. Using TDDFT, we calculated the optical properties of the isomer as well as oligomers and 2-D crystal formed from the isomer as the monomer unit. This molecule has large optical oscillator strength in the visible. Its high-energy and low-energy transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer. These results are useful to describe resonant, coherent transfer of dark excitons in the nanostructures. Research supported by the Air Force Office of Scientific Research.

  16. 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.

  17. Single-Molecule Investigation of Energy Dynamics in a Coupled Plasmon-Exciton System

    Science.gov (United States)

    Imada, Hiroshi; Miwa, Kuniyuki; Imai-Imada, Miyabi; Kawahara, Shota; Kimura, Kensuke; Kim, Yousoo

    2017-07-01

    We investigate the near-field interaction between an isolated free-base phthalocyanine molecule and a plasmon localized in the gap between an NaCl-covered Ag(111) surface and the tip apex of a scanning tunneling microscope. When the tip is located in the close proximity of the molecule, asymmetric dips emerge in the broad luminescence spectrum of the plasmon generated by the tunneling current. The origin of the dips is explained by energy transfer between the plasmon and molecular excitons and a quantum mechanical interference effect, where molecular vibrations provide additional degrees of freedom in the dynamic process.

  18. Excitonic processes at organic heterojunctions

    Science.gov (United States)

    He, ShouJie; Lu, ZhengHong

    2018-02-01

    Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.

  19. Suppressing Energy Loss due to Triplet Exciton Formation in Organic Solar Cells: The Role of Chemical Structures and Molecular Packing

    KAUST Repository

    Chen, Xiankai

    2017-04-21

    In the most efficient solar cells based on blends of a conjugated polymer (electron donor) and a fullerene derivative (electron acceptor),ultrafast formation of charge-transfer (CT) electronic states at the donor-acceptor interfaces and efficient separation of these CT states into free charges, lead to internal quantum efficiencies near 100%. However, there occur substantial energy losses due to the non-radiative recombinations of the charges, mediated by the loweset-energy (singlet and triplet) CT states; for example, such recombinations can lead to the formation of triplet excited electronic states on the polymer chains, which do not generate free charges. This issue remains a major factor limiting the power conversion efficiencies (PCE) of these devices. The recombination rates are, however, difficult to quantify experimentally. To shed light on these issues, here, an integrated multi-scale theoretical approach that combines molecular dynamics simulations with quantum chemistry calculations is employed in order to establish the relationships among chemical structures, molecular packing, and non-radiative recombination losses mediated by the lowest-energy charge-transfer states.

  20. Exciton circular dichroism in channelrhodopsin.

    Science.gov (United States)

    Pescitelli, Gennaro; Kato, Hideaki E; Oishi, Satomi; Ito, Jumpei; Maturana, Andrés Daniel; Nureki, Osamu; Woody, Robert W

    2014-10-16

    Channelrhodopsins (ChRs) are of great interest currently because of their important applications in optogenetics, the photostimulation of neurons. The absorption and circular dichroism (CD) spectra of C1C2, a chimera of ChR1 and ChR2 of Chlamydomonas reinhardtii, have been studied experimentally and theoretically. The visible absorption spectrum of C1C2 shows vibronic fine structure in the 470 nm band, consistent with the relatively nonpolar binding site. The CD spectrum has a negative band at 492 nm (Δε(max) = -6.17 M(-1) cm(-1)) and a positive band at 434 nm (Δε(max) = +6.65 M(-1) cm(-1)), indicating exciton coupling within the C1C2 dimer. Time-dependent density functional theory (TDDFT) calculations are reported for three models of the C1C2 chromophore: (1) the isolated protonated retinal Schiff base (retPSB); (2) an ion pair, including the retPSB chromophore, two carboxylate side chains (Asp 292, Glu 162), modeled by acetate, and a water molecule; and (3) a hybrid quantum mechanical/molecular mechanical (QM/MM) model depicting the binding pocket, in which the QM part consists of the same ion pair as that in (2) and the MM part consists of the protein residues surrounding the ion pair within 10 Å. For each of these models, the CD of both the monomer and the dimer was calculated with TDDFT. For the dimer, DeVoe polarizability theory and exciton calculations were also performed. The exciton calculations were supplemented by calculations of the coupling of the retinal transition with aromatic and peptide group transitions. For the dimer, all three methods and three models give a long-wavelength C2-axis-polarized band, negative in CD, and a short-wavelength band polarized perpendicular to the C2 axis with positive CD, differing in wavelength by 1-5 nm. Only the retPSB model gives an exciton couplet that agrees qualitatively with experiment. The other two models give a predominantly or solely positive band. We further analyze an N-terminal truncated mutant

  1. Charting molecular composition of phosphatidylcholines by fatty acid scanning and ion trap MS3 fragmentation

    DEFF Research Database (Denmark)

    Ekroos, Kim; Ejsing, Christer S.; Bahr, Ute

    2003-01-01

    The molecular composition of phosphatidylcholines (PCs) in total lipid extracts was characterized by a combination of multiple precursor ion scanning on a hybrid quadrupole time-of-flight mass spectrometer and MS3 fragmentation on an ion trap mass spectrometer. Precursor ion spectra for 50 acyl a...

  2. Scanning Tunneling Microscopy Studies of Topological Insulators Grown by Molecular Beam Epitaxy

    Directory of Open Access Journals (Sweden)

    Xue Qikun

    2012-03-01

    Full Text Available We summarize our recent scanning tunneling microscopy (STM study of topological insulator thin films grown by molecular beam epitaxy (MBE, which includes the observation of electron standing waves on topological insulator surface and the Landau quantization of topological surface states. The work has provided valuable information to the understanding of intriguing properties of topological insulators, as predicted by theory.

  3. 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 ...

  4. Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes.

    Science.gov (United States)

    Fofang, Nche T; Park, Tae-Ho; Neumann, Oara; Mirin, Nikolay A; Nordlander, Peter; Halas, Naomi J

    2008-10-01

    Stable Au nanoshell-J-aggregate complexes are formed that exhibit coherent coupling between the localized plasmons of a nanoshell and the excitons of molecular J-aggregates adsorbed on its surface. By tuning the nanoshell plasmon energies across the exciton line of the J-aggregate, plasmon-exciton coupling energies for these complexes are obtained. The strength of this interaction is dependent on the specific plasmon mode of the nanoparticle coupled to the J-aggregate exciton. From a model based on Gans theory, we obtain an expression for the plasmon-exciton hybridized states of the complex.

  5. Bound Exciton Complexes

    Science.gov (United States)

    Meyer, B. K.

    In the preceding chapter, we concentrated on the properties of free excitons. These free excitons may move through the sample and hit a trap, a nonradiative or a radiative recombination center. At low temperatures, the latter case gives rise to either deep center luminescence, mentioned in Sect. 7.1 and discussed in detail in Chap. 9, or to the luminescence of bound exciton complexes (BE or BEC). The chapter continues with the most prominent of these BECs, namely A-excitons bound to neutral donors. The next aspects are the more weakly BEs at ionized donors. The Sect. 7.4 treats the binding or localization energies of BEC from a theoretical point of view, while Sect. 7.5 is dedicated to excited states of BECs, which contain either holes from deeper valence bands or an envelope function with higher quantum numbers. The last section is devoted to donor-acceptor pair transitions. There is no section devoted specifically to excitons bound to neutral acceptors, because this topic is still partly controversially discussed. Instead, information on these A0X complexes is scattered over the whole chapter, however, with some special emphasis seen in Sects. 7.1, 7.4, and 7.5.

  6. Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules.

    Science.gov (United States)

    Kwak, Kyuju; Thanthirige, Viraj Dhanushka; Pyo, Kyunglim; Lee, Dongil; Ramakrishna, Guda

    2017-10-05

    The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of n-hexanethiolate-protected gold clusters ranging from Au25 to Au333. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO-LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules.

  7. Exciton multiplication from first principles.

    Science.gov (United States)

    Jaeger, Heather M; Hyeon-Deuk, Kim; Prezhdo, Oleg V

    2013-06-18

    Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron

  8. Phonon-Driven Oscillatory Plasmonic Excitonic Nanomaterials

    Energy Technology Data Exchange (ETDEWEB)

    Kirschner, Matthew S. [Department; Ding, Wendu [Department; Li, Yuxiu [Center; College; Chapman, Craig T. [Department; Lei, Aiwen [College; Lin, Xiao-Min [Center; Chen, Lin X. [Department; Chemical; Schatz, George C. [Department; Schaller, Richard D. [Department; Center

    2017-12-08

    We demonstrate that coherent acoustic phonons derived from plasmonic nanoparticles can modulate electronic interactions with proximal excitonic molecular species. A series of gold bipyramids with systematically varied aspect ratios and corresponding localized surface plasmon resonance energies, functionalized with a J-aggregated thiacarbocyanine dye molecule, produce two hybridized states that exhibit clear anti-crossing behavior with a Rabi splitting energy of 120 meV. In metal nanoparticles, photoexcitation generates coherent acoustic phonons that cause oscillations in the plasmon resonance energy. In the coupled system, these photo-generated oscillations alter the metal nanoparticle’s energetic contribution to the hybridized system and, as a result, change the coupling between the plasmon and exciton. We demonstrate that such modulations in the hybridization is consistent across a wide range of bipyramid ensembles. We also use Finite-Difference Time Domain calculations to develop a simple model describing this behavior. Such oscillatory plasmonic-excitonic nanomaterials (OPENs) offer a route to manipulate and dynamically-tune the interactions of plasmonic/excitonic systems and unlock a range of potential applications.

  9. Energy and Information Transfer Via Coherent Exciton Wave Packets

    Science.gov (United States)

    Zang, Xiaoning

    Electronic excitons are bound electron-hole states that are generated when light interacts with matter. Such excitations typically entangle with phonons and rapidly decohere; the resulting electronic state dynamics become diffusive as a result. However, if the exciton-phonon coupling can be reduced, it may be possible to construct excitonic wave packets that offer a means of efficiently transmitting information and energy. This thesis is a combined theory/computation investigation to design condensed matter systems which support the requisite coherent transport. Under the idealizing assumption that exciton-phonon entanglement could be completely suppressed, the majority of this thesis focuses on the creation and manipulation of exciton wave packets in quasi-one-dimensional systems. While each site could be a silicon quantum dot, the actual implementation focused on organic molecular assemblies for the sake of computational simplicity, ease of experimental implementation, potential for coherent transport, and promise because of reduced structural uncertainty. A laser design was derived to create exciton wave packets with tunable shape and speed. Quantum interference was then exploited to manipulate these packets to block, pass, and even dissociate excitons based on their energies. These developments allow exciton packets to be considered within the arena of quantum information science. The concept of controllable excitonic wave packets was subsequently extended to consider molecular designs that allow photons with orbital angular momentum to be absorbed to create excitons with a quasi-angular momentum of their own. It was shown that a well-defined measure of topological charge is conserved in such light-matter interactions. Significantly, it was also discovered that such molecules allow photon angular momenta to be combined and later emitted. This amounts to a new way of up/down converting photonic angular momentum without relying on nonlinear optical materials. The

  10. Atomic-scale structure of dislocations revealed by scanning tunneling microscopy and molecular dynamics

    DEFF Research Database (Denmark)

    Christiansen, Jesper; Morgenstern, K.; Schiøtz, Jakob

    2002-01-01

    The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations......, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width...

  11. Molecular images and vibrational spectroscopy of sorbic acid with the scanning tunneling microscope

    Science.gov (United States)

    Smith, Douglas P. E.; Kirk, Michael D.; Quate, Calvin F.

    1987-06-01

    Images of sorbic acid molecules absorbed onto graphite have been taken with a scanning tunneling microscope (STM) operating in liquid helium. Molecular clusters were clearly observed, as was the atomic structure of the graphite substrate. The molecules were seen to diffuse across the substrate at a rate of about 1 Å/min. When dI/dV vs V was measured with the STM probe directly over a sorbic acid molecule, a well-defined spectrum of peaks was obtained whose energies corresponded to the vibrational resonances of the molecule. Large changes in the spectra occurred if the tip was moved a lateral distance of 5 Å.

  12. Soft x-ray excitonics

    Science.gov (United States)

    Moulet, A.; Bertrand, J. B.; Klostermann, T.; Guggenmos, A.; Karpowicz, N.; Goulielmakis, E.

    2017-09-01

    The dynamic response of excitons in solids is central to modern condensed-phase physics, material sciences, and photonic technologies. However, study and control have hitherto been limited to photon energies lower than the fundamental band gap. Here we report application of attosecond soft x-ray and attosecond optical pulses to study the dynamics of core-excitons at the L2,3 edge of Si in silicon dioxide (SiO2). This attosecond x-ray absorption near-edge spectroscopy (AXANES) technique enables direct probing of the excitons’ quasiparticle character, tracking of their subfemtosecond relaxation, the measurement of excitonic polarizability, and observation of dark core-excitonic states. Direct measurement and control of core-excitons in solids lay the foundation of x-ray excitonics.

  13. Tunable excitons in bilayer graphene

    Science.gov (United States)

    Ju, Long; Wang, Lei; Cao, Ting; Taniguchi, Takashi; Watanabe, Kenji; Louie, Steven G.; Rana, Farhan; Park, Jiwoong; Hone, James; Wang, Feng; McEuen, Paul L.

    2017-11-01

    Excitons, the bound states of an electron and a hole in a solid material, play a key role in the optical properties of insulators and semiconductors. Here, we report the observation of excitons in bilayer graphene (BLG) using photocurrent spectroscopy of high-quality BLG encapsulated in hexagonal boron nitride. We observed two prominent excitonic resonances with narrow line widths that are tunable from the mid-infrared to the terahertz range. These excitons obey optical selection rules distinct from those in conventional semiconductors and feature an electron pseudospin winding number of 2. An external magnetic field induces a large splitting of the valley excitons, corresponding to a g-factor of about 20. These findings open up opportunities to explore exciton physics with pseudospin texture in electrically tunable graphene systems​.

  14. Thermal stability and molecular microstructure of heat-induced cereal grains, revealed with Raman molecular microspectroscopy and differential scanning calorimetry.

    Science.gov (United States)

    Khan, Md Majibur Rahman; Yu, Peiqiang

    2013-07-03

    The objectives of the present study were to use Raman molecular microspectroscopy and differential scanning calorimetry (DSC) to reveal molecular thermal stability and thermal degradation behavior of heat-induced cereal grains and reveal the molecular chemistry of the protein structures of cereal grain tissues affected by heat processing and to quantify the protein secondary structures using multicomponent peak modeling Gaussian and Lorentzian methods. Hierarchical cluster analysis (CLA) and principal components analysis (PCA) were also conducted to identify molecular differences in the Raman spectra. Three cereal grain seeds, wheat, triticale, and corn, were used as the model for feed protein in the experiment. The specimens were autoclaved (moist heating) and dry-heated (roasted) at 121 °C for 80 min, respectively. Raman spectroscopy results revealed that there are marked differences in the secondary structures of the proteins subjected to various heating treatments of different cereals. The sensitivity of cereals to moist heating was much higher than the sensitivity to dry heating. The multivariate analyses (CLA and PCA) showed that heat treatment was significantly isolated between the different Raman raw spectra. The DSC study revealed that the thermal degradation behavior of cereals was significantly changed after moist- and dry-heat treatments. The position of the major endothermic peak of dry-heated cereals shifted toward a higher temperature, from 131.7 to 134.0 °C, suggesting the high thermal stability of dry-heated cereals. In contrast, the endothermic peak position was slightly decreased to 132.1 °C in the case of moist autoclaved heating. The digestive behavior and nutritive value of rumen-undegradable protein in animals may be related to the changes of the protein secondary molecular structure and thermal stability of the cereal grain materials, which is attributed by Raman microspectroscopy and DSC endotherm profiles.

  15. Interaction of vasicine with calf thymus DNA: Molecular docking, spectroscopic and differential scanning calorimetric insights

    Science.gov (United States)

    R. S., Sai Murali; R. S., Sai Siddhardha; Rajesh Babu, D.; Venketesh, S.; Basavaraju, R.; Nageswara Rao, G.

    2017-06-01

    The present study brings out the interaction between vasicine, an alkaloid and Adhatoda vasica Nees with double stranded DNA. The physico-chemical interaction between small molecules and nucleic acids is a major area of focus in screening drugs against various cancers. Molecular probing in our study using Molecular Operating Environment (MOE) has revealed interaction of vasicine with DNA double helix. Here we report the interaction of vasicine with Calf thymus DNA. We present for the first time the results obtained from UV-visible, fluorescence spectroscopic and differential scanning calorimetric techniques that suggest a moderate to strong electrostatic, hydrophobic and van der Waals interactions mediating the DNA binding properties of vasicine, leading to disruption of DNA secondary structure.

  16. A morphological study of molecularly imprinted polymers using the scanning electron microscope

    Energy Technology Data Exchange (ETDEWEB)

    Paniagua Gonzalez, Gema [Departamento de Ciencias Analiticas, Facultad de Ciencias, Universidad Nacional de Educacion a Distancia (UNED), 28040 Madrid (Spain)]. E-mail: gpaniagua@pas.uned.es; Fernandez Hernando, Pilar [Departamento de Ciencias Analiticas, Facultad de Ciencias, Universidad Nacional de Educacion a Distancia (UNED), 28040 Madrid (Spain); Durand Alegria, J.S. [Departamento de Ciencias Analiticas, Facultad de Ciencias, Universidad Nacional de Educacion a Distancia (UNED), 28040 Madrid (Spain)

    2006-01-31

    Molecular imprinting is an emerging technique for producing polymers with applications in affinity-based separation, in biomimetic sensors, in catalysis, etc. This variety of uses relies upon the production of polymers with different affinities, specificities, sensitivities and loading capacities. Research into the development of molecular imprinted polymers (MIPs) with new or improved morphologies - which involves modification of the polymerisation process - is therefore underway. This paper reports a comparative study of non-covalent MIPs synthesised by 'bulk' polymerisation using digoxin as template. These were synthesised under different conditions, i.e., changing the functional monomers employed (methacrylic acid or 2-vinylpyridine), the porogens (acetonitrile or dichloromethane) used, and by altering the volume of the latter. The polymerisation process was allowed to proceed either under UV light or in a thermostat-controlled waterbath. The surface morphology (was determined by scanning electron microscopy) and the ability of the different polymers to selectively rebind the template was then evaluated.

  17. Excitonic photoluminescence and photoresponse of ZnS nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Dai, Jun, E-mail: daijun@just.edu.cn [Department of Physics, Jiangsu University of Science and Technology, Zhenjiang 212003 (China); Song, Xing [School of Material Science and Engineering, Jiangsu University, Zhenjiang 212003 (China); Zheng, Hongge [Department of Physics, Jiangsu University of Science and Technology, Zhenjiang 212003 (China); Wu, Chunxia, E-mail: chxwu7771@yahoo.com.cn [School of Material Science and Engineering, Jiangsu University, Zhenjiang 212003 (China)

    2016-05-01

    Single crystal ZnS nanowires are fabricated by vapor phase transport method on sapphire substrate in the presence of Au catalyst. The morphology, composition, and crystal structure are characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). XRD and HRTEM reveal that the ZnS nanowires have perfect single crystal wurtzite structure. The temperature-dependent photoluminescence spectra show that the ZnS nanowires present pure near-bandgap ultraviolet exciton recombination emission at 347 nm. The exciton-related optical properties, including exciton activation energy, temperature-dependent exciton energy and Varshni coefficients describing exciton energy variation, are systematically discussed. In addition, an individual ZnS nanowire-based ultraviolet photodetector is fabricated, which shows good photoresponse ability and fast response rate. The result shows that the ZnS nanowires are particularly suitable for UV photodetectors. - Highlights: • Single crystal ultrathin ZnS nanowires with diameter of 20–100 nm were fabricated by vapor phase transport method. • Exciton-related optical properties were fitted by temperature-dependent photoluminescence spectra. • Single ZnS nanowire ultraviolet photodetector with good photoswitch ability and high photocurrent was demonstrated.

  18. 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 are obs...... energy slightly increases from 21.5 to 23 meV, while its broadening decreases from 5.5 to 3 meV. This is attributed to a strong three-dimensional confinement with improving shape uniformity for decreasing exciton energy. [S0163-1829(99)04739-6]....

  19. Resolving ultrafast exciton migration in organic solids at the nanoscale

    Science.gov (United States)

    Penwell, Samuel B.; Ginsberg, Lucas D. S.; Noriega, Rodrigo; Ginsberg, Naomi S.

    2017-11-01

    Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis.

  20. Exciton dynamics in cuprous oxide

    NARCIS (Netherlands)

    Fishman, D. A.; Revcolevschi, A.; van Loosdrecht, P. H. M.; Stutzmann, M

    2006-01-01

    This work addresses the mid-infrared properties of cuprous oxide and in particular induced absorption due to the presence of excitons. We probe the population of the non-radiative ground state of para-excitons via laser-induced changes of the transmission in the "hydrogenic" 1s-2p/1s-3p transition

  1. 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...

  2. Excitons in solid C60

    Energy Technology Data Exchange (ETDEWEB)

    Shirley, Eric L.; Benedict, Lorin X.; Louie, Steven G.

    1995-10-01

    Exciton levels in undoped, solid C60 are calculated using a model Hamiltonian. We find excitation energies of 1.58 and 1.30 eV for the lowest singlet and triplet exciton, respectively, in comparison with the measured energies of 1.83 and 1.55 eV. Singlet and triplet states have similar energy diagrams, wherein exciton states having T{sub 2g}, T{sub 1g},G{sub g}, and H{sub g} symmetries are separated by up to several tenths of an electron volt. As a function of crystal momentum, exciton energies exhibit dispersion from 20 to 40 meV. Theoretical pressure derivatives of exciton energies are presented.

  3. High-Resolution Scanning Tunneling Microscopy Studies of Molecular Nanostructures on Surfaces

    DEFF Research Database (Denmark)

    Song, Xin

    . First, to study the role of hydrogen bonding in self-assembly, we investigate the monomolecular self-assembled system of pyrene-4,5,9,10-tetrone and phenanthrene- 9,10-dione molecules on Au(111) and HOPG surface respectively and the binary molecular self-assembled system of stearic acid and guanine...... molecules co-adsorbed on HOPG surface. Hydrogen bonding plays a key role in these self-assembled nanostructures and the substrate could also give an effect in the self-assembly. Second, to study the self-assembly across the terrace steps as a defect on surface, the chiral self-assembled supramolecular...... of nanostructures requires deeper insight into the adsorption sites, adsorption configurations, diffusion behaviour and driving forces for self-assembly of different molecules or atoms on different substrates. To study these fundamental issues, scanning tunneling microscopy (STM) has proven to be an ideal choice...

  4. A novel antibody humanization method based on epitopes scanning and molecular dynamics simulation.

    Science.gov (United States)

    Zhang, Ding; Chen, Cai-Feng; Zhao, Bin-Bin; Gong, Lu-Lu; Jin, Wen-Jing; Liu, Jing-Jun; Wang, Jing-Fei; Wang, Tian-Tian; Yuan, Xiao-Hui; He, You-Wen

    2013-01-01

    1-17-2 is a rat anti-human DEC-205 monoclonal antibody that induces internalization and delivers antigen to dendritic cells (DCs). The potentially clinical application of this antibody is limited by its murine origin. Traditional humanization method such as complementarity determining regions (CDRs) graft often leads to a decreased or even lost affinity. Here we have developed a novel antibody humanization method based on computer modeling and bioinformatics analysis. First, we used homology modeling technology to build the precise model of Fab. A novel epitope scanning algorithm was designed to identify antigenic residues in the framework regions (FRs) that need to be mutated to human counterpart in the humanization process. Then virtual mutation and molecular dynamics (MD) simulation were used to assess the conformational impact imposed by all the mutations. By comparing the root-mean-square deviations (RMSDs) of CDRs, we found five key residues whose mutations would destroy the original conformation of CDRs. These residues need to be back-mutated to rescue the antibody binding affinity. Finally we constructed the antibodies in vitro and compared their binding affinity by flow cytometry and surface plasmon resonance (SPR) assay. The binding affinity of the refined humanized antibody was similar to that of the original rat antibody. Our results have established a novel method based on epitopes scanning and MD simulation for antibody humanization.

  5. Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

    Directory of Open Access Journals (Sweden)

    Jakub S. Prauzner-Bechcicki

    2016-11-01

    Full Text Available Titanium dioxide, or titania, sensitized with organic dyes is a very attractive platform for photovoltaic applications. In this context, the knowledge of properties of the titania–sensitizer junction is essential for designing efficient devices. Consequently, studies on the adsorption of organic dyes on titania surfaces and on the influence of the adsorption geometry on the energy level alignment between the substrate and an organic adsorbate are necessary. The method of choice for investigating the local environment of a single dye molecule is high-resolution scanning probe microscopy. Microscopic results combined with the outcome of common spectroscopic methods provide a better understanding of the mechanism taking place at the titania–sensitizer interface. In the following paper, we review the recent scanning probe microscopic research of a certain group of molecular assemblies on rutile titania surfaces as it pertains to dye-sensitized solar cell applications. We focus on experiments on adsorption of three types of prototypical dye molecules, i.e., perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA, phtalocyanines and porphyrins. Two interesting heteromolecular systems comprising molecules that are aligned with the given review are discussed as well.

  6. Nanolithography by scanning probes on calixarene molecular glass resist using mix-and-match lithography

    Science.gov (United States)

    Kaestner, Marcus; Hofer, Manuel; Rangelow, Ivo W.

    2013-07-01

    Going "beyond the CMOS information-processing era," taking advantage of quantum effects occurring at sub-10-nm level, requires novel device concepts and associated fabrication technologies able to produce promising features at acceptable cost levels. Herein, the challenge affecting the lithographic technologies comprises the marriage of down-scaling the device-relevant feature size towards single-nanometer resolution with a simultaneous increase of the throughput capabilities. Mix-and-match lithographic strategies are one promising path to break through this trade-off. Proof-of-concept combining electron beam lithography (EBL) with the outstanding capabilities of closed-loop electric field current-controlled scanning probe nanolithography (SPL) is demonstrated. This combination, whereby also extreme ultraviolet lithography (EUVL) is possible instead of EBL, enables more: improved patterning resolution and reproducibility in combination with excellent overlay and placement accuracy. Furthermore, the symbiosis between EBL (EUVL) and SPL expands the process window of EBL (EUVL) beyond the state of the art, allowing SPL-based pre- and post-patterning of EBL (EUVL) written features at critical dimension levels with scanning probe microscopy-based pattern overlay alignment capability. Moreover, we are able to modify the EBL (EUVL) pattern even after the development step. The ultra-high resolution mix-and-match lithography experiments are performed on the molecular glass resist calixarene using a Gaussian e-beam lithography system operating at 10 keV and a home-developed SPL setup.

  7. 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.

  8. Exciton condensation in strongly correlated electron bilayers

    NARCIS (Netherlands)

    Rademaker, Louk; van den Brink, J.; Zaanen, Jan; Hilgenkamp, H.

    2013-01-01

    We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems, an exciton is the bound state of a double occupied and empty site. In the strong coupling limit, the exciton acts as a hard-core boson. Its physics is captured by the exciton t -J

  9. Dephasing and interaction of excitons CdSe/ZnSe islands

    DEFF Research Database (Denmark)

    Wagner, H. P.; Tranitz, H.-P.; Preis, H.

    2000-01-01

    The dephasing of excitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of dephasing times is observed, indicating the existence of localized excitons with different relaxation times at comparable tra...... transition energies. Polarization-dependent measurements identify the formation of biexcitons. The observed large biexciton binding energy of 22meV increases with decreasing exciton energy, which is attributed to an increasing three-dimensional confinement.......The dephasing of excitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of dephasing times is observed, indicating the existence of localized excitons with different relaxation times at comparable...

  10. 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.

  11. Optics of exciton-plasmon nanomaterials

    Science.gov (United States)

    Sukharev, Maxim; Nitzan, Abraham

    2017-11-01

    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.

  12. Carbon nanotubes as excitonic insulators.

    Science.gov (United States)

    Varsano, Daniele; Sorella, Sandro; Sangalli, Davide; Barborini, Matteo; Corni, Stefano; Molinari, Elisa; Rontani, Massimo

    2017-11-13

    Fifty years ago Walter Kohn speculated that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons-electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show that this excitonic insulator is realized in zero-gap carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings call into question the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulators.

  13. 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

  14. Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

    NARCIS (Netherlands)

    Vlaming, S. M.; Bloemsma, E. A.; Nietiadi, M. Linggarsari; Knoester, J.

    2011-01-01

    Using numerical simulations, we study the effect of disorder on the optical properties of cylindrical aggregates of molecules with strong excitation transfer interactions. The exciton states and the energy transport properties of such molecular nanotubes attract considerable interest for application

  15. Redescription of Cichlidogyrus philander (Monogenea, Ancyrocephalidae using scanning electron microscopy (SEM and molecular analysis

    Directory of Open Access Journals (Sweden)

    Igeh Patience C

    2017-01-01

    Full Text Available The sclerotized structures of monogeneans have traditionally been studied by light microscopy and different staining techniques. Recently, enzymatic digestion followed by scanning electron microscopy (SEM has enabled the examination of structural details not visible with light microscopy. In order to obtain better, and more accurate, morphological information on sclerotized structures not affected by mounting medium or cover slip pressure, the sclerites of Cichlidogyrus philander Douëllou, 1993 (Monogenea, Ancyrocephalidae, collected from Pseudocrenilabrus philander (Weber, 1897, were redescribed using SEM. Parasites were collected from Padda Dam, Gauteng, South Africa and soft tissue was digested to release the sclerotized structures. The digested tissue also provided sufficient genetic material for molecular characterization of this species. Cichlidogyrus philander is characterised by a penis with a sharp, curved, lateral termination, an accessory piece with a hook-like extremity that may appear forked terminally, and lack of a visible vagina. The transverse bars have concave and convex surfaces with ribs on the concave surface. The dorsal bar bears fenestrations at the base of the auricles and the ventral and dorsal gripi are dissimilar. Furthermore, the large first pair of uncinuli shows lateral wings on the left side of the base. On top of this wing, a ball-like structure with a small fenestration is visible. Genetic characters derived from the 28S rDNA, the COI mitochondrial DNA and ITS1 rDNA regions distinguish C. philander from all other Cichlidogyrus sequenced species.

  16. Atomic structure of screw dislocations intersecting the Au(111) surface: A combined scanning tunneling microscopy and molecular dynamics study

    DEFF Research Database (Denmark)

    Engbæk, Jakob; Schiøtz, Jakob; Dahl-Madsen, Bjarke

    2006-01-01

    The atomic-scale structure of naturally occurring screw dislocations intersecting a Au(111) surface has been investigated both experimentally by scanning tunneling microscopy (STM) and theoretically using molecular dynamics (MD) simulations. The step profiles of 166 dislocations were measured using...

  17. 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

  18. Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites

    Science.gov (United States)

    Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.

    2016-03-01

    UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.

  19. Differential scanning calorimetry in life science: thermodynamics, stability, molecular recognition and application in drug design.

    Science.gov (United States)

    Bruylants, G; Wouters, J; Michaux, C

    2005-01-01

    All biological phenomena depend on molecular recognition, which is either intermolecular like in ligand binding to a macromolecule or intramolecular like in protein folding. As a result, understanding the relationship between the structure of proteins and the energetics of their stability and binding with others (bio)molecules is a very interesting point in biochemistry and biotechnology. It is essential to the engineering of stable proteins and to the structure-based design of pharmaceutical ligands. The parameter generally used to characterize the stability of a system (the folded and unfolded state of the protein for example) is the equilibrium constant (K) or the free energy (deltaG(o)), which is the sum of enthalpic (deltaH(o)) and entropic (deltaS(o)) terms. These parameters are temperature dependent through the heat capacity change (deltaCp). The thermodynamic parameters deltaH(o) and deltaCp can be derived from spectroscopic experiments, using the van't Hoff method, or measured directly using calorimetry. Along with isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC) is a powerful method, less described than ITC, for measuring directly the thermodynamic parameters which characterize biomolecules. In this article, we summarize the principal thermodynamics parameters, describe the DSC approach and review some systems to which it has been applied. DSC is much used for the study of the stability and the folding of biomolecules, but it can also be applied in order to understand biomolecular interactions and can thus be an interesting technique in the process of drug design.

  20. Band Gap, Excitons, and Coulomb Interaction in Solid C60

    NARCIS (Netherlands)

    Lof, R.W.; Veenendaal, M.A. van; Jonkman, H.T.; Sawatzky, G.A.; Koopmans, H.

    1992-01-01

    The band gap of solid C60 is found to be 2.3 ± 0.1 eV. The on-site molecular C60 Coulomb interaction (U) as determined from the KVV C60 Auger spectrum is found to be 1.6 ± 0.2 eV. This value of U is shown to lead to Frenkel-type molecular excitons in the 1.5-2 eV range. These results lead us to

  1. 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...

  2. Controlling molecular condensation/diffusion of copper phthalocyanine by local electric field induced with scanning tunneling microscope tip

    Science.gov (United States)

    Nagaoka, Katsumi; Yaginuma, Shin; Nakayama, Tomonobu

    2018-02-01

    We have discovered the condensation/diffusion phenomena of copper phthalocyanine (CuPc) molecules controlled with a pulsed electric field induced by the scanning tunneling microscope tip. This behavior is not explained by the conventional induced dipole model. In order to understand the mechanism, we have measured the electronic structure of the molecule by tunneling spectroscopy and also performed theoretical calculations on molecular orbitals. These data clearly indicate that the molecule is positively charged owing to charge transfer to the substrate, and that hydrogen bonding exists between CuPc molecules, which makes the molecular island stable.

  3. Bosonic cascades of indirect excitons

    Science.gov (United States)

    Nalitov, A. V.; De Liberato, S.; Lagoudakis, P.; Savvidis, P. G.; Kavokin, A. V.

    2017-08-01

    Recently, the concept of the terahertz bosonic cascade laser (BCL) based on a parabolic quantum well (PQW) embedded in a microcavity was proposed. We refine this proposal by suggesting transitions between indirect exciton (IX) states as a source of terahertz emission. We explicitly propose a structure containing a narrow-square QW and a wide-parabolic QW for the realisation of a bosonic cascade. Advantages of this type of structures are in large dipole matrix elements for terahertz transitions and in long exciton radiative lifetimes which are crucial for realisation of threshold and quantum efficiency BCLs.

  4. Proposal of highly efficient photoemitter with strong photon-harvesting capability and exciton superradiance

    Science.gov (United States)

    Matsuda, Takuya; Ishihara, Hajime

    2017-08-01

    We propose a system of highly efficient photoemitters comprising metal-molecule multilayered structures. In the proposed structure, the absorption in the molecular layer is greatly enhanced through quantum interference between the split modes arising from the coupling of the layered excitons and the plasmons sustained by the metal layer. Furthermore, the large interaction volume between surface plasmons and excitons causes exciton superradiance, which results in the extremely efficient photoemission. This finding indicates the possibility of designing highly efficient photoemitters based on simple layered structures.

  5. Dynamic Control of Plasmon-Exciton Coupling in Au Nanodisk–J-Aggregate Hybrid Nanostructure Arrays

    KAUST Repository

    Zheng, Yue Bing

    2009-01-01

    We report the dynamic control of plasmon-exciton coupling in Au nanodisk arrays adsorbed with J-aggregate molecules by incident angle of light. The angle-resolved spectra of an array of bare Au nanodisks exhibit continuous shifting of localized surface plasmon resonances. This characteristic enables the production of real-time, controllable spectral overlaps between molecular and plasmonic resonances, and the efficient measurement of plasmon-exciton coupling as a function of wavelength with one or fewer nanodisk arrays. Experimental observations of varying plasmon-exciton coupling match with coupled dipole approximation calculations.

  6. Exciton Localization in Extended {\\pi}-electron Systems: Comparison of Linear and Cyclic Structures

    CERN Document Server

    Thiessen, Alexander; Jester, Stefan-S; Aggarwal, A Vikas; Idelson, Alissa; Bange, Sebastian; Vogelsang, Jan; Höger, Sigurd; Lupton, John M

    2015-01-01

    We employ five {\\pi}-conjugated model materials of different molecular shape --- oligomers and cyclic structures --- to investigate the extent of exciton self-trapping and torsional motion of the molecular framework following optical excitation. Our studies combine steady-state and transient fluorescence spectroscopy in the ensemble with measurements of polarization anisotropy on single molecules, supported by Monte Carlo simulations. The dimer exhibits a significant spectral red-shift within $\\sim$ 100 ps after photoexcitation which is attributed to torsional relaxation. This relaxation mechanism is inhibited in the structurally rigid macrocyclic analogue. However, both systems show a high degree of exciton localization but with very different consequences: while in the macrocycle the exciton localizes randomly on different parts of the ring, scrambling polarization memory, in the dimer, localization leads to a deterministic exciton position with luminescence characteristics of a dipole. Monte Carlo simulati...

  7. 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.

  8. 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...

  9. Imaging heterogeneous ultrafast exciton dynamics in organic semiconducting thin films

    Science.gov (United States)

    Ginsberg, Naomi S.

    2013-03-01

    In solid state semiconducting molecular materials used in electro-optical applications, relatively long exciton diffusion lengths hold the promise to boost device performance by relaxing proximity constraints on the locations for light absorption and interfacial charge separation. The architecture of such materials determines their optical and electronic properties as a result of spacing- and orientation-dependent Coulomb couplings between adjacent molecules. Exciton character and dynamics are generally inferred from bulk optical measurements, which can present a severe limitation on our understanding of these films because their constituent molecules are not perfectly ordered. Rather, films of small organic molecules are composed of multiple microcrystalline domains, and this deposition-dependent microstructure can have profound impacts on transport properties. Using ultrafast transient absorption microscopy, we track the time evolution of excitons, domain by domain, in solid state thin films of TIPS-pentacene, a small soluble molecule that has recently been used in organic semiconducting devices because of its high hole mobility. The results from this spatially-resolved nonlinear optical spectroscopy support our hypothesis that bulk optical measurements deleteriously average over heterogeneities in both spatial and electronic structure; we have revealed significant inhomogeneity in exciton dynamics. Domains that appear homogeneous in linear optical microscopy are shown to have spatial variation and defects, and notable differences in exciton character and behavior are observed at domain boundaries. To interpret the contrast we observe with ultrafast dynamics, we correlate our data to local linear absorption, polarization analysis, profilometry, and atomic force microscopy. With this combined approach, we aim to ultimately understand fundamental structure-function relationship in molecular materials to provide predictive power to material development and device

  10. Synthetic Control of Exciton Behavior in Colloidal Quantum Dots.

    Science.gov (United States)

    Pu, Chaodan; Qin, Haiyan; Gao, Yuan; Zhou, Jianhai; Wang, Peng; Peng, Xiaogang

    2017-03-08

    Colloidal quantum dots are promising optical and optoelectronic materials for various applications, whose performance is dominated by their excited-state properties. This article illustrates synthetic control of their excited states. Description of the excited states of quantum-dot emitters can be centered around exciton. We shall discuss that, different from conventional molecular emitters, ground-state structures of quantum dots are not necessarily correlated with their excited states. Synthetic control of exciton behavior heavily relies on convenient and affordable monitoring tools. For synthetic development of ideal optical and optoelectronic emitters, the key process is decay of band-edge excitons, which renders transient photoluminescence as important monitoring tool. On the basis of extensive synthetic developments in the past 20-30 years, synthetic control of exciton behavior implies surface engineering of quantum dots, including surface cation/anion stoichiometry, organic ligands, inorganic epitaxial shells, etc. For phosphors based on quantum dots doped with transition metal ions, concentration and location of the dopant ions within a nanocrystal lattice are found to be as important as control of the surface states in order to obtain bright dopant emission with monoexponential yet tunable photoluminescence decay dynamics.

  11. Low temperature exciton-exciton annihilation in amphi-PIPE J-aggregates

    Directory of Open Access Journals (Sweden)

    C. Spitz

    2006-01-01

    Full Text Available The mobility of optically excited excitons on J-aggregates can be demonstrated by the phenomena of exciton-exciton annihilation. In this intensity-dependent process the collision of two excitons results in their annihilation and hence in a shortening of the mean excitation lifetime. By measuring the intensity-dependent fluorescent lifetime in contrast to the predicted immobilization of the excitons at low temperature we could prove the excellent mobility of the excitons at a temperature (4K, which is far below their expected freezing point.

  12. Plasmonic band gap engineering of plasmon-exciton coupling.

    Science.gov (United States)

    Karademir, Ertugrul; Balci, Sinan; Kocabas, Coskun; Aydinli, Atilla

    2014-10-01

    Controlling plasmon-exciton coupling through band gap engineering of plasmonic crystals is demonstrated in the Kretschmann configuration. When the flat metal surface is textured with a sinusoidal grating only in one direction, using laser interference lithography, it exhibits a plasmonic band gap because of the Bragg scattering of surface plasmon polaritons on the plasmonic crystals. The contrast of the grating profile determines the observed width of the plasmonic band gap and hence allows engineering of the plasmonic band gap. In this work, resonant coupling between the molecular resonance of a J-aggregate dye and the plasmonic resonance of a textured metal film is extensively studied through plasmonic band gap engineering. Polarization dependent spectroscopic reflection measurements probe the spectral overlap occurring between the molecular resonance and the plasmonic resonance. The results indicate that plasmon-exciton interaction is attenuated in the band gap region along the grating direction.

  13. Carbon-fiber tips for scanning probe microscopes and molecular electronics experiments

    NARCIS (Netherlands)

    Rubio-Bollinger, G.; Castellanos-Gomez, A.; Bilan, S.; Zotti, L.A.; Arroyo, C.R.; Agraït, N.; Cuevas, J.

    2012-01-01

    We fabricate and characterize carbon-fiber tips for their use in combined scanning tunneling and force microscopy based on piezoelectric quartz tuning fork force sensors. An electrochemical fabrication procedure to etch the tips is used to yield reproducible sub-100-nm apex. We also study electron

  14. 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

  15. Excitons: Molecules in flatland

    Science.gov (United States)

    Yao, Wang

    2015-06-01

    Forming molecules from atoms is commonplace in dense atomic gases. But it now seems that some two-dimensional materials provide a suitable environment for creating complex molecular states from the hydrogen-like electron-hole pairs that form in semiconductors.

  16. 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.

  17. Analytic derivative couplings and first-principles exciton/phonon coupling constants for an ab initio Frenkel-Davydov exciton model: Theory, implementation, and application to compute triplet exciton mobility parameters for crystalline tetracene

    Science.gov (United States)

    Morrison, Adrian F.; Herbert, John M.

    2017-06-01

    Recently, we introduced an ab initio version of the Frenkel-Davydov exciton model for computing excited-state properties of molecular crystals and aggregates. Within this model, supersystem excited states are approximated as linear combinations of excitations localized on molecular sites, and the electronic Hamiltonian is constructed and diagonalized in a direct-product basis of non-orthogonal configuration state functions computed for isolated fragments. Here, we derive and implement analytic derivative couplings for this model, including nuclear derivatives of the natural transition orbital and symmetric orthogonalization transformations that are part of the approximation. Nuclear derivatives of the exciton Hamiltonian's matrix elements, required in order to compute the nonadiabatic couplings, are equivalent to the "Holstein" and "Peierls" exciton/phonon couplings that are widely discussed in the context of model Hamiltonians for energy and charge transport in organic photovoltaics. As an example, we compute the couplings that modulate triplet exciton transport in crystalline tetracene, which is relevant in the context of carrier diffusion following singlet exciton fission.

  18. Analytic derivative couplings and first-principles exciton/phonon coupling constants for an ab initio Frenkel-Davydov exciton model: Theory, implementation, and application to compute triplet exciton mobility parameters for crystalline tetracene.

    Science.gov (United States)

    Morrison, Adrian F; Herbert, John M

    2017-06-14

    Recently, we introduced an ab initio version of the Frenkel-Davydov exciton model for computing excited-state properties of molecular crystals and aggregates. Within this model, supersystem excited states are approximated as linear combinations of excitations localized on molecular sites, and the electronic Hamiltonian is constructed and diagonalized in a direct-product basis of non-orthogonal configuration state functions computed for isolated fragments. Here, we derive and implement analytic derivative couplings for this model, including nuclear derivatives of the natural transition orbital and symmetric orthogonalization transformations that are part of the approximation. Nuclear derivatives of the exciton Hamiltonian's matrix elements, required in order to compute the nonadiabatic couplings, are equivalent to the "Holstein" and "Peierls" exciton/phonon couplings that are widely discussed in the context of model Hamiltonians for energy and charge transport in organic photovoltaics. As an example, we compute the couplings that modulate triplet exciton transport in crystalline tetracene, which is relevant in the context of carrier diffusion following singlet exciton fission.

  19. Advanced electric-field scanning probe lithography on molecular resist using active cantilever

    Science.gov (United States)

    Kaestner, Marcus; Aydogan, Cemal; Ivanov, Tzvetan; Ahmad, Ahmad; Angelov, Tihomir; Reum, Alexander; Ishchuk, Valentyn; Krivoshapkina, Yana; Hofer, Manuel; Lenk, Steve; Atanasov, Ivaylo; Holz, Mathias; Rangelow, Ivo W.

    2015-07-01

    The routine "on demand" fabrication of features smaller than 10 nm opens up new possibilities for the realization of many devices. Driven by the thermally actuated piezoresistive cantilever technology, we have developed a prototype of a scanning probe lithography (SPL) platform which is able to image, inspect, align, and pattern features down to the single digit nanoregime. Here, we present examples of practical applications of the previously published electric-field based current-controlled scanning probe lithography. In particular, individual patterning tests are carried out on calixarene by using our developed table-top SPL system. We have demonstrated the application of a step-and-repeat SPL method including optical as well as atomic force microscopy-based navigation and alignment. The closed-loop lithography scheme was applied to sequentially write positive and negative tone features. Due to the integrated unique combination of read-write cycling, each single feature is aligned separately with the highest precision and inspected after patterning. This routine was applied to create a pattern step by step. Finally, we have demonstrated the patterning over larger areas, over existing topography, and the practical applicability of the SPL processes for lithography down to 13-nm pitch patterns. To enhance the throughput capability variable beam diameter electric field, current-controlled SPL is briefly discussed.

  20. Automated identification and quantification of glycerophospholipid molecular species by multiple precursor ion scanning

    DEFF Research Database (Denmark)

    Ejsing, Christer S.; Duchoslav, Eva; Sampaio, Julio

    2006-01-01

    We report a method for the identification and quantification of glycerophospholipid molecular species that is based on the simultaneous automated acquisition and processing of 41 precursor ion spectra, specific for acyl anions of common fatty acids moieties and several lipid class-specific fragme...... of glycerophospholipids. The automated analysis of total lipid extracts was powered by a robotic nanoflow ion source and produced currently the most detailed description of the glycerophospholipidome....

  1. Antibody-scanning and epitope-tagging methods; molecular mapping of proteins using antibodies.

    Science.gov (United States)

    Toyoda, T; Masunaga, K; Ohtsu, Y; Hara, K; Hamada, N; Kashiwagi, T; Iwahashi, J

    2000-11-01

    Because synthetic short peptides bearing critical binding residues, can chemically mimic the folded antigenic determinants on proteins, short synthetic peptides can generate antibodies that react with cognate sequences in intact folded proteins. According to this mimotope theory, we produced site-specific antibodies by immunization with short peptides which overlapped each other and covered the entire protein, and used them for domain mapping of influenza virus RNA polymerase (antibody-scanning method). We also used a tagged-epitope and its monoclonal antibodies for topology mapping of clathrin light chains in clathrin triskelions by electron microscopy. Both methods using specific epitopes in combination with their antibodies enable us to determine the domains of interesting proteins systematically without the need to generate monoclonal antibodies or mutant proteins.

  2. Influence of exciton-phonons coupling on the exciton binding energy in monolayer transition metal dichalcogenides

    Science.gov (United States)

    Wang, Zi-Wu; Li, Wei-Ping; Xiao, Yao; Li, Run-Ze; Li, Zhi-Qing

    2017-06-01

    We theoretically investigate the correction of exciton binding energy arising from the exciton-optical phonon coupling in monolayer transition metal dichalcogenides (TMDs) using the linear operator and Lee-Low-Pines unitary transformation methods. We take into account not only the exciton coupling with intrinsic longitudinal optical phonon modes but also the surface optical phonon modes induced by polar substrates supporting monolayer TMDs. We find that the exciton binding energies are corrected on a large scale due to these exciton-optical phonon couplings. We discuss the dependences of exciton binding energy on the cut-off wave vector of optical phonon modes, the polarization strength of substrate materials, and the distance between polar substrates and TMDs. These results provide potential explanations for the divergence of the exciton binding energy between the experiment and theory in TMDs.

  3. Charged Frenkel biexcitons in organic molecular crystals

    CERN Document Server

    Agranovich, V M; Kamchatnov, A M

    2001-01-01

    It is known that the energy of the lowest electronic transition in neutral molecules of anthracene, tetracene and other polyacenes is blue shifted in comparison with the corresponding transition energy in mono-valent molecular ions. This effect in molecular crystal may be responsible for the attraction between molecular (Frenkel) exciton and charge carrier. Due to this attraction the bound state of Frenkel exciton and free charge (charged Frenkel exciton) may be formed. The same mechanism can be responsible for formation of charged biexcitons (bound state of two Frenkel excitons and a charge carrier). Calculations are performed for molecular crystals like tetracene by means of one-dimensional lattice model

  4. In silico simulations of tunneling barrier measurements for molecular orbital-mediated junctions: A molecular orbital theory approach to scanning tunneling microscopy

    Energy Technology Data Exchange (ETDEWEB)

    Terryn, Raymond J.; Sriraman, Krishnan; Olson, Joel A., E-mail: jolson@fit.edu; Baum, J. Clayton, E-mail: cbaum@fit.edu [Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901 (United States); Novak, Mark J. [Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, 501 E. Saint Joseph Street, Rapid City, South Dakota 57701 (United States)

    2016-09-15

    A new simulator for scanning tunneling microscopy (STM) is presented based on the linear combination of atomic orbitals molecular orbital (LCAO-MO) approximation for the effective tunneling Hamiltonian, which leads to the convolution integral when applied to the tip interaction with the sample. This approach intrinsically includes the structure of the STM tip. Through this mechanical emulation and the tip-inclusive convolution model, dI/dz images for molecular orbitals (which are closely associated with apparent barrier height, ϕ{sub ap}) are reported for the first time. For molecular adsorbates whose experimental topographic images correspond well to isolated-molecule quantum chemistry calculations, the simulator makes accurate predictions, as illustrated by various cases. Distortions in these images due to the tip are shown to be in accord with those observed experimentally and predicted by other ab initio considerations of tip structure. Simulations of the tunneling current dI/dz images are in strong agreement with experiment. The theoretical framework provides a solid foundation which may be applied to LCAO cluster models of adsorbate–substrate systems, and is extendable to emulate several aspects of functional STM operation.

  5. Excitonic magnetism in d6 perovskites

    Science.gov (United States)

    Afonso, J. Fernández; Kuneš, J.

    2017-03-01

    We use the LDA+U method to study the possibility of exciton condensation in perovskites of transition metals with the d6 electronic configuration such as LaCoO3. For realistic interaction parameters we find several distinct solutions exhibiting a spin-triplet exciton condensate, which gives rise to a local spin density distribution while the ordered moments are vanishingly small. Rhombohedral distortion from the ideal cubic structure suppresses the ordered state, contrary to the spin-orbit coupling which enhances the excitonic condensation energy. We explain the trends observed in the numerical simulations with the help of a simplified strong-coupling model. Our results indicate that LaCoO3 is close to the excitonic instability and suggest ways how to achieve the exciton condensation.

  6. Hydrophilicity and Microsolvation of an Organic Molecule Resolved on the Sub-molecular Level by Scanning Tunneling Microscopy.

    Science.gov (United States)

    Lucht, Karsten; Loose, Dirk; Ruschmeier, Maximilian; Strotkötter, Valerie; Dyker, Gerald; Morgenstern, Karina

    2018-01-26

    Low-temperature scanning tunneling microscopy was used to follow the formation of a solvation shell around an adsorbed functionalized azo dye from the attachment of the first water molecule to a fully solvated molecule. Specific functional groups bind initially one water molecule each, which act as anchor points for additional water molecules. Further water attachment occurs in areas close to these functional groups even when the functional groups themselves are already saturated. In contrast, water molecules surround the hydrophobic parts of the molecule only when the two-dimensional solvation shell closes around them. This study thus traces hydrophilic and hydrophobic properties of an organic molecule down to a sub-molecular length scale. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  7. Atomistic model for excitons: Capturing Strongly Bound Excitons in Monolayer Transition-Metal Dichalcogenides

    Science.gov (United States)

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2015-03-01

    Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. This work has been funded by the Office of Naval Research (ONR), directly and through the Naval Research Laboratory (NRL). F.T and E.S acknowledge support from NRL through the NRC Research Associateship Program and ONR Summer Faculty Program, respectively.

  8. Tryptophan Scanning Mutagenesis Identifies the Molecular Determinants of Distinct Barttin Functions*

    Science.gov (United States)

    Wojciechowski, Daniel; Fischer, Martin; Fahlke, Christoph

    2015-01-01

    CLC-K chloride channels are expressed in the kidney and in the inner ear and require the accessory subunit barttin for proper function and membrane insertion. Barttin exerts multiple functions on CLC-proteins: it modifies protein stability and intracellular trafficking as well as channel activity, ion conduction, and gating. So far, the molecular determinants of these distinct barttin functions have remained elusive. Here we performed serial perturbation mutagenesis to identify the sequence determinants of barttin function. Barttin consists of two transmembrane helices followed by a long intracellular carboxyl terminus, and earlier work demonstrated that the transmembrane core of barttin suffices for most effects on the α-subunit. We individually substituted every amino acid of the predicted transmembrane core (amino acids 9–26 and 35–55) with tryptophan, co-expressed mutant barttin with hClC-Ka or V166E rClC-K1, and characterized CLC-K/barttin channels by patch clamp techniques, biochemistry, and confocal microscopy. The majority of mutations left the chaperone function of barttin, i.e. the effects on endoplasmic reticulum exit and surface membrane insertion, unaffected. In contrast, tryptophan insertion at multiple positions resulted in impaired activity of hClC-Ka/barttin and changes in gating of V166E rClC-K1/barttin. These results demonstrate that mutations in a cluster of hydrophobic residues within transmembrane domain 1 affect barttin-CLC-K interaction and impair gating modification by the accessory subunit. Whereas tight interaction is necessary for functional modification, even impaired association of barttin and CLC-K suffices for normal intracellular trafficking. Our findings allow definition of a likely interaction surface and clarify the mechanisms underlying CLC-K channel modification by barttin. PMID:26063802

  9. Dark excitons in transition metal dichalcogenides

    Science.gov (United States)

    Malic, Ermin; Selig, Malte; Feierabend, Maja; Brem, Samuel; Christiansen, Dominik; Wendler, Florian; Knorr, Andreas; Berghäuser, Gunnar

    2018-01-01

    Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a nondirect gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.

  10. Modification of the luminescent characteristics belonging to the molecule that interacts with the exciton states of the J-aggregate

    Science.gov (United States)

    Ropakova, I. Yu.; Sorokin, A. V.; Zvyagin, A. A.; Malyukin, Yu. V.

    2017-03-01

    A quantum theory for the light absorption and photoluminescence of a molecule (luminophore) interacting with a J-aggregate linear molecular chain is constructed. It is shown that together with the band states of Frenkel excitons in the molecular chain the contribution to light absorption and luminescence comes from local levels that split off from the exciton band as a result of the interaction between the molecular chain and the luminophore. It is also demonstrated that the contribution to light absorption and photoluminescence from local levels is non-linearly dependent on the coupling parameter between the luminophore and the J-aggregate molecular chain.

  11. 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 obtained for mass ratios up to 0.426. The interparticle distances are up to 50 times larger than the corresponding exciton radius. The oscillator strengths are about 104 times greater than those of free excitons, while the exchange corrections for the complex are comparable to those of free excitions...

  12. Excitonic quasimolecules in nanosystems of quantum dots

    Science.gov (United States)

    Pokutnyi, Sergey I.

    2017-09-01

    The theory of excitonic quasimolecules (biexcitons) (formed of spatially separated electrons and holes) in a nanosystem that consists of semiconductor quantum dots synthesized in a borosilicate glass matrix is presented. It is shown that exciton quasimolecule formation is of a threshold character and is possible in nanosystem, if the spacing between the quantum dots surfaces is larger than a certain critical spacing. It was found that the binding energy of the singlet ground state of an exciton quasimolecule, consisting of two semiconductor quantum dots is a significant large values, larger than the binding energy of the biexciton in a semiconductor single crystal by almost two orders of magnitude.

  13. Exciton in closed and opened quantum dot

    Directory of Open Access Journals (Sweden)

    M.V.Tkach

    2007-01-01

    Full Text Available The theory of exciton spectrum in spherically symmetric states for the three- shell closed spherical quantum dot is proposed. The evolution of the exciton spectrum while varying the outer well thickness from zero (stationary spectrum of single closed spherical quantum dot to infinity (quasistationary spectrum of a single open spherical quantum dot is investigated. The mechanism of damping (semiwidth of quasistationary states due to the redistribution over the energy levels of probability of exciton location in the space of two inner shells of nanosystem is studied. It is shown that the three shell closed spherical quantum dot of a rather big thickness of the outer well quite sufficiently and exactly reflects the basic properties of the quasistationary exciton spectrum in a single open spherical quantum dot.

  14. Exciton absorption in narrow armchair graphene nanoribbons

    Energy Technology Data Exchange (ETDEWEB)

    Monozon, B.S. [Physics Department, Marine Technical University, 3 Lotsmanskaya Str., 190008 St. Petersburg (Russian Federation); Schmelcher, P., E-mail: pschmelc@physnet.uni-hamburg.de [Zentrum für Optische Quantentechnologien, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany); The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

    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.

  15. Excitonic physics in a Dirac quantum dot

    Science.gov (United States)

    Raca, V.; Milovanović, M. V.

    2017-11-01

    We present a description of vacuum polarization in a circular Dirac quantum dot in two spatial dimensions assuming α —the relative strength of the Coulomb interaction small enough to render an approximation with a single electron (hole) lowest energy level relevant. Applying this approximation, we find that for αc≈1.05 the lowest level is half filled irrespective of the number of flavors that are present. The ground state can be represented as a superposition of particular (even number) excitonic states which constitute an excitonic cloud that evolves in a crossover manner. The ground state is degenerate with an intervalley excitonic state at αc≈1.05 , a critical strength, that in our approximation marks a point with single electron and exciton resonances.

  16. Atomic lattice excitons: from condensates to crystals

    Energy Technology Data Exchange (ETDEWEB)

    Kantian, A [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria); Daley, A J [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria); Toermae, P [Nanoscience Center, Department of Physics, University of Jyvaeskylae, PO Box 35, FIN-40014 (Finland); Zoller, P [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria)

    2007-11-15

    We discuss atomic lattice excitons (ALEs), bound particle-hole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean set-up, with tunable masses and interactions, to study fundamental properties of excitons including exciton condensation. We also find that for a large effective mass ratio between particles and holes, effective long-range interactions can mediate the formation of an exciton crystal, for which superfluidity is suppressed. Using a combination of mean-field treatments, bosonized theory based on a Born-Oppenheimer approximation, and one-dimensional (1D) numerical computation, we discuss the properties of ALEs under varying conditions, and discuss in particular their preparation and measurement.

  17. Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings

    NARCIS (Netherlands)

    Ding, F.; Akopian, N.; Li, B.; Perinetti, U.; Govorov, A.; Peeters, F.M.; Bof Bufon, C.C.; Deneke, C.; Chen, Y.H.; Rastelli, A.; Schmidt, O.G.; Zwiller, V.

    2010-01-01

    We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an

  18. Exciton-polariton wakefields in semiconductor microcavities

    Energy Technology Data Exchange (ETDEWEB)

    Terças, H., E-mail: hugo.tercas@uibk.ac.at [Physics of Information Group, Instituto de Telecomunicações, Lisbon (Portugal); Institute for Theoretical Physics, University of Innsbruck, Technikerstrasse 25, 6020 Innsbruck (Austria); Mendonça, J.T., E-mail: titomend@ist.utl.pt [Instituto de Física, Universidade de São Paulo, São Paulo SP, 05508-090 Brazil (Brazil); IPFN, Instituto Superior Técnico, 1049-001 Lisboa (Portugal)

    2016-02-22

    We consider the excitation of polariton wakefields due to a propagating light pulse in a semiconductor microcavity. We show that two kinds of wakes are possible, depending on the constituents fraction (either exciton or photon) of the polariton wavefunction. The nature of the wakefields (pure excitonic or polaritonic) can be controlled by changing the speed of propagation of the external pump. This process could be used as a diagnostic for the internal parameters of the microcavity.

  19. 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.

  20. Intrinsic dynamics of weakly and strongly confined excitons in nonpolar nitride-based heterostructures

    OpenAIRE

    Corfdir, Pierre; Levrat, Jacques; Dussaigne, Amélie; Lefebvre, Pierre; Teisseyre, Henryk; Grzegory, Izabella; Suski, Tadeusz; Ganière, Jean-Daniel; Grandjean, Nicolas; Deveaud-Plédran, Benoît

    2011-01-01

    Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm-2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of st...

  1. Automated angle-scanning photoemission end-station with molecular beam epitaxy at KEK-PF BL-1C

    CERN Document Server

    Ono, K; Horiba, K; Oh, J H; Nakazono, S; Kihara, T; Nakamura, K; Mano, T; Mizuguchi, M; Oshima, M; Aiura, Y; Kakizaki, A

    2001-01-01

    In order to satisfy demands to study the electronic structure of quantum nanostructures, a VUV beamline and a high-resolution and high-throughput photoemission end-station combined with a molecular beam epitaxy (MBE) system have been constructed at the BL-1C of the Photon Factory. An angle-resolved photoemission spectrometer, having high energy- and angular-resolutions; VG Microtech ARUPS10, was installed. The total energy resolution of 31 meV at the 60 eV of photon energy is achieved. For the automated angle-scanning photoemission, the electron spectrometer mounted on a two-axis goniometer can be rotated in vacuum by the computer-controlled stepping motors. Another distinctive feature of this end-station is a connection to a MBE chamber in ultahigh vacuum (UHV). In this system, MBE-grown samples can be transferred into the photoemission chamber without breaking UHV. Photoemission spectra of MBE-grown GaAs(0 0 1) surfaces were measured with high-resolution and bulk and surface components are clearly resolved.

  2. Molecular characterization and confocal laser scanning microscopic study of Pygidiopsis macrostomum (Trematoda: Heterophyidae) parasites of guppies Poecilia vivipara.

    Science.gov (United States)

    Borges, J N; Costa, V S; Mantovani, C; Barros, E; Santos, E G N; Mafra, C L; Santos, C P

    2017-02-01

    Pygidiopsis macrostomum and Ascocotyle (Phagicola) pindoramensis (Digenea: Heterophyidae) parasitize guppies as intermediate hosts and, respectively, fish-eating mammals or birds as definitive hosts. Heterophyids have zoonotic potential, and molecular studies associated with morphological and ecological aspects have helped to clarify their taxonomy and phylogeny. Poecilia vivipara naturally parasitized by metacercariae of both species (100% prevalence) exhibit no external signs of parasitism. In this work, four new sequences of P. macrostomum (18S rDNA, 28S rDNA and ITS2 rDNA) and one new sequence of A. (P.) pindoramensis (mtDNA cox-1) are presented. Phylogeny reconstructions linked P. macrostomum to other heterophyids, but the separation of the Heterophyidae and Opisthorchiidae remains unclear. Additionally, we used indirect immunocytochemistry and the phalloidin-fluorescence techniques allied with confocal laser scanning microscopy to describe muscular and neuronal structures of P. macrostomum. A complex arrangement of muscular fibres is associated with the tegument, suckers, gut and reproductive system. Radial fibres around the ventral sucker are thick, branched and extend to the body wall. High-resolution confocal imaging revealed a typical digenean muscular arrangement and important heterophyid morphological traits. These data will support future control measures to reduce the parasitism in guppies reared in fish farming systems, especially for aquarium and experimental purposes. © 2016 John Wiley & Sons Ltd.

  3. Characterization of the molecular distribution of drugs in glassy solid dispersions at the nano-meter scale, using differential scanning calorimetry and gravimetric water vapour sorption techniques

    NARCIS (Netherlands)

    van Drooge, D J; Hinrichs, W L J; Visser, M R; Frijlink, H W

    2006-01-01

    The molecular distribution in fully amorphous solid dispersions consisting of poly(vinylpyrrolidone) (PVP)-diazepam and inulin-diazepam was studied. One glass transition temperature (T-g), as determined by temperature modulated differential scanning calorimetry (TMDSC), was observed in PVP-diazepam

  4. Exciton scattering approach for optical spectra calculations in branched conjugated macromolecules

    Energy Technology Data Exchange (ETDEWEB)

    Li, Hao [Department of Chemistry, University of Houston, Houston, TX 77204 (United States); Wu, Chao [Electronic Structure Lab, Center of Microscopic Theory and Simulation, Frontier Institute of Science and Technology, Xian Jiaotong University, Xian 710054 (China); Malinin, Sergey V. [Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202 (United States); Tretiak, Sergei, E-mail: serg@lanl.gov [Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Chernyak, Vladimir Y., E-mail: chernyak@chem.wayne.edu [Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202 (United States)

    2016-12-20

    The exciton scattering (ES) technique is a multiscale approach based on the concept of a particle in a box and developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, electronic excitations in molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph whose edges and nodes stand for the molecular linear segments and vertices, respectively. Exciton propagation on the linear segments is characterized by the exciton dispersion, whereas exciton scattering at the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized “particle in a box” problems on the graph that represents the molecule. Similarly, unique energy-dependent ES dipolar parameters permit calculations of the corresponding oscillator strengths, thus, completing optical spectra modeling. Both the energetic and dipolar parameters can be extracted from quantum-chemical computations in small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within a considered molecular family could be performed with negligible numerical effort. We demonstrate the ES method application to molecular families of branched conjugated phenylacetylenes and ladder poly-para-phenylenes, as well as structures with electron donor and acceptor chemical substituents. Time-dependent density functional theory (TD-DFT) is used as a reference model for electronic structure. The ES calculations accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.

  5. 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.

  6. A molecular line scan in the Hubble deep field north: Constraints on the co luminosity function and the cosmic H{sub 2} density

    Energy Technology Data Exchange (ETDEWEB)

    Walter, F.; Decarli, R.; Da Cunha, E. [Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany); Sargent, M. [Laboratoire AIM, CEA/DSM-CNRS-Universite Paris Diderot, Irfu/Service d' Astrophysique, CEA Saclay, Orme des Merisiers, F-91191 Gif-sur-Yvette cedex (France); Carilli, C. [NRAO, Pete V. Domenici Array Science Center, P.O. Box O, Socorro, NM 87801 (United States); Dickinson, M.; Daddi, E. [National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719 (United States); Riechers, D. [Cornell University, 220 Space Sciences Building, Ithaca, NY 14853 (United States); Ellis, R. [Astronomy Department, California Institute of Technology, MC105-24, Pasadena, CA 91125 (United States); Stark, D.; Weiner, B. [Steward Observatory, University of Arizona, 933 North Cherry Street, Tucson, AZ 85721 (United States); Aravena, M. [European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago (Chile); Bell, E. [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States); Bertoldi, F. [Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, D-53121 Bonn (Germany); Cox, P.; Downes, D.; Neri, R. [IRAM, 300 rue de la piscine, F-38406 Saint-Martin d' Hères (France); Lentati, L.; Maiolino, R. [Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Menten, K. M., E-mail: walter@mpia.de [Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany); and others

    2014-02-20

    We present direct constraints on the CO luminosity function at high redshift and the resulting cosmic evolution of the molecular gas density, ρ{sub H{sub 2}}(z), based on a blind molecular line scan in the Hubble Deep Field North (HDF-N) using the IRAM Plateau de Bure Interferometer. Our line scan of the entire 3 mm window (79-115 GHz) covers a cosmic volume of ∼7000 Mpc{sup 3}, and redshift ranges z < 0.45, 1.01 < z < 1.89 and z > 2. We use the rich multiwavelength and spectroscopic database of the HDF-N to derive some of the best constraints on CO luminosities in high redshift galaxies to date. We combine the blind CO detections in our molecular line scan (presented in a companion paper) with stacked CO limits from galaxies with available spectroscopic redshifts (slit or mask spectroscopy from Keck and grism spectroscopy from the Hubble Space Telescope) to give first blind constraints on high-z CO luminosity functions and the cosmic evolution of the H{sub 2} mass density ρ{sub H{sub 2}}(z) out to redshifts z ∼ 3. A comparison to empirical predictions of ρ{sub H{sub 2}}(z) shows that the securely detected sources in our molecular line scan already provide significant contributions to the predicted ρ{sub H{sub 2}}(z) in the redshift bins (z) ∼ 1.5 and (z) ∼ 2.7. Accounting for galaxies with CO luminosities that are not probed by our observations results in cosmic molecular gas densities ρ{sub H{sub 2}}(z) that are higher than current predictions. We note, however, that the current uncertainties (in particular the luminosity limits, number of detections, as well as cosmic volume probed) are significant, a situation that is about to change with the emerging ALMA observatory.

  7. Plasmon-excitonic polaritons in superlattices

    Science.gov (United States)

    Kosobukin, V. A.

    2017-05-01

    A theory for propagation of polaritons in superlattices with resonant plasmon-exciton coupling is presented. A periodical superlattice consists of a finite number of cells with closely located a quantum well and a monolayer of metal nanoparticles. Under study is the spectrum of hybrid modes formed of the quasitwo- dimensional excitons of quantum wells and the dipole plasmons of metal particles. The problem of electrodynamics is solved by the method of Green's functions with taking account of the resonant polarization of quantum wells and nanoparticles in a self-consistent approximation. The effective polarizability of spheroidal particles occupying a square lattice is calculated with taking into consideration the local-field effect of dipole plasmons of the layer and their images caused by the excitonic polarization of nearest quantum well. Optical reflection spectra of superlattices with GaAs/AlGaAs quantum wells and silver particles are numerically analyzed. Special attention is paid to the superradiant regime originated in the Bragg diffraction of polaritons in superlattice. Superradiance is investigated separately for plasmons and excitons, and then for hybrid plasmonexcitonic polaritons. It is demonstrated that the broad spectrum of reflectance associated with plasmons depends on the number of cells in superlattice, and it has a narrow spectral dip in the range of plasmon-excitonic Rabi splitting.

  8. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... Because nuclear medicine procedures are able to pinpoint molecular activity within the body, they offer the potential ... or imaging device that produces pictures and provides molecular information. The thyroid scan and thyroid uptake provide ...

  9. 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

  10. Excitonic superfluid phase in double bilayer graphene

    Science.gov (United States)

    Li, J. I. A.; Taniguchi, T.; Watanabe, K.; Hone, J.; Dean, C. R.

    2017-08-01

    A spatially indirect exciton is created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite boson. Such excitons are long-lived, and in the limit of strong interactions are predicted to undergo a Bose-Einstein condensate-like phase transition into a superfluid ground state. Here, we report evidence of an exciton condensate in the quantum Hall effect regime of double-layer structures of bilayer graphene. Interlayer correlation is identified by quantized Hall drag at matched layer densities, and the dissipationless nature of the phase is confirmed in the counterflow geometry. A selection rule for the condensate phase is observed involving both the orbital and valley indices of bilayer graphene. Our results establish double bilayer graphene as an ideal system for studying the rich phase diagram of strongly interacting bosonic particles in the solid state.

  11. Exciton diffusion and dissociation in conjugated polymer/fullerene heterostructures

    NARCIS (Netherlands)

    Markov, D.E.; Amsterdam, E.; Blom, P.W.M.; Sieval, A.B.; Hummelen, J.C.; Heremans, PL; Muccini, M; Hofstraat, H

    2004-01-01

    Time-resolved luminescence spectroscopy has been used to investigate exciton diffusion in thin films of poly(p-phenylene vinylene) (PPV) based derivatives. Exciton density distribution upon photoexcitation in polymer/fullerene heterostructures has been modeled and exciton diffusion length values of

  12. 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....

  13. Exciton broadening in WS2/graphene heterostructures

    Science.gov (United States)

    Hill, Heather M.; Rigosi, Albert F.; Raja, Archana; Chernikov, Alexey; Roquelet, Cyrielle; Heinz, Tony F.

    2017-11-01

    We have used optical spectroscopy to observe spectral broadening of WS2 exciton reflectance peaks in heterostructures of monolayer WS2 capped with mono- to few-layer graphene. The broadening is found to be similar for the A and B excitons and on the order of 5-10 meV. No strong dependence on the number of graphene layers was observed within experimental uncertainty. The broadening can be attributed to charge- and energy-transfer processes between the two materials, providing an observed lower bound for the corresponding time scales of 65 fs.

  14. Anatomy of an Exciton : Vibrational Distortion and Exciton Coherence in H- and J-Aggregates

    NARCIS (Netherlands)

    Tempelaar, Roel; Stradomska, Anna; Knoester, Jasper; Spano, Frank C.

    2013-01-01

    In organic materials, coupling of electronic excitations to vibrational degrees of freedom results in polaronic excited states. Through numerical calculations, we demonstrate that the vibrational distortion field accompanying such a polaron scales as the product of the excitonic interaction field

  15. 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.

  16. Exciton diffusion length in narrow bandgap polymers

    NARCIS (Netherlands)

    Mikhnenko, Oleksandr V.; Azimi, Hamed; Scharber, Markus; Morana, Mauro; Blom, Paul W. M.; Loi, Maria Antonietta

    We developed a new method to accurately extract the singlet exciton diffusion length in organic semiconductors by blending them with a low concentration of methanofullerene[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM). The dependence of photoluminescence (PL) decay time on the fullerene

  17. 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...

  18. 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...

  19. Optical second harmonic generation from Wannier excitons

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm; Cornean, Horia

    2007-01-01

    , a simplified three-band Wannier exciton model of cubic semiconductors is applied and a closed form expression for the complex second harmonic response function including broadening is derived. Our calculated spectra are found to be in excellent agreement with the measured response near the band edge...

  20. 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 two...

  1. New approach for glyco- and lipidomics--molecular scanning of human brain gangliosides by TLC-Blot and MALDI-QIT-TOF MS.

    Science.gov (United States)

    Valdes-Gonzalez, Tania; Goto-Inoue, Naoko; Hirano, Wakako; Ishiyama, Hironobu; Hayasaka, Takahiro; Setou, Mitsutoshi; Taki, Takao

    2011-03-01

    We have developed a TLC-Blot system that makes possible the direct analysis of blotted glycosphingolipids on a polyvinylidene difluoride membrane from a high-performance TLC plate by immunological staining, chemical staining, enzymatic treatment and mass spectrometric (MS) analysis. An ion trap type matrix-assisted laser desorption/ionization-quadrupole ion trap-time of flight (MALDI-QIT-TOF) MS apparatus improved not only the molecular identification but also the analysis of molecular species of lipids on the polyvinylidene difluoride membrane. A new approach for glyco- and lipidomics, molecular scanning technology by a combination of TLC-Blot and MALDI-QIT-TOF MS, was developed and applied to human brain gangliosides separated from the tissues of patients with neural diseases and control patients. The results clearly showed a change of ganglioside composition, in addition to identifying individual ganglioside molecular species, in the hippocampus gray matter of patients with Alzheimer's disease. The results strongly suggested that metabolic changes of gangliosides played an important role in the progression of this disease. The present technology with molecular imaging should provide valuable information for elucidating the significance of molecular species in neuronal functions such as neural transmission, memory, and learning. © 2011 The Authors. Journal of Neurochemistry © 2011 International Society for Neurochemistry.

  2. Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes.

    Science.gov (United States)

    Hu, Zhongjian; Haws, Ryan T; Fei, Zhuping; Boufflet, Pierre; Heeney, Martin; Rossky, Peter J; Vanden Bout, David A

    2017-05-16

    Fluorination represents an important strategy in developing high-performance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains. This conformational variability results in disordered interchain morphology even between a few chains, prohibiting long-range effective interchain coupling. In stark contrast, the experimental and molecular dynamic calculations reveal that backbone fluorination of F-P3EHT leads to an extended rod-like single-chain conformation and hence highly ordered interchain packing in aggregates. Surprisingly, the ordered and close interchain packing in F-P3EHT does not lead to strong excitonic coupling between the chains but rather to dominant intrachain excitonic coupling that greatly reduces the molecular energetic heterogeneity.

  3. Scanning number and brightness yields absolute protein concentrations in live cells: a crucial parameter controlling functional bio-molecular interaction networks.

    Science.gov (United States)

    Papini, Christina; Royer, Catherine A

    2018-02-01

    Biological function results from properly timed bio-molecular interactions that transduce external or internal signals, resulting in any number of cellular fates, including triggering of cell-state transitions (division, differentiation, transformation, apoptosis), metabolic homeostasis and adjustment to changing physical or nutritional environments, amongst many more. These bio-molecular interactions can be modulated by chemical modifications of proteins, nucleic acids, lipids and other small molecules. They can result in bio-molecular transport from one cellular compartment to the other and often trigger specific enzyme activities involved in bio-molecular synthesis, modification or degradation. Clearly, a mechanistic understanding of any given high level biological function requires a quantitative characterization of the principal bio-molecular interactions involved and how these may change dynamically. Such information can be obtained using fluctation analysis, in particular scanning number and brightness, and used to build and test mechanistic models of the functional network to define which characteristics are the most important for its regulation.

  4. Fine structure of the exciton electroabsorption in semiconductor superlattices

    Energy Technology Data Exchange (ETDEWEB)

    Monozon, B.S., E-mail: borismonozon@mail.ru [Physics Department, Marine Technical University, 3 Lotsmanskaya Str., 190008 St.Petersburg (Russian Federation); Schmelcher, P. [Zentrum für Optische Quantentechnologien, The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)

    2017-02-15

    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.

  5. 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 b...

  6. Designing topological defects in 2D materials using scanning probe microscopy and a self-healing mechanism: a density functional-based molecular dynamics study

    Science.gov (United States)

    Popov, Igor; Đurišić, Ivana; Belić, Milivoj R.

    2017-12-01

    Engineering of materials at the atomic level is one of the most important aims of nanotechnology. The unprecedented ability of scanning probe microscopy to address individual atoms opened up the possibilities for nanomanipulation and nanolitography of surfaces and later on of two-dimensional materials. While the state-of-the-art scanning probe lithographic methods include, primarily, adsorption, desorption and repositioning of adatoms and molecules on substrates or tailoring nanoribbons by etching of trenches, the precise modification of the intrinsic atomic structure of materials is yet to be advanced. Here we introduce a new concept, scanning probe microscopy with a rotating tip, for engineering of the atomic structure of membranes based on two-dimensional materials. In order to indicate the viability of the concept, we present our theoretical research, which includes atomistic modeling, molecular dynamics simulations, Fourier analysis and electronic transport calculations. While stretching can be employed for fabrication of atomic chains only, our comprehensive molecular dynamics simulations indicate that nanomanipulation by scanning probe microscopy with a rotating tip is capable of assembling a wide range of topological defects in two-dimensional materials in a rather controllable and reproducible manner. We analyze two possibilities. In the first case the probe tip is retracted from the membrane while in the second case the tip is released beneath the membrane allowing graphene to freely relax and self-heal the pore made by the tip. The former approach with the tip rotation can be achieved experimentally by rotation of the sample, which is equivalent to rotation of the tip, whereas irradiation of the membrane by nanoclusters can be utilized for the latter approach. The latter one has the potential to yield a yet richer diversity of topological defects on account of a lesser determinacy. If successfully realized experimentally the concept proposed here could

  7. Experimental determination of excitonic band structures of single-walled carbon nanotubes using circular dichroism spectra.

    Science.gov (United States)

    Wei, Xiaojun; Tanaka, Takeshi; Yomogida, Yohei; Sato, Naomichi; Saito, Riichiro; Kataura, Hiromichi

    2016-10-05

    Experimental band structure analyses of single-walled carbon nanotubes have not yet been reported, to the best of our knowledge, except for a limited number of reports using scanning tunnelling spectroscopy. Here we demonstrate the experimental determination of the excitonic band structures of single-chirality single-walled carbon nanotubes using their circular dichroism spectra. In this analysis, we use gel column chromatography combining overloading selective adsorption with stepwise elution to separate 12 different single-chirality enantiomers. Our samples show higher circular dichroism intensities than the highest values reported in previous works, indicating their high enantiomeric purity. Excitonic band structure analysis is performed by assigning all observed Eii and Eij optical transitions in the circular dichroism spectra. The results reproduce the asymmetric structures of the valence and conduction bands predicted by density functional theory. Finally, we demonstrate that an extended empirical formula can estimate Eij optical transition energies for any (n,m) species.

  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. 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.

  10. 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.

  11. Phonon-assisted exciton formation in ZnO/(Zn, Mg)O single quantum wells grown on C-plane oriented substrates

    Energy Technology Data Exchange (ETDEWEB)

    Béaur, L. [Université Montpellier 2, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); Bretagnon, T., E-mail: Thierry.Bretagnon@univ-montp2.fr [Université Montpellier 2, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); Guillet, T.; Brimont, C. [Université Montpellier 2, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); Gallart, M. [IPCMS, UMR 7504 CNRS-Université de Strasbourg, 23, rue du Loess B.P. 43F-67034 Strasbourg (France); Gil, B. [Université Montpellier 2, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, F-34095 Montpellier (France); Gilliot, P. [IPCMS, UMR 7504 CNRS-Université de Strasbourg, 23, rue du Loess B.P. 43F-67034 Strasbourg (France); Morhain, C. [CRHEA–CNRS, Rue Bernard Grégory, F-06560 Valbonne (France)

    2013-04-15

    We report on absorption phenomena in ZnO/(Zn, Mg)O quantum wells grown along the c-axis by molecular beam epitaxy. The optical properties of such quantum wells are affected by a huge internal electric field. For wide quantum wells the absorption is driven by Quantum Confined Stark Effect. Phonon-assisted formation of excitons is observed in the case of thin quantum wells. The physical origin of these hot excitons is determined by using both low temperature (T=10 K) photoluminescence excitation spectroscopy and reflectivity measurements. -- Highlights: ► High structural quality ZnO/(Zn, Mg)O quantum wells are growth along the polar c-direction. ► Indirect phonon-assisted formation of excitons in the thin single quantum wells. ► Strong internal electric field present in polar heterostructures prevents the observation of hot excitons.

  12. Roles of binding energy and diffusion length of singlet and triplet excitons in organic heterojunction solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Narayan, Monishka Rita [Centre for Renewable Energy and Low Emission Technology, Charles Darwin University, Darwin, NT 0909 (Australia); Singh, Jai [School of Engineering and IT, Charles Darwin University, Darwin, NT 0909 (Australia)

    2012-12-15

    The influence of binding energy and diffusion length on the dissociation of excitons in organic solids is studied. The binding energy and excitonic Bohr radius of singlet and triplet excitons are calculated and compared using the dissociation energy of 0.3 eV, which is provided by the lowest unoccupied molecular orbital offset in heterojunction organic solar cells. A relation between the diffusion coefficient and diffusion length of singlet and triplet excitons is derived using the Foerster and Dexter transfer processes and are plotted as a function of the donor-acceptor separation. The diffusion length reduces nearly to a zero if the distance between donor and acceptor is increased to more than 1.5 nm. It is found that the donor-acceptor separation needs to be {<=} 1.5 nm for easy dissociation on singlet excitons leading to better conversion efficiency in heterojunction organic solar cells. (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  13. Excitons in atomically thin 2D semiconductors and their applications

    Directory of Open Access Journals (Sweden)

    Xiao Jun

    2017-06-01

    Full Text Available The research on emerging layered two-dimensional (2D semiconductors, such as molybdenum disulfide (MoS2, reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.

  14. Chiral topological excitons in a Chern band insulator

    Science.gov (United States)

    Chen, Ke; Shindou, Ryuichi

    2017-10-01

    A family of semiconductors called Chern band insulators are shown to host exciton bands with nonzero topological Chern integers and chiral exciton edge modes. Using a prototypical two-band Chern insulator model, we calculate a cross-correlation function to obtain the exciton bands and their Chern integers. The lowest exciton band acquires Chern integers such as ±1 and ±2 in the electronic Chern insulator phase. The nontrivial topology can be experimentally observed both by a nonlocal optoelectronic response of exciton edge modes and by a phase shift in the cross-correlation response due to the bulk mode. Our result suggests that magnetically doped HgTe, InAs/GaSb quantum wells, and (Bi,Sb)2Te3 thin films are promising candidates for a platform of topological excitonics.

  15. 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.

  16. Mass measurement with the electron microscope. [Application of scanning transmission electron microscopy in molecular weight determinations of fd phage

    Energy Technology Data Exchange (ETDEWEB)

    Wall, J.S.

    1979-01-01

    The use of electron scattering measurements performed in the electron microscope as a means of measurement of particle molecular weight is described. Various potential sources of errors are identified and estimated where possible. Specimen preparation and observation conditions to minimize errors are described. The fd phage is presented as an example of analysis and an illustration of the accuracy obtainable at low dose.

  17. Engineering and manipulating exciton wave packets

    Science.gov (United States)

    Zang, Xiaoning; Montangero, Simone; Carr, Lincoln D.; Lusk, Mark T.

    2017-05-01

    When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction, and spectral make-up that allows them to be selectively passed, rejected, or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage, and remove structured excitons comprises the foundation for optoexcitonic circuits with application to a wide range of quantum information, energy, and light-flow technologies. The paradigm is demonstrated using both tight-binding and time-domain density functional theory simulations.

  18. 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......-one-dimensional system, enhancing the repulsive interaction between excitons due to Pauli blocking....

  19. Study of exciton transfer in dense quantum dot nanocomposites

    Science.gov (United States)

    Guzelturk, Burak; Hernandez-Martinez, Pedro Ludwig; Sharma, Vijay Kumar; Coskun, Yasemin; Ibrahimova, Vusala; Tuncel, Donus; Govorov, Alexander O.; Sun, Xiao Wei; Xiong, Qihua; Demir, Hilmi Volkan

    2014-09-01

    Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies.Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies. Electronic supplementary

  20. Excitons confined in quantum dots spheroidal prolate; Excitones confinados en puntos cuanticos esferoidales prolatos

    Energy Technology Data Exchange (ETDEWEB)

    Corella M, A.; Rosas, R.A.; Marin, J.L.; Riera, R. [Depto. de Fisica, Universidad de Sonora, A.P. 1626, Hermosillo, Sonora (Mexico)

    2004-07-01

    The variational method is used to solve in approximately way the Schroedinger wave equation associated to a Wannier-Mott exciton confined within a spheroidal quantum dot. The confinement effect on the ground-state energy of the electron-hole pair trapped inside a crystallite with this geometry, and with soft or hard walls, is analyzed. The walls can be modeled as finite or infinite potential barriers with suitable border conditions, which will depend on the considered case. The results of this work are compared with those obtained by other authors through more sophisticated methods. A comparison with experimental data of CdS crystallites embedded in materials of different composition is made, too. For a finite potential barrier, a critical size of the crystallite from which the exciton escapes of the quantum dot, is predicted. This is in opposition with the infinite potential barrier model where the exciton never can leave the region where it is confined. (Author)

  1. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... nuclear medicine procedures are able to pinpoint molecular activity within the body, they offer the potential to ... tells you otherwise, you may resume your normal activities after your nuclear medicine scan. If any special ...

  2. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... a special camera or imaging device that produces pictures and provides molecular information. The thyroid scan and ... and with the help of a computer, create pictures offering details on both the structure and function ...

  3. Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures

    National Research Council Canada - National Science Library

    Rivera, Pasqual; Schaibley, John R; Jones, Aaron M; Ross, Jason S; Wu, Sanfeng; Aivazian, Grant; Klement, Philip; Seyler, Kyle; Clark, Genevieve; Ghimire, Nirmal J; Yan, Jiaqiang; Mandrus, D G; Yao, Wang; Xu, Xiaodong

    2015-01-01

    ..., that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy...

  4. Surprisingly High Conductivity and Efficient Exciton Blocking in Fullerene/Wide-Energy-Gap Small Molecule Mixtures.

    Science.gov (United States)

    Bergemann, Kevin J; Amonoo, Jojo A; Song, Byeongseop; Green, Peter F; Forrest, Stephen R

    2015-06-10

    We find that mixtures of C60 with the wide energy gap, small molecular weight semiconductor bathophenanthroline (BPhen) exhibit a combination of surprisingly high electron conductivity and efficient exciton blocking when employed as buffer layers in organic photovoltaic cells. Photoluminescence quenching measurements show that a 1:1 BPhen/C60 mixed layer has an exciton blocking efficiency of 84 ± 5% compared to that of 100% for a neat BPhen layer. This high blocking efficiency is accompanied by a 100-fold increase in electron conductivity compared with neat BPhen. Transient photocurrent measurements show that charge transport through a neat BPhen buffer is dispersive, in contrast to nondispersive transport in the compound buffer. Interestingly, although the conductivity is high, there is no clearly defined insulating-to-conducting phase transition with increased insulating BPhen fraction. Thus, we infer that C60 undergoes nanoscale (80%) BPhen fractions.

  5. New features of excitonic emission in metal nanoparticle/semiconductor quantum dot nanosystem

    Science.gov (United States)

    Kryuchenko, Yu. V.; Korbutyak, D. V.

    2016-10-01

    We study theoretically the excitonic emission properties of a hybrid nanosystem composed of a spherical metal nanoparticle (NP) and a spherical quantum dot (QD). We show that electromagnetic field (EMF) emitted by a single QD has only dipole, quadrupole, and octupole components, i.e., QD cannot in principle be regarded as an oscillating point dipole, which emits infinite series of multipoles. This leads to a substantial deviation of the characteristics of QD excitonic emission from the emission characteristics of point dipole (molecular fluorophore) located in a vicinity of metal NP at small interparticle distances. The observed fluorescence spectra of the CdTe QD/Ag NP nanostructure are found to be in good agreement with the calculated ones.

  6. The convergence of longitudinal excitons onto the Γ5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons.

    Science.gov (United States)

    Elgawadi, Amal; Gainer, Gordon; Krasinski, Jerzy

    2013-08-21

    The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ∥ c) or perpendicular (K ∥ c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton Γ5 in the K⊥c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ∥ c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton-photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers.

  7. Nuclear Scans

    Science.gov (United States)

    Nuclear scans use radioactive substances to see structures and functions inside your body. They use a special ... images. Most scans take 20 to 45 minutes. Nuclear scans can help doctors diagnose many conditions, including ...

  8. Surface effects on exciton diffusion in non polar ZnO/ZnMgO heterostructures

    Science.gov (United States)

    Sakr, G.; Sartel, C.; Sallet, V.; Lusson, A.; Patriarche, G.; Galtier, P.; Barjon, J.

    2017-12-01

    The diffusion of excitons injected in ZnO/Zn0.92Mg0.08O quantum well heterostructures grown by metal-organic-vapor-phase-epitaxy on non-polar ZnO substrates is investigated at room temperature. Cathodoluminescence linescans in a field-emission-gun scanning-electron-microscope are performed across cleaved cross-sections. A 55 nm diffusion length is assessed for excitons in bulk ZnMgO. When prepared as small angle bevels using focused ion beam (FIB), the effective diffusion length of excitons is shown to decrease down to 8 nm in the thinner part of the slab. This effect is attributed to non-radiative surface recombinations, with a 7  ×  104 cm s‑1 recombination velocity estimated at the FIB-machined ZnMgO surface. The strong reduction of the diffusion extent in such thin lamellae usually used for transmission electron microscopy could be use improve the spatial resolution of cathodoluminescence images, often limited by diffusion processes.

  9. 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.

  10. Anisotropy of exciton migration in poly(p-phenylene vinylene)

    NARCIS (Netherlands)

    Markov, D. E.; Blom, P. W. M.

    The dynamics of the exciton transport in poly(p-phenylene vinylene) (PPV) blended with a low concentration of fullerene molecules is monitored by time-resolved photoluminescence measurements. The diffusion driven motion of excitons toward these scavengers is modeled using a theory based on a random

  11. Temperature dependence of exciton diffusion in conjugated polymers

    NARCIS (Netherlands)

    Mikhnenko, O.V.; Cordella, F.; Sieval, A.B.; Hummelen, J.C.; Blom, P.W.M.; Loi, M.A.

    2008-01-01

    The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a ID diffusion

  12. 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...

  13. Imaging the motion of excitonic complexes in semiconductor quantum wells

    NARCIS (Netherlands)

    Pulizzi, Fabio

    2003-01-01

    The low temperature optical properties of semiconductor quantum wells are dominated by excitonic complexes, i.e. a few charges bound together by the mutual Coulomb interaction. Excitonic complexes have been widely studied in the past not only for their importance in the physics of semiconductors,

  14. 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...

  15. Mapping the exciton diffusion in semiconductor nanocrystal solids.

    Science.gov (United States)

    Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail

    2015-03-24

    Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling.

  16. Interlayer excitons in a bulk van der Waals semiconductor

    DEFF Research Database (Denmark)

    Arora, Ashish; Drueppel, Matthias; Schmidt, Robert

    2017-01-01

    , dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments...

  17. 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...

  18. 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 ...

  19. 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...

  20. Storing excitons in transition-metal dichalcogenides using dark states

    Science.gov (United States)

    Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun

    Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.

  1. Bistable Topological Insulator with Exciton-Polaritons

    Science.gov (United States)

    Kartashov, Yaroslav V.; Skryabin, Dmitry V.

    2017-12-01

    The functionality of many nonlinear and quantum optical devices relies on the effect of optical bistability. Using microcavity exciton-polaritons in a honeycomb arrangement of microcavity pillars, we report the resonance response and bistability of topological edge states. A balance between the pump, loss, and nonlinearity ensures a broad range of dynamical stability and controls the distribution of power between counterpropagating states on the opposite edges of the honeycomb lattice stripe. Tuning energy and polarization of the pump photons, while keeping their momentum constant, we demonstrate control of the propagation direction of the dominant edge state. Our results facilitate the development of practical applications of topological photonics.

  2. Single-photon source based on Rydberg exciton blockade

    Science.gov (United States)

    Khazali, Mohammadsadegh; Heshami, Khabat; Simon, Christoph

    2017-11-01

    Bound states of electron–hole pairs in semiconductors demonstrate a hydrogen-like behavior in their high-lying excited states that are also known as Rydberg exciton states. The strong interaction between excitons in levels with high principal quantum numbers prevents the creation of more than one exciton in a small crystal; resulting in the Rydberg blockade effect. Here, we propose a new kind of solid-state single-photon source based on the recently observed Rydberg blockade effect for excitons in cuprous oxide. Our quantitative estimates based on single and double excitation probability dynamics indicate that GHz rates and values of the second-order correlation function {g}2(0) below the percent level can be simultaneously achievable. These results should pave the way to explore applications of Rydberg excitons in photonic quantum information processing.

  3. Vibronic effects and destruction of exciton coherence in optical spectra of J-aggregates: A variational polaron transformation approach

    Energy Technology Data Exchange (ETDEWEB)

    Bloemsma, E.A.; Silvis, M.H.; Stradomska, A.; Knoester, J., E-mail: j.knoester@rug.nl

    2016-12-20

    Using a symmetry adapted polaron transformation of the Holstein Hamiltonian, we study the interplay of electronic excitation-vibration couplings, resonance excitation transfer interactions, and temperature in the linear absorption spectra of molecular J-aggregates. Semi-analytical expressions for the spectra are derived and compared with results obtained from direct numerical diagonalization of the Hamiltonian in the two-particle basis set representation. At zero temperature, we show that our polaron transformation reproduces both the collective (exciton) and single-molecule (vibrational) optical response associated with the appropriate standard perturbation limits. Specifically, for the molecular dimer excellent agreement with the spectra from the two-particle approach for the entire range of model parameters is obtained. This is in marked contrast to commonly used polaron transformations. Upon increasing the temperature, the spectra show a transition from the collective to the individual molecular features, which results from the thermal destruction of the exciton coherence.

  4. Near-field scanning magneto-optical spectroscopy of Wigner molecules

    Energy Technology Data Exchange (ETDEWEB)

    Mintairov, A. M., E-mail: amintair@nd.edu; Rouvimov, S. [University of Notre Dame, Notre Dame, IN 46556 (United States); Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Saint Petersburg, 194021 (Russian Federation); Kapaldo, J. [University of Notre Dame, Notre Dame, IN 46556 (United States); Merz, J. L.; Kalyygniy, N.; Mintairov, S. A.; Nekrasov, S.; Saly, R.; Vlasov, A. S. [Ioffe Physical-Technical Institute of the Russian Academy of Sciences, Saint Petersburg, 194021 (Russian Federation); Blundell, S. [SPSMS, UMR-E CEA/UJF-Grenoble 1, INAC, Grenoble, FR-38054 (France)

    2016-06-17

    We study the emission spectra of single self-organized InP/GaInP QDs (size 100-220 nm) using high-spatial-resolution, low-temperature (5 K) near-field scanning optical microscope (NSOM) operating at magnetic field strength B=0-10 T. The dots contain up to twenty electrons and represent natural Wigner molecules (WM). We observed vibronic-type shake-up structure in single electron QDs manifesting formation of two electron (2e) WM in photo-excited state. We found that relative intensities of the shake-up components described well by vibronic Frank-Condon factors giving for dots having parabolic confinement energy ħω{sub 0}=1.2-4 meV molecule bond lengths 40-140 nm. We used measurements of magnetic-field-induced shifts to distinguish emission of 2e-WM and singly charged exciton (trion). We also observed magnetic-field-induced molecular-droplet transition for two electron dot, emitting through doubly charge exciton (tetron) at zero magnetic field.

  5. An investigation of exciton behavior in type-II self-assembled GaSb/GaAs quantum dots.

    Science.gov (United States)

    Qiu, Feng; Qiu, Weiyang; Li, Yulian; Wang, Xingjun; Zhang, Yun; Zhou, Xiaohao; Lv, Yingfei; Sun, Yan; Deng, Huiyong; Hu, Shuhong; Dai, Ning; Wang, Chong; Yang, Yu; Zhuang, Qiandong; Hayne, Manus; Krier, A

    2016-02-12

    We report the investigation of exciton dynamics in type-II self-assembled GaSb/GaAs quantum dots. The GaSb/GaAs quantum dots (QDs) were grown using a modified liquid phase epitaxy technique. Statistical size distributions of the uncapped QDs were investigated experimentally by field-emission scanning electron microscopy (SEM) and atomic force microscopy (AFM), and theoretically by an eight-band k  ·  p calculation, which demonstrated a dissolution effect. Furthermore, the low-temperature luminescence spectra of type-II GaSb/GaAs QDs with a thick capping layer exhibit well-resolved emission bands and LO-phonon-assisted transitions in the GaSb wetting layer. However, the luminescence lines quench at temperatures above 250 K, which is attributed to the weak quantum confinement of electrons participating in indirect exciton recombination. It was demonstrated that the room temperature stability of the excitons in type-II GaSb/GaAs QDs could be achieved by growing thin a capping layer, which provides strong quantum confinement in the conduction band and enhances the electron-hole Coulomb interaction, stabilizing the excitons.

  6. Molecular Scanning and Morpho-Physiological Dissection of Component Mechanism in Lens Species in Response to Aluminium Stress.

    Science.gov (United States)

    Singh, Dharmendra; Pal, Madan; Singh, Chandan Kumar; Taunk, Jyoti; Jain, Priyanka; Chaturvedi, Ashish Kumar; Maurya, Sadhana; Karwa, Sourabh; Singh, Rajendra; Tomar, Ram Sewak Singh; Nongthombam, Rita; Chongtham, Nandini; Singh, Moirangthem Premjit

    2016-01-01

    Aluminium (Al) stress was imposed on 285 lentil genotypes at seedling stage under hydroponics to study its effects on morpho-physiological traits where resistant cultigens and wilds showed minimum reduction in root and shoot length and maximum root re-growth (RRG) after staining. Molecular assortment based on 46 simple sequence repeat (SSR) markers clustered the genotypes into 11 groups, where wilds were separated from the cultigens. Genetic diversity and polymorphism information content (PIC) varied between 0.148-0.775 and 0.140-0.739, respectively. Breeding lines which were found to be most resistant (L-7903, L-4602); sensitive cultivars (BM-4, L-4147) and wilds ILWL-185 (resistant), ILWL-436 (sensitive) were grouped into different clusters. These genotypes were also separated on the basis of population structure and Jaccard's similarity index and analysed to study Al resistance mechanism through determination of different attributes like localization of Al and callose, lipid peroxidation, secretion of organic acids and production of antioxidant enzymes. In contrast to sensitive genotypes, in resistant ones most of the Al was localized in the epidermal cells, where its movement to apoplastic region was restricted due to release of citrate and malate. Under acidic field conditions, resistant genotypes produced maximum seed yield/plant as compared to sensitive genotypes at two different locations i.e. Imphal, Manipur, India and Basar, Arunanchal Pradesh, India during 2012-13, 2013-14 and 2014-15. These findings suggest that Al stress adaptation in lentil is through exclusion mechanism and hybridization between the contrasting genotypes from distinct clusters can help in development of resistant varieties.

  7. 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.

  8. Scanning tunneling microscopy and density functional theory investigations on molecular self-assembly of graphene on Ru(0 0 0 1)

    Science.gov (United States)

    Song, Junjie; Zhang, Han-jie; Zhang, Yuxi; Cai, Yiliang; Bao, Shining; He, Pimo

    2016-03-01

    Investigations on the bottom-up fabrication of graphene with 1,3,5-triphenylbenzene as precursor on Ru(0 0 0 1) was carried out using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Upon annealing 1,3,5-triphenylbenzene overlayer on Ru(0 0 0 1) at 550 °C, the precursors dehydrogenated and coalesced into graphitized flakes, and subsequent annealing up to 600 °C results in complete graphene conversion. The migration behavior and close-packing morphology of precursors were captured during STM measurements, and DFT calculations indicated that the inter-molecular interaction is responsible for the accumulation and close-packing of the precursors. The noticeable increment in the dehydrogenation barrier from 1.27 eV for monomer adsorption to 1.62 eV for dimer adsorption is well consistent with the observed drastic reduction of the graphitization temperature at lower precursor coverage, suggesting the crucial influence of inter-molecular vdW interaction on the dehydrogenation process.

  9. 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.

  10. 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.

  11. 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

  12. 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.

  13. Renal scan

    Science.gov (United States)

    ... this page: //medlineplus.gov/ency/article/003790.htm Renal scan To use the sharing features on this ... anaphylaxis . Alternative Names Renogram; Kidney scan Images Kidney anatomy Kidney - blood and urine flow References Chernecky CC, ...

  14. Influence of the sign of the coupling on the temperature dependence of optical properties of one-dimensional exciton models

    Energy Technology Data Exchange (ETDEWEB)

    Cruzeiro, L [CCMAR and FCT, Universidade of Algarve, Campus de Gambelas, 8005-139 Faro (Portugal)

    2008-10-14

    A new physical cause for a temperature-dependent double peak in exciton systems is put forward within a thermal equilibrium approach for the calculation of optical properties of exciton systems. Indeed, it is found that one-dimensional exciton systems with only one molecule per unit cell can have an absorption spectrum characterized by a double peak provided that the coupling between excitations in different molecules is positive. The two peaks, whose relative intensities vary with temperature, are located around the exciton band edges, being separated by an energy of approximately 4V, where V is the average coupling between nearest neighbours. For small amounts of diagonal and off-diagonal disorder, the contributions from the intermediate states in the band are also visible as intermediate structure between the two peaks, this being enhanced for systems with periodic boundary conditions. At a qualitative level, these results correlate well with experimental observations in the molecular aggregates of the thiacarbocyanine dye THIATS and in the organic crystals of acetanilide and N-methylacetamide.

  15. 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.

  16. a 530-590 GHZ Schottky Heterodyne Receiver for High-Resolution Molecular Spectroscopy with Lille's Fast-Scan Fully Solid-State DDS Spectrometer

    Science.gov (United States)

    Pienkina, A.; Margulès, L.; Motiyenko, R. A.; Wiedner, Martina C.; Maestrini, Alain; Defrance, Fabien

    2017-06-01

    Laboratory spectroscopy, especially at THz and mm-wave ranges require the advances in instrumentation techniques to provide high resolution of the recorded spectra with precise frequency measurement that facilitates the mathematical treatment. We report the first implementation of a Schottky heterodyne receiver, operating at room temperature and covering the range between 530 and 590 GHz, for molecular laboratory spectroscopy. A 530-590 GHz non-cryogenic Schottky solid-state receiver was designed at LERMA, Observatoire de Paris and fabricated in partnership with LPN- CNRS (Laboratoire de Photonique et de Nanostructures), and was initially developed for ESA Jupiter Icy Moons Explorer (JUICE), intended to observe Jupiter and its icy moon atmospheres. It is based on a sub-harmonic Schottky diode mixer, designed and fabricated at LERMA-LPN, pumped by a Local Oscillator (LO), consisting of a frequency Amplifier/Multiplier chains (AMCs) from RPG (Radiometer Physics GmBh). The performance of the receiver was demonstrated by absorption spectroscopy of CH_3CH_2CN with Lille's fast-scan DDS spectrometer. A series of test measurements showed the receiver's good sensitivity, stability and frequency accuracy comparable to those of 4K QMC bolometers, thus making room-temperature Schottky receiver a competitive alternative to 4K QMC bolometers to laboratory spectroscopy applications. We will present the first results with such a combination of a compact room temperature Schottky heterodyne receiver and a fast-scan DDS spectrometer. J. Treuttel, L. Gatilova, A. Maestrini et al., 2016, IEEE Trans. Terahertz Science and Tech., 6, 148-155. This work was funded by the French ANR under the Contract No. ANR-13-BS05-0008-02 IMOLABS.

  17. 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 focus is on quantum dots in photonic nanostructures. The fabrication process of reproducible high-quality photonic-crystal structures on electrically gated GaAs samples is presented. This process is employed to investigate light localization in short photonic-crystal waveguides with a dispersion...... relation facilitating a slow-light effect. The effect of the variations in the local density of optical states on electrically tuned quantum dots embedded in photonic structures is investigated. An electric field is employed to induce strain in suspended GaAs structures, where a bidirectional spectral...

  18. Optical Dynamics of Exciton and Polaron Formation in Molecular Aggregates

    NARCIS (Netherlands)

    de Boer, S.; Wiersma, Douwe A.

    1989-01-01

    Results of femtosecond accumulated photon echo, picosecond pump-probe and fluorescence lifetime measurements are reported on aggregates of the dyes pseudoisocyanine (PIC) and substituted thiapyrylium (TPY), embedded in a polycarbonate matrix. It is concluded that in the PIC aggregate, delocalized

  19. Dynamic Coherence in Excitonic Molecular Complexes under Various Excitation Conditions

    CERN Document Server

    Chenu, Aurélia; Mancal, Tomáš

    2013-01-01

    In this paper, we investigate the relevance of dynamic electronic coherence under conditions natural to light-harvesting systems. We formulate the results of a quantum mechanical treatment of a weak light-matter interaction in terms of experimental observable, such as the incident light spectrum and the absorption spectrum of the material, and we derive the description of the incoherent F\\"orster type energy transfer fully from the wave function formalism. We demonstrate that excitation of a coherent superposition of electronic eigenstates of natural light-harvesting complexes by sunlight or by excitation transfer from a neighboring antenna is unlikely and that dynamical coherence therefore cannot play any significant role in natural photosynthesis, regardless of their life time. Dynamical coherence as a transient phenomenon must be strictly distinguished from the effect of excited state delocalization (also termed quantum coherence in the literature) which is established by interaction between the pigments a...

  20. Molecular and Nanoscale Engineering of High Efficiency Excitonic Solar Cells

    Energy Technology Data Exchange (ETDEWEB)

    Jenekhe, Samson A. [Univ. of Washington, Seattle, WA (United States); Ginger, David S. [Univ. of Washington, Seattle, WA (United States); Cao, Guozhong [Univ. of Washington, Seattle, WA (United States)

    2016-01-15

    We combined the synthesis of new polymers and organic-inorganic hybrid materials with new experimental characterization tools to investigate bulk heterojunction (BHJ) polymer solar cells and hybrid organic-inorganic solar cells during the 2007-2010 period (phase I) of this project. We showed that the bulk morphology of polymer/fullerene blend solar cells could be controlled by using either self-assembled polymer semiconductor nanowires or diblock poly(3-alkylthiophenes) as the light-absorbing and hole transport component. We developed new characterization tools in-house, including photoinduced absorption (PIA) spectroscopy, time-resolved electrostatic force microscopy (TR-EFM) and conductive and photoconductive atomic force microscopy (c-AFM and pc-AFM), and used them to investigate charge transfer and recombination dynamics in polymer/fullerene BHJ solar cells, hybrid polymer-nanocrystal (PbSe) devices, and dye-sensitized solar cells (DSSCs); we thus showed in detail how the bulk photovoltaic properties are connected to the nanoscale structure of the BHJ polymer solar cells. We created various oxide semiconductor (ZnO, TiO2) nanostructures by solution processing routes, including hierarchical aggregates and nanorods/nanotubes, and showed that the nanostructured photoanodes resulted in substantially enhanced light-harvesting and charge transport, leading to enhanced power conversion efficiency of dye-sensitized solar cells.

  1. Molecular Determinants Underlying Binding Specificities of the ABL Kinase Inhibitors: Combining Alanine Scanning of Binding Hot Spots with Network Analysis of Residue Interactions and Coevolution.

    Directory of Open Access Journals (Sweden)

    Amanda Tse

    Full Text Available Quantifying binding specificity and drug resistance of protein kinase inhibitors is of fundamental importance and remains highly challenging due to complex interplay of structural and thermodynamic factors. In this work, molecular simulations and computational alanine scanning are combined with the network-based approaches to characterize molecular determinants underlying binding specificities of the ABL kinase inhibitors. The proposed theoretical framework unveiled a relationship between ligand binding and inhibitor-mediated changes in the residue interaction networks. By using topological parameters, we have described the organization of the residue interaction networks and networks of coevolving residues in the ABL kinase structures. This analysis has shown that functionally critical regulatory residues can simultaneously embody strong coevolutionary signal and high network centrality with a propensity to be energetic hot spots for drug binding. We have found that selective (Nilotinib and promiscuous (Bosutinib, Dasatinib kinase inhibitors can use their energetic hot spots to differentially modulate stability of the residue interaction networks, thus inhibiting or promoting conformational equilibrium between inactive and active states. According to our results, Nilotinib binding may induce a significant network-bridging effect and enhance centrality of the hot spot residues that stabilize structural environment favored by the specific kinase form. In contrast, Bosutinib and Dasatinib can incur modest changes in the residue interaction network in which ligand binding is primarily coupled only with the identity of the gate-keeper residue. These factors may promote structural adaptability of the active kinase states in binding with these promiscuous inhibitors. Our results have related ligand-induced changes in the residue interaction networks with drug resistance effects, showing that network robustness may be compromised by targeted mutations

  2. Molecular Determinants Underlying Binding Specificities of the ABL Kinase Inhibitors: Combining Alanine Scanning of Binding Hot Spots with Network Analysis of Residue Interactions and Coevolution

    Science.gov (United States)

    Tse, Amanda; Verkhivker, Gennady M.

    2015-01-01

    Quantifying binding specificity and drug resistance of protein kinase inhibitors is of fundamental importance and remains highly challenging due to complex interplay of structural and thermodynamic factors. In this work, molecular simulations and computational alanine scanning are combined with the network-based approaches to characterize molecular determinants underlying binding specificities of the ABL kinase inhibitors. The proposed theoretical framework unveiled a relationship between ligand binding and inhibitor-mediated changes in the residue interaction networks. By using topological parameters, we have described the organization of the residue interaction networks and networks of coevolving residues in the ABL kinase structures. This analysis has shown that functionally critical regulatory residues can simultaneously embody strong coevolutionary signal and high network centrality with a propensity to be energetic hot spots for drug binding. We have found that selective (Nilotinib) and promiscuous (Bosutinib, Dasatinib) kinase inhibitors can use their energetic hot spots to differentially modulate stability of the residue interaction networks, thus inhibiting or promoting conformational equilibrium between inactive and active states. According to our results, Nilotinib binding may induce a significant network-bridging effect and enhance centrality of the hot spot residues that stabilize structural environment favored by the specific kinase form. In contrast, Bosutinib and Dasatinib can incur modest changes in the residue interaction network in which ligand binding is primarily coupled only with the identity of the gate-keeper residue. These factors may promote structural adaptability of the active kinase states in binding with these promiscuous inhibitors. Our results have related ligand-induced changes in the residue interaction networks with drug resistance effects, showing that network robustness may be compromised by targeted mutations of key mediating

  3. Plasmon-Exciton Resonant Energy Transfer: Across Scales Hybrid Systems

    Directory of Open Access Journals (Sweden)

    Mohamed El Kabbash

    2016-01-01

    Full Text Available The presence of an excitonic element in close proximity of a plasmonic nanostructure, under certain conditions, may lead to a nonradiative resonant energy transfer known as Exciton Plasmon Resonant Energy Transfer (EPRET process. The exciton-plasmon coupling and dynamics have been intensely studied in the last decade; still many relevant aspects need more in-depth studies. Understanding such phenomenon is not only important from fundamental viewpoint, but also essential to unlock many promising applications. In this review we investigate the plasmon-exciton resonant energy transfer in different hybrid systems at the nano- and mesoscales, in order to gain further understanding of such processes across scales and pave the way towards active plasmonic devices.

  4. Excitons in ultrathin organic-inorganic perovskite crystals

    Science.gov (United States)

    Yaffe, Omer; Chernikov, Alexey; Norman, Zachariah M.; Zhong, Yu; Velauthapillai, Ajanthkrishna; van der Zande, Arend; Owen, Jonathan S.; Heinz, Tony F.

    2015-07-01

    We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.

  5. Spin-excitons in heavy-fermion semimetals

    Energy Technology Data Exchange (ETDEWEB)

    Riseborough, Peter S., E-mail: prisebor@temple.edu [Temple University, Philadelphia (United States); Magalhaes, S.G. [Univ. Federal, Fluminense, Niteroi, Rio de Janeiro (Brazil)

    2016-02-15

    Spin-excitons are sharp and dispersive magnetic fluctuations in paramagnetic semiconductors where the dispersion relation lies within the semiconducting gap. Spin-excitons are found in the vicinity of magnetic quantum critical points in semiconductors, much the same as antiparamagnons are precursor fluctuations for quantum critical points in metals. Here we show that this concept of spin-exciton excitations can be extended to heavy-fermion semimetals and provides a natural explanation of the magnetic modes found by inelastic neutron scattering experiments on paramagnetic CeFe{sub 2}Al{sub 10}. - Highlights: • We discuss the theory of spin excitons in heavy-fermion semiconductors as precritical fluctuations. • We show that relatively sharp magnetic in-gap excitations can also occur in semiconductors. • The magnetic excitations are only sharp for a restricted range of center of mass momenta. • They may merge with the quasi-elastic peak associated with incommensurate nesting of electron and hole pockets.

  6. Excitonic giant-dipole potentials in cuprous oxide

    Science.gov (United States)

    Kurz, Markus; Grünwald, Peter; Scheel, Stefan

    2017-06-01

    In this paper we predict the existence of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. In particular, we present a theory of giant-dipole excitons in Cu2O in crossed fields. Within our theoretical approach we perform a pseudoseparation of the center-of-mass motion for the field-dressed excitonic species, thereby obtaining an effective single-particle Hamiltonian for the relative motion. For arbitrary gauge fields we exactly separate the gauge-dependent kinetic-energy terms from the effective single-particle interaction potential. Depending on the applied field strengths and the specific field orientation, the potential for the relative motion of electron and hole exhibits an outer well at spatial separations up to several micrometers and depths up to 380 μ eV , leading to possible permanent excitonic electric dipole moments of around 3 ×106 D.

  7. How bilayer excitons can greatly enhance thermoelectric efficiency

    Science.gov (United States)

    Wu, Kai; Rademaker, Louk; Zaanen, Jan

    2015-03-01

    Presently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer exciton thermocouple. Based on current experimental parameters, a bilayer exciton heterostructures of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.

  8. Quantitative in-situ scanning electron microscope pull-out experiments and molecular dynamics simulations of carbon nanotubes embedded in palladium

    Energy Technology Data Exchange (ETDEWEB)

    Hartmann, S., E-mail: steffen.hartmann@etit.tu-chemnitz.de; Blaudeck, T.; Hermann, S.; Wunderle, B. [Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz (Germany); Hölck, O. [Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz (Germany); Fraunhofer IZM Berlin, Gustav-Meyer-Allee 25, 13355 Berlin (Germany); Schulz, S. E.; Gessner, T. [Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz (Germany); Fraunhofer ENAS Chemnitz, Technologie-Campus 3, 09126 Chemnitz (Germany)

    2014-04-14

    In this paper, we present our results of experimental and numerical pull-out tests on carbon nanotubes (CNTs) embedded in palladium. We prepared simple specimens by employing standard silicon wafers, physical vapor deposition of palladium and deposition of CNTs with a simple drop coating technique. An AFM cantilever with known stiffness connected to a nanomanipulation system was utilized inside a scanning electron microscope (SEM) as a force sensor to determine forces acting on a CNT during the pull-out process. SEM-images of the cantilever attached to a CNT have been evaluated for subsequent displacement steps with greyscale correlation to determine the cantilever deflection. We compare the experimentally obtained pull-out forces with values of numerical investigations by means of molecular dynamics and give interpretations for deviations according to material impurities or defects and their influence on the pull-out data. We find a very good agreement of force data from simulation and experiment, which is 17 nN and in the range of 10–61 nN, respectively. Our findings contribute to the ongoing research of the mechanical characterization of CNT-metal interfaces. This is of significant interest for the design of future mechanical sensors utilizing the intrinsic piezoresistive effect of CNTs or other future devices incorporating CNT-metal interfaces.

  9. Exciton spectrum in multi-shell hexagonal semiconductor nanotube

    Directory of Open Access Journals (Sweden)

    O.M. Makhanets

    2012-10-01

    Full Text Available The theory of exciton spectrum in multi-shell hexagonal semiconductor nanotube is developed within the effective masses and rectangular potentials approximations using the method of effective potential. It is shown that the exciton binding energy for all states non-monotonously depends on the inner wire diameter, approaching several minimal and maximal magnitudes. The obtained theoretical results explain well the experimental positions of luminescence peaks for GaAs/Al0.4Ga0.6As nanotubes.

  10. 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 absorption peak arising from excitons is symmetric. We expect the positive and negative trion absorption line shapes to be identical, independently of the chiral index (n,m)....

  11. 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....

  12. Exciton-plasmon coupling interactions: from principle to applications

    Directory of Open Access Journals (Sweden)

    Cao En

    2018-01-01

    Full Text Available The interaction of exciton-plasmon coupling and the conversion of exciton-plasmon-photon have been widely investigated experimentally and theoretically. In this review, we introduce the exciton-plasmon interaction from basic principle to applications. There are two kinds of exciton-plasmon coupling, which demonstrate different optical properties. The strong exciton-plasmon coupling results in two new mixed states of light and matter separated energetically by a Rabi splitting that exhibits a characteristic anticrossing behavior of the exciton-LSP energy tuning. Compared to strong coupling, such as surface-enhanced Raman scattering, surface plasmon (SP-enhanced absorption, enhanced fluorescence, or fluorescence quenching, there is no perturbation between wave functions; the interaction here is called the weak coupling. SP resonance (SPR arises from the collective oscillation induced by the electromagnetic field of light and can be used for investigating the interaction between light and matter beyond the diffraction limit. The study on the interaction between SPR and exaction has drawn wide attention since its discovery not only due to its contribution in deepening and broadening the understanding of SPR but also its contribution to its application in light-emitting diodes, solar cells, low threshold laser, biomedical detection, quantum information processing, and so on.

  13. 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...

  14. Exciton band structure in bacterial peripheral light-harvesting complexes.

    Science.gov (United States)

    Trinkunas, Gediminas; Zerlauskiene, Oksana; Urbonienė, Vidita; Chmeliov, Jevgenij; Gall, Andrew; Robert, Bruno; Valkunas, Leonas

    2012-05-03

    The variability of the exciton spectra of bacteriochlorophyll molecules in light-harvesting (LH) complexes of photosynthetic bacteria ensures the excitation energy funneling trend toward the reaction center. The decisive shift of the energies is achieved due to exciton spectra formation caused by the resonance interaction between the pigments. The possibility to resolve the upper Davydov sub-band corresponding to the B850 ring and, thus, to estimate the exciton bandwidth by analyzing the temperature dependence of the steady-state absorption spectra of the LH2 complexes is demonstrated. For this purpose a self-modeling curve resolution approach was applied for analysis of the temperature dependence of the absorption spectra of LH2 complexes from the photosynthetic bacteria Rhodobacter (Rba.) sphaeroides and Rhodoblastus (Rbl.) acidophilus. Estimations of the intradimer resonance interaction values as follows directly from obtained estimations of the exciton bandwidths at room temperature give 385 and 397 cm(-1) for the LH2 complexes from the photosynthetic bacteria Rba. sphaeroides and Rhl. acidophilus, respectively. At 4 K the corresponding couplings are slightly higher (391 and 435 cm(-1), respectively). The retained exciton bandwidth at physiological conditions supports the decisive role of the exciton coherence determining light absorption in bacterial light-harvesting antenna complexes.

  15. Exciton-plasmon coupling interactions: from principle to applications

    Science.gov (United States)

    Cao, En; Lin, Weihua; Sun, Mengtao; Liang, Wenjie; Song, Yuzhi

    2018-01-01

    The interaction of exciton-plasmon coupling and the conversion of exciton-plasmon-photon have been widely investigated experimentally and theoretically. In this review, we introduce the exciton-plasmon interaction from basic principle to applications. There are two kinds of exciton-plasmon coupling, which demonstrate different optical properties. The strong exciton-plasmon coupling results in two new mixed states of light and matter separated energetically by a Rabi splitting that exhibits a characteristic anticrossing behavior of the exciton-LSP energy tuning. Compared to strong coupling, such as surface-enhanced Raman scattering, surface plasmon (SP)-enhanced absorption, enhanced fluorescence, or fluorescence quenching, there is no perturbation between wave functions; the interaction here is called the weak coupling. SP resonance (SPR) arises from the collective oscillation induced by the electromagnetic field of light and can be used for investigating the interaction between light and matter beyond the diffraction limit. The study on the interaction between SPR and exaction has drawn wide attention since its discovery not only due to its contribution in deepening and broadening the understanding of SPR but also its contribution to its application in light-emitting diodes, solar cells, low threshold laser, biomedical detection, quantum information processing, and so on.

  16. Quantum Hall drag of exciton condensate in graphene

    Science.gov (United States)

    Liu, Xiaomeng; Watanabe, Kenji; Taniguchi, Takashi; Halperin, Bertrand I.; Kim, Philip

    2017-08-01

    An exciton condensate is a Bose-Einstein condensate of electron and hole pairs bound by the Coulomb interaction. In an electronic double layer (EDL) subject to strong magnetic fields, filled Landau states in one layer bind with empty states of the other layer to form an exciton condensate. Here we report exciton condensation in a bilayer graphene EDL separated by hexagonal boron nitride. Driving current in one graphene layer generates a near-quantized Hall voltage in the other layer, resulting in coherent exciton transport. Owing to the strong Coulomb coupling across the atomically thin dielectric, quantum Hall drag in graphene appears at a temperature ten times higher than previously observed in a GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of Bose-Einstein condensates across Landau levels with different filling factors and internal quantum degrees of freedom. The observed robust exciton condensation opens up opportunities to investigate various many-body exciton phases.

  17. Exciton-plasmon coupling in monolayer molybdenum disulfide

    Science.gov (United States)

    Ziegler, Jed; Newaz, A. K. M.; Bolotin, Kirill; Haglund, Richard

    2013-03-01

    Two-dimensional materials such as monolayer molybdenum disulfide (MoS2) represent a unique platform for investigating the dynamics of exciton-plasmon coupling. We report on the generation and modulation of coherent and incoherent coupled states between excitons in monolayer MoS2 and plasmons in an array of gold nanoparticle deposited onto the surface of MoS2. We study the behavior of these coherent states, termed plexcitons using a combination of photoluminescence, extinction and ultrafast spectroscopies. The close proximity of the two characteristic exciton bands of MoS2 presents multiple coherent coupling configurations, including A-or-B exciton-plasmon, and A-and-B exciton-plasmon interactions. These configurations of plexciton formation that are shown to modulate both the extinction and photoluminescence spectra of the hybrid system. This includes broadband photoluminescence and Fano-type resonances. This behavior is distinct from the spectral response of the MoS2 and plasmonic components of the system. Incoherent exciton-plasmon coupling, achieved by detuning from the plasmon extinction peaks, enhances the interaction of MoS2 with light by focusing the plasmon energy. Depending on which coupling configuration is chosen, our results show that the MoS2/plasmon hybrid systems can act as high efficiency light harvesters, broadband emitters and as tunable visible and NIR photodetectors. Support by Defense Threat Reduction Agency (HDTRA1-1-10-1-0047) and NSF DMR-1056859

  18. Exciton Band Structure in Two-Dimensional Materials.

    Science.gov (United States)

    Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo

    2016-02-12

    Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

  19. 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.

  20. Cooperative scans

    NARCIS (Netherlands)

    M. Zukowski (Marcin); P.A. Boncz (Peter); M.L. Kersten (Martin)

    2004-01-01

    textabstractData mining, information retrieval and other application areas exhibit a query load with multiple concurrent queries touching a large fraction of a relation. This leads to individual query plans based on a table scan or large index scan. The implementation of this access path in most

  1. Characterizing the Hot Spots Involved in RON-MSPβ Complex Formation Using In Silico Alanine Scanning Mutagenesis and Molecular Dynamics Simulation

    Directory of Open Access Journals (Sweden)

    Omid Zarei

    2017-04-01

    Full Text Available Purpose: Implication of protein-protein interactions (PPIs in development of many diseases such as cancer makes them attractive for therapeutic intervention and rational drug design. RON (Recepteur d’Origine Nantais tyrosine kinase receptor has gained considerable attention as promising target in cancer therapy. The activation of RON via its ligand, macrophage stimulation protein (MSP is the most common mechanism of activation for this receptor. The aim of the current study was to perform in silico alanine scanning mutagenesis and to calculate binding energy for prediction of hot spots in protein-protein interface between RON and MSPβ chain (MSPβ. Methods: In this work the residues at the interface of RON-MSPβ complex were mutated to alanine and then molecular dynamics simulation was used to calculate binding free energy. Results: The results revealed that Gln193, Arg220, Glu287, Pro288, Glu289, and His424 residues from RON and Arg521, His528, Ser565, Glu658, and Arg683 from MSPβ may play important roles in protein-protein interaction between RON and MSP. Conclusion: Identification of these RON hot spots is important in designing anti-RON drugs when the aim is to disrupt RON-MSP interaction. In the same way, the acquired information regarding the critical amino acids of MSPβ can be used in the process of rational drug design for developing MSP antagonizing agents, the development of novel MSP mimicking peptides where inhibition of RON activation is required, and the design of experimental site directed mutagenesis studies.

  2. Excitonically Coupled States in Crystalline Coordination Networks.

    Science.gov (United States)

    Haldar, Ritesh; Mazel, Antoine; Joseph, Reetu; Adams, Michael; Howard, Ian A; Richards, Bryce S; Tsotsalas, Manuel; Redel, Engelbert; Diring, Stéphane; Odobel, Fabrice; Wöll, Christof

    2017-10-12

    When chromophores are brought into close proximity, noncovalent interactions (π-π/CH-π) can lead to the formation of excitonically coupled states, which bestow new photophysical properties upon the aggregates. Because the properties of the new states not only depend on the strength of intermolecular interactions, but also on the relative orientation, supramolecular assemblies, where these parameters can be varied in a deliberate fashion, provide novel possibilities for the control of photophysical properties. This work reports that core-substituted naphthalene diimides (cNDIs) can be incorporated into surface-mounted metal- organic structures/frameworks (SURMOFs) to yield optical properties strikingly different from conventional aggregates of such molecules, for example, formed in solution or by crystallization. Organic linkers are used, based on cNDIs, well-known organic chromophores with numerous applications in different optoelectronic devices, to fabricate MOF thin films on transparent substrates. A thorough characterization of the properties of these highly ordered chromophoric assemblies reveals the presence of non-emissive excited states in the crystalline material. Structural modulations provide further insights into the nature of the coupling that gives rise to an excited-state energy level in the periodic structure. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  3. Dark High Density Dipolar Liquid of Excitons.

    Science.gov (United States)

    Cohen, Kobi; Shilo, Yehiel; West, Ken; Pfeiffer, Loren; Rapaport, Ronen

    2016-06-08

    The possible phases and the nanoscale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma, is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature Tc ≈ 4.8 K, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting condensed dark ground state.

  4. Theory of Anisotropic Circular Dichroism of Excitonically Coupled Systems: Application to the Baseplate of Green Sulfur Bacteria.

    Science.gov (United States)

    Lindorfer, Dominik; Renger, Thomas

    2018-03-02

    A simple exciton theory for the description of anisotropic circular dichroism (ACD) spectra of multichromophoric systems is presented that is expected to be of general use for the analysis of structure-function relationships of molecular aggregates such as photosynthetic light-harvesting antennae. The theory is applied to the baseplate of green sulfur bacteria. It is demonstrated that only the combined analysis of ACD and circular dichroism (CD) spectra for the present baseplate bacteriochlorophyll (BChl) a dimer allows for an unambiguous determination of the parameters of the exciton Hamiltonian from experimental data. The analysis of experimental absorption and linear dichroism spectra suggests that either the NMR structure has to be refined or in addition to the dimers seen in the NMR structure and in the CD and ACD spectra, BChl a monomers are present in the baseplate carotenosome sample. A refined dimer structure is presented, explaining all four optical spectra.

  5. Excitons in InP/InAs inhomogeneous quantum dots

    CERN Document Server

    Assaid, E; Khamkhami, J E; Dujardin, F

    2003-01-01

    Wannier excitons confined in an InP/InAs inhomogeneous quantum dot (IQD) have been studied theoretically in the framework of the effective mass approximation. A finite-depth potential well has been used to describe the effect of the quantum confinement in the InAs layer. The exciton binding energy has been determined using the Ritz variational method. The spatial correlation between the electron and the hole has been taken into account in the expression for the wavefunction. It has been shown that for a fixed size b of the IQD, the exciton binding energy depends strongly on the core radius a. Moreover, it became apparent that there are two critical values of the core radius, a sub c sub r sub i sub t and a sub 2 sub D , for which important changes of the exciton binding occur. The former critical value, a sub c sub r sub i sub t , corresponds to a minimum of the exciton binding energy and may be used to distinguish between tridimensional confinement and bidimensional confinement. The latter critical value, a ...

  6. Angular momentum transport with twisted exciton wave packets

    Science.gov (United States)

    Zang, Xiaoning; Lusk, Mark T.

    2017-10-01

    A chain of cofacial molecules with CN or CN h symmetry supports excitonic states with a screwlike structure. These can be quantified with the combination of an axial wave number and an azimuthal winding number. Combinations of these states can be used to construct excitonic wave packets that spiral down the chain with well-determined linear and angular momenta. These twisted exciton wave packets can be created and annihilated using laser pulses, and their angular momentum can be optically modified during transit. This allows for the creation of optoexcitonic circuits in which information, encoded in the angular momentum of light, is converted into excitonic wave packets that can be manipulated, transported, and then reemitted. A tight-binding paradigm is used to demonstrate the key ideas. The approach is then extended to quantify the evolution of twisted exciton wave packets in a many-body, multilevel time-domain density functional theory setting. In both settings, numerical methods are developed that allow the site-to-site transfer of angular momentum to be quantified.

  7. Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes

    CERN Document Server

    Harigaya, Kikuo

    2007-01-01

    Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.

  8. Phonon-assisted absorption of excitons in Cu2O

    Science.gov (United States)

    Schöne, Florian; Stolz, Heinrich; Naka, Nobuko

    2017-09-01

    The basic theoretical foundation for the modeling of phonon-assisted absorption spectra in direct band-gap semiconductors, introduced by Elliott 60 years ago [R. J. Elliott, Phys. Rev. 108, 1384 (1957), 10.1103/PhysRev.108.1384] using second order perturbation theory, results in a square root shaped dependency close to the absorption edge. A careful analysis of the experiments [N. Naka et al., Jpn. J. Appl. Phys. 44, 5096 (2005), 10.1143/JJAP.44.5096] reveals that for the yellow S excitons in Cu2O the lineshape does not follow that square root dependence. The reexamination of the theory shows that the basic assumptions of constant matrix elements and constant energy denominators is invalid for semiconductors with dominant exciton effects like Cu2O , where the phonon-assisted absorption proceeds via intermediate exciton states. The overlap between these and the final exciton states strongly determines the dependence of the absorption on the photon energy. To describe the experimental observed line shape of the indirect absorption of the yellow S exciton states we find it necessary to assume a momentum dependent deformation potential for the optical phonons.

  9. Spectral properties of excitons in the bilayer graphene

    Science.gov (United States)

    Apinyan, V.; Kopeć, T. K.

    2018-01-01

    In this paper, we consider the spectral properties of the bilayer graphene with the local excitonic pairing interaction between the electrons and holes. We consider the generalized Hubbard model, which includes both intralayer and interlayer Coulomb interaction parameters. The solution of the excitonic gap parameter is used to calculate the electronic band structure, single-particle spectral functions, the hybridization gap, and the excitonic coherence length in the bilayer graphene. We show that the local interlayer Coulomb interaction is responsible for the semimetal-semiconductor transition in the double layer system, and we calculate the hybridization gap in the band structure above the critical interaction value. The formation of the excitonic band gap is reported as the threshold process and the momentum distribution functions have been calculated numerically. We show that in the weak coupling limit the system is governed by the Bardeen-Cooper-Schrieffer (BCS)-like pairing state. Contrary, in the strong coupling limit the excitonic condensate states appear in the semiconducting phase, by forming the Dirac's pockets in the reciprocal space.

  10. Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides.

    Science.gov (United States)

    Yu, Hongyi; Liu, Gui-Bin; Gong, Pu; Xu, Xiaodong; Yao, Wang

    2014-05-12

    In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I = 2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I = 2 Dirac cone splits into two degenerate I = 1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin.

  11. MRI Scans

    Science.gov (United States)

    Magnetic resonance imaging (MRI) uses a large magnet and radio waves to look at organs and structures inside your body. Health care professionals use MRI scans to diagnose a variety of conditions, from ...

  12. Bone Scan

    Science.gov (United States)

    ... posts Join Mayo Clinic Connect Bone scan About Advertisement Mayo Clinic does not endorse companies or products. ... a Job Site Map About This Site Twitter Facebook Google YouTube Pinterest Mayo Clinic is a not- ...

  13. Excitonic, vibrational, and van der Waals interactions in electron energy loss spectroscopy.

    Science.gov (United States)

    Mizoguchi, T; Miyata, T; Olovsson, W

    2017-09-01

    The pioneer, Ondrej L. Krivanek, and his collaborators have opened up many frontiers for the electron energy loss spectroscopy (EELS), and they have demonstrated new potentials of the EELS method for investigating materials. Here, inspired by those achievements, we show further potentials of EELS based on the results of theoretical calculations, that is excitonic and van der Waals (vdW) interactions, as well as vibrational information of materials. Concerning the excitonic interactions, we highlight the importance of the two-particle calculation to reproduce the low energy-loss near-edge structure (ELNES), the Na-L 2,3 edge of NaI and the Li-K edge of LiCl and LiFePO 4 . Furthermore, an unusually strong excitonic interaction at the O-K edge of perovskite oxides, SrTiO 3 and LaAlO 3 , is shown. The effect of the vdW interaction in the ELNES is also investigated, and we observe that the magnitude of the vdW effect is approximately 0.1eV in the case of the ELNES from a solid and liquid, whereas its effect is almost negligible in the case of the ELNES from the gaseous phase owing to the long inter-molecular distance. In addition to the "static" information, the influence of the "dynamic" behavior of atoms in materials to EELS is also investigated. We show that measurements of the infrared spectrum are possible by using a modern monochromator system. Furthermore, an estimation of the atomic vibration in core-loss ELNES is also presented. We show the acquisition of vibrational information using the ELNES of liquid methanol and acetic acid, solid Al 2 O 3 , and oxygen gas. Copyright © 2017 Elsevier B.V. All rights reserved.

  14. 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...... excitons and biexcitons are mutually uncorrelated. In contrast, the biexciton phonon scattering is twice as fast and correlated to exciton-phonon scattering, indicating the interaction with similar phonon modes....

  15. On the relation between local and charge-transfer exciton bindingenergies in organic photovoltaic materials

    NARCIS (Netherlands)

    de Gier, Hilde Dorothea; Braam, Henderika; Havenith, Remco

    2015-01-01

    In organic photovoltaic devices two types of excitons can be generated for which different binding energies can be defined: the binding energy of the local exciton generated immediately after light absorption on the polymer and the binding energy of the charge-transfer exciton generated through the

  16. 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.

  17. Exciton localization-delocalization transition in an extended dendrimer

    Energy Technology Data Exchange (ETDEWEB)

    Pouthier, Vincent, E-mail: vincent.pouthier@univ-fcomte.fr [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)

    2013-12-21

    Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} ≈ 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.

  18. Excitonic surface polaritons in luminescence from ZnTe crystals

    Energy Technology Data Exchange (ETDEWEB)

    Brodin, M.S.; Bandura, V.M.; Matsko, M.G. (AN Ukrainskoj SSR, Kiev. Inst. Fiziki)

    1984-10-01

    The form and structure of reflection and exciton-polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal-transverse splitting magnitude ..delta..E/sub LT/ is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k-linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wavelength of the exciting light is investigated.

  19. 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....

  20. 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.

  1. Exciton dynamics in solid-state green fluorescent protein

    Science.gov (United States)

    Dietrich, Christof P.; Siegert, Marie; Betzold, Simon; Ohmer, Jürgen; Fischer, Utz; Höfling, Sven

    2017-01-01

    We study the decay characteristics of Frenkel excitons in solid-state enhanced green fluorescent protein (eGFP) dried from solution. We further monitor the changes of the radiative exciton decay over time by crossing the phase transition from the solved to the solid state. Complex interactions between protonated and deprotonated states in solid-state eGFP can be identified from temperature-dependent and time-resolved fluorescence experiments that further allow the determination of activation energies for each identified process.

  2. 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

  3. EDITORIAL: Focus on Molecular Electronics FOCUS ON MOLECULAR ELECTRONICS

    Science.gov (United States)

    Scheer, Elke; Reineker, Peter

    2008-06-01

    exciton model applies in which the excitation is distributed over several molecules or whether a good description is given by the Frenkel exciton model with the electron and the whole being localized at the same molecular unit. In organic semiconductors the charge transport usually occurs on the basis of holes because of the presence of many defects giving rise to a localization of the electrons. It is therefore a challenge to produce materials with both positive and negative mobile charge carriers. In the 1990s V M Agranovich introduced the idea of hybrid excitons, i.e. of nanostructured materials consisting of both organic and inorganic semiconductors. At the interface between the organic and inorganic parts new excitons can appear, being a superposition of both Frenkel and Wannier excitons and having both the high oscillator strength of the Frenkel and the large optical nonlinearity of the Wannier exciton. The problem is to find optimum combinations of the organic and inorganic parts to enable the hybrid structure concept to work. Micro-cavities also play an important role in the investigation of organic materials resulting in a new state (polariton) as the superposition of a photon and an exciton because of the large exciton-photon interaction. A similar excitation arises because of the interaction between plasmons and photons. A special geometrical shape of a nano-cavity increases the interaction between the electromagnetic radiation and a dipole sitting in the cavity. The interaction between vibronic degrees of freedom and electronic excitations plays an important role for various phenomena such as nonlinear processes, the question of coherence, information on the shape of a potential hypersurface, etc. With the help of femtosecond laser pulses, detailed information on such vibrations can be obtained. Also of great importance is the investigation of the energy transfer in artificial light-harvesting systems, e.g. in dendrimers. Finally the combination of

  4. Polaronic exciton in self-organized assemblies of protonated meso-tetraphenylporphine dimers and water at room temperature

    Science.gov (United States)

    Udal'tsov, Alexander V.

    2016-12-01

    Assemblies consisting of protonated meso-tetraphenylporphine (TPP) dimers and water have been investigated by UV-vis and infrared (IR) spectroscopy and by atomic force microscopy (AFM) in thin layers. Features of electronic absorption spectra of the assemblies are interpreted in terms of hole polaron combined with exciton theory using quantum well with parameters obtained from the dimer structure. It appears to be hole polaron moving defines kinetic energy of polaronic exciton confined in a quantum well when the electron absorbs photon. Hole polaron characteristics such as polaron self-energy, energy of Frank-Condon transitions, and radius of hole polaron moving through water are found to be 1.38 eV, 0.2445 eV, and 0.246 Å, respectively. A doublet at 1944, 1960 cm-1 (0.2412, 0.2432 eV) observed in IR spectra matches the energy of Frank-Condon transitions. Excitation energies estimated using molecular parameters for polaronic excitons in pure water and in the TPP dimers are found in a good agreement with the experimental data.

  5. 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...... of them, the Is heavy-hole exciton, is almost identical to the same state in noninteracting quantum wells, while the other, the heavy-hole interwell exciton, is composed of an electron and a heavy hole in adjacent wells. The interwell exciton leads to a resonant enhancement in the four-wave mixing spectra...

  6. Scanning table

    CERN Multimedia

    1960-01-01

    Before the invention of wire chambers, particles tracks were analysed on scanning tables like this one. Today, the process is electronic and much faster. Bubble chamber film - currently available - (links can be found below) was used for this analysis of the particle tracks.

  7. Scan Statistics

    CERN Document Server

    Glaz, Joseph

    2009-01-01

    Suitable for graduate students and researchers in applied probability and statistics, as well as for scientists in biology, computer science, pharmaceutical science and medicine, this title brings together a collection of chapters illustrating the depth and diversity of theory, methods and applications in the area of scan statistics.

  8. Subdiffusive exciton motion in systems with heavy-tailed disorder

    NARCIS (Netherlands)

    Vlaming, S. M.; Malyshev, V.A.; Eisfeld, A.; Knoester, J.

    2013-01-01

    We study the transport of collective excitations (Frenkel excitons) in systems with static disorder in the transition energies, not limiting ourselves to Gaussian transition energy distributions. Instead, we generalize this model to the wider class of Levy stable distributions, characterized by

  9. Nonmonotonic energy harvesting efficiency in biased exciton chains

    NARCIS (Netherlands)

    Vlaming, S.M.; Malyshev, V.A.; Knoester, J.

    2007-01-01

    We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average

  10. Enhancement of spin propagation due to interlayer exciton condensation

    NARCIS (Netherlands)

    Rademaker, Louk; van den Brink, J.; Hilgenkamp, H.; Zaanen, Jan

    2013-01-01

    We show that an interlayer exciton condensate doped into a strongly correlated Mott insulator exhibits a remarkable enhancement of the bandwidth of the magnetic excitations (triplons). This triplon is visible in the dynamical magnetic susceptibility and can be measured using resonant inelastic x-ray

  11. Spatial inhomogeneity in spectra and exciton dynamics in porphyrin ...

    Indian Academy of Sciences (India)

    2016-08-26

    Aug 26, 2016 ... ... Journal of Chemical Sciences; Volume 128; Issue 11. Spatial inhomogeneity in spectra and exciton dynamics in porphyrin micro-rods and micro-brushes: Confocal microscopy. SHYAMTANU CHATTORAJ KANKAN BHATTACHARYYA. Regular Article Volume 128 Issue 11 November 2016 pp 1717-1724 ...

  12. Exciton Recombination in Formamidinium Lead Triiodide : Nanocrystals versus Thin Films

    NARCIS (Netherlands)

    Fang, Hong-Hua; Protesescu, Loredana; Balazs, Daniel M.; Adjokatse, Sampson; Kovalenko, Maksym V.; Loi, Maria Antonietta

    2017-01-01

    The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI(3)) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is

  13. Charged excitons in doped extended Hubbard model systems

    NARCIS (Netherlands)

    van den Brink, J.; Eder, R; Sawatzky, G.A

    1997-01-01

    We show that the charge transfer excitons in a Hubbard model system including nearest-neighbor Coulomb interactions effectively attain some charge in doped systems and become visible in photoelectron and inverse photoelectron spectroscopies. This shows that the description of a doped system by an

  14. Observation of exciton-polariton ultrafast dynamic Stark effect

    Directory of Open Access Journals (Sweden)

    Snoke David W.

    2013-03-01

    Full Text Available We demonstrate ultrafast phase control of exciton-polaritons in a GaAs/AlGaAs strongly coupled microcavity exploiting the ac Stark effect. Our approach yields meV-scale shifts without carrier generation, providing a powerful tool towards control of polariton BECs.

  15. 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 b...

  16. Creation of Excitons Excited by Light with a Spatial Mode

    Science.gov (United States)

    Syouji, Atsushi; Saito, Shingo; Otomo, Akira

    2017-12-01

    When light is absorbed into matter, its degrees of freedom (i.e., energy, polarization, and phase) are transferred to the matter and conserved. In this study, we demonstrate that elementary excitations in matter, which are one-photon-forbidden transition states, become allowed states because of the phase conservation across the entire cross section of excitation light. In particular, when 1S orthoexcitons of the yellow series in the semiconductor cuprous oxide (Cu2O) were resonantly excited by light with a spatial mode, an increase in the Γ 3 - -phonon-emission peak intensity of the excitons was detected depending on the spatial mode. Using group-theory-based analysis, we show that the irreducible representation of a one-photon-forbidden exciton, which is one of the orthoexcitons, can be transmuted to an allowed state by taking the direct product with the polar vector produced from the spatial mode of the light. Although the transition process of the exciton is locally characterized by the usual quadrupole interaction, the phase conservation at each position at which the sample is irradiated causes the exciton to be in the same spatial-mode state. That causes a change in the transition selection rule. The selection rule relaxation due to the spatial mode of the light was also applied for paraexciton creation.

  17. Permanent Rabi oscillations in coupled exciton-photon systems with PT-symmetry.

    Science.gov (United States)

    Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V

    2016-01-21

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators.

  18. Multi-exciton emission from solitary dopant states of carbon nanotubes.

    Science.gov (United States)

    Ma, Xuedan; Hartmann, Nicolai F; Velizhanin, Kirill A; Baldwin, Jon K S; Adamska, Lyudmyla; Tretiak, Sergei; Doorn, Stephen K; Htoon, Han

    2017-11-02

    By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2nd order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton-exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching. With this work, we bring out multi-excitonic processes of solitary dopant states as a new area to be explored for potential applications in lasing and entangled photon generation.

  19. Is there any Exciton (bottleneck) in an Excitonic Solar Cell: Revisiting the Prospects of Single-Semiconductor OPV

    Science.gov (United States)

    Alam, Muhammad

    2014-03-01

    The discovery dye sensitized and bulk heterojunction (BHJ) solar cells in early 1990s introduced a new class of PV technology that rely on (i) distributed photogeneration of excitons, (ii) dissociation of excitons into free carriers by the heterojunction between two organic semiconductors (OSC), and (iii) collection of free carriers through electron and hole transport layers. The success of the approach is undisputed: the highest efficiency OPV cells have all relied on variants of BHJ approach. Yet, three concerns related to the use of a pair of OSCs, namely, low Voc, process sensitivity, and reliability, suggest that the technology may never achieve efficiency-variability-reliability metrics comparable to inorganic solar cells. This encourages a reconsideration of the prospects of Single semiconductor OPV (SS-OPV), a system presumably doomed by the exciton bottleneck. In this talk, we use an inverted SS-OPV to demonstrate how the historical SS-OPV experiments may have been misinterpreted. No one disputes the signature of excitons in polymer under narrowband excitation, but our experiments show that exciton dissociation need not be a bottleneck for OPV under broadband solar illumination. We demonstrate that an alternate collection-limited theory consistently interprets the classical and new experiments, resolves puzzles such as efficiency loss with increasing light intensity, and voltage-dependent reverse photo-current, etc. The theory and experiments suggest a new ``perovskite-like'' strategy to efficiency-variability-reliability of organic solar cells. The work was supported by the Columbia DOE-EFRC (DE-SC0001085) and NSF-NCN (EEC-0228390).

  20. 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.

  1. Homoconjugation vs. Exciton Coupling in Chiral α,β-Unsaturated Bicyclo[3.3.1]nonane Dinitrile and Carboxylic Acids

    Directory of Open Access Journals (Sweden)

    Gintautas Bagdžiūnas

    2014-07-01

    Full Text Available The chiroptical properties of enantiomerically pure bicyclo[3.3.1]nona-2,6-diene-2,6-dicarbonitrile and related acids were studied by circular dichroism spectroscopy and theoretical computations. A consideration of the molecular structure of the synthesized difunctional compounds revealed that chromophores are predisposed to transannular through-space interaction due to a favourable conformation of the bicyclic skeleton and a rather small interchromophoric distance. Evidence for non-exciton-type coupling between the two acrylonitrile and acrylate moieties in 3 and 4, respectively, was obtained by chiroptical spectroscopy and DFT calculations.

  2. Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction.

    Science.gov (United States)

    Ross, Jason S; Rivera, Pasqual; Schaibley, John; Lee-Wong, Eric; Yu, Hongyi; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jiaqiang; Mandrus, David; Cobden, David; Yao, Wang; Xu, Xiaodong

    2017-02-08

    Semiconductor heterostructures are backbones for solid-state-based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures have enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and a long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe 2 -WSe 2 heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller than the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is 2 orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to the opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

  3. Coherent optical nonlinearities and phase relaxation of quasi-three-dimensional and quasi-two-dimensional excitons in ZnSxSe1 - x/ZnSe structures

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Schätz, A.; Maier, R.

    1997-01-01

    signal is attributed to the response of spin-coupled exciton states, with a decay time given by the inhomogeneous broadening. The photon echo is due to a distribution of localized, noninteracting excitons. We determine the exciton-exciton and exciton-phonon scattering cross sections for different...

  4. 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

  5. Hole, impurity and exciton states in a spherical quantum dot

    Directory of Open Access Journals (Sweden)

    V.I. Boichuk

    2010-01-01

    Full Text Available The 3x3 kp hole Hamiltonian for the wave-function envelopes (effective mass Hamiltonian was used for calculation of discrete states of the hole and acceptor hydrogenic impurity in a spherical Si/SiO2 nanoheterostructure as a function of the quantum dot radius by neglecting the corrugation of constant-energy surfaces. A study was conducted in the case of finite potential well at the separation boundary of the nanoheterosystem. The dependence of the hole energy spectrum on polarization charges, which arise at the separation boundary of the media, and on the dielectric permittivity, was defined. Using the exact electron and hole solutions, the exciton wave-function was constructed and the exciton ground-state energy was defined. The theoretical results have been compared with experimental data.

  6. Linewidths in excitonic absorption spectra of cuprous oxide

    Science.gov (United States)

    Schweiner, Frank; Main, Jörg; Wunner, Günter

    2016-02-01

    We present a theoretical calculation of the absorption spectrum of cuprous oxide (Cu2O ) based on the general theory developed by Y. Toyozawa. An inclusion not only of acoustic phonons but also of optical phonons and of specific properties of the excitons in Cu2O like the central-cell corrections for the 1 S exciton allows us to calculate the experimentally observed linewidths in experiments by T. Kazimierczuk et al. [T. Kazimierczuk, D. Fröhlich, S. Scheel, H. Stolz, and M. Bayer, Nature (London) 514, 343 (2014), 10.1038/nature13832] within the same order of magnitude, which demonstrates a clear improvement in comparison to earlier work on this topic. We also discuss a variety of further effects, which explain the still observable discrepancy between theory and experiment but can hardly be included in theoretical calculations.

  7. Non-Markovian quantum jumps in excitonic energy transfer

    Energy Technology Data Exchange (ETDEWEB)

    Rebentrost, Patrick; Chakraborty, Rupak; 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 excitontransport, which can be seen as an extension of recent environment-assisted quantum transport concepts to the non-Markovian regime. Within the NMQJ method, the Fenna–Matthew–Olson protein is investigated as a prototype for larger photosynthetic complexes.

  8. 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.

  9. 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.

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

    DEFF Research Database (Denmark)

    Birkedal, Dan

    2000-01-01

    The coherent interaction of light and the electronic states of semiconductors near the fundamental bandgap has been a very active topic of research since the advent of ultrafast lasers. While many of the ultrafast nonlinear properties of semiconductors have been well explained within mean field...... 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...... of the secondary emission from quantum wells following ultrafast resonant excitation and find that it provides information on not only the bound biexcitons but also the biexciton continuum. Due to the heterodyne nature of the experimental technique we obtain both amplitude and phase of the coherent emission...

  11. The formation of DNA photodamage: the role of exciton localization.

    Science.gov (United States)

    Rössle, Shaila; Friedrichs, Jana; Frank, Irmgard

    2010-06-21

    The electronic structure during the formation of a cyclobutane pyrimidine dimer (CPD) between two thymine bases is investigated using semi-empirical and first-principles approaches. The dimerization of two isolated thymine bases is found to have no barrier or a very small barrier in agreement with previous studies suggesting low photostability of DNA. The well-known high photostability of DNA can only be explained taking other factors into account. We investigate the role of the exciton location in the particular environment. Different model systems, from isolated thymine bases to an oligonucleotide in aqueous solution, are discussed. Analysis of the frontier orbitals allows one to understand the connection between the location of the exciton, the relative orientation of the thymine bases, and the observed reactivity.

  12. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... News Physician Resources Professions Site Index A-Z Thyroid Scan and Uptake Thyroid scan and uptake uses ... the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is a ...

  13. Thyroid Scan and Uptake

    Science.gov (United States)

    ... News Physician Resources Professions Site Index A-Z Thyroid Scan and Uptake Thyroid scan and uptake uses ... the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is a ...

  14. Triplet energy transfer and triplet exciton recycling in singlet fission sensitized organic heterojunctions

    Science.gov (United States)

    Hamid, Tasnuva; Yambem, Soniya D.; Crawford, Ross; Roberts, Jonathan; Pandey, Ajay K.

    2017-08-01

    Singlet exciton fission is a process where an excited singlet state splits into two triplets, thus leading to generation of multiple excitons per absorbed photon in organic semiconductors. Herein, we report a detailed exciton management approach for multiexciton harvesting over a broadband region of the solar spectrum in singlet fission sensitized organic photodiodes. Through systematic studies on the model cascade of pentacene/rubrene/C60, we found that efficient photocurrent generation from pentacene can still occur despite the presence of a >10nm thick interlayer of rubrene in between the pentacene/C60 heterojunction. Our results show that thin rubrene interlayers of thickness operation a rather interesting result. We discuss the role of rubrene interlayer film discontinuity, triplet exciton reflection from rubrene interlayer and triplet energy transfer from rubrene to pentacene layer followed by diffusion of triplet excitons through rubrene as plausible mechanisms that would enable triplet excitons from pentacene to generate significant photocurrent in a multilayer organic heterojunction.

  15. Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation

    Science.gov (United States)

    Kryjevski, Andrei; Mihaylov, Deyan; Kilina, Svetlana; Kilin, Dmitri

    2017-10-01

    We use a Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory based on density functional theory simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2) and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ˜1.6 at about 3Eg, where Eg is the electronic gap.

  16. Ultrafast quantum beats of anisotropic excitons in atomically thin ReS2.

    Science.gov (United States)

    Sim, Sangwan; Lee, Doeon; Trifonov, Artur V; Kim, Taeyoung; Cha, Soonyoung; Sung, Ji Ho; Cho, Sungjun; Shim, Wooyoung; Jo, Moon-Ho; Choi, Hyunyong

    2018-01-24

    Quantum beats, periodic oscillations arising from coherent superposition states, have enabled exploration of novel coherent phenomena. Originating from strong Coulomb interactions and reduced dielectric screening, two-dimensional transition metal dichalcogenides exhibit strongly bound excitons either in a single structure or hetero-counterpart; however, quantum coherence between excitons is barely known to date. Here we observe exciton quantum beats in atomically thin ReS2 and further modulate the intensity of the quantum beats signal. Surprisingly, linearly polarized excitons behave like a coherently coupled three-level system exhibiting quantum beats, even though they exhibit anisotropic exciton orientations and optical selection rules. Theoretical studies are also provided to clarify that the observed quantum beats originate from pure quantum coherence, not from classical interference. Furthermore, we modulate on/off quantum beats only by laser polarization. This work provides an ideal laboratory toward polarization-controlled exciton quantum beats in two-dimensional materials.

  17. Superfluidity of dipolar excitons in a transition metal dichalcogenide double layer

    Science.gov (United States)

    Berman, Oleg L.; Kezerashvili, Roman Ya.

    2017-09-01

    We study formation and superfluidity of dipolar excitons in double layer heterostructures formed by two transition metal dichalcogenide (TMDC) atomically thin layers. Considering screening effects for an electron-hole interaction via the harmonic oscillator approximation for the Keldysh potential, the analytical expressions for the exciton energy spectrum and the mean field critical temperature Tc for the superfluidity are obtained. It is shown that binding energies of A excitons are larger than for B excitons. The mean field critical temperature for a two-component dilute exciton system in a TMDC double layer is analyzed and shown that the latter is an increasing function of the factor Q , determined by the effective masses of A and B excitons and their reduced mass. Comparison of the calculations for Tc performed by employing the Coulomb and Keldysh interactions demonstrates the importance of screening effects in TMDC.

  18. Harmonic Quantum Coherence of Multiple Excitons in PbS/CdS Core-Shell Nanocrystals

    Science.gov (United States)

    Tahara, Hirokazu; Sakamoto, Masanori; Teranishi, Toshiharu; Kanemitsu, Yoshihiko

    2017-12-01

    The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications. However, the quantum coherence of multiple exciton states in NCs still remains unclear due to a lack of experimental support. Here, we report the first observation of harmonic dipole oscillations in PbS/CdS core-shell NCs using a phase-locked interference detection method for transient absorption. From the ultrafast coherent dynamics and excitation-photon-fluence dependence of the oscillations, we found that multiple excitons cause the harmonic dipole oscillations with ω , 2 ω , and 3 ω oscillations, even though the excitation pulse energy is set to the exciton resonance frequency, ω . This observation is closely related to the quantum coherence of multiple exciton states in NCs, providing important insights into multiple exciton generation mechanisms.

  19. Excitonic transitions in MBE grown h-GaN with cubic inclusions

    Science.gov (United States)

    Strauf, Stefan; Michler, Peter; Gutowski, Jürgen; Selke, Hartmut; Birkle, Udo; Einfeldt, Sven; Hommel, Detlef

    1998-06-01

    Undoped and magnesium doped MBE grown GaN epilayers on sapphire substrates show a particular variety of near-bandgap luminescent transitions. Despite the large lattice mismatch to the substrate, pronounced free- and bound-exciton transitions allow for an estimation of the excitonic binding energies. For the given thickness range (about 1 μm), we find an almost strain-relaxed situation with the main exciton transition energies well corresponding to the bulk values. On their low-energy side, we identify lines having been tentatively assigned to stacking fault excitons, and interface-related exciton transitions correlated to extended defects and/or dislocations in this spatial region. Evidence of cubic inclusions of a size up to 500 nm is doubtless given by observing sharp c-GaN related donor-bound exciton emission and respective structures in transmission electron microscope investigations.

  20. 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.

  1. Excitonic Coherence in Semiconductor Nanostructures Measured by Speckle Analysis

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Hvam, Jørn Märcher

    1999-01-01

    are determined separately, thus distinguishing lifetime from pure dephasing. In particular, the secondary emission of excitons in semiconductor quantum wells is investigated. Here, the combination of static disorder and inelastic scattering leads to a partially coherent emission. The temperature dependence...... is well explained by phonon scattering. Spin-relaxation is found to be dominated by disorder, and is preserving the coherence, while phonon-assisted energy-relaxation is foundto destroy the coherence....

  2. Excitonic condensation in quasi-two-dimensional systems

    Energy Technology Data Exchange (ETDEWEB)

    Crisan, M. [Department of Theoretical Physics, University of Cluj, 400084 Cluj-Napoca (Romania)]. E-mail: mcrisan@phys.ubbcluj.ro; Tifrea, I. [Department of Theoretical Physics, University of Cluj, 400084 Cluj-Napoca (Romania); Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242 (United States)

    2005-10-17

    We present a low energy model for the Bose-Einstein condensation in a quasi-two-dimensional excitonic gas. Using the flow equations of the renormalization group and a {phi}{sup 4} model with the dynamical critical exponent z=2 we calculate the temperature dependence of the critical density, coherence length, magnetic susceptibility, and specific heat. The model can be relevant for the macroscopic coherence observed in GaAs/AlGaAs coupled quantum wells.

  3. Tuning the excitonic and plasmonic properties of copper chalcogenide nanocrystals.

    Science.gov (United States)

    Kriegel, Ilka; Jiang, Chengyang; Rodríguez-Fernández, Jessica; Schaller, Richard D; Talapin, Dmitri V; da Como, Enrico; Feldmann, Jochen

    2012-01-25

    The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions. © 2011 American Chemical Society

  4. Unidirectional flow of lossless exciton-polariton signals

    Science.gov (United States)

    Tan, E. Z.; Liew, T. C. H.

    2018-02-01

    We consider the propagation of intensity signals in the discrete nonlinear driven-dissipative Schrodinger equation, well-known for the description of a variety of systems from coupled arrays of Kerr nonlinear cavities to exciton-polariton arrays. By periodic switching of a driving laser field, we find that the propagation can be engineered to be unidirectional, while the signals fully withstand dissipation and are resilient against disorder. We anticipate that such a mechanism would be relevant for use in photonic circuits.

  5. Time-Lapse Förster Resonance Energy Transfer Imaging by Confocal Laser Scanning Microscopy for Analyzing Dynamic Molecular Interactions in the Plasma Membrane of B Cells.

    Science.gov (United States)

    Sohn, Hae Won; Brzostowski, Joseph

    2018-01-01

    For decades, various Förster resonance energy transfer (FRET) techniques have been developed to measure the distance between interacting molecules. FRET imaging by the sensitized acceptor emission method has been widely applied to study the dynamical association between two molecules at a nanometer scale in live cells. Here, we provide a detailed protocol for FRET imaging by sensitized emission using a confocal laser scanning microscope to analyze the interaction of the B cell receptor (BCR) with the Lyn-enriched lipid microdomain on the plasma membrane of live cells upon antigen binding, one of the earliest signaling events in BCR-mediated B cell activation.

  6. Excitonic spectra in HgGa2Se4 crystals

    Science.gov (United States)

    Syrbu, N. N.; Zalamai, V. V.

    2018-02-01

    Ground and excited states of four excitonic series (A, B, C and D) were discovered in HgGa2Se4 crystals at 10 K. Parameters of excitons and bands were determined. An effective mass of electrons mc is equal to 0.26m0 and masses of holes mv1, mv2 and mv3 are equal to 2.48m0, 2.68m0 and 1.6m0 respectively in Γ point of Brilloin zone. Valence bands splitting by crystal field (Δcf = 70 meV) and spin-orbital interaction (Δso = 250 meV) were estimated in Brillouin zone center. Optical functions (n, ε1 and ε2) for polarizations E⊥c and E||c in electron transitions region (2-6 eV) were calculated by Kramers-Kronig method. The discovered features were discussed on a base of the existing theoretical energetical band structure calculations and excitonic bands symmetries in k = 0 Brillouin zone for chalcopyrite crystals. The resonance Raman scattering was investigated.

  7. Enhanced exciton diffusion length via cooperative quantum transport

    Science.gov (United States)

    Mohseni, Masoud; Abasto, Damian; Lloyd, Seth; Zanardi, Paolo

    2011-03-01

    The energy transfer rate in biomolecular systems is typically calculated from the transition probability of an excitation hopping from one molecule to another using Förster energy transfer based on dipole-dipole interaction of individual molecules in the perturbative regime. However, due to strong interactions of among a group of molecules the excitation can become highly delocalized leading to an effective large dipole moment with an enhanced oscillator strength. Under certain symmetries, this could lead to an enhancement in exicton transfer rate via cooperative donation or acceptance of an excitation. Here, we explore this phenomenon in various multichromophoric geometries, under different symmetries, initial conditions, and dynamics. We study the behavior of the exciton diffusion length under the effects of disorders and environmental fluctuations and quantify the crossover from ballistic to diffusive regimes. Specifically, for a quasi-1 D array of rings containing N chromophores interacting with a bosonic bath, an interplay of time scales dictates the exciton dynamics. In the ``far-field'' regime, environmental interactions are dominating and the system properties are approaching those of the incoherent equilibrium Gibbs state. However, in the ``near-field'' the coherent interactions among dipole aggregates dominate other time scales and exciton diffusion length is enhanced by a factor of √{ N } .

  8. 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 well...... is associated with indirect evidence of the coherence of the collective phase of interwell excitons at temperatures below the critical value....

  9. Molecular Landscape of the Ribosome Pre-initiation Complex during mRNA Scanning: Structural Role for eIF3c and Its Control by eIF5

    Directory of Open Access Journals (Sweden)

    Eiji Obayashi

    2017-03-01

    Full Text Available During eukaryotic translation initiation, eIF3 binds the solvent-accessible side of the 40S ribosome and recruits the gate-keeper protein eIF1 and eIF5 to the decoding center. This is largely mediated by the N-terminal domain (NTD of eIF3c, which can be divided into three parts: 3c0, 3c1, and 3c2. The N-terminal part, 3c0, binds eIF5 strongly but only weakly to the ribosome-binding surface of eIF1, whereas 3c1 and 3c2 form a stoichiometric complex with eIF1. 3c1 contacts eIF1 through Arg-53 and Leu-96, while 3c2 faces 40S protein uS15/S13, to anchor eIF1 to the scanning pre-initiation complex (PIC. We propose that the 3c0:eIF1 interaction diminishes eIF1 binding to the 40S, whereas 3c0:eIF5 interaction stabilizes the scanning PIC by precluding this inhibitory interaction. Upon start codon recognition, interactions involving eIF5, and ultimately 3c0:eIF1 association, facilitate eIF1 release. Our results reveal intricate molecular interactions within the PIC, programmed for rapid scanning-arrest at the start codon.

  10. Picosecond spin relaxations of acceptor-bound exciton and A-band free exciton in wurtzite GaN

    Energy Technology Data Exchange (ETDEWEB)

    Tackeuchi, A.; Otake, H.; Fujita, T.; Kuroda, T. [Department of Applied Physics, Waseda University, Tokyo 169-8555 (Japan); Chinone, T.; Liang, J.H.; Kajikawa, M. [Stanley Electric Company, Ltd., Edanishi 1-3-1, Aoba, Yokohama 225-0014 (Japan)

    2006-07-01

    The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. These spin relaxation times are slightly longer than those of the A-band free excitons of 0.47-0.25 ps in GaN at 150-225 K. The spin relaxation time is found to be proportional to T{sup -0.175}, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  11. Interaction and Dephasing of Excitons in ZnSe Quantum Wires

    DEFF Research Database (Denmark)

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

    1999-01-01

    We study the coherent formation of biexcitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm by transient degenerate four-wave mixing. We observe an increase of the biexciton binding energy with decreasing wire width reaching 30% energy enhancement in the smallest wire structure...... compared to the mesa structure which is attributed to a quenching of the exciton-exciton scattering efficiency by density dependent measurements. The exciton dephasing is found to increase with decreasing wire width which is assigned to an enhanced repulsive exchange interaction between excitons of equal...

  12. The excitonic insulator route through a dynamical phase transition induced by an optical pulse

    Energy Technology Data Exchange (ETDEWEB)

    Brazovskii, S., E-mail: brazov@lptms.u-psud.fr [Université Paris-Saclay, LPTMS, CNRS, Univ. Paris-sud (France); Kirova, N. [Université Paris-Saclay, LPS, CNRS, Univ. Paris-sud (France)

    2016-03-15

    We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor–acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh.

  13. 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.

  14. 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.

  15. Tightly bound indirect exciton in single-layer hybrid organic-inorganic perovskite semiconductor

    Science.gov (United States)

    Li, Jing; Liu, Tao; Liew, Timothy C. H.

    2017-10-01

    We theoretically study the direct and indirect excitons (IXs) in a single-layer hybrid organic-inorganic perovskite (HOIP) semiconductor. Due to the 2D nature, the single-layer HOIP supports the large binding energy of IXs and direct excitons over a wide range of applied electric fields, which exceed the thermal energy of room temperature. Moreover, the ground-state IX has a lower energy than that of direct exciton, which will extend the coherence and relaxation time of IXs. This is beneficial to optoelectronic applications and excitonic information processing devices of IXs.

  16. Plasmonic Structure Enhanced Exciton Generation at the Interface between the Perovskite Absorber and Copper Nanoparticles

    Directory of Open Access Journals (Sweden)

    Sheng Hsiung Chang

    2014-01-01

    Full Text Available The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport.

  17. 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.

  18. Ultrafast dynamics of excitons in delafossite CuScO2 thin films

    Science.gov (United States)

    Liu, Fucai; Makino, T.; Hiraga, H.; Fukumura, T.; Kong, Yongfa; Kawasaki, M.

    2010-05-01

    Ultrafast carrier dynamics were investigated in a delafossite CuScO2, a material with a remarkably large binding energy of exciton (˜0.4 eV), using femtosecond transient transmission spectroscopy. Differential transmission spectra showed dispersive structures in the excitonic resonance energies. We have observed a delayed rise-up on a time scale of 10 ps, suggesting slow carrier cooling. It is followed by a slower decay, time constant of which corresponds to the lifetime of exciton (approximately 0.75 ns). These results were analyzed in terms of the generalized many-body Elliott model, accounting for a screening effect of excitons.

  19. Interaction of Rayleigh waves with 2D dipolar exciton gas: impact of Bose–Einstein condensation

    Science.gov (United States)

    Boev, M. V.; Chaplik, A. V.; Kovalev, V. M.

    2017-12-01

    The theory of the interaction of a two-dimensional gas of indirect dipolar excitons with Rayleigh surface elastic waves has been developed. The absorption and renormalization of the phase velocity of a surface wave, as well as the drag of excitons by the surface acoustic wave and the generation of bulk acoustic waves by a two dimensional gas of dipolar excitons irradiated by external electromagnetic radiation, have been considered. These effects have been studied both in a normal phase at high temperatures and in a condensed phase of the exciton gas. The calculations have been performed in the ballistic and diffusion limits for both phases.

  20. 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...... rate difference between two subsystems within the inhomogeneous distribution is strongly dependent on their energy difference. BIF is strongly affecting the cross-linear polarized FWM response. The signal for positive delay is dominated by the transitions from the one-exciton state X to the two-exciton-states...

  1. Plasmonic Structure Enhanced Exciton Generation at the Interface between the Perovskite Absorber and Copper Nanoparticles

    Science.gov (United States)

    Lin, Kuen-Feng; Chiang, Chien-Hung; Wu, Chun-Guey

    2014-01-01

    The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA) were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport. PMID:25295290

  2. Wigner molecules and charged excitons in near-field magnetophotoluminescence spectra of self-organized InP/GaInP quantum dots

    Science.gov (United States)

    Mintairov, A. M.; Kapaldo, J.; Merz, J. L.; Vlasov, A. S.; Blundell, S. A.

    2017-03-01

    We used high-spatial-resolution, low-temperature near-field scanning optical microscopy (NSOM) operating at magnetic fields B =0 -10 T to study the effects of Wigner localization (WL) on emission spectra of single self-organized InP/GaInP quantum dots (QDs) and investigate the stability of singly (trion) and doubly (tetron) charged exciton complexes in the weak quantum confinement regime. Using NSOM measurements together with configuration interaction calculations, we identify the dots having different electron population N (N =1 -12 ) , quantum confinement (ℏ ω0=0.6 -8 meV ) and size D (D =70 -170 nm ) . For N =2 , we observed a magnetic-field-induced molecular-droplet transition, accompanied by the decomposition of the tetron into a Wigner molecule complex (WMC), and the activation of rotovibronic structure. For N =1 , unusually strong vibronic structure resulting from a trion-type WMC was observed. We have shown that magnetic-field-induced shifts of this structure allow measurement of single particle Fock-Darwin levels and angular momentum transitions of the WMC. In addition, we demonstrated the use of NSOM imaging to probe the charge density distribution and observed anomalous dependence of the image size on the quantum confinement, implying a pairing of electrons or formation of whispering gallery modes in the QD. We demonstrated that InP/GaInP QDs, provide a Wigner-Seitz radius (rs) up to 13, and that the measurements of NSOM magneto-optical spectroscopy using these dots makes it possible to study effects arising from strong Coulomb interaction of a few confined electrons (holes).

  3. Exciton Migration and Amplified Quenching on Two-Dimensional Metal–Organic Layers

    Energy Technology Data Exchange (ETDEWEB)

    Cao, Lingyun; Lin, Zekai; Shi, Wenjie; Wang, Zi; Zhang, Cankun; Hu, Xuefu; Wang, Cheng; Lin, Wenbin (UC); (Xiamen)

    2017-05-10

    The dimensionality dependency of resonance energy transfer is of great interest due to its importance in understanding energy transfer on cell membranes and in low-dimension nanostructures. Light harvesting two-dimensional metal–organic layers (2D-MOLs) and three-dimensional metal–organic frameworks (3D-MOFs) provide comparative models to study such dimensionality dependence with molecular accuracy. Here we report the construction of 2D-MOLs and 3D-MOFs from a donor ligand 4,4',4''-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))tribenzoate (BTE) and a doped acceptor ligand 3,3',3''-nitro-4,4',4''-(benzene-1,3,5-triyl-tris(ethyne-2,1-diyl))tribenzoate (BTE-NO2). These 2D-MOLs and 3D-MOFs are connected by similar hafnium clusters, with key differences in the topology and dimensionality of the metal–ligand connection. Energy transfer from donors to acceptors through the 2D-MOL or 3D-MOF skeletons is revealed by measuring and modeling the fluorescence quenching of the donors. We found that energy transfer in 3D-MOFs is more efficient than that in 2D-MOLs, but excitons on 2D-MOLs are more accessible to external quenchers as compared with those in 3D-MOFs. These results not only provide support to theoretical analysis of energy transfer in low dimensions, but also present opportunities to use efficient exciton migration in 2D materials for light-harvesting and fluorescence sensing.

  4. Time domain investigation on excitonic spectral diffusion in CdSe quantum dots grown on vicinal surface GaAs substrates

    Science.gov (United States)

    Makino, T.; André, R.; Gérard, J.-M.; Romestain, R.; Dang, Le Si; Bartels, M.; Lischka, K.; Schikora, D.

    2004-04-01

    Two monolayers of CdSe sandwiched by ZnSe layers were grown by molecular-beam epitaxy on a vicinal GaAs surface substrate. Small ensembles of these self-assembled quantum dots (QD) have been studied at low temperatures using micro-photoluminescence spectroscopy. We observe reversible spectral diffusion in individual QD luminescence lines. The energy shift is negligibly small at low optical excitation power and amount to about 0.7 meV under higher excitation (0.6 kW/cm2). Because the energy shift is found to be significantly smaller than the exciton-biexciton splitting of CdSe QDs even under high excitation, the conventional spectral filtering could be used to effectively select the exciton-related emission from these QDs.

  5. Fabrication and Characterization of Fullerene-Based Bulk Heterojunction Solar Cells with Porphyrin, CuInS2, Diamond and Exciton-Diffusion Blocking Layer

    Directory of Open Access Journals (Sweden)

    Takeo Oku

    2010-04-01

    Full Text Available Fullerene-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and porphyrin, CuInS2 and diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals.

  6. An approach to glycobiology from glycolipidomics: ganglioside molecular scanning in the brains of patients with Alzheimer's disease by TLC-blot/matrix assisted laser desorption/ionization-time of flight MS.

    Science.gov (United States)

    Taki, Takao

    2012-01-01

    We have established a new approach to glyco- and lipidomics using a thin layer chromatography (TLC)-Blot/matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS) system. This new approach consists of a combination of a method for transferring lipids separated on a TLC-plate to a poly-vinylidene difluoride (PVDF) membrane and direct mass spectrometric analysis of the individual lipids on the membrane by ion trap-type MALDI-TOF MS. This technology was applied to the analysis of individual lipids from the human brain. Then, based on the results of this analysis, ganglioside molecular species in neural diseases were analyzed. The levels of gangliosides GD1b and GT1b were lower in the hippocampal gray matter of patients with Alzheimer's disease than in the hippocampal gray matter of patients with Parkinson's disease or the control patients. The molecular scanning of individual ganglioside molecular species showed a significant reduction of d20 : 1/C18 : 0 ceramide-containing gangliosides in patients with Alzheimer's disease. These findings suggest that Alzheimer's disease is a kind of ganglioside metabolic disease affecting the hippocampal area. A new approach to glycobiology by the TLC-Blot/MALDI-TOF MS system is proposed.

  7. Photo-induced exciton generation in polyvinylpyrrolidone encapsulated Ag2S core-shells: Electrochemical deposition, regular shape and high order of particle size distribution

    Science.gov (United States)

    Mukherjee, Nillohit; Jana, Sumanta; Gopal Khan, Gobinda; Mondal, Anup

    2012-12-01

    Visible light induced frequency switching behavior, exhibited by the electrochemically deposited thin films of polyvinylpyrrolidone (PVP) encapsulated Ag2S nanosphere (core-shell) is shown here. A low frequency (˜40 Hz) pulse was found to be generated upon illumination with 1 Sun due to excitonic transition, which also showed good switching behavior with the "on" and "off" state of the light. Capping of the semiconductor surface by a polymer like PVP reduces the surface states and thus lowers the built in barrier height and the width of depletion region. So, the number of photo generated but non recombining electron-hole pairs (excitons) increases, which put their signature in some unique physical properties like increase in photoluminescence (PL) intensity, light induced frequency switching behavior due to free exciton generation, etc. Here, the depositions were carried out on indium tin oxide coated glass substrates from an aqueous solution of AgNO3, thioacetamide, and PVP. The films were structurally characterized using high resolution X-ray diffraction, field emission scanning electron microscopy, and high resolution transmission electron microscopic techniques. The deposited particles were regular in shape with significantly high order of size distribution. Furrier transform infrared spectroscopy confirmed the presence of PVP as the encapsulating agent. Optical characterization, viz., UV - vis - NIR and NIR-PL revealed noteworthy amount of NIR emission from the deposited material.

  8. Correlative Light and Scanning Electron Microscopy for Observing the Three-Dimensional Ultrastructure of Membranous Cell Organelles in Relation to Their Molecular Components

    Science.gov (United States)

    Koga, Daisuke; Kusumi, Satoshi; Bochimoto, Hiroki; Watanabe, Tsuyoshi; Ushiki, Tatsuo

    2015-01-01

    Although the osmium maceration method has been used to observe three-dimensional (3D) structures of membranous cell organelles with scanning electron microscopy (SEM), the use of osmium tetroxide for membrane fixation and the removal of cytosolic soluble proteins largely impairs the antigenicity of molecules in the specimens. In the present study, we developed a novel method to combine cryosectioning with the maceration method for correlative immunocytochemical analysis. We first immunocytochemically stained a semi-thin cryosection cut from a pituitary tissue block with a cryo-ultramicrotome, according to the Tokuyasu method, before preparing an osmium-macerated specimen from the remaining tissue block. Correlative microscopy was performed by observing the same area between the immunostained section and the adjacent face of the tissue block. Using this correlative method, we could accurately identify the gonadotropes of pituitary glands in various experimental conditions with SEM. At 4 weeks after castration, dilated cisternae of rough endoplasmic reticulum (RER) were distributed throughout the cytoplasm. On the other hand, an extremely dilated cisterna of the RER occupied the large region of the cytoplasm at 12 weeks after castration. This novel method has the potential to analyze the relationship between the distribution of functional molecules and the 3D ultrastructure in different composite tissues. PMID:26374827

  9. Ab initio nonadiabatic dynamics of multichromophore complexes: a scalable graphical-processing-unit-accelerated exciton framework.

    Science.gov (United States)

    Sisto, Aaron; Glowacki, David R; Martinez, Todd J

    2014-09-16

    ("fragmenting") a molecular system and then stitching it back together. In this Account, we address both of these problems, the first by using graphical processing units (GPUs) and electronic structure algorithms tuned for these architectures and the second by using an exciton model as a framework in which to stitch together the solutions of the smaller problems. The multitiered parallel framework outlined here is aimed at nonadiabatic dynamics simulations on large supramolecular multichromophoric complexes in full atomistic detail. In this framework, the lowest tier of parallelism involves GPU-accelerated electronic structure theory calculations, for which we summarize recent progress in parallelizing the computation and use of electron repulsion integrals (ERIs), which are the major computational bottleneck in both density functional theory (DFT) and time-dependent density functional theory (TDDFT). The topmost tier of parallelism relies on a distributed memory framework, in which we build an exciton model that couples chromophoric units. Combining these multiple levels of parallelism allows access to ground and excited state dynamics for large multichromophoric assemblies. The parallel excitonic framework is in good agreement with much more computationally demanding TDDFT calculations of the full assembly.

  10. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... limitations of the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is ... top of page What are some common uses of the procedure? The thyroid scan is used to ...

  11. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... of the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is ... of page What are some common uses of the procedure? The thyroid scan is used to determine ...

  12. Lumbar spine CT scan

    Science.gov (United States)

    CAT scan - lumbar spine; Computed axial tomography scan - lumbar spine; Computed tomography scan - lumbar spine; CT - lower ... The lumbar CT scan is good for evaluating large herniated disks, ... smaller ones. This test can be combined with a myelogram to get ...

  13. Arm CT scan

    Science.gov (United States)

    CAT scan - arm; Computed axial tomography scan - arm; Computed tomography scan - arm; CT scan - arm ... stopping.) A computer creates separate images of the arm area, called slices. These images can be stored, ...

  14. Thoracic spine CT scan

    Science.gov (United States)

    CAT scan - thoracic spine; Computed axial tomography scan - thoracic spine; Computed tomography scan - thoracic spine; CT scan - ... Philadelphia, PA: Elsevier Mosby; 2013:chap 44. US Food and Drug Administration. Computed tomography (CT). Updated August ...

  15. Crystalline Nanoporous Frameworks: a Nanolaboratory for Probing Excitonic Device Concepts.

    Energy Technology Data Exchange (ETDEWEB)

    Allendorf, Mark D.; Azoulay, Jason; Ford, Alexandra Caroline; Foster, Michael E.; El Gabaly Marquez, Farid; Leonard, Francois Leonard; Leong-Hau, Kirsty; Stavila, Vitalie; Talin, Albert Alec; Wong, Brian M.; Brumbach, Michael T.; Van Gough, D.; Lambert, Timothy N.; Rodriguez, Mark A.; Spoerke, Erik David; Wheeler, David Roger; Deaton, Joseph C.; Centrone, Andrea; Haney, Paul; Kinney, R.; Szalai, Veronika; Yoon, Heayoung P.

    2014-09-01

    Electro-optical organic materials hold great promise for the development of high-efficiency devices based on exciton formation and dissociation, such as organic photovoltaics (OPV) and organic light-emitting devices (OLEDs). However, the external quantum efficiency (EQE) of both OPV and OLEDs must be improved to make these technologies economical. Efficiency rolloff in OLEDs and inability to control morphology at key OPV interfaces both reduce EQE. Only by creating materials that allow manipulation and control of the intimate assembly and communication between various nanoscale excitonic components can we hope to first understand and then engineer the system to allow these materials to reach their potential. The aims of this proposal are to: 1) develop a paradigm-changing platform for probing excitonic processes composed of Crystalline Nanoporous Frameworks (CNFs) infiltrated with secondary materials (such as a complimentary semiconductor); 2) use them to probe fundamental aspects of excitonic processes; and 3) create prototype OPVs and OLEDs using infiltrated CNF as active device components. These functional platforms will allow detailed control of key interactions at the nanoscale, overcoming the disorder and limited synthetic control inherent in conventional organic materials. CNFs are revolutionary inorganic-organic hybrid materials boasting unmatched synthetic flexibility that allow tuning of chemical, geometric, electrical, and light absorption/generation properties. For example, bandgap engineering is feasible and polyaromatic linkers provide tunable photon antennae; rigid 1-5 nm pores provide an oriented, intimate host for triplet emitters (to improve light emission in OLEDs) or secondary semiconducting polymers (creating a charge-separation interface in OPV). These atomically engineered, ordered structures will enable critical fundamental questions to be answered concerning charge transport, nanoscale interfaces, and exciton behavior that are inaccessible

  16. Direct observation of free-exciton thermalization in quantum-well structures

    DEFF Research Database (Denmark)

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

    1998-01-01

    . The subsequent relaxation dynamics within the 1s-exciton dispersion is directly monitored by time-resolved studies of the phonon-assisted photoluminescence. It is demonstrated that the free-exciton distribution remains nonthermal for some 100 ps. The observed dynamics is in reasonable agreement with numerical...

  17. Thermal effects in exciton harvesting in biased one-dimensional systems

    NARCIS (Netherlands)

    Vlaming, S. M.; Malyshev, V.A.; Knoester, J.

    2008-01-01

    The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by

  18. Dynamics of exciton diffusion in poly(p-phenylene vinylene)/fullerene heterostructures

    NARCIS (Netherlands)

    Markov, D.E.; Hummelen, J.C.; Blom, P.W.M.; Sieval, A.B.

    The exciton diffusion process in a poly(p-phenylene vinylene)- (PPV-)based derivative is investigated using time-resolved photoluminescence in conjugated polymer/fullerene heterostructures. The decay of the luminescence in the polymer/fullerene heterostructures is governed by exciton diffusion and

  19. The dynamical frustration of interlayer excitons delocalizing in bilayer quantum antiferromagnets

    NARCIS (Netherlands)

    Rademaker, L.; Wu, K.; Hilgenkamp, H.; Zaanen, J.

    2012-01-01

    Using the self-consistent Born approximation we study the delocalization of interlayer excitons in the bilayer Heisenberg quantum antiferromagnet. Under realistic conditions we find that the coupling between the exciton motion and the spin system is strongly enhanced as compared to the case of a

  20. Real-Time Tracking of Singlet Exciton Diffusion in Organic Semiconductors

    NARCIS (Netherlands)

    Kozlov, Oleg V.; de Haan, Foppe; Kerner, Ross A.; Rand, Barry P.; Cheyns, David; Pshenichnikov, Maxim S.

    2016-01-01

    Exciton diffusion in organic materials provides the operational basis for functioning of such devices as organic solar cells and light-emitting diodes. Here we track the exciton diffusion process in organic semiconductors in real time with a novel technique based on femtosecond photoinduced

  1. 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, w...

  2. Bose-Einstein condensation and superfluidity of dipolar excitons in a phosphorene double layer

    Science.gov (United States)

    Berman, Oleg L.; Gumbs, Godfrey; Kezerashvili, Roman Ya.

    2017-07-01

    We study the formation of dipolar excitons and their superfluidity in a phosphorene double layer. The analytical expressions for the single dipolar exciton energy spectrum and wave function are obtained. It is predicted that a weakly interacting gas of dipolar excitons in a double layer of black phosphorus exhibits superfluidity due to the dipole-dipole repulsion between the dipolar excitons. In calculations are employed the Keldysh and Coulomb potentials for the interaction between the charge carriers to analyze the influence of the screening effects on the studied phenomena. It is shown that the critical velocity of superfluidity, the spectrum of collective excitations, concentrations of the superfluid and normal component, and mean-field critical temperature for superfluidity are anisotropic and demonstrate the dependence on the direction of motion of dipolar excitons. The critical temperature for superfluidity increases if the exciton concentration and the interlayer separation increase. It is shown that the dipolar exciton binding energy and mean-field critical temperature for superfluidity are sensitive to the electron and hole effective masses. The proposed experiment to observe a directional superfluidity of excitons is addressed.

  3. Simple Screened Hydrogen Model of Excitons in Two-Dimensional Materials

    DEFF Research Database (Denmark)

    Olsen, Thomas; Latini, Simone; Rasmussen, Filip Anselm

    2016-01-01

    , the description of 2D excitons is complicated by the fact that the screening cannot be assumed to be local. We show that one can consistently define an effective 2D dielectric constant by averaging the screening over the extend of the exciton. For an ideal 2D semiconductor this leads to a simple expression for EB...

  4. The excitonic ground state of the half-filled Peierls insulator

    NARCIS (Netherlands)

    Rice, MJ; Gartstein, YN

    2005-01-01

    We point out that the half-filled Peierls insulator, celebrated for its soliton excitations and its application to trans(polyacetylene), is an excitonic insulator in which collectively bound electron-hole pair excitations (excitons) are mixed into the ground state. Unlike the bound electron pairs of

  5. 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...

  6. Supersonic exciton gratings: coherent inter-polariton scattering in semiconductor microcavities

    DEFF Research Database (Denmark)

    Birkedal, Dan; Vadim, Lyssenko; Hvam, Jørn Märcher

    2002-01-01

    We report on a coherent nonlinear phenomenon in a semiconductor microcavity (SMC), which has no parallel for QW excitons. When two different polariton modes of the SMC are impulsively excited they undergo normal mode oscillations (NMOs) with coherent energy exchange between the exciton and the ca...

  7. Continuum contribution to excitonic four-wave mixing due to interaction-induced nonlinearities

    DEFF Research Database (Denmark)

    Birkedal, Dan; Vadim, Lyssenko; Hvam, Jørn Märcher

    1996-01-01

    We present an experimental and theoretical investigation of ultrafast transient four-wave mixing of GaAs/AlxGa1-xAs quantum wells for coherent excitation of exciton and continuum states. The signal appears at the exciton resonance and is shown to consist of two contributions: an intense spectrally...

  8. Exciton formation and dissociation in mass-asymmetric electron-hole plasmas

    Energy Technology Data Exchange (ETDEWEB)

    Fehske, H [Institut fuer Physik, Ernst-Moritz-Arndt-Universitaet Greifswald, Domstrasse 10a, D-17489 Greifswald (Germany); Filinov, V [Institute for High Energy Density, Russian Academy of Sciences, Izhorskay 13/19, Moscow 127412 (Russian Federation); Bonitz, M [Christian-Albrechts-Universitaet zu Kiel, Institut fuer Theoretische Physik und Astrophysik, Lehrstuhl Statistische Physik, Leibnizstrasse 15, 24098 Kiel (Germany); Fortov, V [Institute for High Energy Density, Russian Academy of Sciences, Izhorskay 13/19, Moscow 127412 (Russian Federation); Levashov, P [Institute for High Energy Density, Russian Academy of Sciences, Izhorskay 13/19, Moscow 127412 (Russian Federation)

    2005-01-01

    First-principle path integral Monte Carlo simulations were performed in order to analyze correlation effects in complex electron-hole plasmas, particularly with regard to the appearance of excitonic bound states. Results are discussed in relation to exciton formation in unconventional semiconductors with large electron hole mass asymmetry.

  9. Physical properties of archaeal tetraether lipid membranes as revealed by differential scanning and pressure perturbation calorimetry, molecular acoustics, and neutron reflectometry: effects of pressure and cell growth temperature.

    Science.gov (United States)

    Zhai, Yong; Chong, Parkson Lee-Gau; Taylor, Leeandrew Jacques-Asa; Erlkamp, Mirko; Grobelny, Sebastian; Czeslik, Claus; Watkins, Erik; Winter, Roland

    2012-03-20

    The polar lipid fraction E (PLFE) is a major tetraether lipid component in the thermoacidophilic archaeon Sulfolobus acidocaldarius. Using differential scanning and pressure perturbation calorimetry as well as ultrasound velocity and density measurements, we have determined the compressibilities and volume fluctuations of PLFE liposomes derived from different cell growth temperatures (T(g) = 68, 76, and 81 °C). The compressibility and volume fluctuation values of PLFE liposomes, which are substantially less than those detected from diester lipid membranes (e.g., DPPC), exhibit small but significant differences with T(g). Among the three T(g)s employed, 76 °C leads to the least compressible and most tightly packed PLFE membranes. This temperature is within the range for optimal cell growth (75-80 °C). It is known that a decrease in T(g) decreases the number of cyclopentane rings in archael tetraether lipids. Thus, our data enable us to present the new view that membrane packing in PLFE liposomes varies with the number of cyclopentane rings in a nonlinear manner, reaching maximal tightness when the tetraether lipids are derived from cells grown at optimal T(g)s. In addition, we have studied the effects of pressure on total layer thickness, d, and neutron scattering length density, ρ(n), of a silicon-D(2)O interface that is covered with a PLFE membrane using neutron reflectometry (NR). At 55 °C, d and ρ(n) are found to be rather insensitive to pressure up to 1800 bar, suggesting minor changes of the thickness of the membrane's hydrophobic core and headgroup orientation upon compression only.

  10. Direct measurement of exciton dissociation energy in polymers

    Directory of Open Access Journals (Sweden)

    J. Toušek

    2017-01-01

    Full Text Available Exciton dissociation energy was obtained based on the comparison of thickness of the space charge region estimated from the measurement of capacitance of prepared Schottky diode and from the measurement of photovoltage spectra. While the capacitance measurements provide information about the total width of the space charge region (SCR the surface photovoltaic effect brings information only about the part of the SCR where electric field is sufficiently high to cause dissociation. For determination of the dissociation energy it is sufficient to find the electric potential in the SCR where the process starts.

  11. Exciton and Hole-Transfer Dynamics in Polymer: Fullerene Blends

    Directory of Open Access Journals (Sweden)

    van Loosdrecht P. H. M.

    2013-03-01

    Full Text Available Ultrafast hole transfer dynamics from fullerene derivative to polymer in bulk heterojunction blends are studied with visible-pump - IR-probe spectroscopy. The hole transfer process is found to occur in 50/300 fs next to the interface, while a longer 15-ps time is attributed to exciton diffusion towards interface in PC71BM domains. High polaron generation efficiency in P3HT blends indicates excellent intercalation between the polymer and the fullerene even at highest PC71BM concentration thereby yielding a valuable information on the blend morphology.

  12. Self-trapping phenomenon in exciton-polariton dynamics

    Science.gov (United States)

    Vasilieva, O. F.; Khadzhi, P. I.

    2013-12-01

    The polariton dynamics in a microcavity in the parametric oscillator mode, when two pump polaritons are transformed into signal and idler polaritons and vice versa, has been studied. A nonlinear differential equation is obtained, which describes the temporal evolution of pump-polariton density, the solution of which is expressed in terms of the Jacobi elliptical functions. The amplitude and period of polaritondensity oscillations are determined by the initial polariton densities and the resonance detuning. The possibility of self-trapping in exciton-polariton dynamics is predicted.

  13. Lateral excitonic switching in vertically stacked quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Jarzynka, Jarosław R.; McDonald, Peter G.; Galbraith, Ian [Institute of Photonics and Quantum Sciences, SUPA, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS (United Kingdom); Shumway, John [Department of Physics, Arizona State University, Tempe, Arizona 85287 (United States)

    2016-06-14

    We show that the application of a vertical electric field to the Coulomb interacting system in stacked quantum dots leads to a 90° in-plane switching of charge probability distribution in contrast to a single dot, where no such switching exists. Results are obtained using path integral quantum Monte Carlo with realistic dot geometry, alloy composition, and piezo-electric potential profiles. The origin of the switching lies in the strain interactions between the stacked dots hence the need for more than one layer of dots. The lateral polarization and electric field dependence of the radiative lifetimes of the excitonic switch are also discussed.

  14. Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films

    OpenAIRE

    Fang, Hong-Hua; Protesescu, Loredana; Balazs, Daniel M.; Adjokatse, Sampson; Kovalenko, Maksym V.; Loi, Maria A.

    2017-01-01

    The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI(3)) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 x 10(-13) cm(-2)) are measured. At high ...

  15. 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"

  16. Exciton quenching close to polymer : vacuum interface of spin-coated films of poly(p-phenylenevinylene) derivative

    NARCIS (Netherlands)

    Mikhnenko, O.V.; Cordella, F.; Sieval, A.B.; Hummelen, J.C.; Blom, P.W.M.; Loi, M.A.

    2009-01-01

    Polymer-fullerene bilayer heterostructures are suited to study excitonic processes in conjugated polymers. Excitons are efficiently quenched at the polymer-fullerene interface, whereas the polymer-vacuum interface is often considered as an exciton-reflecting interface. Here, we report about

  17. Phase Diagram of the Bose Condensation of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells

    DEFF Research Database (Denmark)

    Dremin, A. A.; Timofeev, V. B.; Larionov, A. V.

    2002-01-01

    observed in the experiment was attributed to Bose–Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature interval studied (0.5–3.6) K, the critical exciton density and temperature were determined and a phase diagram outlining the exciton condensate region...

  18. Coherent spin dynamics of an interwell excitonic gas in GaAs/AlGaAs coupled quantum wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Bisti, V. E.; Bayer, M.

    2006-01-01

    The spin dynamics of an interwell exciton gas has been investigated in n-i-n GaAs/AlGaAs coupled quantum wells. The time evolution kinetics of the interwell exciton photoluminescence has been measured under resonant excitation of the 1s heavy-hole intrawell exciton, using a pulsed tunable laser...

  19. Orientation-Dependent Exciton-Plasmon Coupling in Embedded Organic/Metal Nanowire Heterostructures.

    Science.gov (United States)

    Li, Yong Jun; Hong, Yan; Peng, Qian; Yao, Jiannian; Zhao, Yong Sheng

    2017-10-24

    The excitation of surface plasmons by optical emitters based on exciton-plasmon coupling is important for plasmonic devices with active optical properties. It has been theoretically demonstrated that the orientation of exciton dipole can significantly influence the coupling strength, yet systematic study of the coupling process in nanostructures is still hindered by the lack of proper material systems. In this work, we have experimentally investigated the orientation-dependent exciton-plasmon coupling in a rationally designed organic/metal nanowire heterostructure system. The heterostructures were prepared by inserting silver nanowires into crystalline organic waveguides during the self-assembly of dye molecules. Structures with different exciton orientations exhibited varying coupling efficiencies. The near-field exciton-plasmon coupling facilitates the design of nanophotonic devices based on the directional surface plasmon polariton propagations.

  20. An excitonic approach to the intraband THz response of semiconductor nanostructures

    Energy Technology Data Exchange (ETDEWEB)

    Dignam, Marc M.; Sy, Fredrik; Parks, Andrew M. [Department of Physics, Engineering Physics and Astronomy, Queen' s University, Kingston, Ontario, K7L 3N6 (Canada); Wang, Dawei [Electronic Materials Research Laboratory - Key Laboratory of the Ministry of Education, and International Center for Dielectric Research, Xi' an Jiaotong University, Xi' an 710049 (China)

    2014-03-31

    Considerable effort has been devoted in recent years to developing an accurate and computationally-viable theoretical treatment of the THz response of semiconductor nanostructures that are excited by ultrashort optical pulses. Although most approaches, such as the semiconductor Bloch equations, employ an electron-hole basis, we have developed an excitonic approach that has significant advantages in many situations. Our approach includes the exchange interaction between excitons, the effects of the Pauli exclusion principle for the excitons (which are composite Bosons), and the dipole-dipole interactions between excitons. In this paper we review our excitonic formalism and apply it to examine the THz absorption of optically-excited CdSe nanorods and 2D GaAs quantum wells.

  1. Effect of periodic potential on exciton states in semiconductor carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Roslyak, Oleksiy, E-mail: oroslyak@fordham.edu [Department of Physics and Engineering Physics, Fordham University, Bronx, NY 10458 (United States); Piryatinski, Andrei, E-mail: apiryat@lanl.gov [Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States)

    2016-12-20

    We develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet sub-bands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the exciton energy red Stark shift and reduction in the exciton binding energy. Comparison of our results with reported theoretical and experimental studies is provided.

  2. Radiative energy transfer from MoS2 excitons to surface plasmons

    Science.gov (United States)

    Kang, Yimin; Li, Bowen; Fang, Zheyu

    2017-12-01

    In this work, we demonstrated the energy transfer process from few-layer MoS2 to gold dimer arrays via ultrafast pump-probe spectroscopy. With the overlap between the MoS2 exciton and the designed plasmon dipolar modes in the frequency domain, the exciton energy can be radiatively transferred to plasmonic structures, excited the localized surface plasmon resonance, and then enhanced the oscillation of coherent acoustic phonons. Power-dependent differential reflection signals and an analytical model based on the rate equation of exciton density were carried out to quantitatively study the energy transfer process. Our finding explores the energy flow between MoS2 excitons and surface plasmons, and can be contributed to the design of exciton-plasmon structures utilizing ultrathin materials.

  3. Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles

    Energy Technology Data Exchange (ETDEWEB)

    Vistovskyy, V. V., E-mail: vistvv@gmail.com; Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S. [Ivan Franko National University of Lviv, 8 Kyryla i Mefodiya, 79005 Lviv (Ukraine); Myagkota, O. S. [Lviv Polytechnic National University, 12S. Bandera, 79013 Lviv (Ukraine); Gloskovskii, A. [Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg (Germany); Gektin, A. V. [Institute for Scintillation Materials, NAS of Ukraine 60 Lenina Ave, 61001 Kharkiv (Ukraine); Vasil' ev, A. N. [Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, 119991 Moscow (Russian Federation); Rodnyi, P. A. [Saint-Petersburg State Polytechnical University, 29, Polytekhnicheskaya, 195251 Saint-Petersburg (Russian Federation)

    2013-11-21

    The influence of the BaF{sub 2} nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (hν ≤ E{sub g}) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

  4. Excitons in semiconducting quantum filaments of CdS and CdSe with dielectric barriers

    CERN Document Server

    Dneprovskij, V S; Shalygina, O A; Lyaskovskij, V L; Mulyarov, E A; Gavrilov, S A; Masumoto, I

    2002-01-01

    The peculiarities of the luminescence spectra obtained by different polarization and intensity of the pumping excitation and luminescence kinetics of the CdS and CdSe nanocrystals are explained by the exciton transitions in the semiconducting quantum threads with dielectric barriers. The exciton transition energies correspond to the calculated ones with an account of both their dimensional quantization and the effect of the excitons dielectric intensification. It is shown that the excitons transition energies do not change by the change in the quantum threads diameter within the wide range, while the increase in the one-dimensional forbidden zone width of quantum thread by the decrease in its diameter is compensated through the decrease in the excitons binding energy

  5. 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

    Multi-layered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials due to their well-defined morphology, high luminescence efficiencies, and color tunability. We resort to temperature-dependent cw and picosecond photoluminescence (PL......) 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...

  6. Excitonic optical response of carbon chains confined in single-walled carbon nanotubes

    Science.gov (United States)

    Bonabi, Farzad; Brun, Søren J.; Pedersen, Thomas G.

    2017-10-01

    It has been recently shown that long linear carbon chains (carbyne) can be formed inside multiwalled carbon nanotubes (CNTs). Encapsulation of carbyne inside the CNT affects the electronic structure of the chain by the long-range Coulomb interaction. This introduces an indirect band gap in the combined CNT-chain system and results in a change in the optical band gap. We study the excitonic optical response of the combined system using the Bethe-Salpeter and Wannier equations based on density functional theory and tight-binding band structures. The optical properties of isolated CNTs and chains are strongly affected by excitonic effects and the CNT-chain system follows a similar trend. The interaction between the CNT and chain results in new bright excitons as well as charge transfer excitons, where electrons are localized on the CNT and holes on the chain, yielding new dark excitons in the combined system.

  7. Probing long-lived dark excitons in self-assembled quantum dots

    DEFF Research Database (Denmark)

    Johansen, Jeppe; Julsgaard, Brian; Stobbe, Søren

    2010-01-01

    size. The energy dependence is compared to a recent model from the literature, in which the spin flip is due to the combined action of short-range exchange interaction and acoustic phonons. We furthermore observe a pronounced enhancement of the spin-flip rate close to semiconductor-air interfaces......Long-lived dark exciton states are formed in self-assembled quantum dots due to the combination of the angular momentum of electrons and holes. The lifetime of dark excitons are determined by spin-flip processes that transfer dark excitons into radiative bright excitons. We employ time......-resolved spontaneous emission measurements in a modified local density of optical states to unambiguously record the spin-flip rate. Pronounced variations in the spin-flip rate with the quantum dot emission energy are observed demonstrating that the exciton storage time can be extended by controlling the quantum dot...

  8. Rate Equation Analysis of the Dynamics of First-order Exciton Mott Transition

    Science.gov (United States)

    Sekiguchi, Fumiya; Shimano, Ryo

    2017-10-01

    We perform a rate equation analysis of the dynamics of the exciton Mott transition (EMT) assuming a detailed balance between excitons and unbound electron-hole (e-h) pairs. Using the Saha equation and adopting an empirical expression for the band-gap renormalization effect caused by unbound e-h pairs, we show that the ionization ratio of excitons exhibits bistability as a function of the total e-h pair density at low temperatures. We demonstrate that an incubation time emerges in the dynamics of the EMT from the oversaturated exciton gas phase on the verge of the bistable region. The incubation time shows slowing down behavior when the pair density approaches saddle-node bifurcation of the hysteresis curve of the exciton ionization ratio.

  9. Spin dynamics of low-dimensional excitons due to acoustic phonons

    Energy Technology Data Exchange (ETDEWEB)

    Thilagam, A; Lohe, M A [Department of Physics, University of Adelaide, Adelaide 5005 (Australia)

    2006-03-29

    We investigate the spin dynamics of excitons interacting with acoustic phonons in quantum wells, quantum wires and quantum discs by employing a multiband model based on the 4 x 4 Luttinger Hamiltonian. We also use the Bir-Pikus Hamiltonian to model the coupling of excitons to both longitudinal acoustic phonons and transverse acoustic phonons, thereby providing us with a realistic framework in which to determine details of the spin dynamics of excitons. We use a fractional dimensional formulation to model the excitonic wavefunctions and we demonstrate explicitly the decrease of spin relaxation time with dimensionality. Our numerical results are consistent with experimental results of spin relaxation times for various configurations of the GaAs/Al{sub 0.3}Ga{sub 0.7}As material system. We find that longitudinal and transverse acoustic phonons are equally significant in processes of exciton spin relaxations involving acoustic phonons.

  10. Spin dynamics of low-dimensional excitons due to acoustic phonons

    Science.gov (United States)

    Thilagam, A.; Lohe, M. A.

    2006-03-01

    We investigate the spin dynamics of excitons interacting with acoustic phonons in quantum wells, quantum wires and quantum discs by employing a multiband model based on the 4 × 4 Luttinger Hamiltonian. We also use the Bir-Pikus Hamiltonian to model the coupling of excitons to both longitudinal acoustic phonons and transverse acoustic phonons, thereby providing us with a realistic framework in which to determine details of the spin dynamics of excitons. We use a fractional dimensional formulation to model the excitonic wavefunctions and we demonstrate explicitly the decrease of spin relaxation time with dimensionality. Our numerical results are consistent with experimental results of spin relaxation times for various configurations of the GaAs/Al0.3Ga0.7As material system. We find that longitudinal and transverse acoustic phonons are equally significant in processes of exciton spin relaxations involving acoustic phonons.

  11. Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

    Science.gov (United States)

    Pässler, R.; Griebl, E.; Riepl, H.; Lautner, G.; Bauer, S.; Preis, H.; Gebhardt, W.; Buda, B.; As, D. J.; Schikora, D.; Lischka, K.; Papagelis, K.; Ves, S.

    1999-10-01

    High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

  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. Nanophotonic interactions between organic excitons and plasmonic metasurfaces (Conference Presentation)

    Science.gov (United States)

    O'Carroll, Deirdre M.

    2016-09-01

    Thin-film organic semiconductor materials are emerging as energy-efficient, versatile alternatives to inorganic semiconductors for display and solid-state lighting applications. Additionally, thin-film organic laser and photovoltaic technologies, while not yet competitive with inorganic semiconductor-based analogues, can exhibit small device embodied energies (due to comparatively low temperature and low energy-use fabrication processes) which is of interest for reducing overall device cost. To improve energy conversion efficiency in thin-film organic optoelectronics, light management using nanophotonic structures is necessary. Here, our recent work on improving light trapping and light extraction in organic semiconductor thin films using nanostructured silver plasmonic metasurfaces will be presented [1,2]. Numerous optical phenomena, such as absorption induced scattering, out-of-plane waveguiding and morphology-dependent surface plasmon outcoupling, are identified due to exciton-plasmon coupling between the organic semiconductor and the metasurface. Interactions between localized and propagating surface plasmon polaritons and the excitonic transitions of a variety of organic conjugated polymer materials will be discussed and ways in which these interactions may be optimized for particular optoelectronic applications will be presented. [1] C. E. Petoukhoff, D. M. O'Carroll, Absorption-Induced Scattering and Surface Plasmon Out-Coupling from Absorber-Coated Plasmonic Metasurfaces. Nat. Commun. 6, 7899-1-13 (2015). [2] Z. Shen, D. M. O'Carroll, Nanoporous Silver Thin Films: Multifunctional Platforms for Influencing Chain Morphology and Optical Properties of Conjugated Polymers. Adv. Funct. Mater. 25, 3302-3313 (2015).

  14. Excitonic effects in ZnO nanowires and hollow nanotubes

    Science.gov (United States)

    Willander, M.; Lozovik, Y. E.; Zhao, Q. X.; Nur, O.; Hu, Q.-H.; Klason, P.

    2007-02-01

    Energy levels and wave functions of ground and excited states of an exciton are calculated by the method of imaginary time. Energy levels as functions of radius of single and double wall nanotube are studied. Asymptotic behavior of energy levels at large and small values of the radius using perturbation theory and adiabatic approximation is considered. Spatially indirect exciton in semiconductor nanowire is also investigated. Experimental result from high quality reproducible ZnO nanowires grown by low temperature chemical engineering is presented. State of the art high brightness white light emitting diodes (HB-LEDs) are demonstrated from the grown ZnO nano-wires. The color temperature and color rendering index (CRI) of the HB-LEDs values was found to be (3250 K, 82), and (14000 K, 93), for the best LEDs, which means that the quality of light is superior to one obtained from GaN LEDs available on the market today. The role of V Zn and V ° on the emission responsible for the white light band as well as the peak position of this important wide band is thoroughly investigated in a systematic way.

  15. Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films.

    Science.gov (United States)

    Fang, Hong-Hua; Protesescu, Loredana; Balazs, Daniel M; Adjokatse, Sampson; Kovalenko, Maksym V; Loi, Maria Antonietta

    2017-08-01

    The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI 3 ) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron-hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross-section (5.2 × 10 -13 cm -2 ) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap-assisted recombination that turns to bimolecular at high pump fluences. From the temperature-dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap-mediated recombination, which occur in thin films at higher temperatures. © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  16. Boosting the performance of red PHOLEDs by exciton harvesting

    Science.gov (United States)

    Chang, Y.-L.; Wang, Z. B.; Helander, M. G.; Qiu, J.; Lu, Z. H.

    2012-09-01

    Significant development has been made on phosphorescent organic light emitting diodes (PHOLEDs) over the past decade, which eventually resulted in the commercialization of widely distributed active-matrix organic light emitting diode displays for mobile phones. However, higher efficiency PHOLEDs are still needed to further reduce the cost and lower the power consumption for general lighting and LED backlight applications. In particular, red PHOLEDs currently have in general the lowest efficiencies among the three primary colors, due most likely to the energy-gap law. Therefore, a number of groups have of made use of various device configurations, including insertion of a carrier blocking or exciton confining layer, doping the transport layers, as well as employing multiple emissive zone structures to improve the device efficiency. However, these approaches are rather inconvenient for commercial applications. In this work, we have developed a simpler way to boost the performance of red PHOLEDs by incorporating an exciton harvesting green emitter, which transfers a large portion of the energy to the co-deposited red emitter. A high external quantum efficiency (EQE) of 20.6% was achieved, which is among the best performances for red PHOLEDs.

  17. 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.

  18. Efficient Multiple Exciton Generation Observed in Colloidal PbSe Quantum Dots with Temporally and Spectrally Resolved Intraband Excitation

    KAUST Repository

    Ji, Minbiao

    2009-03-11

    We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation In colloidal PbSe QDs. © 2009 American Chemical Society.

  19. Brain PET scan

    Science.gov (United States)

    ... have false results on a PET scan. Blood sugar or insulin levels may affect the test results in people with diabetes . PET scans may be done along with a CT scan. This combination scan is called a PET/CT. Alternative Names Brain positron emission tomography; PET scan - brain References Chernecky ...

  20. Coronary Calcium Scan

    Science.gov (United States)

    ... Back To Health Topics / Coronary Calcium Scan Coronary Calcium Scan Also known as Calcium Scan Test A coronary calcium scan is a CT scan of your heart that detects and measures the amount of calcium in the walls of your coronary arteries. Overview ...

  1. External electric field effect on exciton binding energy in InGaAsP/InP cylindrical quantum wires

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Hailong, E-mail: hlwang@mail.qfnu.edu.cn [College of Physics and Engineering, Qufu Normal University, Qufu 273165 (China); Wang, Wenjuan [College of Physics and Engineering, Qufu Normal University, Qufu 273165 (China); Gong, Qian; Wang, Shumin [State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050 (China)

    2016-12-15

    Exciton binding energies in InGaAsP/InP cylindrical quantum wires are calculated through variational method under the framework of effective-mass envelope-function approximation. It is shown that the variation of exciton binding energy is highly dependent on radius of the wire, material composition and external electric field. Exciton binding energy is a non-monotonic function of wire radius. It increases until it reaches a maximum, and then decreases as the wire radius decreases. With the increase of In composition, the wire radius need increase to reach the maximum value of exciton binding energy. It is also found that the external electric field has little effect on exciton binding energy. However, the excitonic effect will be destroyed when external electric field is large enough. In addition, the Stark shift of exciton binding energy is also calculated.

  2. Excitons into one-axis crystals of zinc phosphide (Zn3P2

    Directory of Open Access Journals (Sweden)

    D.M. Stepanchikov

    2009-01-01

    Full Text Available Theoretical study of excitons spectra is offered in this report as for Zn3P2 crystals. Spectra are got in the zero approach of the theory of perturbations with consideration of both the anisotropy of the dispersion law and the selection rules. The existence of two exciton series was found, which corresponds to two valence bands (hh, lh and the conductivity band (c. It is noteworthy that anisotropy of the dispersion law plus the existence of crystalline packets (layers normal to the main optical axis, both will permit the consideration of two-dimensional excitons too. The high temperature displaying of these 2D-exciton effects is not eliminated even into bulk crystals. The calculated values of the binding energies as well as the oscillator's strength for the optical transitions are given for a volume (3D and for two-dimensional (2D excitons. The model of energy exciton transitions and four-level scheme of stimulated exciton radiation for receiving laser effect are offered.

  3. 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.

  4. Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

    Energy Technology Data Exchange (ETDEWEB)

    Wu, Fengcheng; Lovorn, Timothy; MacDonald, A. H.

    2018-01-01

    We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moire pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moire Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moire potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moire pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.

  5. Excitons and Davydov splitting in sexithiophene from first-principles many-body Green's function theory.

    Science.gov (United States)

    Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen

    2015-09-21

    Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.

  6. Composite Boson Description of a Low-Density Gas of Excitons

    Science.gov (United States)

    Golomedov, A. E.; Lozovik, Yu. E.; Astrakharchik, G. E.; Boronat, J.

    2017-12-01

    Ground-state properties of a fermionic Coulomb gas are calculated using the fixed-node diffusion Monte Carlo method. The validity of the composite boson description is tested for different densities. We extract the exciton-exciton s-wave scattering length by solving the four-body problem in a harmonic trap and mapping the energy to that of two trapped bosons. The equation of state is consistent with the Bogoliubov theory for composite bosons interacting with the obtained s-wave scattering length. The perturbative expansion at low density has contributions physically coming from (a) exciton binding energy, (b) mean-field Gross-Pitaevskii interaction between excitons, and (c) quantum depletion of the excitonic condensate (Lee-Huang-Yang terms for composite bosons). In addition, for low densities we find a good agreement with the Bogoliubov bosonic theory for the condensate fraction of excitons. The equation of state in the opposite limit of large density is found to be well described by the perturbative theory including (a) mixture of two ideal Fermi gases and (b) exchange energy. We find that for low densities both energetic and coherent properties are correctly described by the picture of composite bosons (excitons).

  7. Strong excitonic interactions in the oxygen K-edge of perovskite oxides

    Energy Technology Data Exchange (ETDEWEB)

    Tomita, Kota; Miyata, Tomohiro [Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan); Olovsson, Weine [Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden); Mizoguchi, Teruyasu, E-mail: teru@iis.u-tokyo.ac.jp [Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan)

    2017-07-15

    Excitonic interactions of the oxygen K-edge electron energy-loss near-edge structure (ELNES) of perovskite oxides, CaTiO{sub 3}, SrTiO{sub 3}, and BaTiO{sub 3}, together with reference oxides, MgO, CaO, SrO, BaO, and TiO{sub 2}, were investigated using a first-principles Bethe–Salpeter equation calculation. Although the transition energy of oxygen K-edge is high, strong excitonic interactions were present in the oxygen K-edge ELNES of the perovskite oxides, whereas the excitonic interactions were negligible in the oxygen K-edge ELNES of the reference compounds. Detailed investigation of the electronic structure suggests that the strong excitonic interaction in the oxygen K-edge ELNES of the perovskite oxides is caused by the directionally confined, low-dimensional electronic structure at the Ti–O–Ti bonds. - Highlights: • Excitonic interaction in oxygen-K edge is investigated. • Strong excitonic interaction is found in the oxygen-K edge of perovskite oxides. • The strong excitonic interaction is ascribed to the low-dimensional and confined electronic structure.

  8. A study of polaritonic transparency in couplers made from excitonic materials

    Energy Technology Data Exchange (ETDEWEB)

    Singh, Mahi R.; Racknor, Chris [Department of Physics and Astronomy, Western University, London, Ontario N6A 3K7 (Canada)

    2015-03-14

    We have studied light matter interaction in quantum dot and exciton-polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is also found that transparent states can be switched on and off by applying an external control laser field.

  9. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... of the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is ... code: Phone no: Thank you! Do you have a personal story about radiology? Share your patient story ...

  10. Heart PET scan

    Science.gov (United States)

    ... nuclear medicine scan; Heart positron emission tomography; Myocardial PET scan ... A PET scan requires a small amount of radioactive material (tracer). This tracer is given through a vein (IV), ...

  11. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... of the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid scan is ... taking our brief survey: Survey Do you have a personal story about radiology? Share your patient story ...

  12. Picosecond dynamics of internal exciton transitions in CdSe nanorods

    DEFF Research Database (Denmark)

    Cooke, D. G.; Jepsen, Peter Uhd; Lek, Jun Yan

    2013-01-01

    . The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface states......The picosecond dynamics of excitons in colloidal CdSe nanorods are directly measured via their 1s to 2p-like internal transitions by ultrabroadband terahertz spectroscopy. Broadened absorption peaks from both the longitudinal and transverse states are observed at 8.5 and 11 THz, respectively...

  13. The exciton excitations and relaxation processes in low-dimensional semiconductor heterostructures with quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Aleshkin, V. Ya.; Gavrilenko, L. V.; Gaponova, D. M., E-mail: dmg@ipmras.ru; Krasil’nik, Z. F.; Kryzhkov, D. I. [Russian Academy of Sciences, Institute for Physics of Microstructures (Russian Federation)

    2016-12-15

    The processes associated with the transfer of excitonic excitations between tunnel-uncoupled quantum wells (QW) and the influence of the local electric field were investigated in AlGaAs/GaAs heterostructures by the method of photoluminescence excitation (PLE) spectroscopy at low (4.2 K) temperature. The variation in the intensity of photoluminescence (PL) from the wider QW under resonant excitation of excitonic transition in the adjacent narrow QW has been observed. The difference in the PL maximum position and intensity of the wider QW under resonance excitation of the narrow one is explained by the influence of quantum-confined Stark effect on the process of exciton recombination.

  14. Resonant exciton-phonon coupling in ZnO nanorods at room temperature

    Directory of Open Access Journals (Sweden)

    Soumee Chakraborty

    2011-09-01

    Full Text Available Vibronic and optoelectronic properties, along with detailed studies of exciton-phonon coupling at room temperature (RT for random and aligned ZnO nanorods are reported. Excitation energy dependent Raman studies are performed for detailed analysis of multi-phonon processes in the nanorods. We report here the origin of coupling between free exciton and its associated phonon replicas, including its higher order modes, in the photoluminescence spectra at RT. Resonance of excitonic electron and resonating first order zone center LO phonon, invoked strongly by Frolich interaction, are made responsible for the observed phenomenon.

  15. Particle-in-a-bos model of one-dimensional excitons in conjugated polymers

    DEFF Research Database (Denmark)

    Pedersen, T.G.; Johansen, P.M.; Pedersen, H.C.

    2000-01-01

    A simple two-particle model of excitons in conjugated polymers is proposed as an alternative to usual highly computationally demanding quantum chemical methods. In the two-particle model, the exciton is described as an electron-hole pair interacting via Coulomb forces and confined to the polymer...... of these cases an approximate solution for the general case is obtained. As an application of the model the influence of a static electric field on the electron-hole overlap integral and exciton energy is considered....

  16. Determination of the Exciton Binding Energy in CdSe Quantum Dots

    Energy Technology Data Exchange (ETDEWEB)

    Meulenberg, R; Lee, J; Wolcott, A; Zhang, J; Terminello, L; van Buuren, T

    2009-10-27

    The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using x-ray spectroscopy. Using x-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e. without and exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

  17. 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...

  18. Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon

    Energy Technology Data Exchange (ETDEWEB)

    Thompson, Nicholas J.; Congreve, Daniel N.; Baldo, Marc A., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Energy Frontier Research Center for Excitonics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Goldberg, David; Menon, Vinod M., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Department of Physics, Queens College and Graduate Center, The City University of New York, Flushing, New York 11367 (United States)

    2013-12-23

    Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

  19. Improved value for the silicon free exciton binding energy

    Directory of Open Access Journals (Sweden)

    Martin A. Green

    2013-11-01

    Full Text Available The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date.

  20. Generation of Rabi frequency radiation using exciton-polaritons

    CERN Document Server

    Barachati, Fábio; Kéna-Cohen, Stéphane

    2015-01-01

    We study the use of exciton-polaritons in semiconductor microcavities to generate radiation spanning the infrared to terahertz regions of the spectrum by exploiting transitions between upper and lower polariton branches. The process, which is analogous to difference-frequency generation (DFG), relies on the use of semiconductors with a nonvanishing second-order susceptibility. For an organic microcavity composed of a nonlinear optical polymer, we predict a DFG irradiance enhancement of $2.8\\cdot10^2$, as compared to a bare nonlinear polymer film, when triple resonance with the fundamental cavity mode is satisfied. In the case of an inorganic microcavity composed of (111) GaAs, an enhancement of $8.8\\cdot10^3$ is found, as compared to a bare GaAs slab. Both structures show high wavelength tunability and relaxed design constraints due to the high modal overlap of polariton modes.

  1. Multiple exciton generation in quantum dot-based solar cells

    Directory of Open Access Journals (Sweden)

    Goodwin Heather

    2018-01-01

    Full Text Available Multiple exciton generation (MEG in quantum-confined semiconductors is the process by which multiple bound charge-carrier pairs are generated after absorption of a single high-energy photon. Such charge-carrier multiplication effects have been highlighted as particularly beneficial for solar cells where they have the potential to increase the photocurrent significantly. Indeed, recent research efforts have proved that more than one charge-carrier pair per incident solar photon can be extracted in photovoltaic devices incorporating quantum-confined semiconductors. While these proof-of-concept applications underline the potential of MEG in solar cells, the impact of the carrier multiplication effect on the device performance remains rather low. This review covers recent advancements in the understanding and application of MEG as a photocurrent-enhancing mechanism in quantum dot-based photovoltaics.

  2. Anisotropic plasmons, excitons, and electron energy loss spectroscopy of phosphorene

    Science.gov (United States)

    Ghosh, Barun; Kumar, Piyush; Thakur, Anmol; Chauhan, Yogesh Singh; Bhowmick, Somnath; Agarwal, Amit

    2017-07-01

    In this article, we explore the anisotropic electron energy loss spectrum (EELS) in monolayer phosphorene based on ab initio time-dependent density-functional-theory calculations. Similarly to black phosphorus, the EELS of undoped monolayer phosphorene is characterized by anisotropic excitonic peaks for energies in the vicinity of the band gap and by interband plasmon peaks for higher energies. On doping, an additional intraband plasmon peak also appears for energies within the band gap. Similarly to other two-dimensional systems, the intraband plasmon peak disperses as ωpl∝√{q } in both the zigzag and armchair directions in the long-wavelength limit and deviates for larger wave vectors. The anisotropy of the long-wavelength plasmon intraband dispersion is found to be inversely proportional to the square root of the ratio of the effective masses: ωpl(q y ̂) /ωpl(q x ̂) =√{mx/my } .

  3. Multiple exciton generation in quantum dot-based solar cells

    Science.gov (United States)

    Goodwin, Heather; Jellicoe, Tom C.; Davis, Nathaniel J. L. K.; Böhm, Marcus L.

    2018-01-01

    Multiple exciton generation (MEG) in quantum-confined semiconductors is the process by which multiple bound charge-carrier pairs are generated after absorption of a single high-energy photon. Such charge-carrier multiplication effects have been highlighted as particularly beneficial for solar cells where they have the potential to increase the photocurrent significantly. Indeed, recent research efforts have proved that more than one charge-carrier pair per incident solar photon can be extracted in photovoltaic devices incorporating quantum-confined semiconductors. While these proof-of-concept applications underline the potential of MEG in solar cells, the impact of the carrier multiplication effect on the device performance remains rather low. This review covers recent advancements in the understanding and application of MEG as a photocurrent-enhancing mechanism in quantum dot-based photovoltaics.

  4. Exciton dephasing in single InGaAs quantum dots

    DEFF Research Database (Denmark)

    Leosson, Kristjan; Østergaard, John Erland; Jensen, Jacob Riis

    2000-01-01

    . The homogeneous and inhomogeneous broadening of InGaAs quantum dot luminescence is of central importance for the potential application of this material system in optoelectronic devices. Recent measurements of MOCVD-grown InAs/InGaAs quantum dots indicate a large homogeneous broadening at room temperature due......The homogeneous linewidth of excitonic transitions is a parameter of fundamental physical importance. In self-assembled quantum dot systems, a strong inhomogeneous broadening due to dot size fluctuations masks the homogeneous linewidth associated with transitions between individual states...... to fast dephasing. We present an investigation of the low-temperature homogeneous linewidth of individual PL lines from MBE-grown In0.5Ga0.5As/GaAs quantum dots....

  5. Exciton-polariton flows in cross-dimensional junctions

    Science.gov (United States)

    Winkler, K.; Flayac, H.; Klembt, S.; Schade, A.; Nevinskiy, D.; Kamp, M.; Schneider, C.; Höfling, S.

    2017-05-01

    We study the nonequilibrium exciton-polariton condensation in 1D to 0D and 1D to quasi-2D junctions by means of non-resonant spectroscopy. The shape of our potential landscape allows to probe the resonant transmission of a propagating condensate between a quasi-1D waveguide and cylindrically symmetric states. We observe a distinct mode selection by varying the position of the non-resonant pump laser. Moreover, we study the the case of propagation from a localized trapped condensate state into a waveguide channel. Here, the choice of the position of the injection laser allows us to tune the output in the waveguide. Our measurements are supported by an accurate Ginzburg-Landau modeling of the system shining light on the underlying mechanisms.

  6. Ultrafast Electronic Band Gap Control in an Excitonic Insulator.

    Science.gov (United States)

    Mor, Selene; Herzog, Marc; Golež, Denis; Werner, Philipp; Eckstein, Martin; Katayama, Naoyuki; Nohara, Minoru; Takagi, Hide; Mizokawa, Takashi; Monney, Claude; Stähler, Julia

    2017-08-25

    We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta_{2}NiSe_{5} investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_{C}=0.2  mJ cm^{-2}, the band gap narrows transiently, while it is enhanced above F_{C}. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta_{2}NiSe_{5}, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta_{2}NiSe_{5} with light on the femtosecond time scale.

  7. True Bilayer Exciton Condensate of One-Dimensional Electrons.

    Science.gov (United States)

    Kantian, A; Abergel, D S L

    2017-07-21

    We theoretically predict that a true bilayer exciton condensate, characterized by off-diagonal long-range order and global phase coherence, can be created in one-dimensional solid state electron systems. The mechanism by which this happens is to introduce a single particle hybridization of electron and hole populations, which locks the phase of the relevant mode and hence invalidates the Mermin-Wagner theorem. Electron-hole interactions then amplify this tendency towards off-diagonal long-range order, enhancing the condensate properties by more than an order of magnitude over the noninteracting limit. We show that the temperatures below which a substantial condensate fraction would form could reach hundreds of Kelvin, a benefit of the weak screening in one-dimensional systems.

  8. Microscopic theory of singlet exciton fission. III. Crystalline pentacene

    Energy Technology Data Exchange (ETDEWEB)

    Berkelbach, Timothy C., E-mail: tcb2112@columbia.edu; Reichman, David R., E-mail: drr2103@columbia.edu [Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027 (United States); Hybertsen, Mark S., E-mail: mhyberts@bnl.gov [Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000 (United States)

    2014-08-21

    We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.

  9. Excitonic Wigner crystal and high T sub c ferromagnetism in RB sub 6

    CERN Document Server

    Kasuya, T

    2000-01-01

    The mechanisms for the high T sub c ferromagnetism in La-doped divalent hexaborides DB sub 6 are studied in detail comparing with similar family materials, in particular with YbB sub 6 , EuB sub 6 and Ce monopnictides. It is shown that in DB sub 6 the light-electron-heavy-hole paired excitonic states form the Wigner crystal, or Wigner glass in actual materials, in which the conventional intersite electron exchange interactions similar to that in Ni dominate the pair singlet formation due to the intra pair mixing causing a ferromagnetic spin glass-like ordering of electron spins. In the La-doped system La sub x D sub 1 sub - sub x B sub 6 , the population of molecular La impurity states with giant moments increases as x approaches the optimal value x sub 0 approx 0.005 for high T sub c providing vacant states for the roton-like fluctuations, which cause the high T sub c at the boundary of the delocalization of electron carriers. Therefore, the critical La concentration for delocalization coincides with the opt...

  10. Charge density wave and excitonic magnetic polarons in CeTe sub 2

    CERN Document Server

    Kasuya, T; Takabatake, T

    2000-01-01

    Mechanisms of anomalous magnetic and transport properties in CeTe sub 2 observed recently on single-crystal samples are studied by comparing with the nonmagnetic reference material LaTe sub 2 , as well as other typical low carrier-density systems such as Ce monopnictides, doped Eu chalcogenides and Yb sub 4 As sub 3. The present system is unique on the point of low-carrier semimetal due to CDW of near perfect nesting, which is shown to be nearly independent of the spin-orbit splitting. The large residual resistivity indicates the giant molecular scattering due to excitonic states forming the distorted Wigner crystal, similar to Yb sub 4 As sub 3. At low temperatures, induced magnetic polarons cause unusual novel transport properties with a sharp peak of resistivity without any anomaly on other physical properties. This is attributed to a sharp glassy transition from an antiferromagnetic short-range ordering to the ferromagnetic ordering of the magnetic polarons within each CeTe double layer sandwiching the mo...

  11. Self-trapped exciton configurations in Beryllium Oxide

    Science.gov (United States)

    Botov, M. A.; Kuznetsov, A. Yu; Sobolev, A. B.

    2017-05-01

    The modern radiation technology, nuclear engineering, non-linear optics are associated with radiation-resistant optical material study. Evolution of electronic excitations in these materials is a complex multichannel process which currently has no integrated model. A special role belongs to the low-symmetry single crystals, such as beryllium oxide (BeO). We present theoretical results that advance our understanding of exciton-based channel of electronic excitations relaxation. The four possible self-trapped exciton (STE) configurations in beryllia single crystal have been investigated by using a quantum mechanical approach (Hartree-Fock and B3LYP HF-DFT hybrid functional, as implemented in the CRYSTAL09 code). B3LYP DFT functional with 30% of exact exchange was used (B3LYP30). All calculations were performed using periodic boundary conditions and full SC geometry relaxation. The lattice distortion and charge density distribution for considered defect configurations were obtained. STE-A1 luminescence energy was found to be 6.0 eV for HF and 6.5 eV for B3LYP30; STE-A2 luminescence energy was found to be 9.2 eV for HF and 7.8 eV for B3LYP30. STE-B1 luminescence energy was found to be 5.5 eV for HF, 6.2 eV for B3LYP30; STE-B2 luminescence energy was found to be 4.7 eV for HF.

  12. Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy as a Valuable Tool to Investigate the Ultra-High-Molecular-Weight Polyethylene Wear Mechanisms and Debris in Hip Implants.

    Science.gov (United States)

    Schappo, Henrique; Gindri, Izabelle M; Cubillos, Patrícia O; Maru, Marcia M; Salmoria, Gean V; Roesler, Carlos R M

    2018-01-01

    The use of scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) was investigated to understand the wear mechanisms from a metal-on-polyethylene bearing couple. Morphological features of femoral head acetabular liner, and isolated particles resulting from hip wear testing were evaluated. EDS was proposed to investigate the polymeric nature of the particles isolated from the wear testing. In this work, 28-mm conventional ultra-high-molecular-weight polyethylene acetabular liners paired with metallic heads were tested in a hip wear simulator over 2 million cycles. SEM-EDS was employed to investigate wear mechanisms on hip implant components and associated wear debris. SEM showed worn surfaces for both hip components, and a significant volume of ultra-high-molecular-weight polyethylene wear particles resulting from hip wear testing. Particles were classified into 3 groups, which were then correlated to wear mechanisms. Group I had particles with smooth surfaces, group II consisted of particles with rough surfaces, and group III comprised aggregate-like particles. Group I EDS revealed that particles from groups I and II had a high C/O ratio raising a concern about the particle source. On the other hand, particles from group III had a low C/O ratio, supporting the hypothesis that they resulted from the wear of acetabular liner. Most of particles identified in group III were in the biologically active size range (0.3 to 20 μm). The use of optical and electron microscopy enabled the morphological characterization of worn surfaces and wear debris, while EDS was essential to elucidate the chemical composition of isolated debris. Copyright © 2017 Elsevier Inc. All rights reserved.

  13. A fluorescence scanning electron microscope

    OpenAIRE

    Kanemaru, Takaaki; Hirata, Kazuho; Takasu, Shin-ichi; Isobe, Shin-Ichiro; Mizuki, Keiji; Mataka, Shuntaro; Nakamura, Kei-ichiro

    2010-01-01

    Fluorescence techniques are widely used in biological research to examine molecular localization, while electron microscopy can provide unique ultrastructural information. To date, correlative images from both fluorescence and electron microscopy have been obtained separately using two different instruments, i.e. a fluorescence microscope (FM) and an electron microscope (EM). In the current study, a scanning electron microscope (SEM) (JEOL JXA8600 M) was combined with a fluorescence digital c...

  14. Efficient exciton transport in layers of self-assembled porphyrin derivatives

    NARCIS (Netherlands)

    Huijser, A.; Suijkerbuijk, B.M.J.M.; Klein Gebbink, R.J.M.; Savenije, T.J.; Siebbeles, D.A.

    2008-01-01

    The photosynthetic apparatus of green sulfur bacteria, the chlorosome, is generally considered as a highly efficient natural light-harvesting system. The efficient exciton transport through chlorosomes toward the reaction centers originates from self-assembly of the bacteriochlorophyll molecules.

  15. Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes

    Science.gov (United States)

    Wang, Xu; Alexander-Webber, Jack A.; Jia, Wei; Reid, Benjamin P. L.; Stranks, Samuel D.; Holmes, Mark J.; Chan, Christopher C. S.; Deng, Chaoyong; Nicholas, Robin J.; Taylor, Robert A.

    2016-11-01

    Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states.

  16. Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor-Acceptor Conjugated Polymers.

    Science.gov (United States)

    Ayzner, Alexander L; Mei, Jianguo; Appleton, Anthony; DeLongchamp, Dean; Nardes, Alexandre; Benight, Stephanie; Kopidakis, Nikos; Toney, Michael F; Bao, Zhenan

    2015-12-30

    Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

  17. Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes.

    Science.gov (United States)

    Wang, Xu; Alexander-Webber, Jack A; Jia, Wei; Reid, Benjamin P L; Stranks, Samuel D; Holmes, Mark J; Chan, Christopher C S; Deng, Chaoyong; Nicholas, Robin J; Taylor, Robert A

    2016-11-16

    Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states.

  18. 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.

  19. Particle-in-a-Box Model of Exciton Absorption and Electroabsorption in Conjugated Polymers

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm

    2001-01-01

    The recently proposed particle-in-a-box model of one-dimensional excitons in conjugated polymers is applied in calculations of optical absorption and electroabsorption spectra. It is demonstrated that for polymers of long conjugation length a superposition of single exciton resonances produces a ...... a line shape characterized by a square-root singularity in agreement with experimental spectra near the absorption edge. The effects of finite conjugation length on both absorption and electroabsorption spectra are analyzed.......The recently proposed particle-in-a-box model of one-dimensional excitons in conjugated polymers is applied in calculations of optical absorption and electroabsorption spectra. It is demonstrated that for polymers of long conjugation length a superposition of single exciton resonances produces...

  20. FIR Induced Intrinsic Exciton Transitions in GaAs/AlGaAs Superlattices

    DEFF Research Database (Denmark)

    Dremin, A. A.; Timofeev, V. B.; Birkedal, Dan

    1997-01-01

    Intrinsic transitions of confined excitons in GaAs/AlGaAs superlattices with different barrier widths have been studied with the use of resonant far-infrared absorption under variation of magnetic field perpendicular and tilted with respect to the growth directions. Few resonances have been obser...... observed in spectra induced by FIR radiation by monitoring the changes of the ground state excitonic photoluminescence. The dominant resonances are the electron cyclotron resonance and the intrinsic 1s --> 2p. intrawell transitions of the heavy-hole excitons.......Intrinsic transitions of confined excitons in GaAs/AlGaAs superlattices with different barrier widths have been studied with the use of resonant far-infrared absorption under variation of magnetic field perpendicular and tilted with respect to the growth directions. Few resonances have been...

  1. Phonon induced pure dephasing process of excitonic state in colloidal semiconductor quantum dots

    Science.gov (United States)

    Huang, Tongyun; Han, Peng; Wang, Xinke; Feng, Shengfei; Sun, Wenfeng; Ye, Jiasheng; Zhang, Yan

    2016-04-01

    We present a theoretical study on the pure dephasing process of colloidal semiconductor quantum dots induced by lattice vibrations using continuum model calculations. By solving the time dependent Liouville-von Neumann equation, we present the ultrafast Rabi oscillations between excitonic state and virtual state via exciton-phonon interaction and obtain the pure dephasing time from the fast decayed envelope of the Rabi oscillations. The interaction between exciton and longitudinal optical phonon vibration is found to dominate the pure dephasing process and the dephasing time increases nonlinearly with the reduction of exciton-phonon coupling strength. We further find that the pure dephasing time of large quantum dots is more sensitive to temperature than small quantum dots.

  2. Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor–Acceptor Conjugated Polymers

    KAUST Repository

    Ayzner, Alexander L.

    2015-12-30

    © 2015 American Chemical Society. Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

  3. Unifying Optical Selection Rules for Excitons in Two Dimensions: Band Topology and Winding Numbers

    Science.gov (United States)

    Cao, Ting; Wu, Meng; Louie, Steven G.

    2018-02-01

    We show that band topology can dramatically change the photophysics of two-dimensional semiconductors. For systems in which states near the band extrema are of multicomponent character, the spinors describing these components (pseudospins) can pick up nonzero winding numbers around the extremal k point. In these systems, we find that the strength and required light polarization of an excitonic optical transition are dictated by the optical matrix element winding number, a unique and heretofore unrecognized topological characteristic. We illustrate these findings in three gapped graphene systems—monolayer graphene with inequivalent sublattices and biased bi- and trilayer graphene, where the pseudospin textures manifest into nontrivial optical matrix element winding numbers associated with different valley and photon circular polarization. This winding-number physics leads to novel exciton series and optical selection rules, with each valley hosting multiple bright excitons coupled to light of different circular polarization. This valley-exciton selective circular dichroism can be unambiguously detected using optical spectroscopy.

  4. Observation of Half-Quantum Vortices in an Exciton-Polariton Condensate

    National Research Council Canada - National Science Library

    K. G. Lagoudakis; T. Ostatnický; A. V. Kavokin; Y. G. Rubo; R. André; B. Deveaud-Plédran

    2009-01-01

    Singly quantized vortices have already been observed in many systems, including the superfluid helium, Bose-Einstein condensates of dilute atomic gases, and condensates of exciton-polaritons in the solid state...

  5. Bilayer Excitons in Two-Dimensional Nanostructures for Greatly Enhanced Thermoelectric Efficiency

    Science.gov (United States)

    Wu, Kai; Rademaker, Louk; Zaanen, Jan

    2014-11-01

    Currently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. Bilayer-exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. We suggest an experimental realization of a bilayer-exciton thermocouple. Based on current experimental parameters, a bilayer-exciton heterostructure of p - and n -doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3 . Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9 .

  6. Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures.

    Science.gov (United States)

    Forsberg, Mathias; Hemmingsson, Carl; Amano, Hiroshi; Pozina, Galia

    2015-01-20

    Hybrid samples based on ZnO colloidal nanocrystals (NCs) deposited on AlGaN/GaN quantum well (QW) structures with different top barrier thickness d = 3, 6 and 9 nm are studied by time-resolved photoluminescence. Thermal behavior of the QW exciton lifetime in the hybrids and in the bare QW structures has been compared and it has been found that the QW exciton recombination rate increases in the hybrid having d = 3 nm and decreases in the hybrid with d = 6 nm, while no change has been observed for the structure with d = 9 nm. It is suggested that non-radiative resonance energy transfer from the QW excitons to the ZnO NCs and a variation of the surface potential can both influence the QW exciton lifetime in the hybrids.

  7. Photonic Crystal Architecture for Room-Temperature Equilibrium Bose-Einstein Condensation of Exciton Polaritons

    Science.gov (United States)

    Jiang, Jian-Hua; John, Sajeev

    2014-07-01

    We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense 10-μm-1-cm- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of Cd0.6 Mg0.4Te with a 3D PBG of 9.2% (gap-to-central-frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as ℏΩ=55 meV (i.e., a vacuum Rabi splitting of 2ℏΩ=110 meV). The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For N =106 QWs embedded in the cavity, the collective exciton-photon coupling per QW (ℏΩ/√N =5.4 meV) is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately 10-5m0, where m0 is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to (11aB)-2, where aB≃3.5 nm is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton lifetime to beyond 150 ps at room temperature, sufficient for thermal

  8. Photonic Crystal Architecture for Room-Temperature Equilibrium Bose-Einstein Condensation of Exciton Polaritons

    Directory of Open Access Journals (Sweden)

    Jian-Hua Jiang

    2014-08-01

    Full Text Available We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC. This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG and large exciton density enabled by a multiple quantum-well (QW structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense 10-μm-1-cm- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of Cd_{0.6}  Mg_{0.4}Te with a 3D PBG of 9.2% (gap-to-central-frequency ratio that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as ℏΩ=55  meV (i.e., a vacuum Rabi splitting of 2ℏΩ=110  meV. The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For N=106 QWs embedded in the cavity, the collective exciton-photon coupling per QW (ℏΩ/sqrt[N]=5.4  meV is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately 10^{−5}m_{0}, where m_{0} is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to (11a_{B}^{−2}, where a_{B}≃3.5  nm is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton

  9. Halogenation of SiC for band-gap engineering and excitonic functionalization

    OpenAIRE

    Drissi, L.B.; Ramadan, F. Z.; Lounis, S.

    2017-01-01

    The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their...

  10. Quantum exciton-polariton networks through inverse four-wave mixing

    Science.gov (United States)

    Liew, T. C. H.; Rubo, Y. G.

    2018-01-01

    We demonstrate the potential of quantum operation using lattices of exciton-polaritons in patterned semiconductor microcavities. By introducing an inverse four-wave mixing scheme acting on localized modes, we show that it is possible to develop nonclassical correlations between individual condensates. This allows a concept of quantum exciton-polariton networks, characterized by the appearance of multimode entanglement even in the presence of realistic levels of dissipation.

  11. Exciton-mediated Raman scattering in CdS quantum dot

    Science.gov (United States)

    Guo, Xiaotong; Liu, Cuihong

    2017-09-01

    A theoretical calculation of the differential cross section (DCS) is presented for the exciton-mediated Stokes Raman scattering in CdS quantum dot within the frame work of effective mass approximation at T = 0 K. Numerical calculations reveal that the excitonic effects cause blue shifts in Raman spectroscopy. The magnitude of the Raman shift depends on the quantum confinement, the Coulomb interaction, and the incident photon energy.

  12. Predicting Excitonic Gaps of Semiconducting Single Walled Carbon Nanotubes From a Field Theoretic Analysis

    OpenAIRE

    Konik, Robert M.; Sfeir, Matthew Y.; Misewich, James A.

    2014-01-01

    We demonstrate that a non-perturbative framework for the treatment of the excitations of single walled carbon nanotubes based upon a field theoretic reduction is able to accurately describe experiment observations of the absolute values of excitonic energies. This theoretical framework yields a simple scaling function from which the excitonic energies can be read off. This scaling function is primarily determined by a single parameter, the charge Luttinger parameter of the tube, which is in t...

  13. Excitons in tunnel coupled CdTe and (Cd,Mn)Te quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Terletskii, Oleg; Ryabchenko, Sergiy; Tereshchenko, Oleksandr [Institute of Physics NASU, pr. Nauki 46, 03680 Kyiv (Ukraine); Sugakov, Volodymyr; Vertsimakha, Ganna [Institute for Nuclear Research NASU, pr. Nauki 47, 03680 Kyiv (Ukraine); Karczewski, Grzegorz [Institute of Physics PAS, Al. Lotnikow 32/46, PL-02-668 Warsaw (Poland)

    2017-05-15

    The photoluminescence (PL) from structures containing Cd{sub 0.95}Mn{sub 0.05}Te and CdTe quantum wells (QWs) separated by a narrow (1.94 nm) barrier was studied. The PL lines of comparable intensities from several possible exciton states were observed simultaneously at energy distances substantially exceeding kT. This means that the energy transfer in the studied systems is slower than the radiative recombination of the confined excitons. For the CdTe QW width of about 8.7-9 nm, indirect excitons with the electron and heavy hole chiefly localized in the CdTe and Cd{sub 1-x}Mn{sub x}Te QWs, respectively, were detected in the magnetic field. These indirect excitons have PL energy of about 10-20 meV above the PL line of the direct excitons in the CdTe QW. The observation of the PL from the indirect excitons which are not the lowest excitations in the structure is a distinctive feature of the system. Photoluminescence intensity dependence on the energy and the magnetic field. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  14. 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.

  15. 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.

  16. Local strain-induced band gap fluctuations and exciton localization in aged WS2 monolayers

    Science.gov (United States)

    Krustok, J.; Kaupmees, R.; Jaaniso, R.; Kiisk, V.; Sildos, I.; Li, B.; Gong, Y.

    2017-06-01

    Optical properties of aged WS2 monolayers grown by CVD method on Si/SiO2 substrates are studied using temperature dependent photoluminescence and reflectance contrast spectroscopy. Aged WS2 monolayers have a typical surface roughness about 0.5 nm and, in addition, a high density of nanoparticles (nanocaps) with the base diameter about 30 nm and average height of 7 nm. The A-exciton of aged monolayer has a peak position at 1.951 eV while in as-grown monolayer the peak is at about 24 meV higher energy at room temperature. This red-shift is explained using local tensile strain concept, where strain value of 2.1% was calculated for these nanocap regions. Strained nanocaps have lower band gap energy and excitons will funnel into these regions. At T=10K a double exciton and trion peaks were revealed. The separation between double peaks is about 20 meV and the origin of higher energy peaks is related to the optical band gap energy fluctuations caused by random distribution of local tensile strain due to increased surface roughness. In addition, a wide defect related exciton band XD was found at about 1.93 eV in all aged monolayers. It is shown that the theory of localized excitons describes well the temperature dependence of peak position and halfwidth of the A-exciton band. The possible origin of nanocaps is also discussed.

  17. Effects of magnetic fields on free excitons in CuInSe{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Martin, R.W. [Department of Physics, Strathclyde University, Glasgow (United Kingdom); Babinski, A. [Institute of Experimental Physics, University of Warsaw (Poland); Mudryi, A.V. [Scientific-Practical Material Research Centre, National Academy of Science of Belarus, Minsk (Belarus); Yakushev, M.V.

    2009-05-15

    The effects of magnetic fields up to 20 T were studied in CuInSe{sub 2} single crystals using photoluminescence (PL) at 4.2 K. Diamagnetic shifts of the free A and B excitons measured in the PL spectra in CuInSe{sub 2} at 4.2 K under the magnetic fields were used to estimate the reduced masses (0.095m{sub 0} for the A and 0.098m{sub 0} for the B exciton), binding energies (7.0 meV for the A and 7.2 meV for the B exciton) and Bohr radii (7.6 nm for the A and 7.3 nm for the B exciton) of the free-excitons in CuInSe{sub 2} assuming that both excitons are isotropic and hydrogen-like. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  18. Atomistic theory of excitonic fine structure in InAs/InP nanowire quantum dot molecules

    Science.gov (United States)

    Świderski, M.; Zieliński, M.

    2017-03-01

    Nanowire quantum dots have peculiar electronic and optical properties. In this work we use atomistic tight binding to study excitonic spectra of artificial molecules formed by a double nanowire quantum dot. We demonstrate a key role of atomistic symmetry and nanowire substrate orientation rather than cylindrical shape symmetry of a nanowire and a molecule. In particular for [001 ] nanowire orientation we observe a nonvanishing bright exciton splitting for a quasimolecule formed by two cylindrical quantum dots of different heights. This effect is due to interdot coupling that effectively reduces the overall symmetry, whereas single uncoupled [001 ] quantum dots have zero fine structure splitting. We found that the same double quantum dot system grown on [111 ] nanowire reveals no excitonic fine structure for all considered quantum dot distances and individual quantum dot heights. Further we demonstrate a pronounced, by several orders of magnitude, increase of the dark exciton optical activity in a quantum dot molecule as compared to a single quantum dot. For [111 ] systems we also show spontaneous localization of single particle states in one of nominally identical quantum dots forming a molecule, which is mediated by strain and origins from the lack of the vertical inversion symmetry in [111 ] nanostructures of overall C3 v symmetry. Finally, we study lowering of symmetry due to alloy randomness that triggers nonzero excitonic fine structure and the dark exciton optical activity in realistic nanowire quantum dot molecules of intermixed composition.

  19. Ultrafast transient absorption studies of hematite nanoparticles: the effect of particle shape on exciton dynamics.

    Science.gov (United States)

    Fitzmorris, Bob C; Patete, Jonathan M; Smith, Jacqueline; Mascorro, Xiomara; Adams, Staci; Wong, Stanislaus S; Zhang, Jin Z

    2013-10-01

    Much progress has been made in using hematite (α-Fe2 O3 ) as a potentially practical and sustainable material for applications such as solar-energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge-carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1-3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  20. Exciton dynamics of luminescent defects in aging organic light-emitting diodes

    Science.gov (United States)

    Ingram, Grayson L.; Zhao, Yong-Biao; Lu, Zheng-Hong

    2017-12-01

    Fundamental device physics of exciton dynamics is crucial to the design and fabrication of organic light-emitting diodes (OLEDs) with a long lifetime at high brightness. In this paper, we report a set of analytical equations which describe how and where defects form during exciton-driven degradation of an OLED and their impact on device operation. This set of equations allows us to quantify changes in the exciton and defect populations as a function of time in neat layers of 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) in simple bilayer OLEDs. CBP produces luminescent defects which present a unique opportunity to quantify the exciton capturing dynamics of the defects. Through modeling of the time and current density dependence of both the CBP and defect emission, we clearly identify CBP singlet excitons as the source of OLED degradation. Further analysis of experimental data on devices with precisely positioned exciton capturing layers suggests that defects are formed near organic heterojunctions.

  1. 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 WX 2 (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 WSe 2 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.

  2. The interplay between excitons and trions in a monolayer of MoSe2

    Science.gov (United States)

    Lundt, N.; Cherotchenko, E.; Iff, O.; Fan, X.; Shen, Y.; Bigenwald, P.; Kavokin, A. V.; Höfling, S.; Schneider, C.

    2018-01-01

    The luminescence and absorption properties of transition metal dichalcogenide monolayers are widely determined by neutral and charged excitonic complexes. Here, we focus on the impact of a free carrier reservoir on the optical properties of excitonic and trionic complexes in a MoSe2 monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser, the electron density can be controlled in our sample, which is directly reflected in the contribution of excitons and trions to the luminescence signal. We find significant shifts of both the exciton and trion energies in the presence of an induced electron gas both in power- and in time evolution (on the second to minute scale) in our photoluminescence spectra. In particular, in the presence of the photo-doped carrier reservoir, we observe that the splitting between excitons and trions can be enhanced by up to 4 meV. This behaviour is phenomenologically explained by an interplay between an increased screening of excitons via electrons in our system and a modification of the Fermi level. We introduce a simple but still quantitative treatment of these effects within a variational approach that takes into account both screening and phase space filling effects.

  3. A minimal model for excitons within time-dependent density-functional theory.

    Science.gov (United States)

    Yang, Zeng-hui; Li, Yonghui; Ullrich, Carsten A

    2012-07-07

    The accurate description of the optical spectra of insulators and semiconductors remains an important challenge for time-dependent density-functional theory (TDDFT). Evidence has been given in the literature that TDDFT can produce bound as well as continuum excitons for specific systems, but there are still many unresolved basic questions concerning the role of dynamical exchange and correlation (xc). In particular, the roles of the long spatial range and the frequency dependence of the xc kernel f(xc) for excitonic binding are still not very well explored. We present a minimal model for excitons in TDDFT, consisting of two bands from a one-dimensional (1D) Kronig-Penney model and simple approximate xc kernels, providing an easily accessible model system for studying excitonic effects in TDDFT. For the 1D model system, it is found that adiabatic xc kernels can produce at most two bound excitons, confirming that the long spatial range of f(xc) is not a necessary condition. It is shown how the Wannier model, featuring an effective electron-hole interaction, emerges from TDDFT. The collective, many-body nature of excitons is explicitly demonstrated.

  4. A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes

    Science.gov (United States)

    Sahin Tiras, Kevser; Wang, Yifei; Harmon, Nicholas J.; Wohlgenannt, Markus; Flatte, Michael E.

    In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.

  5. Dynamical patterns of phase transformations from self-trapping of quantum excitons

    Energy Technology Data Exchange (ETDEWEB)

    Yi, Tianyou [South University of Science and Technology of China, Shenzhen, Guangdong 518055 (China); Kirova, Natasha, E-mail: kirova@lps.u-psud.fr [CNRS, LPS, UMR 8626, Univeristé Paris-sud, Orsay 91405 (France); International Institute of Physics, UFRN, 59078-400 Natal, RN (Brazil); Brazovskii, Serguei [CNRS, LPTMS, UMR 8502, Univeristé Paris-sud, Orsay 91405 (France); International Institute of Physics, UFRN, 59078-400 Natal, RN (Brazil)

    2015-03-01

    Phase transitions induced by short optical pulses is a new mainstream in studies of cooperative electronic states. Its special realization in systems with neutral-ionic transformations stands out in a way that the optical pumping goes to excitons rather than to electronic bands. We present a semi-phenomenological modeling of spacio-temporal effects applicable to any system where the optical excitons are coupled to a symmetry breaking order parameter. In our scenario, after a short initial pulse of photons, a quasi-condensate of excitons appears as a macroscopic quantum state which then evolves interacting with other degrees of freedom prone to instability. This coupling leads to self-trapping of excitons; that locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains which evolve through dynamical phase transitions and may persist even after the initiating excitons have recombined. We recover dynamic interplays of fields such as the excitons' wave function, electronic charge transfer and polarization, lattice dimerization.

  6. 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.

  7. 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

  8. Electrical control of the exciton-biexciton splitting in self-assembled InGaAs quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Kaniber, M; Huck, M F; Mueller, K; Clark, E C; Bichler, M; Finley, J J [Walter Schottky Institut, Technische Universitaet Muenchen, Am Coulombwall 4, 85748 Garching (Germany); Troiani, F [S3, Instituto Nanoscienze-CNR, 41125 Modena (Italy); Krenner, H J, E-mail: kaniber@wsi.tum.de [Lehrstuhl fuer Experimentalphysik 1 and Augsburg Center for Innovative Technologies (ACIT), Universitaet Augsburg, Universiaetsstr. 1, 86159 Augsburg (Germany)

    2011-08-12

    The authors demonstrate how lateral electric fields can be used to precisely control the exciton-biexciton splitting in InGaAs quantum dots. By defining split-gate electrodes on the sample surface, optical studies show how the exciton transition can be tuned into resonance with the biexciton by exploiting the characteristically dissimilar DC Stark shifts. The results are compared to model calculations of the relative energies of the exciton and biexciton, demonstrating that the tuning can be traced to a dominance of hole-hole repulsion in the presence of a lateral field. Cascaded decay of the exciton-biexciton system enables the generation of entangled photon pairs without the need to suppress the fine structure splitting of the exciton. Our results demonstrate how the exciton-biexciton system can be electrically controlled.

  9. In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Shize [International Center for Quantum Materials, School of Physics, Peking University and Collaborative Innovation Center of Quantum Matter, Beijing (China); Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Tian, Xuezeng; Wang, Lifen; Wei, Jiake; Qi, Kuo; Li, Xiaomin; Xu, Zhi, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn; Wang, Wenlong; Zhao, Jimin; Bai, Xuedong, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn [Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Wang, Enge, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn [International Center for Quantum Materials, School of Physics, Peking University and Collaborative Innovation Center of Quantum Matter, Beijing (China)

    2014-08-18

    The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

  10. Effect of exciton oscillator strength on upconversion photoluminescence in GaAs/AlAs multiple quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Kojima, Osamu, E-mail: kojima@phoenix.kobe-u.ac.jp; Okumura, Shouhei; Kita, Takashi [Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501 (Japan); Akahane, Kouichi [National Institute of Information and Communications Technology, 4-2-1 Nukui-kitamachi, Koganei, Tokyo 184-8795 (Japan)

    2014-11-03

    We report upconversion photoluminescence (UCPL) in GaAs/AlAs multiple quantum wells. UCPL from the AlAs barrier is caused by the resonant excitation of the excitons in the GaAs well. When the quantum well has sufficient miniband width, UCPL is hardly observed because of the small exciton oscillator strength. The excitation-energy and excitation-density dependences of UCPL intensity show the exciton resonant profile and a linear increase, respectively. These results demonstrate that the observed UCPL caused by the saturated two-step excitation process requires a large number of excitons.

  11. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... concern for you. If you had an intravenous line inserted for the procedure, it will usually be ... procedure that same day that requires an intravenous line. Actual scanning time for a thyroid scan is ...

  12. Thyroid Scan and Uptake

    Medline Plus

    Full Text Available ... the limitations of the Thyroid Scan and Uptake? What is a Thyroid Scan and Uptake? A thyroid ... body converts food to energy. top of page What are some common uses of the procedure? The ...

  13. RBC nuclear scan

    Science.gov (United States)

    ... page: //medlineplus.gov/ency/article/003835.htm RBC nuclear scan To use the sharing features on this page, please enable JavaScript. An RBC nuclear scan uses small amounts of radioactive material to ...

  14. 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.

  15. 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.

  16. Exciton-polaritons in cuprous oxide: Theory and comparison with experiment

    Science.gov (United States)

    Schweiner, Frank; Ertl, Jan; Main, Jörg; Wunner, Günter; Uihlein, Christoph

    2017-12-01

    The observation of giant Rydberg excitons in cuprous oxide (Cu2O ) up to a principal quantum number of n =25 by T. Kazimierczuk et al. [Nature (London) 514, 343 (2014), 10.1038/nature13832] inevitably raises the question whether these quasiparticles must be described within a multipolariton framework since excitons and photons are always coupled in the solid. In this paper we present the theory of exciton-polaritons in Cu2O . To this end we extend the Hamiltonian which includes the complete valence-band structure, the exchange interaction, and the central-cell corrections effects, and which has been recently deduced by F. Schweiner et al. [Phys. Rev. B 95, 195201 (2017), 10.1103/PhysRevB.95.195201], for finite values of the exciton momentum ℏ K . We derive formulas to calculate not only dipole but also quadrupole oscillator strengths when using the complete basis of F. Schweiner et al., which has recently been proven as a powerful tool to calculate exciton spectra. Very complex polariton spectra for the three orientations of K along the axes [001 ] , [110 ] , and [111 ] of high symmetry are obtained and a strong mixing of exciton states is reported. The main focus is on the 1 S ortho-exciton-polariton, for which pronounced polariton effects have been measured in experiments. We set up a 5 ×5 matrix model, which accounts for both the polariton effect and the K -dependent splitting, and which allows treating the anisotropic polariton dispersion for any direction of K . We especially discuss the dispersions for K being oriented in the planes perpendicular to [1 1 ¯0 ] and [111 ] , for which experimental transmission spectra have been measured. Furthermore, we compare our results with experimental values of the K -dependent splitting, the group velocity, and the oscillator strengths of this exciton-polariton. The results are in good agreement. This proves the validity of the 5 ×5 matrix model as a useful theoretical model for further investigations on the 1 S

  17. 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

  18. Halogenation of SiC for band-gap engineering and excitonic functionalization.

    Science.gov (United States)

    Drissi, Lalla Btissam; Ramadan, Fatima Zahra; Lounis, Samir

    2017-09-11

    The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potential not only for solar cell applications, anti-reflection coatings or high-reflective systems but also for a possible realization of excitonic Bose-Einstein condensation. © 2017 IOP Publishing Ltd.

  19. 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.

  20. Impact of the glass transition on exciton dynamics in polymer thin films

    Science.gov (United States)

    Ehrenreich, Philipp; Proepper, Daniel; Graf, Alexander; Jores, Stefan; Boris, Alexander V.; Schmidt-Mende, Lukas

    2017-11-01

    In the development of organic electronics, unlimited design possibilities of conjugated polymers offer a wide variety of mechanical and electronic properties. Thereby, it is crucially important to reveal universal physical characteristics that allow efficient and forward developments of new chemical compounds. In particular for organic solar cells, a deeper understanding of exciton dynamics in polymer films can help to improve the charge generation process further. For this purpose, poly(3-hexylthiophene) (P3HT) is commonly used as a model system, although exciton decay kinetics have found different interpretations. Using temperature-dependent time-resolved photoluminescence spectroscopy in combination with low-temperature spectroscopic ellipsometry, we can show that P3HT is indeed a model system in which excitons follow a simple diffusion/hopping model. Based on our results we can exclude the relevance of hot-exciton emission as well as a dynamic torsional relaxation upon photoexcitation on a ps time scale. Instead, we depict the glass transition temperature of polymers to strongly affect exciton dynamics.

  1. Measuring Exciton Diffusion in Conjugated Polymer Films with Super-resolution Microscopy

    Science.gov (United States)

    Penwell, Samuel; Ginsberg, Lucas; Noriega Manez, Rodrigo; Ginsberg, Naomi

    2015-03-01

    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 microscopy. STED is typically used in biology with well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated STED in conjugated polymer films of MEH-PPV and CN-PPV by taking care to first understand the film's photophysical properties. This new approach provides a way to study exciton diffusion by utilizing subdiffraction optical excitation volumes. In this way, we will obtain a spatiotemporal map of exciton distributions that will help to correlate the energetic landscape to film morphology at the nanoscale. This research is supported in part by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Contract No. DE-AC05-06.

  2. 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.

  3. Halogenation of SiC for band-gap engineering and excitonic functionalization

    Science.gov (United States)

    Drissi, L. B.; Ramadan, F. Z.; Lounis, S.

    2017-11-01

    The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potential not only for solar cell applications, anti-reflection coatings or high-reflective systems but also for a possible realization of excitonic Bose–Einstein condensation.

  4. 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.

  5. Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions

    Energy Technology Data Exchange (ETDEWEB)

    Chang, Shun-Wen [Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089 (United States); Theiss, Jesse; Hazra, Jubin; Aykol, Mehmet; Kapadia, Rehan [Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089 (United States); Cronin, Stephen B. [Department of Physics and Astronomy, University of Southern California, Los Angeles, California 90089 (United States); Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089 (United States); Department of Chemistry, University of Southern California, Los Angeles, California 90089 (United States)

    2015-08-03

    We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In particular, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent. Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field-assisted charge separation. We observe a sharp, well defined E{sub 11} free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally.

  6. Excitonic complexes in natural InAs/GaAs quantum dots

    Science.gov (United States)

    Zieliński, M.; Gołasa, K.; Molas, M. R.; Goryca, M.; Kazimierczuk, T.; Smoleński, T.; Golnik, A.; Kossacki, P.; Nicolet, A. A. L.; Potemski, M.; Wasilewski, Z. R.; Babiński, A.

    2015-02-01

    The quantum confinement in a typical quantum dot (QD) is determined primarily by the nanosystem's dimensions and average composition. We demonstrate, however, that excitonic properties of natural QDs formed in the InAs/GaAs wetting layer are governed predominantly by effects of random fluctuations of the lattice composition. It is shown that the biexciton binding energy is a very sensitive function of the lattice randomness with a nearly flat dependence on the exciton energy. The large variation in different random realizations of a QD structure is shown to lead in some cases to the reversal of the order of excitonic lines. Results of theoretical calculations correspond to statistical properties of neutral excitons and biexcitons as well as trions confined to single natural QDs studied in our microspectroscopic measurements. We observe substantial variation of the biexciton and trion binding energies as well as a correlation of the trion and the biexciton energies. The transition from the negative to the positive binding energy of the trion is also observed, which strongly supports the attribution of the observed trion to the positively charged exciton.

  7. Characterizing and tuning excitons in monolayer and few-layer MoS 2

    Science.gov (United States)

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

    2015-03-01

    We use the GW-BSE method to study excitons arising from transitions in different regions of momentum space in mono- and few-layer MoS2 and consider mechanisms to fundamentally change the features and character of the optical spectra. Our calculations show that sharp spatial variations in dielectric screening make 2D systems, such as MoS2 , computationally challenging, requiring very fine k-space sampling to resolve the structure of excitonic wave functions and converge binding energies. In highly converged calculations, we identify a series of excitons arising from transitions at the K/K' valleys in the Brillouin zone, a higher energy series arising from transitions in the valley of a Mexican hat potential centered at the Γ point, and transitions at the indirect gap from Γ to Λ in few-layer MoS2 . As layer number changes, these states, which have varying character, momentum-space structure and real-space locations, are affected differently by changes in confinement and hybridization. By tuning layer number and strain, we find that we not only can tune the excitation energies but can also change the relative energies of the various excitonic series, allowing for movement of the lowest energy exciton between different regions of the Brillouin zone This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  8. Ultrafast exciton migration in an HJ-aggregate: Potential surfaces and quantum dynamics

    Science.gov (United States)

    Binder, Robert; Polkehn, Matthias; Ma, Tianji; Burghardt, Irene

    2017-01-01

    Quantum dynamical and electronic structure calculations are combined to investigate the mechanism of exciton migration in an oligothiophene HJ aggregate, i.e., a combination of oligomer chains (J-type aggregates) and stacked aggregates of such chains (H-type aggregates). To this end, a Frenkel exciton model is parametrized by a recently introduced procedure [Binder et al., J. Chem. Phys. 141, 014101 (2014)] which uses oligomer excited-state calculations to perform an exact, point-wise mapping of coupled potential energy surfaces to an effective Frenkel model. Based upon this parametrization, the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method is employed to investigate ultrafast dynamics of exciton transfer in a small, asymmetric HJ aggregate model composed of 30 sites and 30 active modes. For a partially delocalized initial condition, it is shown that a torsional defect confines the trapped initial exciton, and planarization induces an ultrafast resonant transition between an HJ-aggregated segment and a covalently bound "dangling chain" end. This model is a minimal realization of experimentally investigated mixed systems exhibiting ultrafast exciton transfer between aggregated, highly planarized chains and neighboring disordered segments.

  9. Exciton-phonon dynamics on complex networks: Comparison between a perturbative approach and exact calculations

    Science.gov (United States)

    Yalouz, Saad; Pouthier, Vincent; Falvo, Cyril

    2017-08-01

    A method combining perturbation theory with a simplifying ansatz is used to describe the exciton-phonon dynamics in complex networks. This method, called PT*, is compared to exact calculations based on the numerical diagonalization of the exciton-phonon Hamiltonian for eight small-sized networks. It is shown that the accuracy of PT* depends on the nature of the network, and three different situations were identified. For most graphs, PT* yields a very accurate description of the dynamics. By contrast, for the Wheel graph and the Apollonian network, PT* reproduces the dynamics only when the exciton occupies a specific initial state. Finally, for the complete graph, PT* breaks down. These different behaviors originate in the interplay between the degenerate nature of the excitonic energy spectrum and the strength of the exciton-phonon interaction so that a criterion is established to determine whether or not PT* is relevant. When it succeeds, our study shows the undeniable advantage of PT* in that it allows us to perform very fast simulations when compared to exact calculations that are restricted to small-sized networks.

  10. Exciton transport in the PE545 complex: insight from atomistic QM/MM-based quantum master equations and elastic network models

    Science.gov (United States)

    Pouyandeh, Sima; Iubini, Stefano; Jurinovich, Sandro; Omar, Yasser; Mennucci, Benedetta; Piazza, Francesco

    2017-12-01

    In this paper, we work out a parameterization of environmental noise within the Haken–Strobl–Reinenker (HSR) model for the PE545 light-harvesting complex, based on atomic-level quantum mechanics/molecular mechanics (QM/MM) simulations. We use this approach to investigate the role of various auto- and cross-correlations in the HSR noise tensor, confirming that site-energy autocorrelations (pure dephasing) terms dominate the noise-induced exciton mobility enhancement, followed by site energy-coupling cross-correlations for specific triplets of pigments. Interestingly, several cross-correlations of the latter kind, together with coupling–coupling cross-correlations, display clear low-frequency signatures in their spectral densities in the 30–70 cm-1 region. These slow components lie at the limits of validity of the HSR approach, which requires that environmental fluctuations be faster than typical exciton transfer time scales. We show that a simple coarse-grained elastic-network-model (ENM) analysis of the PE545 protein naturally spotlights collective normal modes in this frequency range that represent specific concerted motions of the subnetwork of cysteines covalenty linked to the pigments. This analysis strongly suggests that protein scaffolds in light-harvesting complexes are able to express specific collective, low-frequency normal modes providing a fold-rooted blueprint of exciton transport pathways. We speculate that ENM-based mixed quantum classical methods, such as Ehrenfest dynamics, might be promising tools to disentangle the fundamental designing principles of these dynamical processes in natural and artificial light-harvesting structures.

  11. Excitons at the center of the Brillouin zone in CuB{sub 2}O{sub 4} magnetoelectric

    Energy Technology Data Exchange (ETDEWEB)

    Menshenin, V. V., E-mail: menshenin@imp.uran.ru [Russian Academy of Sciences, Mikheev Institute of Metal Physics, Ural Branch (Russian Federation)

    2017-02-15

    The possibility of exciton excitation in CuB{sub 2}O{sub 4} magnetoelectric at point Γ(0, 0, 0) of the Brillouin zone has been analyzed using group theory. All possible orientations of the electric and magnetic fields that permit the excitation of these excitons have been determined.

  12. Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases

    Energy Technology Data Exchange (ETDEWEB)

    Kaindl, Robert A.; Hagele, D.; Carnahan, M. A.; Chemla, D. S.

    2008-09-11

    We report a comprehensive experimental study and detailed model analysis of the terahertz (THz) dielectric response and density kinetics of excitons and unbound electron-hole pairs in GaAs quantum wells. A compact expression is given, in absolute units, for the complex-valued THz dielectric function of intra-excitonic transitions between the 1s and higher-energy exciton and continuum levels. It closely describes the THz spectra of resonantly generated excitons. Exciton ionization and formation are further explored, where the THz response exhibits both intra-excitonic and Drude features. Utilizing a two-component dielectric function, we derive the underlying exciton and unbound pair densities. In the ionized state, excellent agreement is found with the Saha thermodynamic equilibrium, which provides experimental verification of the two-component analysis and density scaling. During exciton formation, in turn, the pair kinetics is quantitatively described by a Saha equilibrium that follows the carrier cooling dynamics. The THz-derived kinetics is, moreover, consistent with time-resolved luminescence measured for comparison. Our study establishes a basis for tracking pair densities via transient THz spectroscopy of photoexcited quasi-2D electron-hole gases.

  13. Nanoroughness localization of excitons in GaAs multiple quantum wells studied by transient four-wave mixing

    DEFF Research Database (Denmark)

    Birkedal, Dan; Vadim, Lyssenko; Pantke, Karl-Heinz

    1995-01-01

    The interface roughness on a nanometer scale plays a decisive role in dephasing of excitons in GaAs multiple quantum wells. The excitonic four-wave mixing signal shows a free polarization decay and a corresponding homogeneously broadened line from areas with interface roughness on a scale larger ...

  14. Lateral redistribution of excitons in CdSe/ZnSe quantum dots

    Science.gov (United States)

    Strassburg, M.; Dworzak, M.; Born, H.; Heitz, R.; Hoffmann, A.; Bartels, M.; Lischka, K.; Schikora, D.; Christen, J.

    2002-01-01

    Lateral redistribution processes of excitons localized in CdSe/ZnSe quantum dot structures are investigated by time-integrated and time-resolved spectroscopy. The photoluminescence properties are governed by lateral energy transfer within a dense ensemble of quantum dots. The quantum dots differ in size and Cd concentration and provide a complex potential landscape with localization sites for excitons. At low temperatures, lateral transfer by tunneling leads to a redshift with increasing delay after pulsed excitation. The mobility edge was determined to 2.561 eV. Above 100 K, thermally activated escape and recapture of excitons cause a strong redshift of the PL maximum in the first 500 ps.

  15. Deterministic Writing and Control of the Dark Exciton Spin Using Single Short Optical Pulses

    Directory of Open Access Journals (Sweden)

    I. Schwartz

    2015-01-01

    Full Text Available We demonstrate that the quantum dot-confined dark exciton forms a long-lived integer spin solid-state qubit that can be deterministically on-demand initiated in a pure state by one optical pulse. Moreover, we show that this qubit can be fully controlled using short optical pulses, which are several orders of magnitude shorter than the life and coherence times of the qubit. Our demonstrations do not require an externally applied magnetic field, and they establish that the quantum dot-confined dark exciton forms an excellent solid-state matter qubit with some advantages over the half-integer spin qubits, such as the confined electron and hole, separately. Since quantum dots are semiconductor nanostructures that allow integration of electronic and photonic components, the dark exciton may have important implications for implementations of quantum technologies consisting of semiconductor qubits.

  16. Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

    Science.gov (United States)

    Miller, Bastian; Steinhoff, Alexander; Pano, Borja; Klein, Julian; Jahnke, Frank; Holleitner, Alexander; Wurstbauer, Ursula

    2017-09-01

    We investigate the photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2 at low temperatures. Surprisingly, we find a doublet structure for such interlayer excitons. Both peaks exhibit long photoluminescence lifetimes of several ten nanoseconds up to 100 ns at low temperatures, which verifies the interlayer nature of both. The peak energy and linewidth of both show unusual temperature and power dependences. In particular, we observe a blue-shift of their emission energy for increasing excitation powers. At a low excitation power and low temperatures, the energetically higher peak shows several spikes. We explain the findings by two sorts of interlayer excitons; one that is indirect in real space but direct in reciprocal space, and the other one being indirect in both spaces. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions

  17. Electrons, holes, and excitons in GaAs polytype quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Climente, Juan I.; Segarra, Carlos; Rajadell, Fernando; Planelles, Josep, E-mail: josep.planelles@uji.es [Departament de Química Física i Analítica, Universitat Jaume I, E-12080 Castelló (Spain)

    2016-03-28

    Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.

  18. Exciton fine structure splitting in InP quantum dots in GaInP.

    Science.gov (United States)

    Ellström, C; Seifert, W; Pryor, C; Samuelson, L; Pistol, M-E

    2007-07-25

    We have investigated the electronic structure of excitons in InP quantum dots in GaInP. The exciton is theoretically expected to have four states. Two of the states are allowed to optically decay to the ground (vacuum) state in the dipole approximation. We see these two lines in photoluminescence (PL) experiments and find that the splitting between the lines (the fine structure splitting) is 150(± 30) µeV. The lines were perpendicularly polarized. We verified that the lines arise from neutral excitons by using correlation spectroscopy. The theoretical calculations show that the polarization of the emission lines are along and perpendicular to the major axis of elongated dots. The fine structure splitting depends on the degree of elongation of the dots and is close to zero for dots of cylindrical symmetry, despite the influence of the piezoelectric polarization, which is included in the calculation.

  19. Exciton absorption spectrum of thin Ag sub 2 ZnI sub 4

    CERN Document Server

    Yunakova, O N; Kovalenko, E N

    2002-01-01

    In Ag sub 2 ZnI sub 4 compound thin films one investigated into the electron spectrum of absorption within 3-6 eV photon energy range. The boundary of interband absorption is determined to correspond to the direct permitted transitions with E sub g = 3.7 eV forbidden gap width. A strong exciton band at E = 3.625 eV (80 K) GAMMA half width temperature run of which within 80-390 K range is governed by exciton-phonon interaction typical for quasi-single-dimensional excitons, is adjacent to the absorption boundary. At T <= 390 K one observes a bend in E(T) and GAMMA(T) dependences associated with generation of the Frenkel defects and followed by transfer of Ag ions to the interstices and vacancies of the compound crystalline lattice

  20. 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 h...... 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.......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...

  1. Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures.

    Science.gov (United States)

    Miller, Bastian; Steinhoff, Alexander; Pano, Borja; Klein, Julian; Jahnke, Frank; Holleitner, Alexander; Wurstbauer, Ursula

    2017-09-13

    We report the observation of a doublet structure in the low-temperature photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe 2 and WSe 2 . Both peaks exhibit long photoluminescence lifetimes of several tens of nanoseconds up to 100 ns verifying the interlayer nature of the excitons. The energy and line width of both peaks show unusual temperature and power dependences. While the low-energy peak dominates the spectra at low power and low temperatures, the high-energy peak dominates for high power and temperature. We explain the findings by two kinds of interlayer excitons being either indirect or quasi-direct in reciprocal space. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.

  2. Memory-assisted exciton diffusion in the chlorosome light-harvesting antenna of green sulfur bacteria

    CERN Document Server

    Fujita, Takatoshi; Saikin, Semion K; Aspuru-Guzik, Alan

    2012-01-01

    Chlorosomes are likely the largest and most efficient natural light-harvesting photosynthetic antenna systems. They are composed of large numbers of bacteriochlorophylls organized into supramolecular aggregates. We explore the microscopic origin of the fast excitation energy transfer in the chlorosome using the recently-resolved structure and atomistic-detail simulations. Despite the dynamical disorder effects on the electronic transitions of the bacteriochlorophylls, our simulations show that the exciton delocalizes over the entire aggregate in about 200 fs. The memory effects associated to the dynamical disorder assist the exciton diffusion through the aggregates and enhance the diffusion coefficients as a factor of two as compared to the model without memory. Furthermore, exciton diffusion in the chlorosome is found to be highly anisotropic with the preferential transfer towards the baseplate, which is the next functional element in the photosynthetic system.

  3. Optoelectronic response and excitonic properties of monolayer MoS{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Ben Amara, Imen [Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université Tunis El Manar Campus Universitaire, 2092 (Tunisia); Ben Salem, Emna, E-mail: bensalem-emna@yahoo.fr [Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université Tunis El Manar Campus Universitaire, 2092 (Tunisia); Institut Préparatoire aux Etudes d' Ingénieurs de Tunis, 2, Rue Jawaher Lel Nahrou-Monfleury, 1008 Tunis (Tunisia); Jaziri, Sihem [Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université Tunis El Manar Campus Universitaire, 2092 (Tunisia); Laboratoire de Physique des Matériaux, Faculté des Sciences de Bizerte, Université de Carthage, 7021 Jarzouna (Tunisia)

    2016-08-07

    Ab initio, electronic energy bands of MoS{sub 2} single layer are reported within the local density functional approximation. The inclusion of spin orbit coupling reveals the presence of two excitons A and B. We also discuss the change of physical properties of MoS{sub 2} from multilayer and bulk counterparts. The nature of the band gap changes from indirect to direct when the thickness is reduced to a single monolayer. The imaginary and real dielectric functions are investigated. Refractive index and birefringence are also reported. The results suggest that MoS{sub 2} is suitable for potential applications in optoelectronic and photovoltaic devices. The ab initio study is essential to propose the crucial parameters for the analytical model used for A-B exciton properties of the monolayer MoS{sub 2}. From a theoretical point of view, we consider how the exciton behavior evolves under environmental dielectrics.

  4. Excitons and Cooper pairs two composite bosons in many-body physics

    CERN Document Server

    Combescot, Monique

    2015-01-01

    This book bridges a gap between two major communities of Condensed Matter Physics, Semiconductors and Superconductors, that have thrived independently. Through an original perspective that their key particles, excitons and Cooper pairs, are composite bosons, the authors raise fundamental questions of current interest: how does the Pauli exclusion principle wield its power on the fermionic components of bosonic particles at a microscopic level and how this affects the macroscopic physics? What can we learn from Wannier and Frenkel excitons and from Cooper pairs that helps us understand "bosonic condensation" of composite bosons and its difference from Bose-Einstein condensation of elementary bosons? The authors start from solid mathematical and physical foundation to derive excitons and Cooper pairs. They further introduce Shiva diagrams as a graphic support to grasp the many-body physics induced by fermion exchange - a novel mechanism not visualized by standard Feynman diagrams. Advanced undergraduate or grad...

  5. Evidence for a topological excitonic insulator in InAs/GaSb bilayers.

    Science.gov (United States)

    Du, Lingjie; Li, Xinwei; Lou, Wenkai; Sullivan, Gerard; Chang, Kai; Kono, Junichiro; Du, Rui-Rui

    2017-12-07

    Electron-hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen-Cooper-Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron-hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase.

  6. Singlet-triplet annihilation limits exciton yield in poly(3-hexylthiophene)

    CERN Document Server

    Steiner, Florian; Lupton, John M

    2014-01-01

    Control of chain length and morphology in combination with single-molecule spectroscopy techniques provide a comprehensive photophysical picture of excited-state losses in the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT). A universal self-quenching mechanism is revealed, based on singlet-triplet exciton annihilation, which accounts for the dramatic loss in fluorescence quantum yield of a single P3HT chain between its solution (unfolded) and bulk-like (folded) state. Triplet excitons fundamentally limit the fluorescence of organic photovoltaic materials, which impacts on the conversion of singlet excitons to separated charge carriers, decreasing the efficiency of energy harvesting at high excitation densities. Interexcitonic interactions are so effective that a single P3HT chain of >100 kDa weight behaves like a two-level system, exhibiting perfect photon-antibunching.

  7. Excitonic characteristics in direct wide-band-gap CuScO2 epitaxial thin films

    Science.gov (United States)

    Hiraga, H.; Makino, T.; Fukumura, T.; Ohtomo, A.; Kawasaki, M.

    2009-11-01

    Thin films of a delafossite compound CuScO2 were grown on spinel MgAl2O4 (111) substrates, yielding in highly crystalline and (0001)-oriented epitaxial structures. Absorption spectra at 20 K revealed a sharp exciton resonance at 3.97 eV, which persisted up to 300 K. Its direct transition band gap at 20 K and exciton binding energies were determined to be about 4.35 and 380 meV, both of which are considerably larger than those of ZnO. In view of its capability of naturally layered structure and p-type doping, this compound will be interesting for exciton physics as well as implementation of heterostructured devices.

  8. Excitonic transition in (Pr,Ca)CoO{sub 3} family

    Energy Technology Data Exchange (ETDEWEB)

    Kunes, Jan; Augustinsky, Pavel [Insitute of Physics, AS CR, Prague (Czech Republic)

    2016-07-01

    The members of (Pr{sub 1-y}Y{sub y}){sub 1-x}Ca{sub x}CoO{sub 3} family exhibit a continuous phase transition accompanied by disappearance of the fluctuating moment of Co and increase of resistivity by several decades. Most intriguing feature of the low temperature phase is breaking of the time reversal symmetry without presence of ordered atomic moments. We will argue that the experimental observations are explained by condensation of atomic size excitons, which gives rise to ordered magnetic multipoles. We will present model calculations performed with dynamical-mean field theory, which demonstrate general features of the excitonic condensation. In addition, we will present results of material specific LDA+U calculations which uncover the excitonic order in Pr{sub 0.5}YCa{sub 0.5}CoO{sub 3} and explain its low temperature behavior.

  9. Production of excitons in grazing collisions of protons with LiF surfaces: An onion model

    Energy Technology Data Exchange (ETDEWEB)

    Miraglia, J. E.; Gravielle, M. S. [Instituto de Astronomia y Fisica del Espacio, Consejo Nacional de Investigaciones Cientificas y Tecnicas and Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Casilla de Correo 67, Sucursal 28, (C1428EGA) Buenos Aires (Argentina)

    2011-12-15

    In this work we evaluate the production of excitons of a lithium fluoride crystal induced by proton impact in the intermediate and high energy regime (from 100 keV to 1 MeV). A simple model is proposed to account for the influence of the Coulomb grid of the target by dressing crystal ions to transform them in what we call onions. The excited states of these onions can be interpreted as excitons. Within this model, total cross section and stopping power are calculated by using the first Born and the continuum distorted-wave (CDW) eikonal initial-state (EIS) approximations. We found that between 7 and 30 excitons per incident proton are produced in grazing collisions with LiF surfaces, becoming a relevant mechanism of inelastic transitions.

  10. Multiband theory of multi-exciton complexes in self-assembled quantum dots

    Science.gov (United States)

    Sheng, Weidong; Cheng, Shun-Jen; Hawrylak, Pawel

    2005-01-01

    We report on a multiband microscopic theory of many-exciton complexes in self-assembled quantum dots. The single particle states are obtained by three methods: single-band effective-mass approximation, the multiband k•p method, and the tight-binding method. The electronic structure calculations are coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave functions of N electrons and N valence holes are expanded in the basis of Slater determinants. The Coulomb matrix elements are evaluated using statically screened interaction for the three different sets of single particle states and the correlated N -exciton states are obtained by the configuration interaction method. The theory is applied to the excitonic recombination spectrum in InAs/GaAs self-assembled quantum dots. The results of the single-band effective-mass approximation are successfully compared with those obtained by using the of k•p and tight-binding methods.

  11. Exciton spin relaxation dynamics in InGaAs /InP quantum wells

    Science.gov (United States)

    Akasaka, Shunsuke; Miyata, Shogo; Kuroda, Takamasa; Tackeuchi, Atsushi

    2004-09-01

    We have investigated the exciton spin relaxation mechanism between 13 and 300K in InGaAs /InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, τs above 40K was found to depend on temperature, T, according to τs∝T-1.1, although the spin relaxation time is constant below 40K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40K, although the carrier density dependence is weak above 40K. These results imply that the main spin relaxation mechanism above and below 40K are the D'yakonov-Perel' process and the Bir-Aronov-Pikus process, respectively.

  12. Quantifying local exciton, charge resonance, and multiexciton character in correlated wave functions of multichromophoric systems

    Science.gov (United States)

    Casanova, David; Krylov, Anna I.

    2016-01-01

    A new method for quantifying the contributions of local excitation, charge resonance, and multiexciton configurations in correlated wave functions of multichromophoric systems is presented. The approach relies on fragment-localized orbitals and employs spin correlators. Its utility is illustrated by calculations on model clusters of hydrogen, ethylene, and tetracene molecules using adiabatic restricted-active-space configuration interaction wave functions. In addition to the wave function analysis, this approach provides a basis for a simple state-specific energy correction accounting for insufficient description of electron correlation. The decomposition scheme also allows one to compute energies of the diabatic states of the local excitonic, charge-resonance, and multi-excitonic character. The new method provides insight into electronic structure of multichromophoric systems and delivers valuable reference data for validating excitonic models.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2017-01-01

    The theoretical description of the timeevolution 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 evelopments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.

  14. Unraveling the Exciton Binding Energy and the Dielectric Constant in Single-Crystal Methylammonium Lead Triiodide Perovskite.

    Science.gov (United States)

    Yang, Zhuo; Surrente, Alessandro; Galkowski, Krzysztof; Bruyant, Nicolas; Maude, Duncan K; Haghighirad, Amir Abbas; Snaith, Henry J; Plochocka, Paulina; Nicholas, Robin J

    2017-04-20

    We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead triiodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of ∼3 meV for the excitonic transitions and a 2s transition that is clearly visible even at zero magnetic field. The exciton binding energy of 16 ± 2 meV in the low-temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room-temperature tetragonal phase, an upper limit for the exciton binding energy of 12 ± 4 meV is estimated from the evolution of 1s-2s splitting at high magnetic field.

  15. Excitons and polaritons in planar heterostructures in external electric and magnetic fields: A multi-sub-level approach

    Science.gov (United States)

    Wilkes, J.; Muljarov, E. A.

    2017-08-01

    Excitons and microcavity polaritons that possess a macroscopic dipole alignment are attractive systems to study. This is due to an enhancement of collective many body effects and an ability to electrostatically control their transport and internal structure. Here, we present an overview of a rigorous calculation of spatially-indirect exciton states in semiconductor coupled quantum wells in externally applied electric and magnetic fields. We also treat dipolaritons that form when such structures are positioned at the antinode of a resonant cavity mode. Our approach is general and can be applied to various planar solid state heterostructures inside optical resonators. It offers a thorough description of the properties of excitons and polaritons that are important for modelling their respective fluids. In particular, we calculate the exciton Bohr radius, binding energy, optical lifetime and magnetic field induced enhancement of the effective mass. We also describe electric and magnetic field control of the exciton and polariton dipole moment and brightness.

  16. Intensity dependent absorption bleaching of high subband excitons in GaAs/AlGaAs multiple quantum wells

    CERN Document Server

    Shin, S H; Lee, E H; Chae, K M; Park, S H; Kim, U

    1998-01-01

    We have investigated the influence of carrier generation on the absorption bleaching of the n=2 and n=3 excitons in GaAs/AlGaAs multiple quantum wells (MQWs). With the excitation near the resonance of the n=1 exciton absorption, the long range coulomb screening and collision broadening had significant effects on the exciton bleaching. At low excitation intensity, the absorption bleaching of the n=2 exciton in 75 A-thick MQWs and that of the n=3 exciton in 150 A-thick MQWs were due to linewidth broadening by the collision broadening effect only. At high excitation intensity, however, the reduction of oscillator strength due to the long range coulomb screening contributed dominantly to absorption bleaching.

  17. Separating homogeneous and inhomogeneous line widths of heavy- and light-hole excitons in weakly disordered semiconductor quantum wells.

    Science.gov (United States)

    Bristow, Alan D; Zhang, Tianhao; Siemens, Mark E; Cundiff, Steven T; Mirin, R P

    2011-05-12

    Optical two-dimensional Fourier-transform spectroscopy is used to study the heavy- and light-hole excitonic resonances in weakly disordered GaAs quantum wells. Homogeneous and inhomogeneous broadening contribute differently to the two-dimensional resonance line shapes, allowing separation of homogeneous and inhomogeneous line widths. The heavy-hole exciton exhibits more inhomogeneous than homogeneous broadening, whereas the light-hole exciton shows the reverse. This situation occurs because of the interplay between the length scale of the disorder and the exciton Bohr radius, which affects the exciton localization and scattering. Utilizing this separation of line widths, excitation-density-dependent measurements reveal that many-body interactions alter the homogeneous dephasing, while disorder-induced dephasing is unchanged.

  18. Three-dimensional localization of excitons in the InAs/GaAs wetting layer - magnetospectroscopic study

    Energy Technology Data Exchange (ETDEWEB)

    Babinski, A.; Golnik, A.; Kossacki, P.; Gaj, J.A. [Institute of Experimental Physics, University of Warsaw (Poland); Borysiuk, J. [Institute of Experimental Physics, University of Warsaw (Poland); Institute of Electronic Materials Technology, Warszawa (Poland); Kret, S. [Institute of Physics, Polish Academy of Sciences, Warszawa (Poland); Raymond, S.; Wasilewski, Z.R. [Institute for Microstructural Sciences, NRC, Ottawa, Ontario (Canada); Potemski, M. [High Magnetic Field Laboratory, CNRS BP-166, Grenoble (France)

    2009-04-15

    Magnetospectroscopic studies of individual excitonic states confined in potential fluctuations in the InAs/GaAs wetting layer (WL) are presented. A neutral exciton and a trion emission have been identified. They split in magnetic field in two components of orthogonal circular polarizations. The respective Zeeman splitting changes linearly with magnetic field up to 10 T. A significant scatter of the effective excitonic g{sup *}-factor is observed, reflecting the distribution of sizes and compositions of potential fluctuations in the WL. The distribution affects also diamagnetic shift of the excitonic emission. The observed properties of the excitons are consistent with a picture of shallow quantum dots formed in the WL due to In composition fluctuations. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  19. Biexcitonic bound and continuum states of homogeneously and inhomogeneously broadened exciton resonances

    DEFF Research Database (Denmark)

    Langbein, W.; Hvam, Jørn Märcher

    2002-01-01

    We investigate the influence of excitonic localization on the corresponding biexcitonic states in GaAs quantum wells by spectrally resolved four-wave mixing. With increasing localization, the biexciton binding energy increases, while the biexciton continuum shifts to higher energies....... The localization leads to an inhomogeneous broadening of the biexciton binding energy and the biexciton continuum edge. Simultaneously, the oscillator strength of the biexciton continuum-edge is reduced. This is interpreted as a result of the different localization of biexcitonic and excitonic states by the random...

  20. Cavity-controlled radiative recombination of excitons in thin-film solar cells

    OpenAIRE

    Vuong, Luat T.; Kozyreff, Gregory; Betancur; Martorell, Jordi

    2009-01-01

    The following article appeared in Vuong, Luat T. ...[et al.]. Cavity-controlled radiative recombination of excitons in thin-film solar cells. Applied physics letters [en línia]. 2009, vol. 95 [Consulta 29/06/2010]. p. 233106-1/233106-3 and may be found at http://apl.aip.org/applab/v95/i23/p233107_s1?view=fulltext We study the performance of photovoltaic devices when controlling the exciton radiative recombination time. We demonstrate that when high-quantum-yield fluorescent photovoltaic ma...

  1. Exciton-dominant Electroluminescence from a Diode of Monolayer MoS2

    Science.gov (United States)

    2014-05-14

    Exciton-dominant electroluminescence from a diode of monolayer MoS2 Yu Ye,1 Ziliang Ye,1 Majid Gharghi,1 Hanyu Zhu,1 Mervin Zhao,1,2 Yuan Wang,1...2014; accepted 29 April 2014; published online 14 May 2014) In two-dimensional monolayer MoS2 , excitons dominate the absorption and emission properties...Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS2 fabricated on a heavily p-type doped silicon substrate

  2. Exciton storage in type-II quantum dots using the optical Aharonov-Bohm effect

    Energy Technology Data Exchange (ETDEWEB)

    Climente, Juan I.; Planelles, Josep, E-mail: josep.planelles@uji.es [Departament de Química Física i Analítica, Universitat Jaume I, E-12080 Castelló (Spain)

    2014-05-12

    We investigate the bright-to-dark exciton conversion efficiency in type-II quantum dots subject to a perpendicular magnetic field. To this end, we take the exciton storage protocol recently proposed by Simonin and co-workers [Phys. Rev. B 89, 075304 (2014)] and simulate its coherent dynamics. We confirm the storage is efficient in perfectly circular structures subject to weak external electric fields, where adiabatic evolution is dominant. In practice, however, the efficiency rapidly degrades with symmetry lowering. Besides, the use of excited states is likely unfeasible owing to the fast decay rates. We then propose an adaptation of the protocol which does not suffer from these limitations.

  3. Spontaneous emission from large quantum dots in nanostructures: Exciton-photon interaction beyond the dipole approximation

    DEFF Research Database (Denmark)

    Stobbe, S.; Kristensen, Philip Trøst; Mortensen, Jakob E.

    2012-01-01

    the interaction between light and spatially extended excitons. In this regime, light and matter degrees of freedom cannot be separated and a complex interplay between the nanostructured optical environment and the exciton envelope function emerges. We illustrate this by specific examples and derive a series...... of important analytical relations, which are useful for applying the formalism to practical problems. In the dipole limit, the decay rate is proportional to the projected local density of optical states, and we obtain the strong and weak confinement regimes as special cases....

  4. Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

    Energy Technology Data Exchange (ETDEWEB)

    Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B., E-mail: benito.alen@csic.es [IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid (Spain); Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A. [Institute for Systems based on Optoelectronics and Microtechnology (ISOM), Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid (Spain)

    2015-11-02

    External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

  5. Real-Time Tracking of Singlet Exciton Diffusion in Organic Semiconductors.

    Science.gov (United States)

    Kozlov, Oleg V; de Haan, Foppe; Kerner, Ross A; Rand, Barry P; Cheyns, David; Pshenichnikov, Maxim S

    2016-02-05

    Exciton diffusion in organic materials provides the operational basis for functioning of such devices as organic solar cells and light-emitting diodes. Here we track the exciton diffusion process in organic semiconductors in real time with a novel technique based on femtosecond photoinduced absorption spectroscopy. Using vacuum-deposited C_{70} layers as a model system, we demonstrate an extremely high diffusion coefficient of D≈3.5×10^{-3}  cm^{2}/s that originates from a surprisingly low energetic disorder of solar cells, light-emitting diodes, and electrically pumped lasers.

  6. Dynamical screening of the exciton resonance in conjugated polymers/carbon nanotubes composites

    Science.gov (United States)

    Lüer, Larry; Hoseinkhani, Sajjad; Meneghetti, Moreno; Lanzani, Guglielmo

    2010-04-01

    We study coherent phonons in polymer-carbon nanotubes composites by sub-10-fs pump-probe spectroscopy. We find that coherent phonons in the polymer network modulate the exciton resonance of the wrapped nanotube. We propose a model of dynamic environmental coupling in which the polymer vibration affects the carbon nanotubes exciton energy by virtue of the modulation of its dielectric screening. Carbon nanotubes act as antenna for the local environment and highlight small changes in the dielectric constant. This shows the extreme sensitivity of carbon nanotubes to their surrounding, a property essential for sensing applications and crucial for understanding composite materials.

  7. Optical nonlinearities of excitonic states in atomically thin 2D transition metal dichalcogenides

    Energy Technology Data Exchange (ETDEWEB)

    Soh, Daniel Beom Soo [Sandia National Lab. (SNL-CA), Livermore, CA (United States). Proliferation Signatures Discovery and Exploitation Department

    2017-08-01

    We calculated the optical nonlinearities of the atomically thin monolayer transition metal dichalcogenide material (particularly MoS2), particularly for those linear and nonlinear transition processes that utilize the bound exciton states. We adopted the bound and the unbound exciton states as the basis for the Hilbert space, and derived all the dynamical density matrices that provides the induced current density, from which the nonlinear susceptibilities can be drawn order-by-order via perturbative calculations. We provide the nonlinear susceptibilities for the linear, the second-harmonic, the third-harmonic, and the kerr-type two-photon processes.

  8. Characteristics of exciton photoluminescence kinetics in low-dimensional silicon structures

    CERN Document Server

    Sachenko, A V; Manojlov, E G; Svechnikov, S V

    2001-01-01

    The time-resolved visible photoluminescence of porous nanocrystalline silicon films obtained by laser ablation have been measured within the temperature range 90-300 K. A study has been made of the interrelationship between photoluminescence characteristics (intensity, emission spectra, relaxation times, their temperature dependencies and structural and dielectric properties (size and shapes of Si nanocrystals, oxide phase of nanocrystal coating, porosity). A photoluminescence model is proposed that describes photon absorption and emission occurring in quantum-size Si nanocrystals while coupled subsystems of electron-hole pairs and excitons take part in the recombination. Possible excitonic Auger recombination mechanism in low-dimensional silicon structures is considered

  9. Collective Behavior of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Timofeev, V. B.; Hvam, Jørn Märcher

    2000-01-01

    Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell excition in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circular polarized light......, the luminescence line of interwell excitions exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon...

  10. 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.

  11. Ionization of Water Clusters is Mediated by Exciton Energy Transfer from Argon Clusters

    Energy Technology Data Exchange (ETDEWEB)

    Golan, Amir; Ahmed, Musahid

    2012-01-25

    The exciton energy deposited in an argon cluster, (Arn ,< n=20>) using VUV radiation is transferred to softly ionize doped water clusters, ((H2O)n, n=1-9) leading to the formation of non-fragmented clusters. Following the initial excitation, electronic energy is channeled to ionize the doped water cluster while evaporating the Ar shell, allowing identification of fragmented and complete water cluster ions. Examination of the photoionization efficiency curve shows that cluster evaporation from excitons located above 12.6 eV are not enough to cool the energized water cluster ion, and leads to their dissociation to (H2O)n-2H+ (protonated) clusters.

  12. Scanning laser Doppler vibrometry

    DEFF Research Database (Denmark)

    Brøns, Marie; Thomsen, Jon Juel

    With a Scanning Laser Doppler Vibrometer (SLDV) a vibrating surface is automatically scanned over predefined grid points, and data processed for displaying vibration properties like mode shapes, natural frequencies, damping ratios, and operational deflection shapes. Our SLDV – a PSV-500H from...

  13. Frequency scanning microstrip antennas

    DEFF Research Database (Denmark)

    Danielsen, Magnus; Jørgensen, Rolf

    1979-01-01

    The principles of using radiating microstrip resonators as elements in a frequency scanning antenna array are described. The resonators are cascade-coupled. This gives a scan of the main lobe due to the phase-shift in the resonator in addition to that created by the transmission line phase...

  14. Molecular semiconductor blends: microstructure, charge carrier transport and application in photovoltaic cells

    Energy Technology Data Exchange (ETDEWEB)

    Opitz, Andreas; Wagner, Julia; Ecker, Bernhard; Bruetting, Wolfgang [Institute of Physics, University of Augsburg (Germany); Hinderhofer, Alexander; Schreiber, Frank [Institute of Applied Physics, University of Tuebingen (Germany)

    2009-07-01

    Blends of organic donor and acceptor materials have the potential of an increase of solar cell efficiency. However, the balance between charge carrier transport in phase-separated structures and exciton dissociation at the donor-acceptor interface has to be optimized. To analyze this relation in more detail the following molecular material combinations were investigated: (i) Copper phthalocyanine (CuPc) combined with fullerene C{sub 60} and (ii) CuPc in combination with perfluorinated CuPc (F{sub 16}CuPc). Measurements by X-ray diffraction and scanning force microscopy indicate the formation of phase-separated nanocrystals for blends of CuPc/C{sub 60} and the formation of mixed crystals in the CuPc/F{sub 16}CuPc system. The formation of mixed crystals is an interesting feature for organic blends which has not yet been explored in organic solar cells. We will discuss the implications of the different molecular arrangements on the optical and electrical properties as well as for the solar cell performance.

  15. Laser Scanning in Forests

    Directory of Open Access Journals (Sweden)

    Håkan Olsson

    2012-09-01

    Full Text Available The introduction of Airborne Laser Scanning (ALS to forests has been revolutionary during the last decade. This development was facilitated by combining earlier ranging lidar discoveries [1–5], with experience obtained from full-waveform ranging radar [6,7] to new airborne laser scanning systems which had components such as a GNSS receiver (Global Navigation Satellite System, IMU (Inertial Measurement Unit and a scanning mechanism. Since the first commercial ALS in 1994, new ALS-based forest inventory approaches have been reported feasible for operational activities [8–12]. ALS is currently operationally applied for stand level forest inventories, for example, in Nordic countries. In Finland alone, the adoption of ALS for forest data collection has led to an annual savings of around 20 M€/year, and the work is mainly done by companies instead of governmental organizations. In spite of the long implementation times and there being a limited tradition of making changes in the forest sector, laser scanning was commercially and operationally applied after about only one decade of research. When analyzing high-ranked journal papers from ISI Web of Science, the topic of laser scanning of forests has been the driving force for the whole laser scanning research society over the last decade. Thus, the topic “laser scanning in forests” has provided a significant industrial, societal and scientific impact. [...

  16. A spatially indirect exciton in vertically coupled quantum dots : 1/Q-expansion

    NARCIS (Netherlands)

    Lozovik, YE; Mur, VD; Narozhny, NB; Petrosyan, AN

    A spatially indirect exciton in vertically coupled quantum dots is considered with the use of 1/Q-expansion, where Q is the dimensionless quantum parameter determined by the ratio of characteristic Coulomb energy of electron-hole interaction to the energy of one-particle transition in a confining

  17. How exciton-vibrational coherences control charge separation in the photosystem II reaction center

    NARCIS (Netherlands)

    Novoderezhkin, V.I.; Romero Mesa, E.; van Grondelle, R.

    2015-01-01

    In photosynthesis absorbed sun light produces collective excitations (excitons) that form a coherent superposition of electronic and vibrational states of the individual pigments. Two-dimensional (2D) electronic spectroscopy allows a visualization of how these coherences are involved in the primary

  18. Exciton relaxation in nanotubular TPPS4 aggregates in water solution and in polymeric matrix

    NARCIS (Netherlands)

    Gulbinas, V.; Karpicz, R.; Augulis, R.; Rotomskis, R.

    2007-01-01

    Exciton properties in meso-Tetra (4-sulphonatophenyl)porphin e (TPPS4) aggregates in water solution and in PVA matrix were investigated at different temperatures by means of transient absorption and fluorescence spectroscopy. Analysis of the steady state and transient absorption spectra shows that

  19. Room temperature excitonic dynamics of non-polar a-plane ZnO epifilms

    Directory of Open Access Journals (Sweden)

    Ja-Hon Lin

    2016-09-01

    Full Text Available Pump polarization dependent carrier dynamics, particularly excitonic dynamics, of non-polar a-plane zinc oxide (ZnO epifilms with two different thicknesses were investigated using time resolved measurements. Unlike the electron and hole dynamics through the above-bandgap excitation, transient differential reflectance (TDR traces revealed similar trends under two orthogonal pump polarization conditions relative to the c-axis (Epu⊥c and Epu∥c of a-ZnO around near-exciton-resonance excitation. By means of a band diagram, the bandgap renormalization (BGR effect can be reasonably explained by the screening of the Coulomb potential energy due to the accumulation of relaxed free carriers that were initially excited through the absorption of two cascaded pump photons via the excitonic level, a process known as two photon absorption (TPA. Thus, the modulation depths of the TPA around zero time delay, due to simultaneous absorption of one pump and one probe photon via the excitonic level, increased linearly with the pump fluence, proportional to the modulation depth resulting from the BGR effects.

  20. Preformed excitons, orbital selectivity, and charge density wave order in 1T-TiSe2

    Science.gov (United States)

    Koley, S.; Laad, M. S.; Vidhyadhiraja, N. S.; Taraphder, A.

    2014-09-01

    Traditional routes to charge density wave (CDW) in transition-metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities, have recently been brought into question. While this calls for exploration of alternative views, a paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between, CDW and superconductive orders in transition-metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic-structure calculations, to an exci-tonic CDW order in 1T-TiSe2. We show that orbital-selective dynamical localization arising from preformed excitonic liquid correlations is somewhat reminiscent of states proposed for d and f band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition-metal dichalcogenides on the border of CDW. This brings the transition-metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in d and f band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.