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Sample records for carbon nanorings excitonic

  1. Aharonov-Bohm effect of excitons in nanorings

    Science.gov (United States)

    Hu, Hui; Zhu, Jia-Lin; Li, Dai-Jun; Xiong, Jia-Jiong

    2001-05-01

    The magnetic field effects on excitons in an InAs nanoring are studied theoretically. By numerically diagonalizing the effective-mass Hamiltonian of the problem that can be separated into terms in center-of-mass and relative coordinates, we calculate the low-lying excitonic energy levels and oscillator strengths as a function of the ring width and the strength of an external magnetic field. It is shown that in the presence of Coulomb correlation, the so-called Aharonov-Bohm effect of excitons exists in a finite (but small) width nanoring. However, when the ring width becomes large, the non-simply-connected geometry of nanorings is destroyed, causing the suppression of the Aharonov-Bohm effect. The analytical results are obtained for a narrow-width nanoring in which the radial motion is the fastest one and adiabatically decoupled from the azimuthal motions. The conditional probability distribution calculated for the low-lying excitonic states allows identification of the presence of the Aharonov-Bohm effect. The linear optical susceptibility is also calculated as a function of the magnetic field, to be compared with the future measurements of optical emission experiments on InAs nanorings.

  2. Aharonov-Bohm effect of excitons in nano-rings

    OpenAIRE

    Hu, Hui; Zhu, Jia-Lin; Li, Dai-Jun; Xiong, Jia-Jiong

    2000-01-01

    The magnetic field effects on excitons in an InAs nano-ring are studied theoretically. By numerically diagonalizing the effective-mass Hamiltonian of the problem, which can be separated into terms in centre-of-mass and relative coordinates, we calculate the low-lying exciton energy levels and oscillator strengths as a function of the width of the ring and the strength of the external magnetic field. The analytical results are obtained for a narrow-width nano-ring in which the radial motion is...

  3. Excitonic Absorption of Semiconductor Nanorings under Terahertz Fields

    Institute of Scientific and Technical Information of China (English)

    ZHANG Tong-Yi; ZHAO Wei; ZHU Shao-Lan

    2005-01-01

    @@ The optical absorption of GaAs nanorings (NRs) under a dc electric field and a terahertz (THz) ac electric field applied in the plane containing the NRs is investigated theoretically. The NRs may enclose some magnetic flux in the presence of a magnetic field perpendicular to the NRs plane. Numerical calculation shows that the excitonic effects are essential to correctly describe the optical absorption in NRs. The applied lateral THz electric field, as well as the dc field leads to reduction, broadening and splitting of the exciton peak. In contrast to the presence of a dc field, significant optical absorption peak arises below the zero-field bandgap in the presence of a THz electric field at a certain frequency. The optical absorption spectrum depends evidently on the frequency and amplitude of the applied THz field and on the magnetic flux threading the NRs. This promises potential applications of NRs for magneto-optical and THz electro-optical sensing.

  4. Anticrossing-induced optical excitonic Aharonov-Bohm effect in strained type-I semiconductor nanorings

    OpenAIRE

    Tadić, M.; Čukarić, N.; Arsoski, V.; Peeters, F. M.

    2010-01-01

    The exciton states in strained (In,Ga)As nanorings embedded in a GaAs matrix are computed. The strain distribution is extracted from the continuum mechanical model, and the exact diagonalization approach is employed to compute the exciton states. Weak oscillations of the ground exciton state energy with the magnetic field normal to the ring are an expression of the excitonic Aharonov-Bohm effect. Those oscillations arise from anticrossings between the ground and the second exciton state and c...

  5. The two-level model of the excitonic Aharonov-Bohm effect in strained self-assembled semiconductor nanorings

    Science.gov (United States)

    Tadić, M.; Arsoski, V.; Čukarić, N.; Peeters, F. M.

    2013-12-01

    The excitonic Aharonov-Bohm oscillations in type-I nanorings are found to be caused by anticrossings between exciton states. These anticrossings are analyzed by a tight-binding-like model of exciton states. The criteria for the existence of the excitonic Aharonov-Bohm oscillations are formulated. For nanorings of realistic width and height, the range of values of the inner radius where the excitonic Aharonov-Bohm oscillations exist is found.

  6. Excitonic Aharonov-Bohm effect: Unstrained versus strained type-I semiconductor nanorings

    Science.gov (United States)

    Tadić, M.; Čukarić, N.; Arsoski, V.; Peeters, F. M.

    2011-09-01

    We study how mechanical strain affects the magnetic field dependence of the exciton states in type-I semiconductor nanorings. Strain spatially separates the electron and hole in (In,Ga)As/GaAs nanorings which is beneficial for the occurrence of the excitonic Aharonov-Bohm (AB) effect. In narrow strained (In,Ga)As/GaAs nanorings the AB oscillations in the exciton ground-state energy are due to anticrossings with the first excited state. No such AB oscillations are found in unstrained GaAs/(Al,Ga)As nanorings irrespective of the ring width. Our results are obtained within an exact numerical diagonalization scheme and are shown to be accurately described by a two-level model with off-diagonal coupling t. The later transfer integral expresses the Coulomb coupling between states of electron-hole pairs. We also found that the oscillator strength for exciton recombination in (In,Ga)As/GaAs nanorings exhibits AB oscillations, which are superimposed on a linear increase with magnetic field. Our results agree qualitatively with recent experiments on the excitonic Aharonov-Bohm effect in type-I (In,Ga)As/GaAs nanorings.

  7. Coulomb interaction effects on the electronic structure of radial polarized excitons in nanorings

    OpenAIRE

    Barticevic, Z.; Pacheco, M.; Simonin, J.; Proetto, C. R.

    2005-01-01

    The electronic structure of radially polarized excitons in structured nanorings is analyzed, with emphasis in the ground-state properties and their dependence under applied magnetic fields perpendicular to the ring plane. The electron-hole Coulomb attraction has been treated rigorously, through numerical diagonalization of the full exciton Hamiltonian in the non-interacting electron-hole pairs basis. Depending on the relative weight of the kinetic energy and Coulomb contributions, the ground-...

  8. Non-circular semiconductor nanorings of type I and II: Emission kinetics in the exciton Aharonov-Bohm effect

    OpenAIRE

    Grochol, Michal; Zimmermann, Roland

    2007-01-01

    Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in type I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectr...

  9. Noncircular semiconductor nanorings of types I and II: Emission kinetics in the excitonic Aharonov-Bohm effect

    Science.gov (United States)

    Grochol, Michal; Zimmermann, Roland

    2007-11-01

    Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in types I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectra are calculated, showing deviations from equilibrium. The presence of a nonradiative exciton decay leads to a quenching of the integrated photoluminescence with magnetic field.

  10. Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II

    OpenAIRE

    Grochol, M.; Grosse, F.; Zimmermann, R.

    2005-01-01

    The optical exciton Aharonov-Bohm effect, i. e. an oscillatory component in the energy of optically active (bright) states, is investigated in nanorings. It is shown that a small effective electron mass, strong confinement of the electron, and high barrier for the hole, achieved e. g. by an InAs nanoring embedded in an AlGaSb quantum well, are favorable for observing the optical exciton Aharonov-Bohm effect. The second derivative of the exciton energy with respect to the magnetic field is uti...

  11. Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II

    Science.gov (United States)

    Grochol, M.; Grosse, F.; Zimmermann, R.

    2006-09-01

    The optical exciton Aharonov-Bohm effect—i.e., an oscillatory component in the energy of optically active (bright) states—is investigated in nanorings. It is shown that a small effective electron mass, strong confinement of the electron, and high barrier for the hole, achieved, e.g., by an InAs nanoring embedded in an AlGaSb quantum well, are favorable for observing the optical exciton Aharonov-Bohm effect. The second derivative of the exciton energy with respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations even for the lowest bright state unambiguously. A connection between the theories for infinitesimal narrow and finite width rings is established. Furthermore, the magnetization is compared to the persistent current, which oscillates periodically with the magnetic field and confirms thus the nontrivial (connected) topology of the wave function in the nanoring.

  12. Aharonov-Bohm effect for an exciton in a finite width nano-ring

    OpenAIRE

    Palmero, F.; Dorignac, J.; Eilbeck, J. C.; Romer, R. A.

    2005-01-01

    We study the Aharonov-Bohm effect for an exciton on a nano-ring using a 2D attractive fermionic Hubbard model. We extend previous results obtained for a 1D ring in which only azimuthal motion is considered, to a more general case of 2D annular lattices. In general, we show that the existence of the localization effect, increased by the nonlinearity, makes the phenomenon in the 2D system similar to the 1D case. However, the introduction of radial motion introduces extra frequencies, different ...

  13. Coulomb-interaction effects on the electronic structure of radially polarized excitons in nanorings

    Science.gov (United States)

    Barticevic, Z.; Pacheco, M.; Simonin, J.; Proetto, C. R.

    2006-04-01

    The electronic structure of radially polarized excitons in structured nanorings is analyzed, with emphasis in the ground-state properties and their dependence under applied magnetic fields perpendicular to the ring plane. The electron-hole Coulomb attraction has been treated rigorously, through numerical diagonalization of the full exciton Hamiltonian in the noninteracting electron-hole pairs basis. Depending on the relative weight of the kinetic energy and Coulomb contributions, the ground-state of polarized excitons has “extended” or “localized” features. In the first case, corresponding to small rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm (AB) oscillations due to the individual orbits of the building particles of the exciton. In the localized regime, corresponding to large rings dominated by the Coulomb interaction, the only remaining AB oscillations are due to the magnetic flux trapped between the electron and hole orbits. This dependence of the exciton, a neutral excitation, on the flux difference confirms this feature as a signature of Coulomb dominated polarized excitons. Analytical approximations are provided in both regimes, which accurately reproduce the numerical results.

  14. Enhancement of the Aharonov-Bohm effect of neutral excitons in semiconductor nanorings with an electric field

    Science.gov (United States)

    Maslov, A. V.; Citrin, D. S.

    2003-03-01

    This work demonstrates that the Aharonov-Bohm effect for excitons, practically indistinguishable from the numerical noise without an applied electric field, becomes clearly evident in the optical absorption once the electric field is applied in the plane containing the nanoring. The enhancement arises as a result of the field-induced delocalization of the relative electron-hole motion around the entire ring. The excitonic effects are essential to describe even qualitatively the absorption spectra.

  15. An exciton on coupled concentric double nanorings in a magnetic field

    Energy Technology Data Exchange (ETDEWEB)

    Song Weiwei; Jin Guojun; Ma Yuqiang [National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093 (China)], E-mail: gjin@nju.edu.cn

    2008-09-10

    We study the influence of coupling between two concentric nanorings, embedded in two different layers separated by an insulating barrier, on the exciton's energy levels by the attractive Fermionic Hubbard model. The hopping coefficients between different rings are derived and the carriers' cyclotron movement caused by the magnetic field are taken into account. We find the amplitude of the Aharonov-Bohm oscillation is suppressed by the emergence of the Coulomb interaction. The coupling leads to a decrease of the energy of the excited states and this decrease varies significantly for different types of configuration for the electron and the hole. The study of the energy dependence on the radii of the two rings shows that the lowest energy level is determined by the small ring and the deviations of the exciton's energy from that of the single ring decreases with increasing magnetic field. The results also show that the energy of the AB oscillation can be modulated by changing the thickness of the spacer.

  16. An exciton on coupled concentric double nanorings in a magnetic field

    Science.gov (United States)

    Song, Weiwei; Jin, Guojun; Ma, Yu-qiang

    2008-09-01

    We study the influence of coupling between two concentric nanorings, embedded in two different layers separated by an insulating barrier, on the exciton's energy levels by the attractive Fermionic Hubbard model. The hopping coefficients between different rings are derived and the carriers' cyclotron movement caused by the magnetic field are taken into account. We find the amplitude of the Aharonov-Bohm oscillation is suppressed by the emergence of the Coulomb interaction. The coupling leads to a decrease of the energy of the excited states and this decrease varies significantly for different types of configuration for the electron and the hole. The study of the energy dependence on the radii of the two rings shows that the lowest energy level is determined by the small ring and the deviations of the exciton's energy from that of the single ring decreases with increasing magnetic field. The results also show that the energy of the AB oscillation can be modulated by changing the thickness of the spacer.

  17. An exciton on coupled concentric double nanorings in a magnetic field

    International Nuclear Information System (INIS)

    We study the influence of coupling between two concentric nanorings, embedded in two different layers separated by an insulating barrier, on the exciton's energy levels by the attractive Fermionic Hubbard model. The hopping coefficients between different rings are derived and the carriers' cyclotron movement caused by the magnetic field are taken into account. We find the amplitude of the Aharonov-Bohm oscillation is suppressed by the emergence of the Coulomb interaction. The coupling leads to a decrease of the energy of the excited states and this decrease varies significantly for different types of configuration for the electron and the hole. The study of the energy dependence on the radii of the two rings shows that the lowest energy level is determined by the small ring and the deviations of the exciton's energy from that of the single ring decreases with increasing magnetic field. The results also show that the energy of the AB oscillation can be modulated by changing the thickness of the spacer

  18. Controlled synthesis of cadmium carbonate nanowires, nanoribbons, nanorings and sphere like architectures via hydrothermal method

    International Nuclear Information System (INIS)

    Crystalline nanowires, nanoribbons, nanorings and sphere like architectures of cadmium carbonate have been synthesized with the spontaneous self-assembly of nanocrystals in aqueous solution under hydrothermal condition. The powder X-ray diffraction (PXRD) patterns of these materials exhibit phase pure hexagonal structure. The perfect circular nanorings with radius 375-437 nm, as a new member of nanostructured cadmium carbonate family are being reported for the first time. The width of the cadmium carbonate nanowires/nanoribbons and nanorings, respectively are found to be in the range 11-30 and 26-50 nm as observed by transmission electron microscope (TEM). The effect of temperature and concentration of urea on the cadmium carbonate morphology is discussed. The plausible growth mechanism for the formation of nanorings is also proposed.

  19. Exciton diffusion, end quenching, and exciton-exciton annihilation in individual air-suspended carbon nanotubes

    OpenAIRE

    Ishii A.; Yoshida, M.; Kato, Y. K.

    2015-01-01

    Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependen...

  20. Exciton diffusion, end quenching, and exciton-exciton annihilation in individual air-suspended carbon nanotubes

    OpenAIRE

    Ishii, A; Yoshida, M.; Kato, Y. K.

    2014-01-01

    Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependen...

  1. Carbon nanorings with inserted acenes: breaking symmetry in excited state dynamics.

    Science.gov (United States)

    Franklin-Mergarejo, R; Alvarez, D Ondarse; Tretiak, S; Fernandez-Alberti, S

    2016-01-01

    Conjugated cycloparaphenylene rings have unique electronic properties being the smallest segments of carbon nanotubes. Their conjugated backbones support delocalized electronic excitations, which dynamics is strongly influenced by cyclic geometry. Here we present a comparative theoretical study of the electronic and vibrational energy relaxation and redistribution in photoexcited cycloparaphenylene carbon nanorings with inserted naphthalene, anthracene, and tetracene units using non-adiabatic excited-state molecular dynamics simulations. Calculated excited state structures reflect modifications of optical selection rules and appearance of low-energy electronic states localized on the acenes due to gradual departure from a perfect circular symmetry. After photoexcitation, an ultrafast electronic energy relaxation to the lowest excited state is observed on the time scale of hundreds of femtoseconds in all molecules studied. Concomitantly, the efficiency of the exciton trapping in the acene raises when moving from naphthalene to anthracene and to tetracene, being negligible in naphthalene, and ~60% and 70% in anthracene and tetracene within the first 500 fs after photoexcitation. Observed photoinduced dynamics is further analyzed in details using induced molecular distortions, delocatization properties of participating electronic states and non-adiabatic coupling strengths. Our results provide a number of insights into design of cyclic molecular systems for electronic and light-harvesting applications. PMID:27507429

  2. Carbon nanorings with inserted acenes: breaking symmetry in excited state dynamics

    Science.gov (United States)

    Franklin-Mergarejo, R.; Alvarez, D. Ondarse; Tretiak, S.; Fernandez-Alberti, S.

    2016-08-01

    Conjugated cycloparaphenylene rings have unique electronic properties being the smallest segments of carbon nanotubes. Their conjugated backbones support delocalized electronic excitations, which dynamics is strongly influenced by cyclic geometry. Here we present a comparative theoretical study of the electronic and vibrational energy relaxation and redistribution in photoexcited cycloparaphenylene carbon nanorings with inserted naphthalene, anthracene, and tetracene units using non-adiabatic excited-state molecular dynamics simulations. Calculated excited state structures reflect modifications of optical selection rules and appearance of low-energy electronic states localized on the acenes due to gradual departure from a perfect circular symmetry. After photoexcitation, an ultrafast electronic energy relaxation to the lowest excited state is observed on the time scale of hundreds of femtoseconds in all molecules studied. Concomitantly, the efficiency of the exciton trapping in the acene raises when moving from naphthalene to anthracene and to tetracene, being negligible in naphthalene, and ~60% and 70% in anthracene and tetracene within the first 500 fs after photoexcitation. Observed photoinduced dynamics is further analyzed in details using induced molecular distortions, delocatization properties of participating electronic states and non-adiabatic coupling strengths. Our results provide a number of insights into design of cyclic molecular systems for electronic and light-harvesting applications.

  3. Localized Excitons in Carbon Nanotubes.

    Science.gov (United States)

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

    2015-03-01

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

  4. Polarized excitons in nanorings and the optical Aharonov-Bohm effect

    Science.gov (United States)

    Govorov, A. O.; Ulloa, S. E.; Karrai, K.; Warburton, R. J.

    2002-08-01

    The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between ``bright'' exciton ground states and novel ``dark'' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.

  5. Spontaneous exciton dissociation in carbon nanotubes

    OpenAIRE

    Kumamoto, Y.; Yoshida, M.; Ishii, A; Yokoyama, A.; Shimada, T; Kato, Y. K.

    2013-01-01

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. Correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating during their relaxation into the lowest exciton state. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator st...

  6. Polarized excitons in nanorings and the `optical' Aharonov-Bohm effect

    OpenAIRE

    Govorov, A. O.; Ulloa, S. E.; Karrai, K.; Warburton, R. J.

    2002-01-01

    The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a new mechanism for the topological phase of a neutral particle, a polarized exciton confined to a se...

  7. Excitons in nanorings. The signature of quantum phases in optical emission

    Science.gov (United States)

    Govorov, Alexander; Ulloa, Sergio

    2002-03-01

    When a quantum charged particle moves along a closed trajectory in an external magnetic field, the Aharonov-Bohm (AB) effect can occur, caused by quantum interference between paths with different phases. However, excitons in solids are neutral and hence a cursory analysis would not predict the AB effect. Nevertheless, we demonstrate here that a strong magnetic interference effect for neutral particles can exist, such as the polarizable excitons in quantum-ring structures. This phenomenon is akin to the Aharonov-Anandan quantum phases acquired under a cyclic evolution of the Hamiltonian. With increasing normal magnetic field, the ground state of the exciton acquires a nonzero angular momentum due to a net microscopic phase in the electron-hole pair. Under these conditions, the photoluminescence becomes suppressed in well-defined intervals of magnetic field, which depend on the degree of correlation of the two-particle motion. We demonstrate theoretically this novel magnetic interference effect using models of type-II quantum dots and InAs self-assembled quantum rings [1]. 1. A. Lorke et al., Phys. Rev. Lett. 84, 2223 (2000).

  8. Cross-polarized excitons in carbon nanotubes.

    Science.gov (United States)

    Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R

    2008-05-13

    Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E(22) van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active "bright" excitonic band polarized parallel to the tube axis and several optically "weak" cross-polarized excitons. The rest are optically (near) forbidden "dark" transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E(11) transition are more localized along the circumference of a tube, compared with others related to the E(12) and E(21) cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

  9. 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...

  10. 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...

  11. 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)

  12. Theory of Exciton Energy Transfer in Carbon Nanotube Composites

    OpenAIRE

    Davoody, A. H.; F Karimi; Arnold, M. S.; Knezevic, I.

    2016-01-01

    We compute the exciton transfer (ET) rate between semiconducting single-wall carbon nanotubes (SWNTs). We show that the main reasons for the wide range of measured ET rates reported in the literature are 1) exciton confinement in local quantum wells stemming from disorder in the environment and 2) exciton thermalization between dark and bright states due to intratube scattering. The SWNT excitonic states are calculated by solving the Bethe-Salpeter equation using tight-binding basis functions...

  13. Exciton resonances quench the photoluminescence of zigzag carbon nanotubes

    OpenAIRE

    Reich, Stephanie; Thomsen, Christian; Robertson, John

    2005-01-01

    We show that the photoluminescence intensity of single-walled carbon nanotubes is much stronger in tubes with large chiral angles - armchair tubes - because exciton resonances make the luminescence of zigzag tubes intrinsically weak. This exciton-exciton resonance depends on the electronic structure of the tubes and is found more often in nanotubes of the +1 family. Armchair tubes do not necessarily grow preferentially with present growth techniques; they just have stronger luminescence. Our ...

  14. Exciton-Plasmon Interactions in Individual Carbon Nanotubes

    OpenAIRE

    Bondarev, I.V.; Woods, L. M.; Popescu, A.

    2010-01-01

    We use the macroscopic quantum electrodynamics approach suitable for absorbing and dispersing media to study the properties and role of collective surface excitations --- excitons and plasmons --- in single-wall and double-wall carbon nanotubes. We show that the interactions of excitonic states with surface electromagnetic modes in individual small-diameter (

  15. Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes.

    Science.gov (United States)

    Hofmann, Matthias S; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J; Högele, Alexander

    2016-05-11

    We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes. PMID:27105355

  16. 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....

  17. Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes

    OpenAIRE

    Hofmann, Matthias S.; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J.; Högele, Alexander

    2016-01-01

    We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wal...

  18. Nonlinear Photoluminescence Spectroscopy of Carbon Nanotubes with Localized Exciton States

    OpenAIRE

    Iwamura, Munechiyo; Akizuki, Naoto; Miyauchi, Yuhei; Mouri, Shinichiro; Shaver, Jonah; Gao, Zhenghong; Cognet, Laurent; Lounis, Brahim; Matsuda, Kazunari

    2014-01-01

    International audience We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong saturation behavior was observed regardless of the origin of the local states, and foun...

  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. Origin of low-energy photoluminescence peaks in single carbon nanotubes: K -momentum dark excitons and triplet dark excitons

    OpenAIRE

    Matsunaga, Ryusuke; Matsuda, Kazunari; Kanemitsu, Yoshihiko

    2010-01-01

    We performed photoluminescence (PL) spectroscopy on single carbon nanotubes to investigate the satellite PL peaks, which are much lower in energy than the lowest (E11) bright exciton peak. From the temperature and tube-diameter dependences of the PL spectra, we clarified two origins of the low-energy PL peaks. The weak peak, lying about 130 meV below the E11 bright exciton state, is well explained by the phonon sideband of the K-momentum dark exciton states above the lowest-bright exciton sta...

  1. Photoluminescence clamping with few excitons in a single-walled carbon nanotube

    OpenAIRE

    Xiao, Y. -F.; Nhan, T. Q.; Wilson, M. W. B.; Fraser, J. M.

    2009-01-01

    Single air-suspended carbon nanotubes (length 2 - 5 microns) exhibit high optical quantum efficiency (7 - 20%) for resonant pumping at low intensities. Under ultrafast excitation, the photoluminescence dramatically saturates for very low injected exciton numbers (2 to 6 excitons per pulse per SWCNT). This PL clamping is attributed to highly efficient exciton-exciton annihilation over micron length scales. Stochastic modeling of exciton dynamics and femtosecond excitation correlation spectrosc...

  2. Mechanism of exciton dephasing in a single carbon nanotube studied by photoluminescence spectroscopy

    OpenAIRE

    Yoshikawa, Kohei; Matsunaga, Ryusuke; Matsuda, Kazunari; Kanemitsu, Yoshihiko

    2009-01-01

    We studied the temperature and chirality dependence of the photoluminescence (PL) linewidth of single carbon nanotubes to clarify the mechanism of exciton dephasing. The PL linewidth of a single carbon nanotube broadened linearly with increasing temperature, indicating that the linewidth and exciton dephasing are determined through exciton-phonon interactions. From the chirality dependence of the PL linewidth, we concluded that exciton dephasing is caused by both the longitudinal acoustic and...

  3. 3D Bridged Carbon Nanoring/Graphene Hybrid Paper as a High-Performance Lateral Heat Spreader.

    Science.gov (United States)

    Zhang, Jianwei; Shi, Gang; Jiang, Cai; Ju, Su; Jiang, Dazhi

    2015-12-01

    Graphene paper (GP) has attracted great attention as a heat dissipation material due to its unique thermal transfer property exceeding the limit of graphite. However, the relatively poor thermal transfer properties in the normal direction of GP restricts its wider applications in thermal management. In this work, a 3D bridged carbon nanoring (CNR)/graphene hybrid paper is constructed by the intercalation of polymer carbon source and metal catalyst particles, and the subsequent in situ growth of CNRs in the confined intergallery spaces between graphene sheets through thermal annealing. Further investigation demonstrates that the CNRs are covalently bonded to the graphene sheets and highly improve the thermal transport in the normal direction of the CNR/graphene hybrid paper. This full-carbon architecture shows excellent heat dissipation ability and is much more efficient in removing hot spots than the reduced GP without CNR bridges. This highly thermally conductive CNR/graphene hybrid paper can be easily integrated into next generation commercial high-power electronics and stretchable/foldable devices as high-performance lateral heat spreader materials. This full-carbon architecture also has a great potential in acting as electrodes in supercapacitors or hydrogen storage devices due to the high surface area. PMID:26476622

  4. Exciton decay dynamics in individual carbon nanotubes at room temperature

    OpenAIRE

    Gokus, Tobias; Hartschuh, Achim; Harutyunyan, Hayk; Allegrini, Maria; Hennrich, Frank; Kappes, Manfred; Green, Alexander A.; Hersam, Mark C.; Araujo, Paulo T.; Jorio, Ado

    2008-01-01

    We studied the exciton decay dynamics of individual semiconducting single-walled carbon nanotubes at room temperature using time-resolved photoluminescence spectroscopy. The photoluminescence decay from nanotubes of the same (n,m) type follows a single exponential decay function, however, with lifetimes varying between about 1 and 40 ps from nanotube to nanotube. A correlation between broad photoluminescence spectra and short lifetimes was found and explained by defects promoting both nonradi...

  5. Aharonov-Bohm effects on bright and dark excitons in carbon nanotubes

    International Nuclear Information System (INIS)

    A short-range part of the Coulomb interaction causes splitting and shift of excitons due to exchange interaction and mixing between different valleys in semiconducting carbon nanotubes. In the absence of a magnetic flux only a single exciton is optically active (bright) and all others are inactive (dark). Two bright excitons appear in the presence of an Aharonov- Bohm magnetic flux

  6. Optical absorption in semiconductor nanorings under electric and magnetic fields

    Science.gov (United States)

    Zhang, Tong-Yi; Cao, Jun-Cheng; Zhao, Wei

    2005-01-01

    The optical absorption in semiconductor nanorings under a lateral DC field and a perpendicular magnetic field is numerically simulated by coherent wave approach. The exciton dominated optical absorption is compared with the free-carrier interband absorption to demonstrate the key role of Coulomb interaction between electron and hole. The influence of the lateral DC field and the perpendicular magnetic field on the optical absorption are discussed in detail. It shows that the lateral DC field can significantly enhance the Aharonov-Bohm effect of the neutral excitons in semiconductor nanorings.

  7. Exciton-assisted optomechanics with suspended carbon nanotubes

    International Nuclear Information System (INIS)

    We propose a framework for inducing strong optomechanical effects in a suspended carbon nanotube based on deformation potential (DP) exciton–phonon coupling. The excitons are confined using an inhomogeneous axial electric field which generates optically active quantum dots with a level spacing in the milli-electronvolt range and a characteristic size in the 10 nm range. A transverse field induces a tunable parametric coupling between the quantum dot and the flexural modes of the nanotube mediated by electron–phonon interactions. We derive the corresponding excitonic DPs and show that this interaction enables efficient optical ground-state cooling of the fundamental mode and could allow us to realize the strong and ultra-strong coupling regimes of the Jaynes–Cummings and Rabi models. (paper)

  8. Many-body effects and excitonic features in 2D biphenylene carbon.

    Science.gov (United States)

    Lüder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-14

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future. PMID:26772582

  9. Observation of Charged Excitons in Hole-Doped Carbon Nanotubes Using Photoluminescence and Absorption Spectroscopy

    OpenAIRE

    Matsunaga, Ryusuke; Matsuda, Kazunari; Kanemitsu, Yoshihiko

    2011-01-01

    We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong elec...

  10. Observation of charged excitons in hole-doped carbon nanotubes using photoluminescence and absorption spectroscopy

    OpenAIRE

    Matsunaga, Ryusuke; Matsuda, Kazunari; Kanemitsu, Yoshihiko

    2010-01-01

    We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong elec...

  11. Many-body effects and excitonic features in 2D biphenylene carbon

    International Nuclear Information System (INIS)

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon’s excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future

  12. Many-body effects and excitonic features in 2D biphenylene carbon

    Energy Technology Data Exchange (ETDEWEB)

    Lüder, Johann, E-mail: johann.luder@physics.uu.se; Puglia, Carla; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara [Department of Physics and Astronomy, Uppsala University, P.O. Box 516, 751 20 Uppsala (Sweden); Ottosson, Henrik [Department of Chemistry–BMC, Uppsala University, P.O. Box 576, 751 23 Uppsala (Sweden)

    2016-01-14

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon’s excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  13. A cycloparaphenylene nanoring with graphenic hexabenzocoronene sidewalls.

    Science.gov (United States)

    Lu, Dapeng; Wu, Haotian; Dai, Yafei; Shi, Hong; Shao, Xiang; Yang, Shangfeng; Yang, Jinlong; Du, Pingwu

    2016-06-01

    Herein we report the synthesis of a novel hexabenzocoronene-containing cycloparaphenylene carbon nanoring, cyclo[12]-paraphenylene[2]-2,11-hexabenzocoronenylene, by metal-mediated cross-coupling reactions. The nanoring was accomplished by rationally designed palladium-catalyzed coupling of diborylhexabenzocoronene and L-shaped cyclohexane units, followed by nickel-mediated C-Br/C-Br coupling and the aromatization of cyclohexane moieties. The structure was confirmed by NMR and HR-MS. Especially, the cycloparaphenylene structure is firstly observed by STM. The photophysical properties were studied using UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, and theoretical calculations. PMID:27172905

  14. Excitonic signatures in the optical response of single-wall carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Voisin, Christophe; Berger, Sebastien; Cassabois, Guillaume; Roussignol, Philippe [Laboratoire Pierre Aigrain, Ecole Normale Superieure, CNRS UMR8551, UPMC, Universite Paris Diderot, 24 rue Lhomond, 75005 Paris (France); Berciaud, Stephane [IPCMS, UMR 7504, CNRS Universite de Strasbourg, 23 rue du Loess, 67034 Strasbourg (France); Yan, Hugen; Hone, James; Heinz, Tony F. [Physics, Mechanical Engineering and Electrical Engineering Departments, Columbia University, New York, NY (United States); Lauret, Jean-Sebastien [Laboratoire de Photonique Quantique et Moleculaire, Ecole Normale Superieure de Cachan, CNRS UMR 8537, Institut Dalembert, 61 Avenue Wilson, Cachan (France)

    2012-05-15

    The optical properties of single-wall carbon nanotubes (SWNTs) are dominated by the excitonic character of the transitions even at room temperature. The very peculiar properties of these excitons arise from both the one-dimensional (1D) nature of carbon nanotubes and from the electronic properties of graphene from which nanotubes are made. We first propose a brief qualitative review of the structure of the excitonic manifold and emphasize the role of dark states. We describe recent experimental investigations of this excitonic structure by means of temperature dependent PL measurements. We investigate the case of upper sub-bands and show that high-order optical transitions remain excitonic for large diameter nanotubes. A careful investigation of Rayleigh scattering spectra at the single nanotube level reveals clear exciton-phonon side-bands and Lorentzian line profiles for all semi-conducting nanotubes. In contrast, metallic nanotubes show an ambivalent behavior which is related to the reduced excitonic binding energy. Schematic of the exciton manifold in single-wall carbon nanotubes. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  15. Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single Molecule Reactions

    CERN Document Server

    Cognet, Laurent; Rocha, John-David R; Doyle, Condell D; Tour, James M; Weisman, R Bruce

    2007-01-01

    Single-molecule chemical reactions with individual single-walled carbon nanotubes were observed through near-infrared photoluminescence microscopy. The emission intensity within distinct submicrometer segments of single nanotubes changes in discrete steps after exposure to acid, base, or diazonium reactants. The steps are uncorrelated in space and time, and reflect the quenching of mobile excitons at localized sites of reversible or irreversible chemical attack. Analysis of step amplitudes reveals an exciton diffusional range of about 90 nanometers, independent of nanotube structure. Each exciton visits approximately 104 atomic sites during its lifetime, providing highly efficient sensing of local chemical and physical perturbations.

  16. Effects of environmental and exciton screening in single-walled carbon nanotubes

    International Nuclear Information System (INIS)

    The ground-state exciton binding energy for single-walled carbon nanotubes (SWCNTs) in vacuum calculated ignoring the screening of Coulomb interaction appears to be much greater than the corresponding band gap. The most essential contributions to the screening of electron-hole (e-h) interaction potential in semiconducting SWCNTs, which return the ground-state exciton binding energy into the energy gap, are considered. Our estimates on the screening effects and exciton binding energies are in satisfactory agreement with the corresponding experimental data for concrete nanotubes.

  17. Defect-Induced Photoluminescence from Dark Excitonic States in Individual Single-Walled Carbon Nanotubes

    OpenAIRE

    Harutyunyan, Hayk; Gokus, Tobias; Green, Alexander A.; Hersam, Mark C.; Allegrini, Maria; Hartschuh, Achim

    2009-01-01

    We show that new low-energy photoluminescence (PL) bands can be created in the spectra of semiconducting single-walled carbon nanotubes by intense pulsed excitation. The new bands are attributed to PL from different nominally dark excitons that are “brightened” because of a defect-induced mixing of states with different parity and/or spin. Time-resolved PL studies on single nanotubes reveal a significant reduction of the bright exciton lifetime upon brightening of the dark excitons. The lowes...

  18. Radiative lifetimes and coherence lengths of one-dimensional excitons in single-walled carbon nanotubes

    OpenAIRE

    Miyauchi, Yuhei; Hirori, Hideki; Matsuda, Kazunari; Kanemitsu, Yoshihiko

    2009-01-01

    We evaluated the radiative lifetimes and the one-dimensional exciton coherence lengths in single-walled carbon nanotubes (SWNTs). The radiative lifetimes determined from simultaneous measurements of photoluminescence (PL) lifetimes and PL quantum yields range from ~3 to 10 ns, and slightly increase with the tube diameter. The exciton coherence lengths in SWNTs are of the order of 10 nm, as deduced from the experimentally obtained radiative lifetimes, and they are about ten times larger than t...

  19. Exciton diffusion in air-suspended single-walled carbon nanotubes

    OpenAIRE

    Moritsubo, S.; Murai, T.; Shimada, T; Murakami, Y.; Chiashi, S.; Maruyama, S.; Kato, Y. K.

    2010-01-01

    Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer...

  20. Exciton Diffusion in Air-Suspended Single-Walled Carbon Nanotubes

    OpenAIRE

    Moritsubo, S.; Murai, T.; Shimada, T; Murakami, Y.; Chiashi, S.; Maruyama, S.; Kato, Y. K.

    2010-01-01

    Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer...

  1. Photoluminescence sidebands of carbon nanotubes below the bright singlet excitonic levels: Coupling between dark excitons and K-point phonons

    OpenAIRE

    Murakami, Yoichi; Lu, Benjamin; Kazaoui, Said; Minami, Nobutsugu; Okubo, Tatsuya; Maruyama, Shigeo

    2008-01-01

    We performed detailed photoluminescence (PL) spectroscopy studies of three different types of single-walled carbon nanotubes (SWNTs) by using samples that contain essentially only one chiral type of SWNT, (6,5), (7,5), or (10,5). The observed PL spectra unambiguously show the existence of an emission sideband at ~ 145 meV below the lowest singlet excitonic (E11) level, whose identity and origin are now under debate. We find that the energy separation between the E11 level and the sideband is ...

  2. Calculation Method for Exciton Wavefunctions with Electron--Hole Exchange Interaction: Application to Carbon Nanotubes

    Science.gov (United States)

    Ajiki, Hiroshi

    2013-05-01

    A new method for calculating exciton wavefunctions in the presence of a long-range electron--hole (e--h) exchange interaction (EXI) is presented. The e--h EXI arises, for example, for cross-polarized excitons in a single-walled carbon nanotube (SWNT). Cross-polarized excitons have previously been calculated as an eigenvalue problem of a Bethe--Salpeter equation (BSE) within the Tamm--Dancoff-type approximation (TDA). The resulting wavefunctions provide quite different absorption spectra in comparison with those calculated in the self-consistent-field method [S. Uryu and T. Ando, J. Phys.: Conf. Ser. 302 (2011) 012004]. Although the self-consistent-field method is more reliable, exciton wavefunctions cannot be obtained from this method. A general method is derived here to obtain exciton wavefunctions that take the e--h EXI into account within the TDA, and the method is applied to the cross-polarized excitons of a SWNT. The absorption spectra calculated from the resulting exciton wavefunctions agree well with the spectra calculated from the self-consistent-field method within a rotating-wave approximation.

  3. Predicting excitonic gaps of semiconducting single-walled carbon nanotubes from a field theoretic analysis

    Science.gov (United States)

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

    2015-02-01

    We demonstrate that a nonperturbative 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 turn a function of the tube chirality, dielectric environment, and the tube's dimensions, thus expressing disparate influences on the excitonic energies in a unified fashion. We test this theory explicitly on the data reported by Dukovic et al. [Nano Lett. 5, 2314 (2005), 10.1021/nl0518122] and Sfeir et al. [Phys. Rev. B 82, 195424 (2010), 10.1103/PhysRevB.82.195424] and so demonstrate the method works over a wide range of reported excitonic spectra.

  4. Electric-Field Induced Activation of Dark Excitonic States in Carbon Nanotubes.

    Science.gov (United States)

    Uda, T; Yoshida, M; Ishii, A; Kato, Y K

    2016-04-13

    Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric-field effects on various excitonic states by simultaneously measuring photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. Field-induced exciton dissociation can explain the photocurrent threshold field, and the edge of the E11 continuum states has been identified by extrapolating to zero threshold. PMID:26999284

  5. Quantum tunneling through graphene nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Wu Zhenhua; Zhang, Z Z; Chang Kai [SKLSM, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, 100083 Beijing (China); Peeters, F M, E-mail: kchang@red.semi.ac.cn [Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen (Belgium)

    2010-05-07

    We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

  6. Quantum tunneling through graphene nanorings

    International Nuclear Information System (INIS)

    We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

  7. Quantum tunneling through graphene nanorings

    Science.gov (United States)

    Wu, Zhenhua; Zhang, Z. Z.; Chang, Kai; Peeters, F. M.

    2010-05-01

    We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

  8. Prolonged spontaneous emission and dephasing of localized excitons in air-bridged carbon nanotubes

    Science.gov (United States)

    Sarpkaya, Ibrahim; Zhang, Zhengyi; Walden-Newman, William; Wang, Xuesi; Hone, James; Wong, Chee W.; Strauf, Stefan

    2013-07-01

    The bright exciton emission of carbon nanotubes is appealing for optoelectronic devices and fundamental studies of light-matter interaction in one-dimensional nanostructures. However, to date, the photophysics of excitons in carbon nanotubes is largely affected by extrinsic effects. Here we perform time-resolved photoluminescence measurements over 14 orders of magnitude for ultra-clean carbon nanotubes bridging an air gap over pillar posts. Our measurements demonstrate a new regime of intrinsic exciton photophysics with prolonged spontaneous emission times up to T1=18 ns, about two orders of magnitude better than prior measurements and in agreement with values hypothesized by theorists about a decade ago. Furthermore, we establish for the first time exciton decoherence times of individual nanotubes in the time domain and find fourfold prolonged values up to T2=2.1 ps compared with ensemble measurements. These first observations motivate new discussions about the magnitude of the intrinsic dephasing mechanism while the prolonged exciton dynamics is promising for applications.

  9. 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.

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

    Science.gov (United States)

    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. PMID:27127916

  11. Tunable bandgaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons

    OpenAIRE

    Spataru, Catalin D.; Léonard, François

    2010-01-01

    Doping of semiconductors is essential in modern electronic and photonic devices. While doping is well understood in bulk semiconductors, the advent of carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications raises some key questions about the role and impact of doping at low dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps and exciton binding energies can be dramatically reduced upon experimentally relevant doping, and can be tuned gradua...

  12. Direct Experimental Evidence of Exciton-Phonon Bound States in Carbon Nanotubes

    OpenAIRE

    Plentz, Flavio; Henrique B. Ribeiro; Jorio, Ado; Pimenta, Marcos A.; Strano, Michael S.

    2005-01-01

    We present direct experimental observation of exciton-phonon bound states in the photoluminescence excitation spectra of isolated single walled carbon nanotubes in aqueous suspension. The photoluminescence excitation spectra from several distinct single-walled carbon nanotubes show the presence of at least one sideband related to the tangential modes, lying {200 meV} above the main absorption/emission peak. Both the energy position and line shapes of the sidebands are in excellent agreement w...

  13. Stark effect of excitons in individual air-suspended carbon nanotubes

    OpenAIRE

    Yoshida, M.; Kumamoto, Y.; Ishii, A; Yokoyama, A.; Kato, Y. K.

    2014-01-01

    We investigate electric-field induced redshifts of photoluminescence from individual single-walled carbon nanotubes. The shifts scale quadratically with field, while measurements with different excitation powers and energies show that effects from heating and relaxation pathways are small. We attribute the shifts to the Stark effect and characterize nanotubes with different chiralities. By taking into account exciton binding energies for air-suspended tubes, we find that theoretical predictio...

  14. Stark effect of excitons in individual air-suspended carbon nanotubes

    OpenAIRE

    Yoshida, M.; Kumamoto, Y.; Ishii, A; Yokoyama, A.; Kato, Y. K.

    2014-01-01

    We investigate electric-field induced redshifts of photoluminescence from individual single-walled carbon nanotubes. The shifts scale quadratically with field, while measurements with different excitation powers and energies show that effects from heating and relaxation pathways are small. We attribute the shifts to the Stark effect, and characterize nanotubes with different chiralities. By taking into account exciton binding energies for air-suspended tubes, we find that theoretical predicti...

  15. Identification of excitonic phonon sideband by photoluminescence spectroscopy of single-walled carbon-13 nanotubes

    OpenAIRE

    Miyauchi, Yuhei; Maruyama, Shigeo

    2005-01-01

    We have studied photoluminescence (PL) and resonant Raman scatterings of single-walled carbon nanotubes (SWNTs) consisting of carbon-13 (SW13CNTs) synthesized from a small amount of isotopically modified ethanol. There was almost no change in the Raman spectra shape for SW13CNTs except for a downshift of the Raman shift frequency by the square-root of the mass ratio 12/13. By comparing photoluminescence excitation (PLE) spectra of SW13CNTs and normal SWNTs, the excitonic phonon sideband due t...

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

    Institute of Scientific and Technical Information of China (English)

    YU Gui-Li; JIA Yong-Lei

    2009-01-01

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

  17. Exciton Mobility and Localized Defects in Single Carbon Nanotubes Studied with Tip-Enhanced Near-Field Optical Microscopy

    OpenAIRE

    Georgi, Carsten

    2011-01-01

    In this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-enhanced near-field optical microscopy (TENOM). This technique provides a sub-diffraction spatial resolution of 15 nm on the basis of strong local signal enhancement, which allows for nanoscale imaging of the photoluminescence (PL) intensity and energy along single semiconducting SWNTs. Thereby, the mobility of excitons and their interaction with defects and spatial exciton energy variations can be directly vi...

  18. Nonlinear Photoluminescence Excitation Spectroscopy of Carbon Nanotubes: Exploring the Upper Density Limit of One-Dimensional Excitons

    OpenAIRE

    Murakami, Y.; Kono, J.

    2008-01-01

    We have studied emission properties of high-density excitons in single-walled carbon nanotubes through nonlinear photoluminescence excitation spectroscopy. As the excitation intensity was increased, all emission peaks arising from different chiralities showed clear saturation in intensity. Each peak exhibited a saturation value that was independent of the excitation wavelength, indicating that there is an upper limit on the exciton density for each nanotube species. We developed a theoretical...

  19. Ultrafast vibrations of gold nanorings

    DEFF Research Database (Denmark)

    Kelf, T; Tanaka, Y; Matsuda, O;

    2011-01-01

    We investigate the vibrational modes of gold nanorings on a silica substrate with an ultrafast optical technique. By comparison with numerical simulations, we identify several resonances in the gigahertz range associated with axially symmetric deformations of the nanoring and substrate. We...... elucidate the corresponding mode shapes and find that the substrate plays an important role in determining the mode damping. This study demonstrates the need for a plasmonic nano-optics approach to understand the optical excitation and detection mechanisms for the vibrations of plasmonic nanostructures....

  20. Interband optical spectra of magnetoexcitons in semiconductor nanorings: Electron-hole spatial correlation

    Science.gov (United States)

    Citrin, D. S.; Maslov, A. V.

    2005-08-01

    An analytic model [R. A. Römer and M. E. Raikh, Phys. Rev. B 62, 7045 (2000); K. Moulopoulos and M. Constantinou, Phys. Rev. B 70, 235327 (2004)] for magnetoexcitons in nanoscale semiconductor rings is extended to calculate directly the linear optical properties. The spectroscopic properties exhibit pronounced Φ0=hc/e excitonic Aharonov-Bohm oscillations in the threading magnetic flux Φ when the ring radius R is less than the effective exciton Bohr radius a0 . The electron-hole spatial correlation induced by an optical field as a function of nanoring radius and threading magnetic flux is studied.

  1. Non-radiative Exciton Decay in Single-walled Carbon Nanotubes

    Science.gov (United States)

    Harrah, Mark; Swan, Anna

    2010-03-01

    Experiments have shown step-wise changes in the fluorescence intensity from single-walled carbon nanotubes [1,2]. It has been proposed that the underlying mechanism for the step-wise changes is diffusion-limited quenching of excitons at defects [1]. This property has been used to demonstrate single-molecule detection for biological applications [3]. We perform a Monte-Carlo simulation of nanotube fluorescence with a diffusion-limited quenching model. The fluorescence intensity is seen to depend on the mean-square distance between defects, implying a nonlinear dependence on the number of defects. The intensity for consecutive defect counts can overlap depending on the positions of the defects. [4pt] [1] Cognet, L. et al. Science 316, 1465-1468 (2007).[0pt] [2] Jin, H. et al. Nano Lett. 8, 4299-4304 (2008).[0pt] [3] Heller, D. A. et al. Nature Nanotech. 4, 114-120 (2009).

  2. Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions

    Science.gov (United States)

    Ihly, Rachelle; Mistry, Kevin S.; Ferguson, Andrew J.; Clikeman, Tyler T.; Larson, Bryon W.; Reid, Obadiah; Boltalina, Olga V.; Strauss, Steven H.; Rumbles, Garry; Blackburn, Jeffrey L.

    2016-06-01

    Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

  3. Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions

    Energy Technology Data Exchange (ETDEWEB)

    Ihly, Rachelle; Mistry, Kevin S.; Ferguson, Andrew J.; Clikeman, Tyler T.; Larson, Bryon W.; Reid, Obadiah; Boltalina, Olga V.; Strauss, Steven H.; Rumbles, Garry; Blackburn, Jeffrey L.

    2016-04-25

    Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

  4. α-Fe{sub 2}O{sub 3}@C nanorings as anode materials for high performance lithium ion batteries

    Energy Technology Data Exchange (ETDEWEB)

    Li, Le; Li, Zhenzhen; Fu, Wenming; Li, Fagen [Department of Physics, Faculty of Science, Ningbo University, Ningbo (China); Wang, Jun, E-mail: wjnaf@ustc.edu [Department of Physics, Faculty of Science, Ningbo University, Ningbo (China); Wang, Wenzhong [School of Science, Minzu University of China, Beijing 100081 (China)

    2015-10-25

    α-Fe{sub 2}O{sub 3}@C core–shell nanorings are prepared by a facile large-scale two-step route incorporating a hydrothermal method and a carbon coated progress. Its structure and morphology are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscope, and thermogravimetry. It is found that the as-prepared composite is composed of α-Fe{sub 2}O{sub 3}@C nanorings of about 148 nm in outer diameter, 50 nm in thickness, and 115 nm in length. These α-Fe{sub 2}O{sub 3}@C nanorings are enwrapped with ∼3 nm thick carbon shell. And the electrodes exhibit longer cycle life (815 mAhg{sup −1} after cycling 160 times) at high current rate (1000 mAg{sup −1}) compared with that of bare α-Fe{sub 2}O{sub 3} nanorings (810 mAhg{sup −1} after cycling 30 times). The improved performance of the composite is attributed to the bondage from carbon shell, which can enhance the electronic conductivity and structural stability of α-Fe{sub 2}O{sub 3} nanorings. - Highlights: • α-Fe{sub 2}O{sub 3}@C core–shell nanorings are prepared by a facile two-step route. • The α-Fe{sub 2}O{sub 3}@C nanorings are firstly reported as anode materials for LIBs. • The nanorings show a high capacity of 815 mAhg{sup −1} at 1 Ag{sup -1} after 160 cycles.

  5. One-Pot Silver Nanoring Synthesis

    Directory of Open Access Journals (Sweden)

    Drogat Nicolas

    2009-01-01

    Full Text Available Abstract Silver colloidal nanorings have been synthesized by reducing silver ions with NaBH4 in trisodium citrate buffers. pH increase, by addition of NaOH, was used to speed up reduction reaction. The UV–vis absorption spectra of resulting silver nanorings showed two peaks accounting for transverse and longitudinal surface plasmon resonance, at ≈400 nm, and between 600 and 700 nm, respectively. The shapes of these silver nanoparticles (nanorings depended on AgNO3/NaBH4 ratio, pH and reaction temperature. Particles were analysed by transmission electron microscopy, scanning electron microscopy and X-ray diffraction. A reaction pathway is proposed to explain silver nanoring formation.

  6. Plasmon mode excitation and photoluminescence enhancement on silver nanoring

    CERN Document Server

    Kuchmizhak, A A; Kulchin, Yu N; Vitrik, O B

    2015-01-01

    We demonstrate a simple and high-performance laser-assisted technique for silver nanoring fabrication, which includes the ablation of the Ag film by focused nanosecond pulses and subsequent reactive ion polishing. The nanoring diameter and thickness can be controlled by optimizing both the pulse energy and the metal film thickness at laser ablation step, while the subsequent reactive ion polishing provides the ability to fabricate the nanoring with desirable height. Scattering patterns of s-polarized collimated laser beam obliquely illuminating the nanoring demonstrate the focal spot inside the nanoring shifted from its center at a distance of ~ 0.57Rring. Five-fold enhancement of the photoluminescence signal from the Rhodamine 6G organic dye near the Ag nanoring was demonstrated. This enhancement was attributed to the increase of the electromagnetic field amplitude near the nanoring surface arising from excitation of the multipole plasmon modes traveling along the nanoring. This assumption was confirmed by d...

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

    OpenAIRE

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

    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 applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculati...

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

    Science.gov (United States)

    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.

    2010-08-01

    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 applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.

  9. Observation of excited-state excitons and band-gap renormalization in hole-doped carbon nanotubes using photoluminescence excitation spectroscopy

    OpenAIRE

    Kimoto, Yoshio; Okano, Makoto; Kanemitsu, Yoshihiko

    2013-01-01

    The higher Rydberg states of the E11 exciton in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) were studied using one- and two-photon photoluminescence excitation spectroscopy. Increasing the hole-dopant concentration resulted in a redshift of the first excited state (2g) and a blueshift of the ground state (1u) of the E11 exciton. From the redshift of higher Rydberg states, we found that a reduction of the band-gap energy occurs in hole-doped SWCNTs. These findings show that ...

  10. Magnetic phase diagram of graphene nanorings in an electric field

    International Nuclear Information System (INIS)

    Magnetic properties of graphene nanorings are investigated in the presence of an electric field. Within the formalism of Hubbard model, the graphene nanorings of various geometric configurations are found to exhibit rich phase diagram. For a nanoring system which has degenerate states at the Fermi level, the system is shown to undergo an abrupt phase transition from the antiferromagnetic to a nonmagnetic state in an electric field applied cross its zigzag edges. However, the nanoring is found to always stay in the antiferromagnetic state when the electric field is applied cross its armchair edges. For the other nanoring system with a finite single-particle gap, the magnetic moments of its antiferromagnetic ground state is seen to decrease gradually to zero with the electric field applied cross the zigzag edges. When the electric field is applied cross the armchair edges, the nanoring is shown to undergo several magnetic phase transitions before settling itself in a nonmagnetic ordering. (paper)

  11. Ordered arrays of lead zirconium titanate nanorings

    International Nuclear Information System (INIS)

    Periodic arrays of nanorings of morphotropic phase boundary lead zirconium titanate (PZT) have been successfully fabricated using a novel self-assembly technique: close-packed monolayers of latex nanospheres were deposited onto Pt-coated silicon substrates, and then plasma cleaned to form ordered arrays of isolated nanospheres, not in contact with each other. Subsequent pulsed laser deposition of PZT, high angle argon ion etching and thermal annealing created the arrays of isolated nanorings, with diameters of ∼100 nm and wall thicknesses of ∼10 nm. Energy dispersive x-ray analysis confirms that the rings are compositionally morphotropic phase boundary PZT, and high resolution transmission electron microscopy imaging of lattice fringes demonstrates some periodicities consistent with perovskite rather than pyrochlore material. The dimensions of these nanorings, and the expected 'soft' behaviour of the ferroelectric material from which they are made, means that they offer the most likely opportunity to date for observing whether or not vortex arrangements of electrical dipoles, analogous to those seen in ferromagnetic nanostructures, actually exist

  12. Size-Independent Energy Transfer in Biomimetic Nanoring Complexes.

    Science.gov (United States)

    Parkinson, Patrick; Kamonsutthipaijit, Nuntaporn; Anderson, Harry L; Herz, Laura M

    2016-06-28

    Supramolecular antenna-ring complexes are of great interest due to their presence in natural light-harvesting complexes. While such systems are known to provide benefits through robust and efficient energy funneling, the relationship between molecular structure, strain (governed by nuclear coordinates and motion), and energy dynamics (arising from electronic behavior) is highly complex. We present a synthetic antenna-nanoring system based on a series of conjugated porphyrin chromophores ideally suited to explore such effects. By systematically varying the size of the acceptor nanoring, we reveal the interplay between antenna-nanoring binding, local strain, and energy dynamics on the picosecond time scale. Binding of the antenna unit creates a local strain in the nanoring, and this strain was measured as a function of the size of the nanoring, by UV-vis-NIR titration, providing information on the conformational flexibility of the system. Strikingly, the energy-transfer rate is independent of nanoring size, indicating the existence of strain-localized acceptor states, spread over about six porphyrin units, arising from the noncovalent antenna-nanoring association. PMID:27176553

  13. Design of multiple nanoring-based metallic nanophotonic superlens

    Science.gov (United States)

    Choi, Jong-ryul; Oh, Youngjin; Kim, Donghyun

    2012-01-01

    In this study, we investigate metallic nanophotonic lenses that consist of multiple nano-ring apertures. Emphasis was placed on a nanophotonic lens of simple two nano-ring apertures modeled on a BK7 substrate with capping and adhesion layers. Electromagnetic fields were numerically calculated in response to metal, substrate, and ambient changes when the distance of two nano-ring apertures varies at an aperture depth and width fixed at 50 and 350 nm. The results indicate that control over fairly broad ranges of focal lengths and intensities can be achieved, which is expected to be useful to implement an optical element for various nanoscale imaging applications.

  14. Multi-excitonic effects on optical spectra of semiconducting carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Watanabe, Kouta; Asano, Kenichi; Ogawa, Tetsuo, E-mail: watanabe@acty.phys.sci.osaka-u.ac.j [Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)

    2009-02-01

    We calculated the wavefunction and the binding energy of the biexciton in semiconducting carbon nanotubes and studied the spectral weights of the photoluminescence spectra and the biexciton contribution to the two-photon absorption spectra. The wavefunction and the binding energy are calculated by means of the the Lanczos method.

  15. Biosensors based on GaN nanoring optical cavities

    Science.gov (United States)

    Kouno, Tetsuya; Takeshima, Hoshi; Kishino, Katsumi; Sakai, Masaru; Hara, Kazuhiko

    2016-05-01

    Biosensors based on GaN nanoring optical cavities were demonstrated using room-temperature photoluminescence measurements. The outer diameter, height, and thickness of the GaN nanorings were approximately 750–800, 900, and 130–180 nm, respectively. The nanorings functioned as whispering-gallery-mode (WGM)-type optical cavities and exhibited sharp resonant peaks like lasing actions. The evanescent component of the WGM was strongly affected by the refractive index of the ambient environment, the type of liquid, and the sucrose concentration of the analyzed solution, resulting in shifts of the resonant wavelengths. The results indicate that the GaN nanorings can potentially be used in sugar sensors of the biosensors.

  16. Direct observation of strain segregation ZnO nanorings using x-ray diffraction

    International Nuclear Information System (INIS)

    A zinc oxide nanoring is transformed through coaxial, uniradius, and epitaxial coiling of a single-crystalline nano-ribbon. The polar charge on ZnO nanoribbon's ±(001) surfaces, resulting from the termination by zinc and oxygen atomic layers, respectively, tends to fold itself if the surface charges are uncompensated during growth. It is believed that the mechanical configuration of such a nanoring reflects a balance among the polar charge, surface area, and elastic deformation. Although the research into functional oxide and one-dimensional nanostructures has rapidly expanded because of their unique and novel applications in optics, optoelectronics, catalysis, and piezoelectricity, the lattice deforms in response to the coiling stress remains largely unknown. We report here x-ray diffraction measurements of individual ZnO nanorings that clearly show the segregation of lattice strains under coiling stresses. Our studies were carried out at the 2-ID-D beamline, a hard x-ray microprobe facility, of the Advanced Photon Source of Argonne National Laboratory. Using advanced zone-plate optics, we focused 10 keV synchrotron x-rays, selected with a double-crystal Si (111) monochromator, into a 150 nm x 150 nm spot with a flux density of 4.5 x 104 photons/sec/nm2/0.01%BW, allowing the structure of an inhomogeneous nanomaterial to be analyzed. Single-crystal nanorings of ZnO were grown by a solid-vapor process from a mixture of ZnO, indium oxide, and lithium carbonate powders at the temperature of 1400 C. The as-synthesized nanorings were transferred onto a silicon nitride membrane coated with a gold mesh of 120 (micro)m/per grid and 1 (micro)m thick. Scanning electron microscopy was used to characterize the lateral sizes of the rings and coordinate them against the mesh. To ensure the measurement on an individual nanoring, a ring is selected to be a specimen only if it is several tens of microns separated from others. The x-ray absorption image of the gold mesh was

  17. Theory of core excitons

    Energy Technology Data Exchange (ETDEWEB)

    Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.

    1980-01-01

    The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)

  18. Theory of core excitons

    International Nuclear Information System (INIS)

    The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material

  19. Dynamic susceptibility of onion in ferromagnetic elliptical nanoring

    Science.gov (United States)

    Mu, Congpu; Song, Jiefang; Xu, Jianghong; Wen, Fusheng

    2016-06-01

    Micromagnetic simulation was performed to investigate the equilibrium state and dynamic susceptibility spectra of magnetic elliptical nanoring. There are two equilibrium states (onion and vortex) obtained in elliptical nanoring. The onion state can be used to record information in MRAM. And it is important to investigate the dynamic susceptibility spectra of onion state, which is closely related to writing and reading speed of magnetic memory devices. Those results show that two or three resonance peaks are found under different thickness of elliptical nanoring with onion state, respectively. The low resonance frequency of two resonance peaks is increasing with the arm width of the elliptical ring, but is decreasing with the thickness. However, the high frequency of two resonance peaks is decreasing with the arm width of the elliptical ring.

  20. An approach to fabrication of metal nanoring arrays.

    Science.gov (United States)

    Bayati, Maryam; Patoka, Piotr; Giersig, Michael; Savinova, Elena R

    2010-03-01

    Fabrication of tailored nanomaterials with desired structure and properties is the greatest challenge of modern nanotechnology. Herein, we describe a wet chemical method for the preparation of large area metal nanoring arrays. This method is based on self-assembly of polystyrene sphere template on a flat substrate and wicking/reducing metal precursor into the interstices between the template and the substrate. In this article, platinum, gold, and copper nanorings were fabricated by applying 505 nm polystyrene spheres onto highly oriented pyrolytic graphite (HOPG) and Si(100) substrates, followed by reducing the templated metal salt with NaBH(4). AFM images reveal formation of arrays of metal nanorings comprising metal nanoparticles with the average ring height of 5.7 +/- 0.8 nm and diameter of 167.3 +/- 8.9 nm. XPS confirms that these structures are metallic. PMID:20104920

  1. Enhanced Magnetoabsorption Oscillations in Semiconductor Nanorings

    Science.gov (United States)

    Citrin, David; Maslov, Alex

    2003-03-01

    We present calculations of the interband optical absorption of semiconductor nanoscale rings including essential excitonic effects. We show that an in-plane electric field can significantly enhance the depth of magnetoabsorption oscillations due to Aharonov-Bohm like effects in the optical properties.

  2. Subwavelength electromagnetic energy transport by stack of metallic nanorings

    Science.gov (United States)

    Jafari, M. R.; Ebrahimi, F.; Nooshirvani, M.

    2010-09-01

    In this paper, we consider linear ordered stack of metallic nanorings in the presence of Aharonov-Bohm magnetic flux for externally tunable electromagnetic energy transport below the diffraction limit. Using random phase approximation, we demonstrate that such structure supports propagating surface plasmon modes with negative group velocities and with magnetic flux dependent frequencies. Our results for dispersion relations, bandwidths, and tunabilities of surface plasmon modes give an explicit demonstration that nanoring based subwavelength waveguides are potential candidate for electromagnetic energy transport below the diffraction limit in the terahertz part of spectrum.

  3. Control of spin-polarised currents in graphene nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Saiz-Bretín, M.; Munárriz, J. [GISC, Departamento de Física de Materiales, Universidad Complutense, E-28040 Madrid (Spain); Malyshev, A.V. [GISC, Departamento de Física de Materiales, Universidad Complutense, E-28040 Madrid (Spain); Ioffe Physical–Technical Institute, St-Petersburg (Russian Federation); Domínguez-Adame, F., E-mail: adame@fis.ucm.es [GISC, Departamento de Física de Materiales, Universidad Complutense, E-28040 Madrid (Spain); Department of Physics, University of Warwick, Coventry, CV4 7AL (United Kingdom)

    2015-09-25

    We study electronic transport in systems comprising square graphene nanorings with a ferromagnetic insulator layer on top of them. The rings are connected symmetrically or asymmetrically to contacts. The proximity exchange interaction of electrons with magnetic ions results in spin-dependent transport properties. When a nanoring is connected asymmetrically, the occurrence of Fano-like antiresonances in the transmission coefficient can induce abrupt changes in the spin polarisation under minute variations of the Fermi energy. We also demonstrate that the spin polarisation can be efficiently controlled by a side-gate voltage. This opens a possibility to use these effects for fabricating tunable sources of polarised electrons.

  4. Micromagnetic simulation of Fe asymmetric nanorings

    International Nuclear Information System (INIS)

    During the last decade several methods to control the vortex chirality in nanodots have been proposed. One of them, the introduction of asymmetry in the geometry of the dots, originates interesting effects on the magnetic behavior of the particle. However, asymmetry in core-free structures is also interesting to investigate because of the reproducibility of their magnetic properties. In this work we report systematic changes in the coercivity and remanence in asymmetric nanorings. The angular dependence is also addressed. For specific geometries and magnetic field direction newly reversal modes appear associated with important changes in the coercivity and remanence of the rings. - Highlights: → We report that the existence of asymmetry strongly influences the coercivity and the remanence. → Magnetization reversal is driven by the nucleation of a C state and propagation of a vortex state. → We also conclude that the lack of a core contributes to the stability of the vortex state. → Asymmetry can be useful for tailoring specific magnetic characteristics of these systems.

  5. Exciton laser rate equations

    OpenAIRE

    Garkavenko A. S.

    2011-01-01

    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.

  6. Excitons in Electrostatic Traps

    OpenAIRE

    Hammack, A. T.; Gippius, N. A.; Andreev, G. O.; Butov, L. V.; Hanson, M.; Gossard, A. C.

    2005-01-01

    We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively confine excitons in the electrostatic traps. We present calculations for various classes of electrostatic ...

  7. Electric field geometries dominate quantum transport coupling in silicon nanoring

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Tsung-Han, E-mail: askaleeg@gmail.com, E-mail: sfhu.hu@gmail.com; Hu, Shu-Fen, E-mail: askaleeg@gmail.com, E-mail: sfhu.hu@gmail.com [Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan (China)

    2014-03-28

    Investigations on the relation between the geometries of silicon nanodevices and the quantum phenomenon they exhibit, such as the Aharonov–Bohm (AB) effect and the Coulomb blockade, were conducted. An arsenic doped silicon nanoring coupled with a nanowire by electron beam lithography was fabricated. At 1.47 K, Coulomb blockade oscillations were observed under modulation from the top gate voltage, and a periodic AB oscillation of ΔB = 0.178 T was estimated for a ring radius of 86 nm under a high sweeping magnetic field. Modulating the flat top gate and the pointed side gate was performed to cluster and separate the many electron quantum dots, which demonstrated that quantum confinement and interference effects coexisted in the doped silicon nanoring.

  8. Aufgerollte Nanoröhren auf III-V-Halbleiterbasis

    OpenAIRE

    Deneke, Christoph

    2005-01-01

    Diese Arbeit beschäftigt sich mit der Herstellung, Charakterisierung und Funktionalisierung von dreidimensionalen Mikro- und Nanoobjekten aus zweidimensionalen Schichten. So werden Bänder, Ringe und freistehende Membranen aus verspannten Bischichten hergestellt, die durch einen selektiven Ätzprozeß von ihrem Substrat abgelöst werden. Weiter wird eine Reihe von selbstorganisierenden, halbleiter-basierten Nanoröhren (RUNT) gezeigt, die durch Aufrollen solcher verspannten Bischichten entstehen....

  9. Confined exciton spectroscopy

    International Nuclear Information System (INIS)

    Full text: In this work, the exciton is considered as a sensor of the electronic and optical properties of materials such as semiconductors, which have size compared to the exciton De Broglie wavelength, approximately 20 nm, depending on the semiconductor. Examples of electron-phonon, electron-electron, photon-electron, exciton-polariton, phonon-plasmon, are presented, under different confinement conditions such as quantum wells, superlattices

  10. Multiple exciton generation induced enhancement of the photoresponse of pulsed-laser-ablation synthesized single-wall-carbon-nanotube/PbS-quantum-dots nanohybrids

    Science.gov (United States)

    Ka, Ibrahima; Le Borgne, Vincent; Fujisawa, Kazunori; Hayashi, Takuya; Kim, Yoong Ahm; Endo, Morinobu; Ma, Dongling; El Khakani, My Ali

    2016-02-01

    The pulsed laser deposition method was used to decorate appropriately single wall carbon nanotubes (SWCNTs) with PbS quantum dots (QDs), leading to the formation of a novel class of SWCNTs/PbS-QDs nanohybrids (NHs), without resorting to any ligand engineering and/or surface functionalization. The number of laser ablation pulses (NLp) was used to control the average size of the PbS-QDs and their coverage on the SWCNTs’ surface. Photoconductive (PC) devices fabricated from these SWCNTs/PbS-QDs NHs have shown a significantly enhanced photoresponse, which is found to be PbS-QD size dependent. Wavelength-resolved photocurrent measurements revealed a strong photoconductivity of the NHs in the UV-visible region, which is shown to be due to multiple exciton generation (MEG) in the PbS-QDs. For the 6.5 nm-diameter PbS-QDs (with a bandgap (Eg) = 0.86 eV), the MEG contribution of the NHs based PC devices was shown to lead to a normalized internal quantum efficiency in excess of 300% for photon energies ≥4.5Eg. While the lowest MEG threshold in our NHs based PC devices is found to be of ~2.5Eg, the MEG efficiency reaches values as high as 0.9 ± 0.1.

  11. Multiple exciton generation induced enhancement of the photoresponse of pulsed-laser-ablation synthesized single-wall-carbon-nanotube/PbS-quantum-dots nanohybrids

    Science.gov (United States)

    Ka, Ibrahima; Le Borgne, Vincent; Fujisawa, Kazunori; Hayashi, Takuya; Kim, Yoong Ahm; Endo, Morinobu; Ma, Dongling; El Khakani, My Ali

    2016-01-01

    The pulsed laser deposition method was used to decorate appropriately single wall carbon nanotubes (SWCNTs) with PbS quantum dots (QDs), leading to the formation of a novel class of SWCNTs/PbS-QDs nanohybrids (NHs), without resorting to any ligand engineering and/or surface functionalization. The number of laser ablation pulses (NLp) was used to control the average size of the PbS-QDs and their coverage on the SWCNTs’ surface. Photoconductive (PC) devices fabricated from these SWCNTs/PbS-QDs NHs have shown a significantly enhanced photoresponse, which is found to be PbS-QD size dependent. Wavelength-resolved photocurrent measurements revealed a strong photoconductivity of the NHs in the UV-visible region, which is shown to be due to multiple exciton generation (MEG) in the PbS-QDs. For the 6.5 nm-diameter PbS-QDs (with a bandgap (Eg) = 0.86 eV), the MEG contribution of the NHs based PC devices was shown to lead to a normalized internal quantum efficiency in excess of 300% for photon energies ≥4.5Eg. While the lowest MEG threshold in our NHs based PC devices is found to be of ~2.5Eg, the MEG efficiency reaches values as high as 0.9 ± 0.1. PMID:26830452

  12. Multiple exciton generation induced enhancement of the photoresponse of pulsed-laser-ablation synthesized single-wall-carbon-nanotube/PbS-quantum-dots nanohybrids.

    Science.gov (United States)

    Ka, Ibrahima; Le Borgne, Vincent; Fujisawa, Kazunori; Hayashi, Takuya; Kim, Yoong Ahm; Endo, Morinobu; Ma, Dongling; El Khakani, My Ali

    2016-01-01

    The pulsed laser deposition method was used to decorate appropriately single wall carbon nanotubes (SWCNTs) with PbS quantum dots (QDs), leading to the formation of a novel class of SWCNTs/PbS-QDs nanohybrids (NHs), without resorting to any ligand engineering and/or surface functionalization. The number of laser ablation pulses (NLp) was used to control the average size of the PbS-QDs and their coverage on the SWCNTs' surface. Photoconductive (PC) devices fabricated from these SWCNTs/PbS-QDs NHs have shown a significantly enhanced photoresponse, which is found to be PbS-QD size dependent. Wavelength-resolved photocurrent measurements revealed a strong photoconductivity of the NHs in the UV-visible region, which is shown to be due to multiple exciton generation (MEG) in the PbS-QDs. For the 6.5 nm-diameter PbS-QDs (with a bandgap (Eg) = 0.86 eV), the MEG contribution of the NHs based PC devices was shown to lead to a normalized internal quantum efficiency in excess of 300% for photon energies ≥4.5Eg. While the lowest MEG threshold in our NHs based PC devices is found to be of ~2.5Eg, the MEG efficiency reaches values as high as 0.9 ± 0.1. PMID:26830452

  13. Impurity effects on energy levels and far-infrared spectra of nanorings

    Science.gov (United States)

    Hui, Pan; Jia-Lin, Zhu

    2003-11-01

    The effects of a positively charged impurity on the energy levels and far-infrared spectra of one and two electrons in semiconductor nanorings under magnetic fields are studied. The effects of the nanoring size and the impurity position are also discussed. It is shown that the electron-electron interaction and electron-impurity one in nanorings are strongly dependent on the nanoring size and the impurity position. Based on the studies of the impurity and field effects, the impurity-induced Aharonov-Bohm oscillations of the far-infrared spectra are found. The results predict a possibility of observing phenomena related to electron-impurity interaction in a nanoring in the future.

  14. Impurity effects on energy levels and far-infrared spectra of nanorings

    International Nuclear Information System (INIS)

    The effects of a positively charged impurity on the energy levels and far-infrared spectra of one and two electrons in semiconductor nanorings under magnetic fields are studied. The effects of the nanoring size and the impurity position are also discussed. It is shown that the electron-electron interaction and electron-impurity one in nanorings are strongly dependent on the nanoring size and the impurity position. Based on the studies of the impurity and field effects, the impurity-induced Aharonov-Bohm oscillations of the far-infrared spectra are found. The results predict a possibility of observing phenomena related to electron-impurity interaction in a nanoring in the future

  15. Impurity effects on energy levels and far-infrared spectra of nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Pan Hui; Zhu Jialin [Department of Physics, Tsinghua University, Beijing 100084 (China)

    2003-11-05

    The effects of a positively charged impurity on the energy levels and far-infrared spectra of one and two electrons in semiconductor nanorings under magnetic fields are studied. The effects of the nanoring size and the impurity position are also discussed. It is shown that the electron-electron interaction and electron-impurity one in nanorings are strongly dependent on the nanoring size and the impurity position. Based on the studies of the impurity and field effects, the impurity-induced Aharonov-Bohm oscillations of the far-infrared spectra are found. The results predict a possibility of observing phenomena related to electron-impurity interaction in a nanoring in the future.

  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. Exciton-polariton laser

    Science.gov (United States)

    Moskalenko, S. A.; Tiginyanu, I. M.

    2016-05-01

    We present a review of the investigations realized in the last decades of the phenomenon of the Bose-Einstein condensation (BEC) in the system of two-dimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity exciton-polariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of exciton-polaritons in semiconductor microcavities is related to the ultra-low threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.

  18. The role of dipolar interactions for the magnetic properties of ferromagnetic nanoring

    International Nuclear Information System (INIS)

    We investigate numerically the effects of the dipolar interactions on magnetic properties in small ferromagnetic nanorings using a Monte Carlo technique. Our simulated results show that the strength of dipolar interaction in the magnetic nanoring has an important influence on the magnetization reversal processes and further the coercivity and the remanence. As the dipolar interaction increases, the transition of magnetization reversal processes from the onion-rotation state to the vortex state can occur, which results in an increase in coercivity and a decrease in remanence. On the other hand, it is found that the coercivity and the remanence depend more strongly on the strength of dipolar coupling for the relatively small size nanoring than for the large size nanoring in width. This can be attributed to the stable vortex state without core in smaller width nanoring in contrast to the metastable vortex state with core in larger width nanoring, induced by strong dipolar interactions. Additionally, the temperature dependence of coercivity and remanence in magnetic nanoring is also studied at a fixed dipolar interaction.

  19. Coherent phonon coupled with exciton in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser

    Energy Technology Data Exchange (ETDEWEB)

    Kobayashi, Takayoshi, E-mail: kobayashi@ils.uec.ac.jp [Advanced Ultrafast Laser Research Center and Department of Engineering Science, Faculty of Informatics and Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585 (Japan); JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075 (Japan); Department of Electrophysics, National Chiao-Tung University, Hsinchu 30010, Taiwan (China); Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871 (Japan); Nie, Zhaogang; Du, Juan; Xue, Bing [Advanced Ultrafast Laser Research Center and Department of Engineering Science, Faculty of Informatics and Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585 (Japan); JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075 (Japan)

    2014-08-01

    The vibrational wavepackets dynamics of single-walled carbon nanotubes (SWCNTs) are studied through the modulation of the transition probability in the visible spectral range of the systems. The modulations corresponding to the radial breathing mode (RBM), observed in the time traces for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed. The vibrational modes of the coherent phonon spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. The present study pointed out that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to derivative type dependence of the absorbed photon energy spectrum due to molecular-phase modulation, Raman (and Raman-like) gain and loss processes, and molecular phase modulation, respectively. - Highlights: • Vibrational dynamics are studied through the modulation of transition probability. • Probe λ dependence of amplitude is due to complex third-order susceptibility. • Coherent phonon dynamics are induced by Raman loss and gain. • Molecular phase modulation by vibration introduces a periodical shift of spectrum.

  20. Exciton Lifetime Paradoxically Enhanced by Dissipation and Decoherence: Toward Efficient Energy Conversion of a Solar Cell

    Science.gov (United States)

    Yamada, Yasuhiro; Yamaji, Youhei; Imada, Masatoshi

    2015-11-01

    Energy dissipation and decoherence are at first glance harmful to acquiring the long exciton lifetime desired for efficient photovoltaics. In the presence of both optically forbidden (namely, dark) and allowed (bright) excitons, however, they can be instrumental, as suggested in photosynthesis. By simulating the quantum dynamics of exciton relaxations, we show that the optimized decoherence that imposes a quantum-to-classical crossover with the dissipation realizes a dramatically longer lifetime. In an example of a carbon nanotube, the exciton lifetime increases by nearly 2 orders of magnitude when the crossover triggers a stable high population in the dark excitons.

  1. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.; Spiegelberg, C.; Vadim, Lyssenko; Stanton, C.; Hvam, Jørn Märcher; Timofeev, V. B.; Bayer, M.

    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 ...

  2. Multiscale photosynthetic exciton transfer

    CERN Document Server

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

    2012-01-01

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

  3. Excitonic surface lattice resonances

    Science.gov (United States)

    Humphrey, A. D.; Gentile, M. J.; Barnes, W. L.

    2016-08-01

    Electromagnetic resonances are important in controlling light at the nanoscale. The most studied such resonance is the surface plasmon resonance that is associated with metallic nanostructures. Here we explore an alternative resonance, the surface exciton-polariton resonance, one based on excitonic molecular materials. Our study is based on analytical and numerical modelling. We show that periodic arrays of suitable molecular nanoparticles may support surface lattice resonances that arise as a result of coherent interactions between the particles. Our results demonstrate that excitonic molecular materials are an interesting alternative to metals for nanophotonics; they offer the prospect of both fabrication based on supramolecular chemistry and optical functionality arising from the way the properties of such materials may be controlled with light.

  4. Machine Learning Exciton Dynamics

    CERN Document Server

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

    2015-01-01

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

  5. Confined electronic states and their modulations in graphene nanorings

    Science.gov (United States)

    Zhu, Jia-Lin; Wang, Xingyuan; Yang, Ning

    2012-09-01

    Confined electronic states in quantum rings formed by spatially modulated finite Dirac gap (FDGQR) in graphene are systematically studied by series-expansion method, and are compared with those in infinite-mass-boundary and one-dimensional quantum rings. The shape-size effect of FDGQR is illustrated to be distinct from that in graphene quantum dots. The Aharonov-Bohm effect in FDGQR is clearly shown by the energy spectrum and the optical-transition probabilities. The FDGQR coupled with the electrostatic-potential induced nanoring is found useful for modulating the Dirac electronic states and the optical-transition probabilities. These results may help us to understand and to control the quantum behaviors of confined electronic states in graphene.

  6. Polarization dependent switching of asymmetric nanorings with a circular field

    Directory of Open Access Journals (Sweden)

    Nihar R. Pradhan

    2016-01-01

    Full Text Available We experimentally investigated the switching from onion to vortex states in asymmetric cobalt nanorings by an applied circular field. An in-plane field is applied along the symmetric or asymmetric axis of the ring to establish domain walls (DWs with symmetric or asymmetric polarization. A circular field is then applied to switch from the onion state to the vortex state, moving the DWs in the process. The asymmetry of the ring leads to different switching fields depending on the location of the DWs and direction of applied field. For polarization along the asymmetric axis, the field required to move the DWs to the narrow side of the ring is smaller than the field required to move the DWs to the larger side of the ring. For polarization along the symmetric axis, establishing one DW in the narrow side and one on the wide side, the field required to switch to the vortex state is an intermediate value.

  7. Formation of In2O3 nanorings on Si substrates

    Science.gov (United States)

    Hsin, C. L.; Yu, S. Y.; Huang, C. W.; Wu, W. W.

    2010-11-01

    A new approach to form the In2O3 nanorings (NRs) has been proven by tailoring the difference between property of metal and metal oxide. The formation process of the In2O3 NRs is proposed to be resulted form a subtle competition between the oxidation and evaporation of indium at the rim and center, respectively. Patterned In2O3 NRs have been grown on (001) Si substrates in combination with nanosphere lithography. The size and morphology of the NRs can be controlled by the size of polystyrene nanospheres and the thickness of indium layer. The optical property measurements showed that the In2O3 NRs are sensitive in absorption and emission of light between 600 and 622 nm in wavelength. The patterned In2O3 NRs on silicon are advantageous for fabricating optical-response photonic devices at the desired locations and direct integration to the silicon-based photonic devices with current processing technology.

  8. Quasienergy Spectroscopy of Excitons

    DEFF Research Database (Denmark)

    Johnsen, Kristinn; Jauho, Antti-Pekka

    1999-01-01

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

  9. Triplet exciton dynamics

    International Nuclear Information System (INIS)

    Results are presented of electron spin echo experiments combined with laser flash excitation on triplet states of aromatic molecules. Some of the theoretical and experimental aspects of the photoexcited triplet state are discussed in detail and the electron spin echo spectrometers and laser systems are described. All the experiments described in this thesis were performed at liquid helium temperatures. An account is given of the ESE experiments performed on the photoexcited, non-radiative, triplet state of pentacene in napthalene. This is an example of the ESE technique being used to ascertain the zero-field splitting parameters, the populating and depopulating rates, and the orientation of the pentacene molecules in the naphthalene host. A combination of high resolution laser flash excitation and electron-spin echoes in zero-magnetic field allowed the author to observe directly k(vector)→k(vector)' exciton scattering processes in the one-dimensional triplet excitons in tetrachlorobenzene for the first time. Additional experimental data about exciton scattering is provided and a study of the orientational dependence of the spin-lattice relaxation of the triplet excitons in an external magnetic field is described. (Auth.)

  10. Exciton solitary waves (exolitons)

    International Nuclear Information System (INIS)

    The soliton theory is briefly explained with regard to cooperative phenomena in one-dimensional systems. The study of the dynamics of a one-dimensional lattice shows that nonlinear phonon interaction results in the production of a solitary wave. The procedure is indicated for the mathematical solution of the problem of the exciton-phonon interaction. (M.S.)

  11. Electronic energy levels of nanorings with impurities and Aharonov-Bohm effects

    Science.gov (United States)

    Ramírez, S.

    2011-10-01

    By modeling impurities along a nanoring as general potential forms the Schrödinger equation for ballistic electrons is shown to separate in cylindrical coordinates. We find an analytical eigenvalue equation for N delta-function-barrier impurities in the presence of magnetic flux. Previous calculations of the electronic states of a one-dimensional (1D) and two-dimensional (2D) nanoring for only one or two impurities modeled by equal square barriers is explicitly extended to three and four different or equal impurities modeled as delta-barrier, square-barrier, or delta-well potential forms. This is shown to be generalizable to any number N. Effects on the energy spectra due to magnetic flux and different kinds and numbers of impurities are compared in 1D and 2D nanorings.

  12. Effects of lateral asymmetry on electronic structure of strained semiconductor nanorings in a magnetic field

    Science.gov (United States)

    Milošević, M. M.; Tadić, M.; Peeters, F. M.

    2008-11-01

    The influence of lateral asymmetry on the electronic structure and optical transitions in elliptical strained InAs nanorings is analyzed in the presence of a perpendicular magnetic field. Two-dimensional rings are assumed to have elliptical inner and outer boundaries oriented in mutually orthogonal directions. The influence of the eccentricity of the ring on the energy levels is analyzed. For large eccentricity of the ring, we do not find any Aharonov-Bohm effect, in contrast to circular rings. Rather, the single-particle states of the electrons and the holes are localized as in two laterally coupled quantum dots formed in the lobes of the nanoring. Our work indicates that the control of shape is important for the existence of the Aharonov-Bohm effect in semiconductor nanorings.

  13. Effects of lateral asymmetry on electronic structure of strained semiconductor nanorings in a magnetic field

    International Nuclear Information System (INIS)

    The influence of lateral asymmetry on the electronic structure and optical transitions in elliptical strained InAs nanorings is analyzed in the presence of a perpendicular magnetic field. Two-dimensional rings are assumed to have elliptical inner and outer boundaries oriented in mutually orthogonal directions. The influence of the eccentricity of the ring on the energy levels is analyzed. For large eccentricity of the ring, we do not find any Aharonov-Bohm effect, in contrast to circular rings. Rather, the single-particle states of the electrons and the holes are localized as in two laterally coupled quantum dots formed in the lobes of the nanoring. Our work indicates that the control of shape is important for the existence of the Aharonov-Bohm effect in semiconductor nanorings.

  14. Exciton-polariton mediated superconductivity

    OpenAIRE

    Laussy, Fabrice P.; Kavokin, Alexey V; Shelykh, Ivan A

    2010-01-01

    We revisit the exciton mechanism of superconductivity in the framework of microcavity physics, replacing virtual excitons as a binding agent of Cooper pairs by excitations of an exciton-polariton Bose-Einstein condensate. We consider a model microcavity where a quantum well with a two-dimensional electron gas is sandwiched between two undoped quantum wells, where a polariton condensate is formed. We show that the critical temperature for superconductivity dramatically increases with the conde...

  15. Enhancing second harmonic generation in gold nanoring resonators filled with lithium niobate.

    Science.gov (United States)

    Lehr, Dennis; Reinhold, Jörg; Thiele, Illia; Hartung, Holger; Dietrich, Kay; Menzel, Christoph; Pertsch, Thomas; Kley, Ernst-B; Tünnermann, Andreas

    2015-02-11

    Plasmonic nanorings provide the unique advantage of a pronounced plasmonic field enhancement inside their core. If filled with a polarizable medium, it may significantly enhance its optical effects. Here, we demonstrate this proposition by filling gold nanorings with lithium niobate. The generated second harmonic signal is compared to the signal originating from an unpatterned lithium niobate surface. Measurements and simulation confirm an enhancement of about 20. Applications requiring nanoscopic localized light sources like fluorescence spectroscopy or quantum communication will benefit from our findings. PMID:25584636

  16. Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

    DEFF Research Database (Denmark)

    Jia, Chun-Jiang; Sun, Ling-Dong; Luo, Feng; Han, Xiao-Dong; Heyderman, Laura J.; Yan, Zheng-Guang; Yan, Chun-Hua; Zheng, Kun; Zhang, Ze; Takano, Mikio; Hayashi, Naoaki; Eltschka, Matthias; Klaeui, Mathias; Ruediger, Ulrich; Kasama, Takeshi; Gontard, Lionel Cervera; Dunin-Borkowski, Rafal E.; Tzvetkov, George; Raabe, Joerg

    2008-01-01

    able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures...... and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings...

  17. Excitons in asymmetric quantum wells

    Science.gov (United States)

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

    2016-09-01

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

  18. Highly tunable plasmonic nanoring arrays for nanoparticle manipulation and detection

    Science.gov (United States)

    Sergides, M.; Truong, V. G.; Chormaic, S. Nic

    2016-09-01

    The advancement of trapping and detection of nano-objects at very low laser powers in the near-infra-red region (NIR) is crucial for many applications. Singular visible-light nano-optics based on abrupt phase changes have recently demonstrated a significant improvement in molecule detection. Here, we propose and demonstrate tunable plasmonic nanodevices, which can improve both the trapping field enhancement and detection of nano-objects using singular phase drops in the NIR range. The plasmonic nanostructures, which consist of gaps with dimensions 50 nm × 50 nm connecting nanorings in arrays is discussed. These gaps act as individual detection and trapping sites. The tunability of the system is evident from extinction and reflection spectra while increasing the aperture size in the arrays. Additionally, in the region where the plasmonic nano-array exhibits topologically-protected, near-zero reflection behaviour, the phase displays a rapid change. Our experimental data predict that, using this abrupt phase changes, one can improve the detection sensitivity by 10 times compared to the extinction spectra method. We finally report experimental evidence of 100 nm polystyrene beads trapping using low incident power on these devices. The overall design demonstrates strong capability as an optical, label-free, non-destructive tool for single molecule manipulation where low trapping intensity, minimal photo bleaching and high sensitivity is required.

  19. Aharonov-Bohm oscillation modes in double-barrier nanorings

    Science.gov (United States)

    Zhu, Jia-Lin; Yu, Xiquan; Dai, Zhensheng; Hu, Xiao

    2003-02-01

    The energy spectrum and Aharonov-Bohm (AB) effect in a two-dimensional nanoring interrupted by two identical barriers are studied, and a way of labeling a state according to the node numbers of the wave function in the absence of magnetic flux is introduced. It is found that a magnetic flux φ can modify both the phase and amplitude of wave functions due to the presence of the barriers. AB oscillations are strongly affected by the double barriers, and there are two modes of strong AB oscillations, named O and X modes. The energy levels of O and X modes are occasionally degenerate at φ=0 and 0.5, respectively, and the corresponding wave functions of both degenerate states are localized and can be greatly modified by a small change of φ. The O mode of AB oscillations, which does not exist in the parallel double-barrier ring usually used in experiments, presents an interesting picture and suggests other related phenomena.

  20. Aharonov-Bohm oscillation modes in double-barrier nanorings

    International Nuclear Information System (INIS)

    The energy spectrum and Aharonov-Bohm (AB) effect in a two-dimensional nanoring interrupted by two identical barriers are studied, and a way of labeling a state according to the node numbers of the wave function in the absence of magnetic flux is introduced. It is found that a magnetic flux φ can modify both the phase and amplitude of wave functions due to the presence of the barriers. AB oscillations are strongly affected by the double barriers, and there are two modes of strong AB oscillations, named O and X modes. The energy levels of O and X modes are occasionally degenerate at φ=0 and 0.5, respectively, and the corresponding wave functions of both degenerate states are localized and can be greatly modified by a small change of φ. The O mode of AB oscillations, which does not exist in the parallel double-barrier ring usually used in experiments, presents an interesting picture and suggests other related phenomena

  1. Wafer-scale arrays of epitaxial ferroelectric nanodiscs and nanorings

    International Nuclear Information System (INIS)

    Wafer-scale arrays of well-ordered Pb(Zr0.2Ti0.8)O3 nanodiscs and nanorings were fabricated on the entire area (10 mm x 10 mm) of the SrRuO3 bottom electrode on an SrTiO3 single-crystal substrate using the laser interference lithography (LIL) process combined with pulsed laser deposition. The shape and size of the nanostructures were controlled by the amount of PZT deposited through the patterned holes and the temperature of the post-crystallization steps. X-ray diffraction and transmission electron microscopy confirmed that (001)-oriented PZT nanostructures were grown epitaxially on the SrRuO3(001) bottom electrode layer covering the (001)-oriented single-crystal substrate. The domain structures of PZT nano-islands were characterized by reciprocal space mapping using synchrotron x-ray radiation. Ferroelectric properties of each PZT nanostructure were characterized by scanning force microscopy in the piezoresponse mode.

  2. Exciton Transport in Organic Semiconductors

    Science.gov (United States)

    Menke, Stephen Matthew

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

  3. 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...

  4. Excitons in semiconductor nano structures

    International Nuclear Information System (INIS)

    A formation of excitons is studied in semiconductor nano structures where electrons and holes are spatially separated by a potential barrier. The disorder present within electron-hole interface is due to structural imperfections which are unavoidable in the course of fabrication. The exciton density is calculated as a function of the disorder and of the hopping integral value. (Author)

  5. Excitonic condensation in bilayer systems

    Science.gov (United States)

    Su, Jung-Jung

    Among the many examples of Bose condensation considered in physics, electron-hole-pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because of controversy about condensate properties. In this thesis, we studied the various aspects of spontaneous symmetry broken state of exciton in bilayer using mean field theory. We calculated the photoluminescence of excitonic condensation created by laser. We developed a one-dimensional toy model of excitonic supercurrent using mean field theory plus non-equilibrium Green's function (NEGF) which give qualitatively consistent results with experiments. We proposed graphene bilayer as a novel system for excitonic condensation to occur and estimate it to exist even at temperature as high as room temperature.

  6. Eco-friendly synthesis of colloidal silver nanospheres, nanorings and nanonetworks

    NARCIS (Netherlands)

    Singh, A.K.; Rai, A.K.; Bicanic, D.D.

    2009-01-01

    Colloidal silver nanospheres, nanorings, and nanonetworks were synthesized by the nanosecond pulsed laser ablation of a silver metal plate in a pure distilled water (at room temperature) using the fundamental (1064 nm), second harmonic (532 nm), and third harmonic (355 nm) wavelengths of the Nd:YAG

  7. Energy levels and far-infrared spectra of oval-shaped nanorings

    Science.gov (United States)

    Gutiérrez, W.; García, L. F.; Mikhailov, I. D.

    2014-05-01

    The evolution of the Aharonov-Bohm oscillation of low-lying states and far infrared spectrum associated to variation of the path curvature for electron motion along nanorings with centerlines in a form of a set of Cassini ovals, whose shape is changed continuously from a single elongated loop to two separated loops is theoretically investigated.

  8. Energy levels and far-infrared spectra of oval-shaped nanorings

    International Nuclear Information System (INIS)

    The evolution of the Aharonov-Bohm oscillation of low-lying states and far infrared spectrum associated to variation of the path curvature for electron motion along nanorings with centerlines in a form of a set of Cassini ovals, whose shape is changed continuously from a single elongated loop to two separated loops is theoretically investigated

  9. Energy levels and far-infrared spectra of oval-shaped nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Gutiérrez, W.; García, L. F.; Mikhailov, I. D. [Escuela de Física, Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

    2014-05-15

    The evolution of the Aharonov-Bohm oscillation of low-lying states and far infrared spectrum associated to variation of the path curvature for electron motion along nanorings with centerlines in a form of a set of Cassini ovals, whose shape is changed continuously from a single elongated loop to two separated loops is theoretically investigated.

  10. Facile Hydrothermal Synthesis of Fe3O4/C Core-Shell Nanorings for Efficient Low-Frequency Microwave Absorption.

    Science.gov (United States)

    Wu, Tong; Liu, Yun; Zeng, Xiang; Cui, Tingting; Zhao, Yanting; Li, Yana; Tong, Guoxiu

    2016-03-23

    Using elliptical iron glycolate nanosheets as precursors, elliptical Fe3O4/C core-shell nanorings (NRs) [25 ± 10 nm in wall thickness, 150 ± 40 nm in length, and 1.6 ± 0.3 in long/short axis ratio] are synthesized via a one-pot hydrothermal route. The surface-poly(vinylpyrrolidone) (PVP)-protected-glucose reduction/carbonization/Ostwald ripening mechanism is responsible for Fe3O4/C NR formation. Increasing the glucose/precursor molar ratio can enhance carbon contents, causing a linear decrease in saturation magnetization (Ms) and coercivity (Hc). The Fe3O4/C NRs reveal enhanced low-frequency microwave absorption because of improvements to their permittivity and impedance matching. A maximum RL value of -55.68 dB at 3.44 GHz is achieved by Fe3O4/C NRs with 11.95 wt % C content at a volume fraction of 17 vol %. Reflection loss (RL) values (≤-20 dB) are observed at 2.11-10.99 and 16.5-17.26 GHz. Our research provides insights into the microwave absorption mechanism of elliptical Fe3O4/C core-shell NRs. Findings indicate that ring-like and core-shell nanostructures are promising structures for devising new and effective microwave absorbers. PMID:26915716

  11. Design und Optimierung organischer Nanoröhrchen (für die potentielle Anwendung in kostengünstigen organischen Photovoltaik-Bauteilen)

    OpenAIRE

    Rastedt, Maren

    2013-01-01

    Organische Nanoröhrchen wurden mit Hilfe des Templat-Benetzungsverfahren aus unterschiedlichsten Verbindungsklassen hergestellt, optimiert und auf ihre Eigenschaften hin untersucht. Neben Nanoröhrchen aus Tbf-Derivaten, die zu Wellenleitung und Fluoreszenz befähigt sind, wurden auch Nanoröhrchen aus Farbstoffen sowie aus Komponenten klassischer BHJ-Solarzellen hergestellt. Für die letztgenannte Gruppe wurde ein Verfahren entwickelt, mit dem es möglich ist, auch Core-Shell-Nanoröhrchen herzust...

  12. Including excitons in semiconductor solar cell modelling

    OpenAIRE

    Burgelman, Marc; Minnaert, Ben

    2005-01-01

    Excitons are marginally important in classical semiconductor device physics, and their treatment is not included in standard solar cell modelling. However, in organic semiconductors and solar cells, the role of excitons is essential, as the primary effect of light absorption is exciton generation, and free electrons and holes are created by exciton dissociation. First steps to include excitons in solar cell modelling were presented by Green 1996 and Zhang 1998. Their model was restricted to a...

  13. Onset of exciton-exciton annihilation in single layer black phosphorus

    OpenAIRE

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Klopotowski, L.; Tabis, W.; Vignolle, B.; Maude, D. K.; Plochocka, P.

    2016-01-01

    The exciton dynamics in monolayer black phosphorus is investigated over a very wide range of photoexcited exciton densities using time resolved photoluminescence. At low excitation densities, the exciton dynamics is successfully described in terms of a double exponential decay. With increasing exciton population, a fast, non-exponential component develops as exciton-exciton annihilation takes over as the dominant recombination mechanism under high excitation conditions. Our results identify a...

  14. Sensitivity and tunability of heptamer clusters composed of asymmetric split nanorings

    International Nuclear Information System (INIS)

    We investigate the multiple Fano resonances in plasmonic heptamer clusters comprising of asymmetric nanorings (H-ANR) with angular split gaps by FDTD simulations. Evolution of the extinction spectra is demonstrated for various angular gaps. By modifying the angular gaps, we show large tunability of the Fano resonances on both the frequency position and the modulation depth. Sensing properties of the H-ANR are studied by the shift of Fano resonances in different dielectric environments or with different thicknesses of p-mercaptoaniline (pMA) thin films. As for the robustness of the H-ANR, the Fano resonances may be affected by the displacement of the center nanoring, depending on the displacement direction. Our study shows that the H-ANR structure has high tunability and is a good candidate for optical sensing. (paper)

  15. Radiation effects from first principles : the role of excitons in electronic-excited processes.

    Energy Technology Data Exchange (ETDEWEB)

    Wong, Bryan Matthew

    2009-09-01

    Electron-hole pairs, or excitons, are created within materials upon optical excitation or irradiation with X-rays/charged particles. The ability to control and predict the role of excitons in these energetically-induced processes would have a tremendous impact on understanding the effects of radiation on materials. In this report, the excitonic effects in large cycloparaphenylene carbon structures are investigated using various first-principles methods. These structures are particularly interesting since they allow a study of size-scaling properties of excitons in a prototypical semi-conducting material. In order to understand these properties, electron-hole transition density matrices and exciton binding energies were analyzed as a function of size. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in these structures. Based on overall trends in exciton binding energies and their spatial delocalization, we find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these systems.

  16. 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.

  17. Aharonov-Bohm oscillations in disordered nanorings with quantum dots: Effect of electron-electron interactions

    Science.gov (United States)

    Semenov, Andrew G.; Zaikin, Andrei D.

    2010-01-01

    We investigate the effect of electron-electron interactions on Aharonov-Bohm (AB) current oscillations in nanorings formed by a chain of metallic quantum dots. We demonstrate that electron-electron interactions cause electron dephasing thereby suppressing the amplitude of AB oscillations at all temperatures down to T=0. The crossover between thermal and quantum dephasing is found to be controlled by the ring perimeter. Our predictions can be directly tested in future experiments.

  18. Aharonov-Bohm oscillations in disordered nanorings with quantum dots: Effect of electron-electron interactions

    OpenAIRE

    Semenov, Andrew G.; Zaikin, Andrei D.

    2009-01-01

    We investigate the effect of electron-electron interactions on Aharonov-Bohm (AB) current oscillations in nanorings formed by a chain of metallic quantum dots. We demonstrate that electron-electron interactions cause electron dephasing thereby suppressing the amplitude of AB oscillations at all temperatures down to T=0. The crossover between thermal and quantum dephasing is found to be controlled by the ring perimeter. Our predictions can be directly tested in future experiments.

  19. Highly ordered protein nanorings designed by accurate control of glutathione S-transferase self-assembly.

    Science.gov (United States)

    Bai, Yushi; Luo, Quan; Zhang, Wei; Miao, Lu; Xu, Jiayun; Li, Hongbin; Liu, Junqiu

    2013-07-31

    Protein self-assembly into exquisite, complex, yet highly ordered architectures represents the supreme wisdom of nature. However, precise manipulation of protein self-assembly behavior in vitro is a great challenge. Here we report that by taking advantage of the cooperation of metal-ion-chelating interactions and nonspecific protein-protein interactions, we achieved accurate control of the orientation of proteins and their self-assembly into protein nanorings. As a building block, we utilized the C2-symmetric protein sjGST-2His, a variant of glutathione S-transferase from Schistosoma japonicum having two properly oriented His metal-chelating sites on the surface. Through synergic metal-coordination and non-covalent interactions, sjGST-2His self-assembled in a fixed bending manner to form highly ordered protein nanorings. The diameters of the nanorings can be regulated by tuning the strength of the non-covalent interaction network between sjGST-2His interfaces through variation of the ionic strength of the solution. This work provides a de novo design strategy that can be applied in the construction of novel protein superstructures. PMID:23865524

  20. Nanorings and nanocrescents formed via shaped nanosphere lithography: a route toward large areas of infrared metamaterials

    International Nuclear Information System (INIS)

    This paper presents a new approach to nanosphere lithography, which overcomes undesirable manufacturing issues such as complex tilted-rotary evaporation and ion beam milling. A key innovation in this process is the use of non-conductive edge strips placed on top of the samples prior to metal removal. Such elements help to direct the flow of reactive ions during plasma etching and produce well-ordered arrays of metallic nanorings and nanocrescents over large areas of ∼1 cm2. The obtained highly uniform nanocrescent array exhibits an electric resonance of 1.7 μm and a magnetic resonance of 3 μm. The absorption resonances of the fabricated nanorings depend on their diameters and shift toward shorter wavelengths (λ = 1.7 μm for do = 308 nm) as compared to larger rings (λ = 2.2 μm do = 351 nm). FDTD-based simulations match well with the experimental results. This ‘shaped nanosphere lithography’ approach creates opportunities to generate nanorings and nanocrescents that promise potential applications in chemical and biological sensing, for surface enhanced spectroscopy and in the field of infrared metamaterials. (paper)

  1. Exciton dispersion in molecular solids

    International Nuclear Information System (INIS)

    The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron–hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier–Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe–Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron–hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators. (topical review)

  2. 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.

  3. Interwell excitons in GaAs superlattices

    DEFF Research Database (Denmark)

    Birkedal, Dan; Sayed, Karim El; Sanders, G.; Spiegelberg, C.; Lyssenko, V. G.; Stanton, C.; Hvam, Jørn Märcher; Timofeev, V. B.; Bayer, M.

    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...

  4. YBa2Cu3O7-δ nanorings to probe fluxoid quantization in High Critical Temperature Superconductors

    Science.gov (United States)

    Arpaia, R.; Charpentier, S.; Toskovic, R.; Bauch, T.; Lombardi, F.

    2014-11-01

    We have realized YBa2Cu3O7-δ (YBCO) nanorings and measured the magnetoresistance R(B) close to the superconducting transition. The large oscillations that we have measured can be interpreted in terms of vortex dynamics triggering the nanowires to the resistive state. The Fast Fourier Transform spectrum of the magnetoresistance oscillations shows a single sharp peak for nanorings with narrower loop arm width: this peak can be univocally associated to a h/2e periodicity as predicted for optimally doped YBCO. Moreover it is a clear evidence of a uniform vorticity of the order parameter inside the rings, confirming a high degree of homogeneity of our nanostructures. This result gives a boost to further investigations of YBCO nanorings at different dopings within the superconducting dome, where in the underdoped regime a R(B) periodicity different from the conventional h/2e has been predicted.

  5. Exciton size and quantum transport in nanoplatelets

    International Nuclear Information System (INIS)

    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

  6. Nanobubble and nanodroplet template growth of particle nanorings versus nanoholes in drying nanofluids and polymer films

    Science.gov (United States)

    Darwich, S.; Mougin, K.; Vidal, L.; Gnecco, E.; Haidara, H.

    2011-03-01

    Here we demonstrate how confined nanobubbles and nanodroplets, which can either form spontaneously at the suspension/substrate interface, or can more interestingly be purposely introduced in the system, allow assembly of nanoparticles (NPs) into nanoring-like structures with a flexible control of both the size and distribution. As with most wetting-mediated nanopatterning methods, this approach provides an alternative to direct replication from templates. The formation of two-dimensional ring-shaped nanostructures was obtained by drying a nanocolloidal gold (Au) suspension drop confining nanobubbles (or nanodroplets) that are settled at a solid substrate. AFM investigation of the dry nanostructures showed the formation of isolated Au NPs rings having diameters ranging from 200 nm to 500 nm along the dewetting-drying path of the suspension drop. The flexibility of these wetting processes for the variation of the spatial features of the nanoring (size and shape resolution) essentially depends on physical parameters such as the nanobubble/nanodroplet size and concentration, the wettability, and the evaporation rate of the nanofluid drop on the substrate. Furthermore, we show that the underpinning mechanism of this evaporation-assisted assembly of Au NPs into supported functional nanoring patterns is fairly similar to that at work in the spontaneous formation of nanoholes in drying polymer thin films. Finally, the method proves to be a simple and flexible nanofabrication tool to be extended to various nanosize objects, towards specific optical and sensing applications.'Here we demonstrate how confined nanobubbles and nanodroplets, which can either form spontaneously at the suspension/substrate interface, or can more interestingly be purposely introduced in the system, allow assembly of nanoparticles (NPs) into nanoring-like structures with a flexible control of both the size and distribution. As with most wetting-mediated nanopatterning methods, this approach provides

  7. Spatially indirect excitons in coupled quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Lai, Chih-Wei Eddy

    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){sup 2} were

  8. Contrastive analysis of multiple exciton generation theories

    Science.gov (United States)

    Tan, Hengyu; Chang, Qing

    2015-10-01

    Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.

  9. Strain-mediated deterministic control of 360° domain wall motion in magnetoelastic nanorings

    Science.gov (United States)

    Liang, Cheng-Yen; Sepulveda, Abdon E.; Hoff, Daniel; Keller, Scott M.; Carman, Gregory. P.

    2015-11-01

    This study provides numerical simulations for deterministic 360° magnetization rotation of the transverse domain walls in a nickel nano-ring (outer diameter: 500 nm, inner diameter: 300 nm, and thickness: 10 nm) on a lead zirconate titanate (Pb[ZrxTi1-x]O3 0 sequential voltage application to successive pairs of electrodes and thus can be operated at quasi-static speeds and does not rely on magnetization dynamics to achieve 360° magnetization rotation. These analytical models provide support for developing new devices such as nanoscale multiferroic driven electromagnetic motors.

  10. Synthesis of Five-Porphyrin Nanorings by Using Ferrocene and Corannulene Templates.

    Science.gov (United States)

    Liu, Pengpeng; Hisamune, Yutaka; Peeks, Martin D; Odell, Barbara; Gong, Juliane Q; Herz, Laura M; Anderson, Harry L

    2016-07-11

    The smallest and most strained member of a family of π-conjugated cyclic porphyrin oligomers was synthesized by using pentapyridyl templates based on ferrocene and corannulene. Both templates are effective for directing the synthesis of the butadiyne-linked cyclic pentamer, despite the fact that the radii of their N5 donor sets are too small by 0.5 Å and 0.9 Å, respectively (from DFT calculations). The five-porphyrin nanoring exhibits a structured absorption spectrum and its fluorescence extends to 1200 nm, reflecting strong π conjugation and Herzberg-Teller vibronic coupling. PMID:27213825

  11. Asymmetric split nanorings for Fano induced plasmonic sensor in visible region

    Energy Technology Data Exchange (ETDEWEB)

    Ding, Yao, E-mail: dingyao@cqcet.com [General Education College, Chongqing College of Electronic Engineering, Chongqing 401331 (China); Liao, Zhongwei, E-mail: liaozw@cqu.edu.cn [Soft Matter and Interdisciplinary Research Institute, College of Physics, Chongqing University, Chongqing 401331 (China)

    2015-05-01

    Fano resonance exhibits high sensitivity and promising applications in the field of ultra-sensitive plasmonic sensor. In this work, the Fano lineshape in spectra of gold rectangular split nanorings (RSNRs) is investigated using the finite element method. The simulation results figure out the Fano lineshape could be modulated by the positions of split gap in RSNRs for symmetry breaking, which is explained by the plasmonic hybridization theory. Furthermore, the high order bonding plasmon mode H in absorption spectra exhibits high sensitivity in visible region. Our investigations here are beneficial for the design and application of ultra-sensitive LSPR sensor in visible region.

  12. Two-exciton excited states of J-aggregates in the presence of exciton-exciton annihilation

    Science.gov (United States)

    Levinsky, B.; Fainberg, B. D.; Nesterov, L. A.; Rosanov, N. N.

    2016-07-01

    We study decay of two-exciton states of a J-aggregate that is collective in nature. We use mathematical formalism based on effective non-Hermitian Hamiltonian suggested in nuclear theory. We show that decay of two-exciton states is strongly affected by the interference processes in the exciton-exciton annihilation. Our evaluations of the imaginary part of the effective Hamiltonian show that it exceeds the spacing between real energies of the two-exciton states that gives rise to the transition to the regime of overlapping resonances supplying the system by the new collectivity - the possibility of coherent decay in the annihilation channel. The decay of two-exciton states varies from twice bimolecular decay rate to the much smaller values that is associated with population trapping. We have also considered the corresponding experiment in the framework of our approach, the picture of which appears to be more complex and richer than it was reasoned before.

  13. Excitation of surface-radiation exciton-polariton modes at strong attenuations of excitons

    International Nuclear Information System (INIS)

    Formation of a photoluminescence spectrum of surface-radiation exciton-polariton modes is analysed at strong exciton attenuations when criteria of applicability of Boltzmann kinetic equation are broken for function of polariton distribution. (author)

  14. EXCITON DYNAMICS IN ORGANIC MOLECULAR CRYSTALS

    OpenAIRE

    A. Matsui; Mizuno, K.; Kobayashi, M.

    1985-01-01

    Dynamical behavior of Frenkel excitons in aromatic hydrocarbon crystals, pyrene, α-perylene, β-perylene, and tetracene are overviewed based on the published references and in terms of the self-trap depth. Then pressure-induced instability in exciton states (the change in the self-trap depth) in α-perylene and anthracene is demonstrated and discussed. Finally a quasi-free exciton state is suggested to be the origin of the luminescence in anthracene at room temperature.

  15. Exciton Binding Energy of Monolayer WS2

    OpenAIRE

    Bairen Zhu; Xi Chen; Xiaodong Cui

    2015-01-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach toward measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71eV around K valley in the Brillouin zone. The trion binding energy of 34meV, two-photon absorption cross s...

  16. Study of Squeezed Excitons in Polar Semiconductors

    Institute of Scientific and Technical Information of China (English)

    YIN Miao; CHENG Ze; WU Zi-Xia; PING Yun-Xia

    2009-01-01

    Some properties of excitons in polar semiconductors are studied theoretically by means of squeezed state variational approach. This method makes it possible to consider bilinear terms of the phonon operators as well as linear terms arising from the Lee-Low-Pines (LLP)-like transformation. The exciton ground state energy and binding energy are calculated numerically. It is shown that the squeezing effect is significant in the case of strong exciton-phonon coupling region.

  17. Charge-transfer excitons in DNA.

    Science.gov (United States)

    Conwell, E M; McLaughlin, P M; Bloch, S M

    2008-02-21

    There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data. PMID:18232682

  18. Excitons in the rare gas solids

    International Nuclear Information System (INIS)

    Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs

  19. Plasmon and Exciton Coupling and Purcell Enhancement

    Science.gov (United States)

    Rice, Quinton; Rigo, Maria Veronica; Fudala, Rafal; Cho, Hyoyeong; Kim, Wan-Joong; Rich, Ryan; Tabibi, Bagher; Gryczynski, Zygmunt; Gryczynski, Ignacy; Yu, William; Seo, Jaetae

    2014-05-01

    The photoluminescence from plasmon-coupled exciton is of great interest for optoelectronic applications, because of the large quantum yield with localized field enhancement and reduced nonradiative transition. The Coulomb interaction through plasmon-exciton coupling results in the Purcell enhancement of quantum dots (QDs) in the vicinity of metal nanoparticles (MNPs). With plasmon-exciton coupling, the radiative and non-radiative decay rates and the coupling rates compete with each other. The coupling rate is closely related to the coupling distance between QDs and MNPs. The optimized coupling distance scales the re-excitation density of localized fields and the plasmon-exciton coupling rates. If the plasmon-exciton coupling rate is much faster than the radiative and non-radiative transitions of excitons, the re-excitations of excitons by the localized plasmonic field and the reduction of non-radiative transitions may occur. This presentation includes plasmon-exciton coupling dynamics, large enhancement and temporal properties of PL, and dipole-PL polarization fidelity of hybrid optical materials of plasmonic nanometals and excitonic semiconductor QDs. The work at Hampton University was supported by the National Science Foundation (NSF HRD-1137747), and Army Research Office (ARO W911NF-11-1-0177). The work at University of North Texas was supported by National Institutes of Health (NIH R01EB12003, and 5R21CA14897 (Z.G.)).

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

    Science.gov (United States)

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

    2016-06-01

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

  2. Excitons in monocline zinc diphosphide. A-exciton series and Fano factor

    International Nuclear Information System (INIS)

    The spectra of the A-exciton series absorption by the various wave vector directions and radiation polarization states are studied in the monocline zinc diphosphide - β-ZnP2. The oscillators forces, which are subjected to the Fn ∝ n-3 dependence of the S-type, characteristic for the exciton states, are determined for the observed transitions. It is assumed, that the A-exciton series is conditioned partially by the permitted dipole transitions of the orthoexciton into the nS-state. The Fano antiresonances originate in the absorption spectra or the resonances of the A-exciton series by availability of the E radiation vector constituent on the crystallographic axis c. They are conditioned by the configuration interaction of the series A discrete exciton states with the exciton-phonon spectrum continuum, conditioned by the indirect transitions into the 1S-zone of the singlet exciton with the phonon emission

  3. Radiative recombination of excitons in amorphous semiconductors

    International Nuclear Information System (INIS)

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

  4. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

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

    2010-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...

  5. DNA-mediated excitonic upconversion FRET switching

    Science.gov (United States)

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-01

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  6. Coherent spontaneous radiation of Frenkel excitons

    Energy Technology Data Exchange (ETDEWEB)

    Andrianov, S.N.; Samartsev, V.V.; Sheibut, Yu.E.

    1988-02-01

    We have constructed a mathematical model that describes the coherent spontaneous radiation of a macroscopically filled exciton mode. We have demonstrated qualitatively ways in which a coherent subsystem of excitons can be formed. When this occurs under the influence of an external coherent source, exciton free induction occurs, while in the case of a spontaneous transition of the excitons to a coherent state through freezing of the reservoir of intermolecular interactions superradiance occurs. We have concluded that superradiance in the system of Frenkel excitons is possible under the influence of the non-Dicke mechanisms of self-induction of correlations, when the dipole transition moments of the individual atoms are coupled into a single macroscopic effective spin through their electrostatic interaction, this being analogous to the ordering of magnetic dipole moments in a ferromagnet.

  7. Effects of magnetic impurities on electron transmission in a quantum nanoring

    Directory of Open Access Journals (Sweden)

    L Eslami

    2010-03-01

    Full Text Available In this paper we study the Aharonov-Bohm oscillations of transmission coefficient for an electron passing through a quantum nanoring with two identical magnetic impurities using quantum waveguide theory. It is shown that the Aharonov-Bohm oscillations are independent of the coupling constant between the electron and magnetic impurities for the singlet spin state of impurities, while for the other spin states of impurities the Aharonov-Bohm oscillations decrease rapidly with an increasing coupling constant. For a triplet spin state of impurities, we can use this system as a quantum NOT gate with an efficiency upper than 50% by adjusting the coupling constant between the electron and magnetic impurities and the magnetic flux passing through the ring.

  8. Magnetic-field effects in graphene nanorings: armchair versus zigzag edge terminations

    Science.gov (United States)

    Yannouleas, Constantine; Romanovsky, Igor; Landman, Uzi

    2013-03-01

    Dirac quasiparticles in narrow graphene nanorings exhibit characteristic differences in their behavior depending on the shape (e.g., trigonal vs. hexagonal) and the type of edge terminations (armchair vs. zigzag). The differences are manifested in the tight-binding single-particle spectra as a function of the magnetic field B and in the patterns of the Aharonov-Bohm oscillations. The symmetry of shape leads to the appearance of three-member (triangles) or six-member (hexagons) braid bands.[2] With the exception of the formation of the braid bands, the characteristic differences maintain in the energy spectra of the continuous Dirac-Weyl equation for a circular ring of finite width. These differences will be further analyzed with the help of a relativistic superlattice model. Supported by the U.S. D.O.E. (FG05-86ER-45234)

  9. Ultrasonic synthesis, characterization and formation mechanism of aggregated nanorings of EuF3

    Institute of Scientific and Technical Information of China (English)

    WU Dapeng; WANG Xinjun; BAI Zhengyu; JIANG Kai

    2008-01-01

    The aggregated nanorings of EuF3 were synthesized via ultrasonic irritation in aqueous solution. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD pattern proved that the crystalline phase of the EuF3 rngs was hexagonal. The SEM and TEM images indicated that the as-prepared EuF3 nanocrys-tals had ring-like morphology and were aggregated by numerous small crystallites (about 10-15 nm in diameter); the outer diameter of the rings was in the range of 200-300 nm, while the inner diameter was in the range of 50-80 nm with a thickness of 30-40 nm. Moreover, the time-depend experiments were carried out to disclose the formation mechanism of the as-prepared ring-like nanostructures.

  10. Width dependent transition of quantized spin-wave modes in Ni80Fe20 square nanorings

    International Nuclear Information System (INIS)

    We investigated optically induced ultrafast magnetization dynamics in square shaped Ni80Fe20 nanorings with varying ring width. Rich spin-wave spectra are observed whose frequencies showed a strong dependence on the ring width. Micromagnetic simulations showed different types of spin-wave modes, which are quantized upto very high quantization number. In the case of widest ring, the spin-wave mode spectrum shows quantized modes along the applied field direction, which is similar to the mode spectrum of an antidot array. As the ring width decreases, additional quantization in the azimuthal direction appears causing mixed modes. In the narrowest ring, the spin-waves exhibit quantization solely in azimuthal direction. The different quantization is attributed to the variation in the internal field distribution for different ring width as obtained from micromagnetic analysis and supported by magnetic force microscopy.

  11. Topological effects and particle-physics analogies beyond the massless Dirac-Weyl fermion in graphene nanorings

    OpenAIRE

    Romanovsky, Igor; Yannouleas, Constantine; Landman, Uzi

    2013-01-01

    Armchair and zigzag edge terminations in planar hexagonal and trigonal graphene nanorings are shown to underlie one-dimensional topological states associated with distinctive energy gaps and patterns (e.g., linear dispersion of the energy of an hexagonal ring with an armchair termination versus parabolic dispersion for a zigzag terminated one) in the bands of the tight-binding spectra as a function of the magnetic field. A relativistic Dirac-Kronig-Penney model analysis of the tight-binding A...

  12. Excitons in the Fractional Quantum Hall Effect

    Science.gov (United States)

    Laughlin, R. B.

    1984-09-01

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

  13. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

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

    2010-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is...

  14. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

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

    2011-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is...

  15. Phonon-mediated Josephson oscillations in excitonic and polaritonic condensates

    OpenAIRE

    Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I.A.

    2010-01-01

    We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton- exciton interactions as well as regimes of self- sustained re...

  16. Dependence of Resonance Energy Transfer on Exciton Dimensionality

    OpenAIRE

    Junis Rindermann, Jan; Pozina, Galia; Monemar, Bo; Hultman, Lars; Amano, Hiroshi; Lagoudakis, Pavlos G.

    2011-01-01

    We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of locali...

  17. Optical properties of near-surface exciton quantum wells

    OpenAIRE

    N. Atenco Analco; B. Flores Desirena; A. Silva Castillo; F. Pérez Rodríguez

    2001-01-01

    An overview of theoretical investigations on near-surface semiconductor quantum wells, whose optical properties are considerably affected by the interaction of the exciton with the sample surface is given. Near-surface quantum wells with both weak and strong quantum confinement of excitons are considered. When the exciton quantum well is very close to the sample surface, exciton dynamics is determined not only by characteristics of the quantum well, but also by the interaction of the exciton ...

  18. Extended domains of organized nanorings of silver grains as surface-enhanced Raman scattering sensors for molecular detection

    Energy Technology Data Exchange (ETDEWEB)

    Bechelany, M; Brodard, P; Philippe, L; Michler, J, E-mail: mikhael.bechelany@empa.c, E-mail: pierre.brodard@empa.c [Laboratory for Mechanics of Materials and Nanostructures, EMPA, Swiss Federal Laboratories for Materials Testing and Research, Feuerwerkerstrasse 39, CH-3602 Thun (Switzerland)

    2009-11-11

    The possibility to synthesize large areas of silver grains organized in nanorings using a simple technique based on nanosphere lithography and electroless plating as a metal deposition method is described for the first time. In addition, we present a systematic SERS study of the obtained long-range ordered silver nanodots and nanorings. The possibility to precisely control the size, the interdistance and the morphology of these nanostructures allows us to systematically investigate the influence of these parameters on SERS. We show that the best possible SERS substrates should not only present optimal sizes, interdistances and shapes, but also a grain-like structure composed of sub-100 nm grains in order to maximize the number of hot-spots. In addition, we show that grains arranged in nanorings present higher enhancement factors (E{sub F} = 5.5 x 10{sup 5}) as compared to similar arrays made of nanodots. A wide range of applications, including real-time monitoring of catalytic surface reactions, environmental and security monitoring as well as clinical and pharmaceutical screening, can be envisaged for these SERS substrates.

  19. Catalyst free self-assembled growth of InN nanorings on stepped Si (5 5 3) surface

    Energy Technology Data Exchange (ETDEWEB)

    Chauhan, Amit Kumar Singh; Kumar, Mukesh; Gupta, Govind, E-mail: govind@nplindia.org

    2015-08-01

    Highlights: • Novel technique to synthesis self-assembled InN nanorings by low energy N. • Anisotropic strain relaxation driven self-assembly lead to formation of nanorings. • Strong downward band bending in VB-spectra signify large electron accumulation. - Abstract: A novel technique for synthesis of high crystalline quality self-assembled InN nanorings, by nitriding the bulk deposited In/Si (5 5 3)-1 × 4 system using low energy N{sub 2}{sup +} ions at 520 °C, has been demonstrated. Scanning electron microscopy images reveal the formation of ring shaped InN structures with average size ∼500 nm. Anisotropic strain relaxation via self assembly could be the driving force for the formation of these InN rings on reconstructed Si (5 5 3) surface. High resolution X-ray diffraction analysis indicates high crystalline quality of these wurtzite InN nanostructures with c-plane. A strong downward band bending was observed in X-ray photoelectron spectroscopy valence band spectra which signify the large electron accumulation in the InN nanostructures.

  20. Extended domains of organized nanorings of silver grains as surface-enhanced Raman scattering sensors for molecular detection

    Science.gov (United States)

    Bechelany, M.; Brodard, P.; Philippe, L.; Michler, J.

    2009-11-01

    The possibility to synthesize large areas of silver grains organized in nanorings using a simple technique based on nanosphere lithography and electroless plating as a metal deposition method is described for the first time. In addition, we present a systematic SERS study of the obtained long-range ordered silver nanodots and nanorings. The possibility to precisely control the size, the interdistance and the morphology of these nanostructures allows us to systematically investigate the influence of these parameters on SERS. We show that the best possible SERS substrates should not only present optimal sizes, interdistances and shapes, but also a grain-like structure composed of sub-100 nm grains in order to maximize the number of hot-spots. In addition, we show that grains arranged in nanorings present higher enhancement factors (EF = 5.5 × 105) as compared to similar arrays made of nanodots. A wide range of applications, including real-time monitoring of catalytic surface reactions, environmental and security monitoring as well as clinical and pharmaceutical screening, can be envisaged for these SERS substrates.

  1. Extended domains of organized nanorings of silver grains as surface-enhanced Raman scattering sensors for molecular detection

    International Nuclear Information System (INIS)

    The possibility to synthesize large areas of silver grains organized in nanorings using a simple technique based on nanosphere lithography and electroless plating as a metal deposition method is described for the first time. In addition, we present a systematic SERS study of the obtained long-range ordered silver nanodots and nanorings. The possibility to precisely control the size, the interdistance and the morphology of these nanostructures allows us to systematically investigate the influence of these parameters on SERS. We show that the best possible SERS substrates should not only present optimal sizes, interdistances and shapes, but also a grain-like structure composed of sub-100 nm grains in order to maximize the number of hot-spots. In addition, we show that grains arranged in nanorings present higher enhancement factors (EF = 5.5 x 105) as compared to similar arrays made of nanodots. A wide range of applications, including real-time monitoring of catalytic surface reactions, environmental and security monitoring as well as clinical and pharmaceutical screening, can be envisaged for these SERS substrates.

  2. Excitonic molecules in type-II superlattices

    Science.gov (United States)

    Tsuchiya, T.; Katayama, S.; Ando, T.

    1998-01-01

    Excitonic molecules in GaAs/AlAs type-II superlattices are numerically investigated. In spite of large difference of electronic structures between type-II and type-I superlattices, variational calculations show that the configuration of particles is similar to that in type-I superlattices. This is because the layer width is smaller than the extent of excitonic wavefunctions in the direction parallel to the layers in the present superlattices.

  3. Exciton transport by surface acoustic waves

    Science.gov (United States)

    Rudolph, J.; Hey, R.; Santos, P. V.

    2007-05-01

    Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.

  4. Orientation of luminescent excitons in layered nanomaterials

    Science.gov (United States)

    Schuller, Jon A.; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid

    2013-04-01

    In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling--MoS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS2 mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

  5. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jørn Märcher; Bacher, G.; Kümmell, T.; Forchel, A.

    1998-01-01

    within the wire due to the electron trapping in surface states and exciton localization. The exciton-exciton scattering efficiency, determined by the density dependence of the exciton dephasing, is found to increase with decreasing win width. This is assigned to the reduced phase space in a quasi......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...

  6. Ferromagnetic nanorings

    International Nuclear Information System (INIS)

    Ferromagnetic metal rings of nanometre range widths and thicknesses exhibit fundamentally new spin states, switching behaviour and spin dynamics, which can be precisely controlled via geometry, material composition and applied field. Following the discovery of the 'onion state', which mediates the switching to and between vortex states, a range of fascinating phenomena has been found in these structures. In this overview of our work on ring elements, we first show how the geometric parameters of ring elements determine the exact equilibrium spin configuration of the domain walls of rings in the onion state, and we show how such behaviour can be understood as the result of the competition between the exchange and magnetostatic energy terms. Electron transport provides an extremely sensitive probe of the presence, spatial location and motion of domain walls, which determine the magnetic state in individual rings, while magneto-optical measurements with high spatial resolution can be used to probe the switching behaviour of ring structures with very high sensitivity. We illustrate how the ring geometry has been used for the study of a wide variety of magnetic phenomena, including the displacement of domain walls by electric currents, magnetoresistance, the strength of the pinning potential introduced by nanometre size constrictions, the effect of thermal excitations on the equilibrium state and the stochastic nature of switching events

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2016-01-14

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

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

    International Nuclear Information System (INIS)

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

  9. Nonlinear exciton transfer in protein helices

    International Nuclear Information System (INIS)

    We study the transfer of vibronic excitation energy in helical forms of proteins. The steric structure of the helix protein is modelled by a three-dimensional network of oscillators representing peptide groups. The covalent and hydrogen bonds between the peptide groups are described by pair interaction potentials. Each peptide group possesses one internal vibrational (excitonic) degree of freedom embodying the amide-I mode. The transfer dynamics of an amide-I exciton along the helix is expressed in terms of a tight-binding system. In the first part of this paper we study a reduced system arising when the vibrations of the covalent bonds are neglected. For the resulting system consisting of the exciton coupled to the hydrogen bond vibrations oriented along the helix axis we construct polaron solutions. Subsequently we investigate the mobility of the polarons within the complete protein matrix including deformations of the covalent bonds too. In particular we show that, during a phase of adaptation going along with internal energy exchange between the exciton and the bond vibrations, a relaxation into a new steady regime takes place. The newly reached equilibrium state is characterized by a localized exciton breather and is attributed local deformations of the steric peptide cage in the form of phonobreathers. Finally, coherent motion of an exciton breather is initiated through suitable injection of kinetic energy. In this way the long-range transfer of vibronic amide-I energy in the steric protein cage is provided. Interestingly, the α-helix possesses better facilities in supporting mobile localized excitons compared to the 3-10-helix form of proteins

  10. An Analytical Solution for Exciton Generation, Reaction, and Diffusion in Nanotube and Nanowire-Based Solar Cells.

    Science.gov (United States)

    Bellisario, Darin O; Paulson, Joel A; Braatz, Richard D; Strano, Michael S

    2016-07-21

    Excitonic solar cells based on aligned or unaligned networks of nanotubes or nanowires offer advantages with respect of optical absorption, and control of excition and electrical carrier transport; however, there is a lack of predictive models of the optimal orientation and packing density of such devices to maximize efficiency. Here-in, we develop a concise analytical framework that describes the orientation and density trade-off on exciton collection computed from a deterministic model of a carbon nanotube (CNT) photovoltaic device under steady-state operation that incorporates single- and aggregate-nanotube photophysics published earlier (Energy Environ Sci, 2014, 7, 3769). We show that the maximal film efficiency is determined by a parameter grouping, α, representing the product of the network density and the effective exciton diffusion length, reflecting a cooperativity between the rate of exciton generation and the rate of exciton transport. This allows for a simple, master plot of EQE versus film thickness, parametric in α allowing for optimal design. This analysis extends to any excitonic solar cell with anisotropic transport elements, including polymer, nanowire, quantum dot, and nanocarbon photovoltaics. PMID:27357970

  11. Generation of multiple excitons in Ag2S quantum dots: Single high-energy versus multiple-photon excitation

    KAUST Repository

    Sun, Jingya

    2014-02-20

    We explored biexciton generation via carrier multiplication (or multiple-exciton generation) by high-energy photons and by multiple-photon absorption in Ag2S quantum dots (QDs) using femtosecond broad-band transient absorption spectroscopy. Irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S QDs, two distinct characteristic time constants of 9.6-10.2 and 135-175 ps are obtained for the nonradiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely, tightly bound and weakly bound excitons. More importantly, the lifetimes of multiple excitons in Ag 2S QDs were about 1 and 2 orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices. © 2014 American Chemical Society.

  12. Nonequilibrium optical properties of bare and functionalized carbon nanotubes

    OpenAIRE

    Langlois, Benjamin

    2014-01-01

    This thesis is devoted to the experimental study by femtosecond pump-probe spectroscopy of micellar solutions of carbon nanotubes. The properties of carbon nanotubes are greatly affected by their 1D geometry. Quantum confinement of electrons in 1D leads to enhanced coulomb interactions giving rise to strongly bound excitons. Many-body effects between excitons is investigated by the moment method analysis of the transient absorption spectra. The relaxation of excitons is controlled by diffusio...

  13. Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

    CERN Document Server

    Kühn, Oliver

    2011-01-01

    The theoretical description and the properties of Frenkel excitons in non-covalently bonded molecular aggregates are reviewed from a multi-exciton perspective of dissipative quantum dynamics. First, the photophysical and quantum chemical characterization of the monomeric dye building blocks is discussed, including the important aspect of electron-vibrational coupling within the Huang-Rhys model. Supplementing the model by the Coulombic interactions between monomers, the description of aggregates in terms of excitonic or vibrational-excitonic bands follows. Besides of giving rise to complex absorption and emission line shapes, exciton-vibrational interaction is responsible for energy and phase relaxation and thereby limits the size of coherent excitations in larger aggregates. Throughout, emphasis is put on the electronic three-level model as a minimum requirement to describe nonlinear spectroscopies including effects of two-exciton states such as excited state absorption and exciton-exciton annihilation. The ...

  14. Observation of Two-Exciton States in Perylene Bisimide Aggregates

    OpenAIRE

    Lochbrunner S.; Seidel M.; Würthner F.; Wolter S.

    2013-01-01

    The behavior of excitons on perylene bisimide aggregates is investigated at high excitation densities by femtosecond absorption spectroscopy. Indications for a significant population in the two-exciton manifold are found.

  15. Observation of Two-Exciton States in Perylene Bisimide Aggregates

    Directory of Open Access Journals (Sweden)

    Lochbrunner S.

    2013-03-01

    Full Text Available The behavior of excitons on perylene bisimide aggregates is investigated at high excitation densities by femtosecond absorption spectroscopy. Indications for a significant population in the two-exciton manifold are found.

  16. Attractive Coulomb interaction of two-dimensional Rydberg excitons

    Science.gov (United States)

    Shahnazaryan, V.; Shelykh, I. A.; Kyriienko, O.

    2016-06-01

    We analyze theoretically the Coulomb scattering processes of highly excited excitons in the direct-band-gap semiconductor quantum wells. We find that contrary to the interaction of ground-state excitons, the electron and hole exchange interaction between excited excitons has an attractive character both for s - and p -type two-dimensional (2D) excitons. Moreover, we show that similar to the three-dimensional highly excited excitons, the direct interaction of 2D Rydberg excitons exhibits van der Waals-type long-range interaction. The results predict the linear growth of the absolute value of exchange interaction strength with an exciton principal quantum number and point the way towards enhancement of optical nonlinearity in 2D excitonic systems.

  17. Resonant Transmission through Serially Connected Hexagonal Nanorings with Magnetic Flux Effects

    Science.gov (United States)

    Hedin, Eric; Joe, Yong

    Nanostructures composed of six quantum dots (QDs) connected in a ring are linked together in a linear chain with each ring separated by a coupling segment from adjoining rings. A tight-binding model is used to obtain the electron transmission through an arbitrary number of rings in series as a function of energy, external magnetic field, coupling parameters, and QD site energy values. Modifications of the transmission band structure as a function of external field, due to the Aharonov-Bohm and Zeeman effects, demonstrate control over the conductance properties of the linear chain of nano-rings. Resonant transmission effects (with electron energy equal to the QD site energy values) show a complex dependence upon an interplay of magnetic flux, inter-ring coupling, and the strength of the coupling between the ring system and the external leads. For specific values of lead and ring couplings, nearly full transmission (ballistic transport) is seen to occur across a broad energy range, independent of the number of rings in series. Partially supported by BSU ASPiRE program.

  18. Topological effects and particle physics analogies beyond the massless Dirac-Weyl fermion in graphene nanorings

    Science.gov (United States)

    Romanovsky, Igor; Yannouleas, Constantine; Landman, Uzi

    2013-04-01

    Armchair and zigzag edge terminations in planar hexagonal and trigonal graphene nanorings are shown to underlie one-dimensional topological states associated with distinctive energy gaps and patterns (e.g., linear dispersion of the energy of an hexagonal ring with an armchair termination versus parabolic dispersion for a zigzag terminated one) in the bands of the tight-binding spectra as a function of the magnetic field. A relativistic Dirac-Kronig-Penney model analysis of the tight-binding Aharonov-Bohm behavior reveals that the graphene quasiparticle in an armchair hexagonal ring is a condensed-matter realization of an ultrarelativistic fermion with a position-dependent mass term, akin to the zero-energy fermionic solitons with fractional charge familiar from quantum-field theory and from the theory of polyacetylene. The topological origins of the above behavior are highlighted by contrasting it with the case of a trigonal armchair ring, where we find that the quasiparticle excitations behave as familiar Dirac fermions with a constant mass. Furthermore, the spectra of a zigzag hexagonal ring correspond to the low-kinetic-energy nonrelativistic regime of a lepton-like massive fermion. A one-dimensional relativistic Lagrangian formalism coupling a fermionic and a scalar bosonic field via a Yukawa interaction, in conjunction with the breaking of the Z2 reflectional symmetry of the scalar field, is shown to unify the above dissimilar behaviors.

  19. Colloidal gold nanorings for improved photodynamic therapy through field-enhanced generation of reactive oxygen species

    Science.gov (United States)

    Hu, Yue; Yang, Yamin; Wang, Hongjun; Du, Henry

    2013-02-01

    Au nanostructures that exhibit strong localized surface plasmon resonance (SPR) have excellent potential for photo-medicine, among a host of other applications. Here, we report the synthesis and use of colloidal gold nanorings (GNRs) with potential for enhanced photodynamic therapy of cancer. The GNRs were fabricated via galvanic replacement reaction of sacrificial Co nanoparticles in gold salt solution with low molecular weight (Mw = 2,500) poly(vinylpyrrolidone) (PVP) as a stabilizing agent. The size and the opening of the GNRs were controlled by the size of the starting Co particles and the concentration of the gold salt. UV-Vis absorption measurements indicated the tunability of the SPR of the GNRs from 560 nm to 780 nm. MTT assay showed that GNRs were non-toxic and biocompatible when incubated with breast cancer cells as well as the healthy counterpart cells. GNRs conjugated with 5-aminolevulinic acid (5-ALA) photosensitizer precursor led to elevated formation of reactive oxygen species and improved efficacy of photodynamic therapy of breast cancer cells under light irradiation compared to 5-ALA alone. These results can be attributed to significantly enhance localized electromagnetic field of the GNRs.

  20. Au nanorings for enhancing absorption and backscattering monitored with optical coherence tomography

    Energy Technology Data Exchange (ETDEWEB)

    Tseng, Hung-Yu; Lee, Cheng-Kuang; Wu, Shou-Yen; Chi, Ting-Ta; Yang, Kai-Min; Wang, Jyh-Yang; Kiang, Yean-Woei; Yang, C C [Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, No 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan, Republic of China (China); Tsai, Meng-Tsan [Department of Electrical Engineering, Chang Gung University, No 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan, 33302 Taiwan (China); Wu, Yang-Che [Graduate Institute of Clinical Dentistry, School of Dentistry, National Taiwan University, No 1 Changde St, Taipei, 10002 Taiwan (China); Chou, Han-Yi E; Chiang, Chun-Pin, E-mail: ywkiang@ntu.edu.tw, E-mail: ccy@cc.ee.ntu.edu.tw [Graduate Institute of Oral Biology, School of Dentistry, National Taiwan University, No 1 Changde St, Taipei, 10002 Taiwan (China)

    2010-07-23

    Preparation of a high-concentration Au nanoring (NR) water solution and its applications to the enhancement of image contrast in optical coherence tomography (OCT) and the generation of the photothermal effect in a bio-sample through localized surface plasmon (LSP) resonance are demonstrated. Au NRs are first fabricated on a sapphire substrate with colloidal lithography and secondary sputtering of Au, and then transferred into a water solution through a liftoff process. By controlling the NR geometry, the LSP dipole resonance wavelength in tissue can cover a spectral range of 1300 nm for OCT scanning of deep tissue penetration. The extinction cross sections of the fabricated Au NRs in water are estimated to give levels of 10{sup -10}-10{sup -9} cm{sup 2} near their LSP resonance wavelengths. The fabricated Au NRs are then delivered into pig adipose samples for OCT scanning. It is observed that, when resonant Au NRs are delivered into such a sample, LSP resonance-induced Au NR absorption results in a photothermal effect, making the opaque pig adipose cells transparent. Also, the delivered Au NRs in the intercellular substance enhance the image contrast of OCT scanning through LSP resonance-enhanced scattering. By continuously OCT scanning a sample, both photothermal and image contrast enhancement effects are observed. However, by continually scanning a sample with a low scan frequency, only the image contrast enhancement effect is observed.

  1. Nonlinear Photoluminescence Properties of Trions in Hole-doped Single-walled Carbon Nanotubes

    OpenAIRE

    Akizuki, Naoto; Iwamura, Munechiyo; Mouri, Shinichiro; Miyauchi, Yuhei; Kawasaki, Tomohiro; Watanabe, Hiroshi; Suemoto, Tohru; Watanabe, Kouta; Asano, Kenichi; Matsuda, Kazunari

    2014-01-01

    We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreov...

  2. Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.

    Science.gov (United States)

    Hader, Kilian; May, Volkhard; Lambert, Christoph; Engel, Volker

    2016-05-11

    Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton-exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before. PMID:27120976

  3. Ultrafast exciton relaxation in quasi-one-dimensional perylene derivatives

    OpenAIRE

    Engel, Egbert

    2006-01-01

    This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addresse...

  4. Non-conservation of excitons in finite molecular chain

    Energy Technology Data Exchange (ETDEWEB)

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

    2010-11-15

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

  5. Excitonic condensation in systems of strongly correlated electrons

    OpenAIRE

    Kunes, Jan

    2015-01-01

    The idea of exciton condensation in solids was introduced in 1960's with the analogy to superconductivity in mind. While exciton supercurrents have been realized only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. ...

  6. Instantaneous Rayleigh scattering from excitons localized in monolayer islands

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Leosson, Kristjan; Jensen, Jacob Riis; Hvam, Jørn Märcher; Zimmermann, R.

    2000-01-01

    resonance is observed. Instead, when exciting only a subsystem of the exciton resonance, in our case excitons localized in quantum well regions of a specific monolayer thickness, the rise has an instantaneous component. This is due to the spatial nonuniformity of the initially excited exciton polarization...

  7. Self-trapped excitons in circular cacteriochlorophyll antenna complexes

    International Nuclear Information System (INIS)

    Fluorescence line narrowing and hole-burning spectroscopic studies of excitons in the LH2 pigment-protein complex, which is a part of the light harvesting system of purple bacteria, are combined with straightforward numerical simulations of the emission spectra based on exciton polaron model. The analysis provides evidence for self-trapping of all the excitons, except the lowest one

  8. Energy barriers for bit-encoding states based on 360° domain walls in ultrathin ferromagnetic nanorings

    Science.gov (United States)

    Muratov, C. B.; Osipov, V. V.; Vanden-Eijnden, E.

    2015-05-01

    A numerical thermal stability study of the bit-encoding states in a proposed multi-level magnetic storage element based on an ultrathin ferromagnetic nanoring is presented. The material parameters and the ring dimensions for which there are five distinct metastable magnetization configurations separated by energy barriers exceeding 50kBT at room temperature are identified. The results are obtained, using the string method for the study of rare events to locate the transition states separating the metastable states and to identify the most likely thermally activated pathways.

  9. Exciton dynamics in perturbed vibronic molecular aggregates.

    Science.gov (United States)

    Brüning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V

    2016-07-01

    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. PMID:26798840

  10. Polarizable Optical Bistability of Frenkel Excitons

    Directory of Open Access Journals (Sweden)

    O. Derevyanchuk

    2003-06-01

    Full Text Available Optical spectra of molecular crystals with two molecules in their elementary cell are investigated. Depending on the polarization angle one or two exciton excitation bands can be realized in the spectrum. The bands are separated by the terminal frequency interval. The exciton absorption bands genesis and the peculiarities of the function of their form at the change of the laser radiation polarization have been analyzed and the regularity of arising bistable states in organic benzol-type crystals has been established.

  11. Control of exciton transport using quantum interference

    Science.gov (United States)

    Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.

    2015-12-01

    It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.

  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. Excitonic optical bistability in n-type doped semiconductors

    International Nuclear Information System (INIS)

    A resonant monochromatic pump laser generates coherent excitons in an n-type doped semiconductor. Both exciton-exciton and exciton-donor interactions come into play. The former interaction can give rise to the appearance of optical bistability which is heavily influenced by the latter one. When optical bistability occurs at a fixed laser frequency both its holding intensity and hysteresis loop size are shown to decrease with increasing donor concentration. Two possibilities are suggested for experimentally determining one of the two parameters of the system - the exciton-donor coupling constant and the donor concentration, if the other parameter is known beforehand. (author). 36 refs, 2 figs

  14. Coherent interactions between phonons and exciton or exciton-polariton condensates

    OpenAIRE

    Vishnevsky, D. V.; Gippius, N. A.; Shelykh, I.A.; Solnyshkov, D. D.; Malpuech, G.

    2011-01-01

    We analyse the interaction of exciton and exciton-polariton condensates in semiconductor microcavity with a coherent acoustic wave. An analytical solution for the dispersion of excitations of coupled condensate-phonon system is found in the approximation of k-independent interactions. Accounting for k-dependence results in a stronger modification of the dispersion, and even in the appearance of the "roton instability" region.

  15. Multistability of cavity exciton-polaritons affected by the thermally generated exciton reservoir

    OpenAIRE

    Vishnevsky, D. V.; Solnyshkov, D. D.; Gippius, N. A.; Malpuech, G.

    2011-01-01

    Until now, the generation of an excitonic reservoir in a cavity polariton system under quasi-resonant pumping has always been neglected. We show that in microcavities having a small Rabi splitting (typically GaAs cavities with a single quantum well), this reservoir can be efficiently populated by polariton-phonon scattering. We consider the influence of the exciton reservoir on the energy shifts of the resonantly pumped polariton modes. We show that the presence of this reservoir effectively ...

  16. 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...

  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 f...

  18. 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.

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

    Institute of Scientific and Technical Information of China (English)

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

    2005-01-01

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

  20. Cancer cell uptake behavior of Au nanoring and its localized surface plasmon resonance induced cell inactivation

    International Nuclear Information System (INIS)

    Au nanorings (NRIs), which have the localized surface plasmon resonance (LSPR) wavelength around 1058 nm, either with or without linked antibodies, are applied to SAS oral cancer cells for cell inactivation through the LSPR-induced photothermal effect when they are illuminated by a laser of 1065 nm in wavelength. Different incubation times of cells with Au NRIs are considered for observing the variations of cell uptake efficiency of Au NRI and the threshold laser intensity for cell inactivation. In each case of incubation time, the cell sample is washed for evaluating the total Au NRI number per cell adsorbed and internalized by the cells based on inductively coupled plasma mass spectrometry measurement. Also, the Au NRIs remaining on cell membrane are etched with KI/I2 solution to evaluate the internalized Au NRI number per cell. The threshold laser intensities for cell inactivation before washout, after washout, and after KI/I2 etching are calibrated from the circular area sizes of inactivated cells around the illuminated laser spot center with various laser power levels. By using Au NRIs with antibodies, the internalized Au NRI number per cell increases monotonically with incubation time up to 24 h. However, the number of Au NRI remaining on cell membrane reaches a maximum at 12 h in incubation time. The cell uptake behavior of an Au NRI without antibodies is similar to that with antibodies except that the uptake NRI number is significantly smaller and the incubation time for the maximum NRI number remaining on cell membrane is delayed to 20 h. By comparing the threshold laser intensities before and after KI/I2 etching, it is found that the Au NRIs remaining on cell membrane cause more effective cancer cell inactivation, when compared with the internalized Au NRIs. (paper)

  1. The Orientation of Luminescent Excitons in Layered Organic Nanomaterials

    Science.gov (United States)

    Schuller, Jon; Karavelli, Sinan; He, Keliang; Yang, Shyuan; Shan, Jie; Kymissis, John; Zia, Rashid

    2012-02-01

    A fundamental understanding of optoelectronics in organic semiconductors is complicated by the diversity of excitons which can exist within a single material system. Measurements that distinguish between different exciton types are crucial for a complete understanding of organic materials. By fitting experimental curves of angle-, polarization-, and energy-dependent PL to analytical Purcell calculations we quantify the relative dipole moments for in-plane and out-of-plane oriented excitons in organic and inorganic layered nanomaterials. In mono- and bi-layers of Molybdenum Disulfide (MoS2) and Graphene Oxide the luminescence arises only from in-plane oriented excitons. In the perylene derivative PTCDA, however, we show that PL arises from both in-plane and out-of-plane excitons. We observe a difference in emission frequency between the dipole orientations which indicates the existence of two distinct exciton species: an in-plane oriented Frenkel exciton and an out-of-plane oriented Charge Transfer exciton. Based on these results we devise and implement a method for isolating luminescence from either exciton species. We observe different temporal dynamics for the two distinct excitons, highlighting the power of this technique for fundamental studies of organic materials.

  2. 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.

  3. Microcavity controlled coupling of excitonic qubits

    CERN Document Server

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

    2012-01-01

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

  4. 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...... and holes are described as in-plane point charges with a finite extension in the perpendicular direction, and their interaction is screened by a dielectric function with a nonlinear q dependence which is computed ab initio. The screened interaction is used in a generalized Mott-Wannier model to...... structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function, and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless...

  5. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    CERN Document Server

    Olšina, Jan; Wang, Chen; Cao, Jianshu

    2014-01-01

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

  6. 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.

  7. Excitons in atomically thin black phosphorus

    OpenAIRE

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.

    2016-01-01

    Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in plane anisotropy. The emission energy de...

  8. Macroscopically ordered state in exciton system

    OpenAIRE

    Butov, L. V.; Gossard, A. C.; Chemla, D. S.

    2002-01-01

    Macroscopically ordered arrays of vortices in quantum liquids, such as superconductors, He-II, and atom Bose-Einstein Condensates (BEC), demonstrate macroscopic coherence in flowing superfluids [1-4]. Despite of the rich variety of systems where quantum liquids reveal macroscopic ordering, experimental observation of a macroscopically ordered electronic state in semiconductors has remained a challenging unexplored problem. A system of excitons is a promising candidate for the realization of m...

  9. Semiconductor excitons in strong terahertz fields

    Energy Technology Data Exchange (ETDEWEB)

    Steiner, J.T.; Kira, M.; Koch, S.W. [Department of Physics and Material Sciences Center, Philipps-University, Marburg (Germany)

    2009-02-15

    A microscopic theory for the nonlinear terahertz response of excited semiconductor systems is presented. It is shown that the measurable quantities in typical terahertz transmission/reflection experiments contain large ponderomotive contributions which mask the signatures from terahertz-induced many-body transitions. A scheme is developed to remove the ponderomotive contributions and applied to isolate the signatures of Rabi oscillations of an exciton population. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  10. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    OpenAIRE

    Kenny, Elise P.; Kassal, Ivan

    2015-01-01

    Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the ...

  11. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    OpenAIRE

    Olšina, Jan; Dijkstra, Arend G.; Wang, Chen; Cao, Jianshu

    2014-01-01

    Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined by the non-canoni...

  12. Ultrafast dynamics of excitons in tetracene single crystals

    Science.gov (United States)

    Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich

    2014-03-01

    Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

  13. Ultrafast dynamics of excitons in tetracene single crystals

    Energy Technology Data Exchange (ETDEWEB)

    Birech, Zephania; Schwoerer, Heinrich, E-mail: heso@sun.ac.za [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa); Schwoerer, Markus [Department of Physics, University of Bayreuth, Bayreuth (Germany); Schmeiler, Teresa; Pflaum, Jens [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)

    2014-03-21

    Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

  14. 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.

  15. Excitonic gap formation and condensation in the bilayer graphene structure

    Science.gov (United States)

    Apinyan, V.; Kopeć, T. K.

    2016-09-01

    We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2009-02-01

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

  17. Wannier-Mott excitons in semiconductors with a superlattice

    International Nuclear Information System (INIS)

    The effect of the motion of a Wannier-Mott exciton in semiconductors with a superlattice formed by heterojunctions on the exciton binding energy and wave function is analyzed. This effect arises as a result of the fact that the dispersion laws of the electron and hole that form an exciton in a superlattice differ from the quadratic law. The investigated one-dimensional superlattice consists of alternating semiconductor layers with different energy positions of the conduction and valence bands, i.e., with one-dimensional wells and barriers. The exciton state in a superlattice consisting of quantum dots is analyzed. It is demonstrated that the closer the electron and hole effective masses, the greater the dependence of the binding energy on the exciton quasi-momentum. The possibility of replacing the tunneling excitation transfer between superlattice cells with the dipole-dipole one at certain exciton quasi-wave vector values is investigated

  18. Fractional Solitons in Excitonic Josephson Junctions.

    Science.gov (United States)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770

  19. Fractional Solitons in Excitonic Josephson Junctions

    Science.gov (United States)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-10-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.

  20. Development and understanding of multifunctional gold nanorings for photodynamic therapy of cancer

    Science.gov (United States)

    Hu, Yue

    Gold nanostructures of various geometries and dimensions are being broadly explored in the fast expanding field of nanomedicine due to the unusual combination of biocompatibility, versatility of surface functionalization, and localized surface-plasmon resonance (LSPR). Examples of relevant applications include biomedical imaging, clinical diagnostics and therapeutics. Gold nanorings (Au NRs) are of particular interest because of their intricate structure and LSPR tunability over a wide wavelength range. A solution-based synthesis approach is yet to be developed, however, to allow exploration of Au NRs for basic and applied studies. This dissertation focuses on the fabrication, characterization, and evaluation of colloidal Au NRs for enhanced photodynamic therapy (PDT) of breast cancer cells. We have developed a novel synthesis strategy and associated pathways toward Au NRs by galvanic replacement reaction of Co nanoparticles (Co NPs) in HAuCl4 solution. We have shown that surface-chemistry mediated particle-particle interaction of the Co NPs that leads to their 1D chain-like assembly as sacrificial template is critically important for the growth of Au NRs. Otherwise, Au nanoshells or Au nanotubes may result. The Au NRs exhibit tunable LSPR wavelengths from visible to near-infrared (NIR) region via varying the aspect ratio, in agreement with our theoretical calculations using the finite-difference time domain (FDTD) method. A layer-by-layer (LbL) assembly method was used to load Al(III) phthalocyanine chloride tetrasulfonic acid (AlPcS4) photosensitizer (PS) onto the surface of Au NRs. We have revealed that the photosensitivity of AlPcS4 can be quenched unless desorbed from the surface of Au NRs in the cellular compartment. An eight-fold increase in PDT killing efficiency of human breast cancer cells (MDA-MB-231) has been achieved using LbL-assembled AlPcS4-Au NR complexes in comparison to AlPcS4 only or the mixture of AlPcS 4 and Au NRs. This strategy allows

  1. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons

    Science.gov (United States)

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-03-01

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ~250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

  2. Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling

    OpenAIRE

    Moore, Barry; Autschbach, Jochen

    2012-01-01

    The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intens...

  3. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

    OpenAIRE

    Selig, Malte; Berghäuser, Gunnar; Raja, Archana; Nagler, Philipp; Schüller, Christian; Heinz, Tony F.; Korn, Tobias; Chernikov, Alexey; Malic, Ermin; Knorr, Andreas

    2016-01-01

    Atomically thin transition metal dichalcogenides (TMDs) are direct-gap semiconductors with strong light-matter and Coulomb interaction. The latter accounts for tightly bound excitons, which dominate the optical properties of these technologically promising materials. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emiss...

  4. Excitonic Properties in GaAs Parabolic Quantum Dots

    OpenAIRE

    Jaziri, S.; Bennaceur, R.

    1995-01-01

    Certain classes of semiconductor quantum dots being actually fabricated exhibit a nearly parabolic confinement for both the electron and the hole. In undoped quantum dots, excitonic effects are important. In this work, first we present theoretical results on exciton properties in parabolic quantum dots: resonance energy, binding energy and oscillator strength. Then, we investigate the effects of external electric and magnetic fields on exciton in quantum dots.

  5. Exciton ionization in multilayer transition-metal dichalcogenides

    DEFF Research Database (Denmark)

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

    2016-01-01

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

  6. Optical spectroscopy of near-surface excitonic states

    OpenAIRE

    H. Azucena Coyotécatl; N.R. Grigorieva; B.A. Kazennov; Novikov, B. V.; Madrigal, J.; F. Pérez Rodríguez; A.V. Sel´kin

    1999-01-01

    We have studied the low temperature excitonic reflectance and luminescence spectra of CdS1-xSex solid solution with a near-surface excitonic potential well formed by excessive Se. The reflectance spectra were obtained at different angles of incidence for boths-and p polarization components of light. New striking spectral features due to exciton confinement and quantization in the surface region were observed. For various samples, the generalized Morse surface potential was proved to be a good...

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

    DEFF Research Database (Denmark)

    Kalt, H.; Hoffmann, J.; Umlauff, M.; Langbein, Wolfgang Werner; Hvam, Jørn Märcher

    1998-01-01

    cw or time-resolved phonon-sideband spectroscopy. A thermal exciton distribution evolves on a 100 ps timescale at low exciton densities when only scattering with acoustic phonons is possible. Exciton-exciton scattering accelerates the relaxation dynamics at high densities. Filling of the free heavy......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 by......-hole exciton states and thermal population of the light-hole exciton states is observed. Measurements of the polarization degree after excitation with a sigma(+)-polarized picosecond pulse show that light-hole excitons with zero in-plane momentum change sign of their spin with each emission of an LO phonon....

  8. Revealing and Characterizing Dark Excitons Through Coherent Multidimensional Spectroscopy

    CERN Document Server

    Tollerud, Jonathan O; Davis, Jeffrey A

    2016-01-01

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

  9. Disorder-enhanced exciton delocalization in an extended dendrimer.

    Science.gov (United States)

    Pouthier, Vincent

    2014-08-01

    The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory. PMID:25215792

  10. Binding energies of indirect excitons in double quantum well systems

    Science.gov (United States)

    Rossokhaty, Alex; Schmult, Stefan; Dietsche, Werner; von Klitzing, Klaus; Kukushkin, Igor

    2011-03-01

    A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~ 5 × 109 cm-2 and results in their destruction.

  11. Spin-wave spectra and stability of the in-plane vortex state in two-dimensional magnetic nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Mamica, S., E-mail: mamica@amu.edu.pl [Faculty of Physics, Adam Mickiewicz University in Poznań, ul. Umultowska 85, 61-614 Poznań (Poland)

    2013-12-21

    We study theoretically two-dimensional nanorings assumed to have the in-plane vortex magnetic configuration. Using a discrete dipole model we examine the spectrum of normal spin-wave modes vs. the dipolar-to-exchange interaction ratio. We systematize the spin-wave excitations by their azimuthal and radial wave numbers. The lowest-frequency mode, the fundamental (quasiuniform) mode, and the mode hybridization are analyzed; the discussion of the influence of effective pinning at the ring boundaries is provided as well. We study the stability of the in-plane vortex state and discuss the role of the size of the ring and the type of lattice arrangement of the magnetic moments within it. To facilitate comparison with our results we provide the relationships between microscopic parameters, used in our model, and those used in the case of continuous medium.

  12. Width dependent transition of quantized spin-wave modes in Ni{sub 80}Fe{sub 20} square nanorings

    Energy Technology Data Exchange (ETDEWEB)

    Banerjee, Chandrima; Saha, Susmita; Barman, Saswati; Barman, Anjan, E-mail: abarman@bose.res.in [Thematic Unit of Excellence on Nanodevice Technology, Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700098 (India); Rousseau, Olivier [CEMS-RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198 (Japan); Otani, YoshiChika [CEMS-RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198 (Japan); Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581 (Japan)

    2014-10-28

    We investigated optically induced ultrafast magnetization dynamics in square shaped Ni{sub 80}Fe{sub 20} nanorings with varying ring width. Rich spin-wave spectra are observed whose frequencies showed a strong dependence on the ring width. Micromagnetic simulations showed different types of spin-wave modes, which are quantized upto very high quantization number. In the case of widest ring, the spin-wave mode spectrum shows quantized modes along the applied field direction, which is similar to the mode spectrum of an antidot array. As the ring width decreases, additional quantization in the azimuthal direction appears causing mixed modes. In the narrowest ring, the spin-waves exhibit quantization solely in azimuthal direction. The different quantization is attributed to the variation in the internal field distribution for different ring width as obtained from micromagnetic analysis and supported by magnetic force microscopy.

  13. Symmetry-related motional enhancement of exciton magnetic moment

    Energy Technology Data Exchange (ETDEWEB)

    Tronc, P. [Laboratoire d' Optique, Ecole Superieure de Physique et Chimie Industrielles, Paris (France)

    2010-05-15

    A dramatic motional enhancement of heavy- and light-hole exciton magnetic moment in zinc-blende semiconductors under a magnetic field applied parallel to the [001] direction has been put into evidence when the exciton moves along the same direction [J. J. Davies et al., Phys. Rev. Lett. 97, 187403 (2006)]. The authors of the paper assigned the effect to a mixing between the 1S and 2P exciton states arising from the cubic term in the Luttinger Hamiltonian expansion in momentum. Such exciton states do not take into account the exact crystal structure since they are just eigenstates of the angular momentum. In addition, the Luttinger Hamiltonian does not take into account the full magnetic-field effect since it does not include the gauge transformations under the symmetry operations of the structure under the field. By determining the exact symmetry of the exciton states, it is shown here that, under a field parallel to the [001] direction, the Zeeman Splitting value vanishes at the {gamma} point and, due to accidental quasi-degeneracy in energy between dark and bright exciton states, becomes finite when the exciton moves parallel to the field. A perturbation model allows fitting experimental data and explains the exciton magnetic-moment enhancement with kinetic energy. On the contrary, under a field parallel to the [110] direction with the exciton moving parallel to the field, no accidental degeneracy probably takes place between exciton states. As a consequence, the concept of Zeeman Splitting is not relevant since no energy level is degenerate. In addition, a possible quasi-degeneracy between the excitons recombining with the {sigma}{sup +} and {sigma}{sup -} polarizations, respectively, would not allow coupling their two states, hence would not change notably the experimental results. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  14. Plasmonic nanoring fabrication tuned to pitch: Efficient, deterministic, and large scale realization of ultra-small gaps for next generation plasmonic devices

    International Nuclear Information System (INIS)

    A double-patterning process for scalable, efficient, and deterministic nanoring array fabrication is presented. It enables gaps and features below a size of 20 nm. A writing time of 3 min/cm2 makes this process extremely appealing for scientific and industrial applications. Numerical simulations are in agreement with experimentally measured optical spectra. Therefore, a platform and a design tool for upcoming next generation plasmonic devices like hybrid plasmonic quantum systems are delivered.

  15. Bose Condensation of Interwell Excitons in Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Timofeev, V. B.; Ni, P. A.; Dubonos, S. V.; Hvam, Jørn Märcher; Soerensen, K.

    2002-01-01

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

  16. Tunable exciton Aharonov-Bohm effect in a quantum ring

    International Nuclear Information System (INIS)

    We studied the optical Aharonov-Bohm effect for an exciton in a semiconductor quantum ring. A perpendicular electric field applied to a quantum ring with large height, is able to tune the exciton ground state energy such that it exhibits a weak observable Aharonov-Bohm oscillations. This Aharonov-Bohm effect is tunable in strength and period.

  17. Excitonic luminescence upconversion in a two-dimensional semiconductor

    Science.gov (United States)

    Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong

    2016-04-01

    Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton-phonon coupling. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1' optical phonon, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.

  18. 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. PMID:25798712

  19. Excitonic Contribution on Light Emitting Properties of Nano silicon

    International Nuclear Information System (INIS)

    We developed a phenomenological model by integrating the effects of excitonic energy states, localized surface states and quantum confinement to obtain an analytical expression for the room temperature photoluminescence (PL) intensity. The binding energy and oscillator strength of strongly confined excitons has been calculated to examine its contribution on optical band gap and electronic properties of silicon quantum dots. The band gap decreases as much as 0.23 eV with excitonic effects for the smallest dot. The effects of exciton states explain almost accurately the experimental PL data. Our model provides the mechanism for controlling the PL intensity by controlling model parameters. The results for the size dependence of the optical band gap and oscillator strength are presented the role excitonic effects on optical and electronic properties are discussed. (author)

  20. Exciton Correlations in Intramolecular Singlet Fission.

    Science.gov (United States)

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

    2016-06-15

    We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases. PMID:27183040

  1. Multiple exciton generation effect in quantum dots

    International Nuclear Information System (INIS)

    Full text: Recently among various investigations in the field of nanophysics a great intensification in studying of multiple exciton generation (MEG) effect in quantum dots at absorption of photons with the energy several times higher than the gap in electronic spectra (hν>>Eg) has been made [1-2]. There have been proposed four theoretical approaches to explain the MEG effect. However, the statistic approach basing on the Fermi theory of multiple pi-mesons generation (1950) is the most adequate related to others. In this work: 1. the results of statistic approach have been generalized; 2. the microscopic mechanism basing on theory of electronic 'shaking' has been proposed; 3. the problem of deviation of mean multiplicity of MEG effect from the Poisson law of fluctuations has been investigated that is a evidence of strong non-linear and non-equilibrium character of exciton generation process; 4. the role of interface electronic states of quantum dot and ligand has been considered by means of perturbation theory of closed states related to surface Tamm electronic states of quantum dot and legend; 5. the problem of size optimization of quantum dot to receive the maximum multiplicity has been studied; 6. the efficiency of MEG effect for the third generation of solar cells has been calculated on the basis of proposed theory. (authors)

  2. 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.

  3. Luminescence, autolocalization and decay of excitons on defects in ionic crystals (CsBr)

    International Nuclear Information System (INIS)

    The review is made of investigations on specific features of excitons in alkali haloid crystals: existence of free and autolocalized excitons, autolocalizing excitons with defects production, defects recombination with luminescent exciton production, cubic crystals (mainly CsBr) taken as an exsample

  4. 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.

  5. Dynamical process of exciton-exciton scattering in CuI thin films

    International Nuclear Information System (INIS)

    We have investigated the time-resolved photoluminescence (PL) spectra of CuI thin films under intense excitation conditions in the time region up to 25 ps. In the time-integrated PL spectra, we have clearly observed the PL band originated from the inelastic scattering of excitons, so-called P emission. The time-resolved PL spectra obtained by using an optical Kerr gating method exhibit that the peak energy of the P-emission band temporally changes in a picosecond region, which reflects the variation of the effective temperature of excitonic system. In the time profile of the P-emission band, we found the following two characteristic properties. One is that the decay time hardly depends on the excitation power. The other is that the rise time becomes considerably faster with increasing excitation power. Moreover, we have found that the inverse of the rise time of the P emission exhibits an almost quadratic dependence on the excitation power

  6. Dipole excitons in coupled quantum wells: toward an equilibrium exciton condensate

    OpenAIRE

    Snoke, David W.

    2012-01-01

    In recent years, experiments by several groups have demonstrated spontaneous coherence in polariton systems, which can be viewed as a type of nonequilibrium Bose-Einstein condensation. In these systems, the polariton lifetime is longer than, but not much longer than, the polariton-polariton scattering time which leads to the thermalization. By contrast, over the past twenty years several groups have pursued experiments in a different system, which has very long exciton lifetime, up to 30 micr...

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

    CERN Document Server

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

    2015-01-01

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

  8. Excitation of exciton states on a curved surface

    Science.gov (United States)

    Silotia, Poonam; Prasad, Vinod

    2016-05-01

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

  9. Evidence of Excitonic Optical Tamm States using Molecular Materials

    CERN Document Server

    Núñez-Sánchez, S; Murshidy, M M; Abdel-Hady, A G; Serry, M Y; Adawi, A M; Rarity, J G; Oulton, R; Barnes, W L

    2015-01-01

    We report the first experimental observation of an Excitonic Optical Tamm State supported at the interface between a periodic multilayer dielectric structure and an organic dye-doped polymer layer. The existence of such states is enabled by the metal-like optical properties of the excitonic layer based on aggregated dye molecules. Experimentally determined dispersion curves, together with simulated data, including field profiles, allow us to identify the nature of these new modes. Our results demonstrate the potential of organic excitonic materials as a powerful means to control light at the nanoscale, offering the prospect of a new alternative type of nanophotonics based on molecular materials.

  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. 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.

  12. Control of Exciton Photon Coupling in Nano-structures

    Science.gov (United States)

    Liu, Xiaoze

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

  13. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    CERN Document Server

    Kenny, Elise P

    2015-01-01

    Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. We also caution ...

  14. Exciton transfer under dichotomic noise: GME treatment

    International Nuclear Information System (INIS)

    The exciton energy transfer between molecules in a dimer is investigated using a model, in which the influence of the phonons is described by a dichotomic stochastic process with colored noise giving rise to modulations of the molecular excitation energies. The solution of equations of motion for the density matrix of the system (obtained algebraically on a computer) is used to determine (also algebraically) the form of the memory functions which enter the Generalized Master Equation. The behaviour of the roots and their weights in the memory function is discussed thoroughly. Connection of the so called coherence time to stochastic parameters is treated analytically. Different limiting cases are investigated. (author). 14 refs, 9 figs

  15. Exciton dynamics in emergent Rydberg lattices

    CERN Document Server

    Bettelli, S; Fernholz, T; Adams, C S; Lesanovsky, I; Ates, C

    2013-01-01

    The dynamics of excitons in a one-dimensional ensemble with partial spatial order are studied. During optical excitation, cold Rydberg atoms spontaneously organize into regular spatial arrangements due to their mutual interactions. This emergent lattice is used as the starting point to study resonant energy transfer triggered by driving a $nS$ to $n^\\prime P$ transition using a microwave field. The dynamics are probed by detecting the survival probability of atoms in the $nS$ Rydberg state. Experimental data qualitatively agree with our theoretical predictions including the mapping onto XXZ spin model in the strong-driving limit. Our results suggest that emergent Rydberg lattices provide an ideal platform to study coherent energy transfer in structured media without the need for externally imposed potentials.

  16. Optical properties of Rydberg excitons and polaritons

    Science.gov (United States)

    Zielińska-Raczyńska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David

    2016-02-01

    We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu20 crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.

  17. Dynamics of Optically Driven Exciton and Quantum Decoherence

    Institute of Scientific and Technical Information of China (English)

    JIN Guang-Ri; LIU Yu-Xi; ZHOU Duan-Lu; SUN Chang-Pu

    2003-01-01

    By using the normal ordering method, we study the state evolution of an optically driven excitons in aquantum well immersed in a leaky cavity, which was introduced by Yu-Xi Liu et al. [Phys. Rev. A63 (2001) 033816]. Theinfluence of the externallaser field on the quantum decoherence of a mesoscopically superposed state of the excitons isinvestigated. Our result shows that, the classical field can compensate the energy dissipation of the excitons. Althoughthe decoherence rate of the excitonic Schrodinger cat state does not depend on the external field, the phase of thedecoherence factor can be well controlled by adjusting the amplitude of the external field as well as the detuning betweenthe field and the transition frequency of the atom.

  18. Vibrational exciton-mediated quantum state transfert: a simple model

    CERN Document Server

    Pouthier, Vincent J C

    2012-01-01

    A communication protocol is proposed in which quantum state transfer is mediated by a vibrational exciton. We consider two distant molecular groups grafted on the sides of a lattice. These groups behave as two quantum computers where the information in encoded and received. The lattice plays the role of a communication channel along which the exciton propagates and interacts with a phonon bath. Special attention is paid for describing the system involving an exciton dressed by a single phonon mode. The Hamiltonian is thus solved exactly so that the relevance of the perturbation theory is checked. Within the nonadiabatic weak-coupling limit, it is shown that the system supports three quasi-degenerate states that define the relevant paths followed by the exciton to tunnel between the computers. When the model parameters are judiciously chosen, constructive interferences take place between these paths. Phonon-induced decoherence is minimized and a high-fidelity quantum state transfer occurs over a broad temperat...

  19. Excitonic condensation in systems of strongly correlated electrons

    Czech Academy of Sciences Publication Activity Database

    Kuneš, Jan

    2015-01-01

    Roč. 27, č. 33 (2015), s. 333201. ISSN 0953-8984 Institutional support: RVO:68378271 Keywords : electronic correlations * exciton * Bose-Einstein condensation Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.346, year: 2014

  20. Charge Transfer Excitons at van der Waals Interfaces.

    Science.gov (United States)

    Zhu, Xiaoyang; Monahan, Nicholas R; Gong, Zizhou; Zhu, Haiming; Williams, Kristopher W; Nelson, Cory A

    2015-07-01

    The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation. PMID:26001297

  1. Superconductivity in CuCl/Si superlattices: excitonic pairing?

    Science.gov (United States)

    Rhim, S. H.; Saniz, Rolando; Weinert, Michael; Freeman, A. J.

    2013-03-01

    Two-dimensional (2D) hetero-bonded semiconductor interfaces have been suggested as candidate geometries where excitonic superconductivity [2] - and the greatly enhanced where TC compared to phonon mechanisms mediation - can be realized. Among experimental efforts, epitaxially grown CuCl on Si (111) has reportedly exhibited excitonic superconductivity at 60 ~150 K. Our first-principles calculations confirm 2D metallicity at the interfaces due to charge transfer by valence mismatch. [3] The excitonic mechanism is investigated by calculating the kernel function, K (ω) , for the average of the electronic contributions to the effective interaction.[4] The attractive interaction found in the CuCl/Si superlattice indicates the feasibility of excitonic pairing for a certain frequency range. US Department of Energy (DE-FG02-05ER45372)

  2. Excellent microwave-absorbing properties of elliptical Fe₃O₄ nanorings made by a rapid microwave-assisted hydrothermal approach.

    Science.gov (United States)

    Liu, Yun; Cui, Tingting; Wu, Tong; Li, Yana; Tong, Guoxiu

    2016-04-22

    High-quality elliptical polycrystalline Fe3O4 nanorings (NRs) with continuously tunable size have been synthesized in large amounts via a rapid microwave-assisted hydrothermal approach. The surface-protected glucose reducing/etching/Ostwald ripening mechanism is responsible for the formation of NRs. Ring size can be modulated by selecting iron glycolate nanosheets with various sizes as precursors. The size-dependent magnetic behavior of the NRs was observed. Our research gives insights into the understanding of the microwave absorption mechanism of elliptical Fe3O4 NRs. Owing to their large specific surface area, shape anisotropy, and closed ring-like configuration, elliptical polycrystalline Fe3O4 NRs exhibited significantly enhanced microwave absorption performance compared with Fe3O4 circular NRs, nanosheets, microspheres, nanospindles, and nanotubes. An optimal reflection loss value of -41.59 dB is achieved at 5.84 GHz and R(L) values (≤-20 dB) are observed at 3.2-10.4 GHz. Some new mechanisms including multiple scattering, oscillation resonance absorption, microantenna radiation, and interference are also crucial to the enhanced absorption properties of NRs. These findings indicate that ring-like nanostructures are a promising structure for devising new and effective microwave absorbers. PMID:26962718

  3. Combination of photothermal and photodynamic inactivation of cancer cells through surface plasmon resonance of a gold nanoring

    Science.gov (United States)

    Chu, Chih-Ken; Tu, Yi-Chou; Hsiao, Jen-Hung; Yu, Jian-He; Yu, Chih-Kang; Chen, Shih-Yang; Tseng, Po-Hao; Chen, Shuai; Kiang, Yean-Woei; Yang, C. C.

    2016-03-01

    We demonstrate effective inactivation of oral cancer cells SAS through a combination of photothermal therapy (PTT) and photodynamic therapy (PDT) effects based on localized surface plasmon resonance (LSPR) around 1064 nm in wavelength of a Au nanoring (NRI) under femtosecond (fs) laser illumination. The PTT effect is caused by the LSPR-enhanced absorption of the Au NRI. The PDT effect is generated by linking the Au NRI with the photosensitizer of sulfonated aluminum phthalocyanines (AlPcS) for producing singlet oxygen through the LSPR-enhanced two-photon absorption (TPA) excitation of AlPcS. The laser threshold intensity for cancer cell inactivation with the applied Au NRI linked with AlPcS is significantly lower when compared to that with the Au NRI not linked with AlPcS. The comparison of inactivation threshold intensity between the cases of fs and continuous laser illuminations at the same wavelength and with the same average power confirms the crucial factor of TPA under fs laser illumination for producing the PDT effect.

  4. Atomistic modelling of radiation effects: Towards dynamics of exciton relaxation

    OpenAIRE

    Shluger, A. L.; Gavartin, J. L.; Szymanski, M. A.; Stoneham, A. M.

    2000-01-01

    This brief review is focused on recent results of atomistic modelling and simulation of exciton related processes in ionic materials. We present an analysis of thermal fluctuations of the electrostatic potential in cubic ionic crystals and their relation to formation of a tail in the electron density of states and localisation of electronic states. Then the possible 'fast' mechanism of formation of F-H pairs in KBr as a result of decomposition of relaxing excitons is discussed. We briefly des...

  5. An exciton-polariton mediated all-optical router

    OpenAIRE

    Flayac, H.; Savenko, I. G.

    2013-01-01

    We propose an all-optical nonlinear router based on a double barrier gate connected to periodically modulated guides. A semiconductor microcavity is driven nonresonantly in-between the barriers to form an exciton-polariton condensate on a discrete state that is subject to the exciton blueshift. The subsequent coherent optical signal is allowed to propagate through a guide provided that the condensate energy is resonant with a miniband or is blocked if it faces a gap. While a symmetric sample ...

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

    OpenAIRE

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

    2016-01-01

    Massive bosonic particles realise a rich variety of collective quantum phenomena where their underlying fermionic structure is hardly observed. For example, Bose-Einstein condensation of atomic gases is quantitatively understood by neglecting the atoms fermionic nature. Semiconductor excitons, i.e. Coulomb-bound electron-hole pairs, constitute a class of composite bosons which contrasts with this behaviour. Indeed, Combescot and co-workers have predicted that the excitons underlying fermionic...

  7. Frequency combs with weakly lasing exciton-polariton condensates

    OpenAIRE

    Rayanov, K.; Altshuler, B. L.; Rubo, Y. G.; Flach, S.

    2015-01-01

    We predict the spontaneous modulated emission from a pair of exciton-polariton condensates due to coherent (Josephson) and dissipative coupling. We show that strong polariton-polariton inter- action generates complex dynamics in the weak-lasing domain way beyond Hopf bifurcations. As a result, the exciton-polariton condensates exhibit self-induced oscillations and emit an equidistant frequency comb light spectrum. A plethora of possible emission spectra with asymmetric peak dis- tributions ap...

  8. Enhanced Multiple Exciton Generation in Amorphous Silicon Nanoparticles

    OpenAIRE

    Kryjevski, Andrei; Kilin, Dmitri

    2014-01-01

    Multiple exciton generation (MEG) in nanometer-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms strongly depends on the degree of the core structural disorder as shown by the many-body perturbation theory (MBPT) calculations based on the density functional theory (DFT) simulations. Working to the second order in the electron-photon coupling and in the screened Coulomb interaction we calculate quantum efficiency (QE), the average number of excitons created...

  9. Femtosecond THz Studies of Intra-Excitonic Transitions

    Energy Technology Data Exchange (ETDEWEB)

    Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.

    2007-10-02

    Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.

  10. Phase coherence and spectral functions in the two-dimensional excitonic systems

    International Nuclear Information System (INIS)

    The nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose–Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered

  11. Bose–Einstein condensation of excitons in Cu2O: progress over 30 years

    International Nuclear Information System (INIS)

    Experiments on Bose–Einstein condensation (BEC) of excitons in the semiconductor Cu2O started over 30 years ago, as one of the first serious attempts at exciton BEC. Early claims were based on spectroscopic signatures and transport data which have since been reinterpreted, in large part because the Auger recombination process for excitons was not well understood. Understanding of the Auger process has advanced, and recent experiments have made significant progress toward exciton BEC. We review the history of experiments on exciton BEC in Cu2O, the Auger recombination process, and the prospects for observing exciton BEC in this system in the near future. (review article)

  12. Resonantly excited high-density exciton gas studiedvia broadbandTHz spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Huber, Rupert; Kaindl, Robert A.; Schmid, Ben A.; Chemla, Daniel S.

    2005-06-25

    We report the density-dependent crossover of a resonantly photoexcited exciton gas from insulating to conducting phases. Broadband terahertz spectroscopy gives direct access to the exciton binding energy via intra-excitonic 1s-2p transitions. A strong shift, broadening, and ultimately the disappearance of this resonance occurs with decreasing inter-particle distance. Densities of excitons and unbound electron-hole pairs are followed quantitatively using a model of the composite free-carrier and exciton terahertz conductivity. Comparison with near-infrared absorption changes illustrates a significantly enhanced energy shift and broadening of the intra-excitonic resonance.

  13. Benchmarking Calculations of Excitonic Couplings between Bacteriochlorophylls.

    Science.gov (United States)

    Kenny, Elise P; Kassal, Ivan

    2016-01-14

    Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. Understanding these uncertainties can guard against striving for unrealistic precision; at the same time, detailed benchmarks can allow important qualitative questions-which do not depend on the precise values of the simulation parameters-to be addressed with greater confidence about the conclusions. PMID:26651217

  14. Plasmon transmission through excitonic subwavelength gaps

    Science.gov (United States)

    Sukharev, Maxim; Nitzan, Abraham

    2016-04-01

    We study the transfer of electromagnetic energy across a subwavelength gap separating two co-axial metal nanorods. In the absence of spacer in the gap separating the rods, the system exhibits strong coupling behavior between longitudinal plasmons in the two rods. The nature and magnitude of this coupling are studied by varying various geometrical parameters. As a function of frequency, the transmission is dominated by a split longitudinal plasmon peak. The two hybrid modes are the dipole-like "bonding" mode characterized by a peak intensity in the gap and a quadrupole-like "antibonding" mode whose amplitude vanishes at the gap center. When the length of one rod is varied, this mode spectrum exhibits the familiar anti-crossing behavior that depends on the coupling strength determined by the gap width. When off-resonant 2-level emitters are placed in the gap, almost no effect on the frequency dependent transmission is observed. In contrast, when the molecular system is resonant with the plasmonic line shape, the transmission is strongly modified, showing characteristics of strong exciton-plasmon coupling. Most strongly modified is the transmission near the lower frequency "bonding" plasmon mode. The presence of resonant molecules in the gap affects not only the molecule-field interaction but also the spatial distribution of the field intensity and the electromagnetic energy flux across the junction.

  15. Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.

    Science.gov (United States)

    Al-Otaify, Ali; Kershaw, Stephen V; Gupta, Shuchi; Rogach, Andrey L; Allan, Guy; Delerue, Christophe; Binks, David J

    2013-10-21

    The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model. PMID:23999734

  16. Intermolecular exciton-exciton annihilation in phospholipid vesicles doped with [Ru(bpy)2dppz]2+

    Science.gov (United States)

    De la Cadena, Alejandro; Pascher, Torbjörn; Davydova, Dar'ya; Akimov, Denis; Herrmann, Felix; Presselt, Martin; Wächtler, Maria; Dietzek, Benjamin

    2016-01-01

    The ultrafast photophysics of [Ru(bpy)2dppz]2+ (dppz = dipyrido[3,2-a:2‧,3‧-c]-phenazine) embedded into the walls of phospholipid vesicles has been studied by femtosecond time-resolved pump-probe spectroscopy. While [Ru(bpy)2dppz]2+ has been studied intensively with respect to its intramolecular charge transfer processes, which are associated with the well known light-switch effect, this study focuses on intermolecular energy transfer processes taking place upon dense packing of the complexes into a phospholipid membrane composed of dipalmitoyl-L-α-phosphatidylglycerol, which can be thought of as a simplistic model of a cellular membrane. The data indicate additional quenching of excited [Ru(bpy)2dppz]2+ upon increasing the pump-pulse intensity. Hence, the observed photophysics, which is assigned to the presence of intermolecular exciton-exciton annihilation at high pump-intensities, might be related to the ultrafast photophysics of [Ru(bpy)2dppz]2+ when used as a chromophore to stain cells, an effect that may be taken into account during the employment of novel cellular markers based on Ru polypyridine complexes.

  17. The local surface plasmon resonance property and refractive index sensitivity of metal elliptical nano-ring arrays

    International Nuclear Information System (INIS)

    In this paper, we systematically investigate the optical property and refractive index sensitivity (RIS) of metal elliptical nano-ring (MENR) arranged in rectangle lattice by finite-difference time-domain method. Eight kinds of considered MENRs are divided into three classes, namely fixed at the same outer size, at the same inner size, and at the same middle size. All MENR arrays show a bonding mode local surface plasmon resonance (LSPR) peak in the near-infrared region under longitudinal and transverse polarizations, and lattice diffraction enhanced LSPR peaks emerge, when the LSPR peak wavelength (LSPRPW) matches the effective lattice constant of the array. The LSPRPW is determined by the charge moving path length, the parallel and cross interactions induced by the stable distributed charges, and the moving charges inter-attraction. High RIS can be achieved by small particle distance arrays composed of MENRs with big inner size and small ring-width. On the other hand, for a MENR array, the comprehensive RIS (including RIS and figure of merit) under transverse polarization is superior to that under longitudinal polarization. Furthermore, on condition that compared arrays are fixed at the same lattice constant, the phenomenon that the RIS of big ring-width MENR arrays may be higher than that of small ring-width MENR arrays only appears in the case of compared arrays with relatively small lattice constant and composed of MENRs fixed at the same inner size simultaneously. Meanwhile, the LSPRPW of the former MENR arrays is also larger than that of the latter MENR arrays. Our systematic results may help experimentalists work with this type of systems

  18. Nonmonotonic energy harvesting efficiency in biased exciton chains.

    Science.gov (United States)

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

    2007-10-21

    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 time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well. PMID:17949203

  19. Dynamics of exciton formation and relaxation in photoexcited semiconductors

    Science.gov (United States)

    Janković, Veljko; Vukmirović, Nenad

    2015-12-01

    We investigate the dynamics of the exciton formation and relaxation on a picosecond time scale following a pulsed photoexcitation of a semiconductor. The study is conducted in the framework of the density matrix theory complemented with the dynamics controlled truncation scheme. We truncate the phonon branch of the resulting hierarchy of equations and propose the form of coupling among single-phonon-assisted and higher-order phonon-assisted density matrices so as to ensure the energy and particle-number conservation in a closed system. Time scales relevant for the exciton formation and relaxation processes are determined from numerical investigations performed on a one-dimensional model for the values of model parameters representative of a typical organic and inorganic semiconductor. The exciton dynamics is examined for different values of central frequency of the exciting field, temperature, and microscopic model parameters, such as the strengths of carrier-carrier and carrier-phonon couplings. We find that for typical organic semiconductor parameters, formation of bound excitons occurs on a several-hundred-femtosecond time scale, while their subsequent relaxation and equilibration take at least several picoseconds. These time scales are consistent with recent experimental studies of the exciton formation and relaxation in conjugated polymer-based materials.

  20. Fundamental processes of exciton scattering at organic solar-cell interfaces: One-dimensional model calculation

    Science.gov (United States)

    Masugata, Yoshimitsu; Iizuka, Hideyuki; Sato, Kosuke; Nakayama, Takashi

    2016-08-01

    Fundamental processes of exciton scattering at organic solar-cell interfaces were studied using a one-dimensional tight-binding model and by performing a time-evolution simulation of electron–hole pair wave packets. We found the fundamental features of exciton scattering: the scattering promotes not only the dissociation of excitons and the generation of interface-bound (charge-transferred) excitons but also the transmission and reflection of excitons depending on the electron and hole interface offsets. In particular, the dissociation increases in a certain region of an interface offset, while the transmission shows resonances with higher-energy bound-exciton and interface bound-exciton states. We also studied the effects of carrier-transfer and potential modulations at the interface and the scattering of charged excitons, and we found trap dissociations where one of the carriers is trapped around the interface after the dissociation.

  1. Excitonic and vibronic structure of absorption spectra of Me-PTCDI and PTCDA crystals

    International Nuclear Information System (INIS)

    The excitonic and vibronic spectra (exciton + one quantum of intramolecular vibration) of Me-PTCDI and PTCDA crystals are studied in the case of strong mixing of a Frenkel exciton (FE) and charge-transfer excitons (CTEs). The linear optical susceptibility is calculated in the framework of dynamical theory of vibronic spectra. The absorption spectra of both crystals have been modelled. The positions, maximal values and integral intensity of the absorption peaks which correspond to the bound exciton-phonon states and to unbound (many-particle) states have been calculated using FE and CTEs's parameters of Me-PTCDI and PTCDA crystals. The calculated spectra show: (i) the possible recovering of excitonic and vibronic regions; (ii) bigger integral intensity of many-particle states in the case of weak exciton-phonon coupling; (iii) the dominant role of the bound states in the case of intermediate and strong exciton-phonon coupling

  2. Roles of Hund's rule coupling in excitonic density-wave states

    OpenAIRE

    Kaneko, Tatsuya; Ohta, Yukinori

    2014-01-01

    Excitonic density-wave states realized by the quantum condensation of electron-hole pairs (or excitons) are studied in the two-band Hubbard model with Hund's rule coupling and the pair hopping term. Using the variational cluster approximation, we calculate the grand potential of the system and demonstrate that Hund's rule coupling always stabilizes the excitonic spin-density-wave state and destabilizes the excitonic charge-density-wave state and that the pair hopping term enhances these effec...

  3. Exciton self-trapping into diatomic and triatomic molecular complexes in xenon cryocrystals

    International Nuclear Information System (INIS)

    The recent study of molecular trapped centers in Xe cryocrystals was extended on triatomic self-trapped excitons. Time- and spectrally-resolved molecular luminescence was measured in the temperature range 5-60 K. The processes of intrinsic exciton self-rapping into diatomic and triatomic molecular complexes and extrinsic exciton trapping at lattice imperfections were separated by selective photoexcitation of Xe cryocrystals by synchrotron radiation. The temperature dependencies of triplet lifetimes of molecular exciton subbands were measured for the first time

  4. Transport, Aharonov-Bohm, and Topological Effects in Graphene Molecular Junctions and Graphene Nanorings

    OpenAIRE

    Yannouleas, Constantine; Romanovsky, Igor; Landman, Uzi

    2015-01-01

    The unique ultra-relativistic, massless, nature of electron states in two-dimensional extended graphene sheets, brought about by the honeycomb lattice arrangement of carbon atoms in two-dimensions, provides ingress to explorations of fundamental physical phenomena in graphene nanostructures. Here we explore the emergence of new behavior of electrons in atomically precise segmented graphene nanoribbons (GNRs) and graphene rings with the use of tight-binding calculations, non-equilibrium Green'...

  5. Carrier cooling and exciton formation in GaSe

    International Nuclear Information System (INIS)

    The initial cooling of hot carriers and the subsequent exciton formation in GaSe are studied by time-resolved photoluminescence (PL) using femtosecond up-conversion techniques. From the time-resolved PL spectra of this layered III-VI semiconductor two different energy relaxation channels are derived. After an initial subpicosecond cooling due to Froehlich-type interaction of carriers with longitudinal optical E'(22) phonons a slower regime follows, which is dominated by deformation potential interaction with the nonpolar optical A1'(12) phonons. The coupling constant for nonpolar optical phonon scattering is derived. The subsequent formation of excitons is studied at different carrier densities and detection energies. A cross section for the free-exciton formation is determined based on a rate equation model. copyright 1997 The American Physical Society

  6. Eliashberg theory of excitonic insulating transition in graphene

    Energy Technology Data Exchange (ETDEWEB)

    Wang Jingrong; Liu Guozhu, E-mail: gzliu@ustc.edu.cn [Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026 (China)

    2011-04-20

    A sufficiently strong Coulomb interaction may open an excitonic fermion gap and thus drive a semimetal-insulator transition in graphene. In this paper, we study the Eliashberg theory of excitonic transition by coupling the fermion gap equation self-consistently to the equation of the vacuum polarization function. Including the fermion gap into the polarization function increases the effective strength of the Coulomb interaction because it reduces the screening effects due to the collective particle-hole excitations. Although this procedure does not change the critical point, it leads to a significant enhancement of the dynamical fermion gap in the excitonic insulating phase. The validity of the Eliashberg theory is justified by showing that the vertex corrections are suppressed at the large N limit.

  7. Correlation effect of Rabi oscillations of excitons in quantum dots

    International Nuclear Information System (INIS)

    We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree-Fock-type Coulomb interactions between excitons

  8. Exciton coupling induces vibronic hyperchromism in light-harvesting complexes

    International Nuclear Information System (INIS)

    The recently suggested possibility that weak vibronic transitions can be excitonically enhanced in light-harvesting complexes is studied in detail. A vibronic exciton dimer model that includes ground-state vibrations is investigated using the multi-configuration time-dependent Hartree method with a parameter set typical to photosynthetic light-harvesting complexes. The absorption spectra are discussed based on the Coulomb coupling, the detuning of the site energies, and the number of vibrational modes. Fluorescence spectra calculations show that the spectral densities obtained from the low-temperature fluorescence line-narrowing measurements of light-harvesting systems need to be corrected for the effects of excitons. For the J-aggregate configuration, as in most light-harvesting complexes, the true spectral density has a larger amplitude than that obtained from the measurement

  9. Competition between excitonic gap generation and disorder scattering in graphene

    International Nuclear Information System (INIS)

    We study the disorder effect on the excitonic gap generation caused by strong Coulomb interaction in graphene. By solving the self-consistently coupled equations of dynamical fermion gap m and disorder scattering rate Γ, we have found a critical line on the plane of interaction strength λ and disorder strength g. The phase diagram is divided into two regions: in the region with large λ and small g, m≠0 and Γ=0; in the other region, m=0 and Γ≠0 for nonzero g. In particular, there is no coexistence of finite fermion gap and finite scattering rate. These results imply a strong competition between excitonic gap generation and disorder scattering. This conclusion does not change when an additional contact four-fermion interaction is included. For sufficiently large λ, the growing disorder may drive a quantum phase transition from an excitonic insulator to a metal.

  10. Controlled Spin Transport in Planar Systems Through Topological Exciton

    CERN Document Server

    Abhinav, Kumar

    2015-01-01

    It is shown that a charge-neutral spin-1 exciton, possibly realizable only in planar systems like graphene and topological insulators, can be effectively used for controlled spin transport in such media. The effect of quantum and thermal fluctuations yield a parametric excitation threshold for its realization. This planar exciton differs from the conventional ones, as it owes its existence to the topological Chern-Simons (CS) term. The parity and time-reversal violating CS term can arise from quantum effects in systems with parity-breaking mass-gap. The spinning exciton naturally couples to magnetic field, leading to the possibility of controlled spin transport. Being neutral, it is immune to a host of effect, which afflicts spin transport through charged fermions.

  11. Bose Condensation of Interwell Excitons in Double Quantum Wells

    DEFF Research Database (Denmark)

    Larionov, A. V.; Timofeev, V. B.; Ni, P. A.; Dubonos, S. V.; Hvam, Jørn Märcher; Soerensen, K.

    2002-01-01

    The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n–i–n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring...... quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and...... observation of luminescence. For weak pumping (less than 50 µW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the...

  12. Large Range Manipulation of Exciton Species in Monolayer WS2

    CERN Document Server

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

    2016-01-01

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

  13. Exciton dynamics in organic light-emitting diodes

    Science.gov (United States)

    Kim, Kwangsik; Won, Taeyoung

    2012-11-01

    In this paper, we present a numerical simulation for the optoelectronic material and device characterization in organic light-emitting diodes (OLEDs). Our model includes a Gaussian density of states to account for the energetic disorder in the organic semiconductors and the Fermi-Dirac statistics to account for the charge-hopping process between uncorrelated sites. The motivation for this work is the extraction of the emission profile and the source spectrum of a given OLED structure. The physical model covers all the key physical processes in OLEDs: namely, charge injection, transport and recombination, exciton diffusion, transfer, and decay. The exciton model includes generation, diffusion, energy transfer, and annihilation. We assume that the light emission originates from an oscillation and is thus embodied as excitons and is embedded in a stack of multilayers. The outcoupled emission spectrum is numerically calculated as a function of viewing angle, polarization, and dipole orientation. We also present simulated current-voltage and transient results.

  14. Reflectance modulation by free-carrier exciton screening in semiconducting nanotubes

    Science.gov (United States)

    Pinto, Fabrizio

    2013-07-01

    A model of exciton screening by photo-generated free charges in semiconducting single-walled carbon nanotubes is considered to interpret recent data from the only experiment on this phenomenon reported in the literature. The potential of electron-hole interactions on the nanotube surface is computed starting from the derived full two-dimensional expression. The error of screened potential numerical computations is analyzed in detail by also including strategies for convergence acceleration and computing time optimization. The two-dimensional Wannier equation on the nanotube surface is solved by means of variational methods and convergence to published results in the unscreened case is demonstrated. The effect of screening charges on the exciton energy is estimated numerically by taking advantage of memoization algorithms. We show that a firm connection can be made between the present description and data readily available from future similar reflectivity experiments to constrain the linear density of photo-generated carriers. Applications of dielectric function modulation to dispersion force manipulation and nanodevice actuation are briefly discussed.

  15. Tunable exciton funnel using Moiré superlattice in twisted van der Waals bilayer.

    Science.gov (United States)

    Wu, Menghao; Qian, Xiaofeng; Li, Ju

    2014-09-10

    A spatially varying bandgap drives exciton motion and can be used to funnel energy within a solid (Nat. Photonics 2012, 6, 866-872). This bandgap modulation can be created by composition variation (traditional heterojunction), elastic strain, or in the work shown next, by a small twist between two identical semiconducting atomic sheets, creating an internal stacking translation u(r) that varies gently with position r and controls the local bandgap Eg(u(r)). Recently synthesized carbon/boron nitride (Nat. Nanotechnol. 2013, 8, 119) and phosphorene (Nat. Nanotechnol. 2014, 9, 372) may be used to construct this twisted semiconductor bilayer that may be regarded as an in-plane crystal but an out-of-plane molecule, which could be useful in solar energy harvesting and electroluminescence. Here, by first-principles methods, we compute the bandgap map and delineate its material and geometric sensitivities. Eg(u(r)) is predicted to have multiple local minima ("funnel centers") due to secondary or even tertiary periodic structures in-plane, leading to a hitherto unreported pattern of multiple "exciton flow basins". A compressive strain or electric field will further enhance Eg-contrast in different regions of the pseudoheterostructure so as to absorb or emit even broader spectrum of light. PMID:25110970

  16. Temperature effects in the absorption spectra and exciton luminescence in ammonium halides

    International Nuclear Information System (INIS)

    Warm-up behavior of the first maximum exciton absorption bands in ammonium halides is explored. Under phase transition occurs offset of bands, bound both with changing a parameter of lattice, and efficient mass of exciton. Warm-up dependency of quantum leaving a luminescence of self-trapped excitons in ammonium halides is measured. (author)

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

    DEFF Research Database (Denmark)

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

    2016-01-01

    We present a generalized hydrogen model for the binding energies (EB) and radii of excitons in two-dimensional (2D) materials that sheds light on the fundamental differences between excitons in two and three dimensions. In contrast to the well-known hydrogen model of three-dimensional (3D) excitons...

  18. 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

  19. Realization of an all optical exciton-polariton router

    International Nuclear Information System (INIS)

    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

  20. 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.

  1. Strong-field terahertz-optical mixing in excitons

    CERN Document Server

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

    2002-01-01

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

  2. Enhanced energy transport in genetically engineered excitonic networks

    Science.gov (United States)

    Park, Heechul; Heldman, Nimrod; Rebentrost, Patrick; Abbondanza, Luigi; Iagatti, Alessandro; Alessi, Andrea; Patrizi, Barbara; Salvalaggio, Mario; Bussotti, Laura; Mohseni, Masoud; Caruso, Filippo; Johnsen, Hannah C.; Fusco, Roberto; Foggi, Paolo; Scudo, Petra F.; Lloyd, Seth; Belcher, Angela M.

    2016-02-01

    One of the challenges for achieving efficient exciton transport in solar energy conversion systems is precise structural control of the light-harvesting building blocks. Here, we create a tunable material consisting of a connected chromophore network on an ordered biological virus template. Using genetic engineering, we establish a link between the inter-chromophoric distances and emerging transport properties. The combination of spectroscopy measurements and dynamic modelling enables us to elucidate quantum coherent and classical incoherent energy transport at room temperature. Through genetic modifications, we obtain a significant enhancement of exciton diffusion length of about 68% in an intermediate quantum-classical regime.

  3. 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.

  4. Nongeneric dispersion of excitons in the bulk of WSe2

    Science.gov (United States)

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

    2016-08-01

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

  5. Nano-rings with a handle – Synthesis of substituted cycloparaphenylenes

    Directory of Open Access Journals (Sweden)

    Anne-Florence Tran-Van

    2014-08-01

    Full Text Available The research of cycloparaphenylenes (CPPs, the smallest armchair carbon nanotube, has been a quest for the past decades which experienced a revival in 2008 when the first synthesis was achieved. Since then CPPs with various ring sizes have been realized. The incorporation of substituents and the synthesis of CPPs with building blocks different from phenyl rings bear challenges of their own. Such structures, however, are highly interesting, as they allow for an incorporation of CPPs as defined nano-objects for other applications. Therefore, this review provides a status report about the current efforts in synthesizing CPPs beyond the parent unsubstituted oligo-phenylene structure.

  6. 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...

  7. Crossover from polariton lasing to exciton lasing in a strongly coupled ZnO microcavity

    Science.gov (United States)

    Lai, Ying-Yu; Chou, Yu-Hsun; Lan, Yu-Pin; Lu, Tien-Chang; Wang, Shing-Chung; Yamamoto, Yoshihisa

    2016-02-01

    Unlike conventional photon lasing, in which the threshold is limited by the population inversion of the electron-hole plasma, the exciton lasing generated by exciton-exciton scattering and the polariton lasing generated by dynamical condensates have received considerable attention in recent years because of the sub-Mott density and low-threshold operation. This paper presents a novel approach to generate both exciton and polariton lasing in a strongly coupled microcavity (MC) and determine the critical driving requirements for simultaneously triggering these two lasing operation in temperature mechanisms in strongly coupled MCs and verify a new method with which to trigger dual laser emission based on exciton and polariton.

  8. Exciton-Phonon Scattering in CdSe/ZnSe Quantum Dots

    Institute of Scientific and Technical Information of China (English)

    张立功; 申德振; 范希武; 吕少哲

    2002-01-01

    A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantum dots with a ZnCdSe quantum well. We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantum dots and two-dimensional exciton in the CdSe wetting layer. The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantum dots in comparison with that in the two-dimensional wetting layer, which indicates the decrease of exciton and optical phonon coupling in the CdSe quantum dots.

  9. All-optical depletion of dark excitons from a semiconductor quantum dot

    International Nuclear Information System (INIS)

    Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the dark exciton population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion

  10. 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...

  11. Luminescence and decay of excitons in lead tungstate crystals

    Czech Academy of Sciences Publication Activity Database

    Laguta, V. V.; Nikl, Martin; Zazubovich, S.

    2007-01-01

    Roč. 42, - (2007), s. 515-520. ISSN 1350-4487 Grant ostatní: Estonian Science Foundation(EE) 6548 Institutional research plan: CEZ:AV0Z10100521 Keywords : luminescence * excitons defects, * lead tungstate Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.054, year: 2007

  12. Theory of Ultrafast Exciton Motion in Photosynthetic Antennae

    Science.gov (United States)

    Renger, Thomas; May, Volkhard

    1998-03-01

    Ultrafast exciton motion and its coupling to protein vibrations in photosynthetic antennae are investigated by means of a density matrix approach (O. Kühn, Th. Renger, T. Pullerits, J. Voigt, V. May, Ann. Rev. Photochem. Photobiol. (in press).). First we consider the Fenna Matthews Olson (FMO) photosynthetic antenna complex of Chlorobium Tepidum. Using the same approach and the same parameters, linear absorption spectra and ultrafast pump--probe and transient anisotropy spectra have been succesfully simulated (Th. Renger, V. May, J. Phys. Chem. B (submitted).). The model allows to utilize exciton relaxation data as a probe for a global--shape estimation of the spectral density of low--frequency protein vibrations. In a second approach concentrating on a Chla/Chlb hetero--dimer of the Light--Harvesting--Complex of the Photosystem II of higher plants an unified microscopic description is offered for coherent vibrational dynamics, excited state absorption, and exciton-exciton annihilation processes. The theory explains the intensity dependent ultrafast nonlinear optical response recently measured in a pump--probe experiment. The presence of non--Markovian effects in the dissipative dynamics is demonstrated (Th. Renger, V. May, Phys. Rev. Lett. 78), 3406 (1996), Th. Renger, V. May, J. Phys. Chem. B 101, 7211 (1997).

  13. Highly anisotropic and robust excitons in monolayer black phosphorus.

    Science.gov (United States)

    Wang, Xiaomu; Jones, Aaron M; Seyler, Kyle L; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-06-01

    Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices. PMID:25915195

  14. Exploration of exciton delocalization in organic crystalline thin films

    Science.gov (United States)

    Hua, Kim; Manning, Lane; Rawat, Naveen; Ainsworth, Victoria; Furis, Madalina

    The electronic properties of organic semiconductors play a crucial role in designing new materials for specific applications. Our group recently found evidence for a rotation of molecular planes in phthalocyanines that is responsible for the disappearance of a delocalized exciton in these systems for T >150K.................()().......1 In this study, we attempt to tune the exciton delocalization of small organic molecules using strain effects and alloying different molecules in the same family. The exciton behavior is monitored using time- and polarization resolved photolumniscence (PL) spectroscopy as a function of temperature. Specifically, organic crystalline thin films of octabutoxy phthalocyanine (H2OBPc), octyloxy phthalocyanines and H-bonded semiconductors such as the quinacridone and indigo derivatives are deposited on flexible substrates (i.e. Kapton and PEN) using an in-house developed pen-writing method.........2 that results in crystalline films with macroscopic long range order. The room temperature PL studies show redshift and changes in polarization upon bending of the film. Crystalline thin films of alloyed H2OBPc and octabutoxy naphthalocyanine with ratios ranging from 1:1 to 100:1 fabricated on both sapphire and flexible substrates are also explored using the same PL spectroscopy to elucidate the behaviors of delocalized excitons. .1N. Rawat, et al., J Phys Chem Lett 6, 1834 (2015). 2R. L. Headrick, et al., Applied Physics Letters 92, 063302 (2008). NSF DMR-1056589, NSF DMR-1062966.

  15. Optical properties of localized excitons in semiconductor nanostructures

    DEFF Research Database (Denmark)

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

    2002-01-01

    liniebredde ved lav temperatur. Ved stigende temperatur vokser den homogene liniebredde imidlertid hastigt, og en analyse af temperaturafhængigheden antyder, at exciton LO-fonon vekselvirkning i kvantepunkterne er den dominerende spredningsmekanisme ved højere temperaturer i den type kvantepunkter, der her er...

  16. Finite life time effects in the coherent exciton transfer

    International Nuclear Information System (INIS)

    The paper addresses a specific problem in the exciton transfer in molecular aggregates, namely the influence of the finite life time effects, on the memory functions entering the Generalized Master Equation (GME) which connect different sites of the system. 7 refs, 2 figs

  17. Hamiltonian multiplex interaction based on excitons effect in semiconductor QCs

    Directory of Open Access Journals (Sweden)

    Arezu Jahanshir

    2014-11-01

    Full Text Available The subject of modern technology has been the focus of extensive theoretical investigations in semiconducting nanostructures which we know as quantum dots (QCs. The possibility of monitoring and controlling the properties of QCs attracted considerable attention to these objects, as an important basic system in future technology. So, the quantum-mechanical effects play a significant role in the description of the formation mechanism QCs, determination of mass spectrum, binding energy and other characteristics. Based on QFT and by using oscillator representation method (ORM and operator product expansion technique developed in QFT, we study the properties of electron-hole QDs, determine mass spectrum and peruse spin-spin interactions in exciton system and its multiple pair systems. This method has applications to calculate the binding energy of exciton system in ground and excited states with semi-nuclear structure in semiconductor QCs or cold atomic few-body systems and develop the general calculation’s theory of few-body systems based on the Coulomb interaction between particles by forming excitonic pairs in semiconductor QCs. We investigate the binding energy of exciton bound states. It is shown that fermion particles have a very small mass, and after bonding together by dynamically force, constituent particles become massive, which is analogous to what happens in QCD.

  18. On the possibility of excitonic magnetism in Ir double perovskites

    Czech Academy of Sciences Publication Activity Database

    Pajskr, K.; Novák, Pavel; Pokorný, Vladislav; Kolorenč, Jindřich; Arita, R.; Kuneš, Jan

    2016-01-01

    Roč. 93, č. 3 (2016), "035129-1"-"035129-6". ISSN 1098-0121 R&D Projects: GA ČR GA13-25251S Institutional support: RVO:68378271 Keywords : spin-orbit coupling * double perovskite * excitonic magnetism Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.736, year: 2014

  19. Excitonic Doppler-Rabi Oscillations in a Moving Organic Slab

    Institute of Scientific and Technical Information of China (English)

    2005-01-01

    It is theoretically shown that excitonic Doppler-Rabi oscillations can occur in an organic slab moving along the axis of a high-Q cavity. Due to the √N enhancement of the vacuum Rabi frequency, this effect can be more easily observed than that in a moving two-level atom.

  20. Order-disorder oscillations in exciton-polariton superfluids.

    Science.gov (United States)

    Saito, Hiroki; Aioi, Tomohiko; Kadokura, Tsuyoshi

    2013-01-11

    The dynamics of an exciton-polariton superfluid resonantly pumped in a semiconductor microcavity are investigated by mean-field theory. Modulational instability develops into crystalline order, and then, ordered and disordered states alternately form. It is found that the crystalline order at rest can coexist with superflow. PMID:23383923

  1. Mori approach to exciton memories in initially unrelaxed excitonphonon systems: direct and reorganized perturbation expansions

    Science.gov (United States)

    Čápek, V.

    1993-11-01

    Starting from the Mori formalism, memory functions of excitons coupled to harmonic phonons in periodic crystals with linear exciton-phonon coupling are calculated in two limiting cases: that of the naxrow unrenormalized exciton band with only site local coupling and that of the weak exciton-phonon coupling. Mutual correspondence as well as that with results of analogous works is discussed. By a mathematical trick, perturbational series for memory kernel of initially unrela-xed excitons interacting locally with harmonic phonons is then reorganized. Using that, full agreement is achieved with previous results obtained by Generalized Master Equations for initially relaxed excitons in case of narrow exciton band width. For the weak exciton-phonon coupling case, appreciable reduction of damping of the quasicoherent channel may be achieved on account of polaron effects. Crucial role of the exciton bandwidth beyond the lowest order is found in both cases for the low-temperature exciton diffusivity. Some predictions on the temperature dependence of the phonon-scattering limited exciton diffusion constant are given.

  2. Using dark states for exciton storage in transition-metal dichalcogenides.

    Science.gov (United States)

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2016-01-27

    We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule 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 a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. PMID:26704568

  3. Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots

    Science.gov (United States)

    Damtie, Fikeraddis A.; Wacker, Andreas

    2016-03-01

    We study the exciton dynamics in an optically excited nanocrystal quantum dot. Multiple exciton formation is more efficient in nanocrystal quantum dots compared to bulk semiconductors due to enhanced Coulomb interactions and the absence of conservation of momentum. The formation of multiple excitons is dependent on different excitation parameters and the dissipation. We study this process within a Lindblad quantum rate equation using the full many-particle states. We optically excite the system by creating a single high energy exciton ESX in resonance to a double exciton EDX. With Coulomb electron-electron interaction, the population can be transferred from the single exciton to the double exciton state by impact ionisation (inverse Auger process). The ratio between the recombination processes and the absorbed photons provide the yield of the structure. We observe a quantum yield of comparable value to experiment assuming typical experimental conditions for a 4 nm PbS quantum dot.

  4. Electron energy loss spectroscopy of excitons in two-dimensional-semiconductors as a function of temperature

    Science.gov (United States)

    Tizei, Luiz H. G.; Lin, Yung-Chang; Lu, Ang-Yu; Li, Lain-Jong; Suenaga, Kazu

    2016-04-01

    We have explored the benefits of performing monochromated Electron Energy Loss Spectroscopy (EELS) in samples at cryogenic temperatures. As an example, we have observed the excitonic absorption peaks in single layer Transition Metal Dichalcogenides. These peaks appear separated by small energies due to spin orbit coupling. We have been able to distinguish the split for MoS2 below 300 K and for MoSe2 below 220 K. However, the distinction between peaks is only clear at 150 K. We have measured the change in absorption threshold between 150 K and 770 K for MoS2 and MoSe2. We discuss the effect of carbon and ice contamination in EELS spectra. The increased spectral resolution available made possible with modern monochromators in electron microscopes will require the development of stable sample holders which reaches temperatures far below that of liquid nitrogen.

  5. Electron energy loss spectroscopy of excitons in two-dimensional-semiconductors as a function of temperature

    KAUST Repository

    Tizei, Luiz H. G.

    2016-04-21

    We have explored the benefits of performing monochromated Electron Energy Loss Spectroscopy(EELS) in samples at cryogenic temperatures. As an example, we have observed the excitonic absorption peaks in single layer Transition Metal Dichalcogenides. These peaks appear separated by small energies due to spin orbit coupling. We have been able to distinguish the split for MoS2 below 300 K and for MoSe2 below 220 K. However, the distinction between peaks is only clear at 150 K. We have measured the change in absorption threshold between 150 K and 770 K for MoS2 and MoSe2. We discuss the effect of carbon and ice contamination in EELSspectra. The increased spectral resolution available made possible with modern monochromators in electron microscopes will require the development of stable sample holders which reaches temperatures far below that of liquid nitrogen.

  6. 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...... exciton collective phase at temperatures below a critical Tc. A theoretical analysis of the interwell exciton spin-flip dynamics has been developed....

  7. Gate-controlled generation of optical pulse trains using individual carbon nanotubes

    OpenAIRE

    Jiang, M; Kumamoto, Y.; Ishii, A; Yoshida, M.; Shimada, T; Kato, Y. K.

    2014-01-01

    In single-walled carbon nanotubes, electron–hole pairs form tightly bound excitons because of limited screening. These excitons display a variety of interactions and processes that could be exploited for applications in nanoscale photonics and optoelectronics. Here we report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically induced carrier accumulation quenches photoluminescence, while a voltag...

  8. Coherent dynamics of exciton orbital angular momentum transferred by optical vortex pulses

    Science.gov (United States)

    Shigematsu, K.; Yamane, K.; Morita, R.; Toda, Y.

    2016-01-01

    The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge (ℓ ) dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different ℓ values under the same excitation conditions. The OAM decay times of the ℓ =1 component are usually longer than those of the ℓ =0 component. The ℓ -dependent OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing.

  9. Effect of the Pauli exclusion principle on the singlet exciton yield in conjugated polymers

    Science.gov (United States)

    Thilagam, A.

    2016-03-01

    Optical devices fabricated using conjugated polymer systems give rise to singlet exciton yields which are high compared to the statistically predicted estimate of 25 % obtained using simple recombination schemes. In this study, we evaluate the singlet exciton yield in conjugated polymers systems by fitting to a model that incorporates the Pauli exclusion principle. The rate equations which describe the exciton dynamics include quantum dynamical components (both density and spin-dependent) which arise during the spin-allowed conversion of composite intra-molecular excitons into loosely bound charge-transfer (CT) electron-hole pairs. Accordingly, a crucial mechanism by which singlet excitons are increased at the expense of triplet excitons is incorporated in this work. Non-ideal triplet excitons which form at high densities, are rerouted via the Pauli exclusion mechanism to form loosely bound CT states which subsequently convert to singlet excitons. Our derived expression for the yield in singlet exciton incorporates the purity measure and provides a realistic description of the carrier dynamics at high exciton densities.

  10. ULTRAHIGH MAGNETIC FIELD OPTICAL STUDY OF SINGLE-WALLED CARBON NANOTUBES FILM

    Directory of Open Access Journals (Sweden)

    Mukhtar Effendi

    2012-02-01

    Full Text Available Excitons in Single-Walled Carbon Nanotubes (SWNTs have emerged as an ideal candidate for exploring one-dimensional (1-D exciton physics. Exciton states which dominate optical properties of SWNTs even at room temperature, are not clarify yet. The optical absorption spectra of aligned SWNTs films under ultra high magnetic fields up to 190 T are examined to investigate this issue. Shifting and splitting of the absorption peaks due to Aharonov-Bohm effect was observed clearly above 80 T in the configuration where the magnetic fields were applied in parallel to the alignment of SWNTs. The lowest singlet exciton state has been determined through the analysis of energy splitting of excitons by the application of magnetic fields.   Keywords: blue shift, optically active, optically inactive, red shift, single-turn coil system

  11. Exciton collision and fluorescence quenching in R-phycoerythrin

    Science.gov (United States)

    Zheng, Xiguang; Wang, He Z.; Zhao, Fuli; Gao, Zhaolan; Yu, Zhenxin; Zhu, Jinchang; Xia, Andong; Jiang, Lijin

    1994-08-01

    Single ps pulse spectroscopic techniques have been employed to investigate excited-state dynamics of R-phycoerythrin (R-PE) isolated from two red algae Polysiphonia urceolata and Porphyra yezoensis, respectively, at various excitation photon densities in a range of 8 X 1014 $AP 1 X 1017 photons/cm2. The fluorescence quantum yields of the R-PE from the two algae are found to decrease in logarithm with increasing of excitation intensity due to S1-S1 exciton collision. A fast component in addition to a slow one recorded at low excitation density and a large initial anisotropy are observed at higher excitation density. Different dynamical characteristics of exciton collision between the cases of Polysiphonia urceolata and Porphyra yezoensis are shown and proposed to be owing to different numbers of excited pigments available and chromophore architectrue in R-PE from different biological sources.

  12. Organic photovoltaic cell incorporating electron conducting exciton blocking layers

    Energy Technology Data Exchange (ETDEWEB)

    Forrest, Stephen R.; Lassiter, Brian E.

    2014-08-26

    The present disclosure relates to photosensitive optoelectronic devices including a compound blocking layer located between an acceptor material and a cathode, the compound blocking layer including: at least one electron conducting material, and at least one wide-gap electron conducting exciton blocking layer. For example, 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure increased power conversion efficiency is achieved, compared to an analogous device using a conventional blocking layers shown to conduct electrons via damage-induced midgap states.

  13. Direct measurement of exciton valley coherence in monolayer WSe2

    Science.gov (United States)

    Hao, Kai; Moody, Galan; Wu, Fengcheng; Dass, Chandriker Kavir; Xu, Lixiang; Chen, Chang-Hsiao; Sun, Liuyang; Li, Ming-Yang; Li, Lain-Jong; MacDonald, Allan H.; Li, Xiaoqin

    2016-07-01

    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.

  14. Luminescence of excitons in mesoscopic ZnO particles

    International Nuclear Information System (INIS)

    Luminescence and time-resolved luminescence spectra of ZnO particles with the size of ∼20 nm have been investigated under band-to-band excitation. In the ultraviolet spectral region, there appear broad luminescence bands at 367.6, 368.8, 374.5 and 381.9 nm at 10 K. Decay profiles at 369 and 374 nm exhibit two decay components. The decay times are ∼20 and ∼100 ps for 369 nm, and ∼40 and ∼400 ps for 374 nm. These luminescence bands are attributable to the radiative recombination of free and bound excitons. Based on the present results, the effects of surface states and defects on excitons in mesoscopic ZnO particles have been discussed

  15. Exciton localization in solution-processed organolead trihalide perovskites

    Science.gov (United States)

    He, Haiping; Yu, Qianqian; Li, Hui; Li, Jing; Si, Junjie; Jin, Yizheng; Wang, Nana; Wang, Jianpu; He, Jingwen; Wang, Xinke; Zhang, Yan; Ye, Zhizhen

    2016-03-01

    Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium-lead-halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

  16. Chemical potential and compressibility of quantum Hall bilayer excitons,.

    Energy Technology Data Exchange (ETDEWEB)

    Skinner, Brian

    2016-02-25

    I consider a system of two parallel quantum Hall layers with total filling factor 0 or 1. When the distance between the layers is small enough, electrons and holes in opposite layers can form inter-layer excitons, which have a finite effective mass and interact via a dipole-dipole potential. I present results for the chemical potential u of the resulting bosonic system as a function of the exciton concentration n and the interlayer separation d. I show that both u and the interlayer capacitance have an unusual nonmonotonic dependence on d, owing to the interplay between an increasing dipole moment and an increasing effective mass with increasing d. Finally, I discuss the transition between the superfluid and Wigner crystal phases, which is shown to occur at d x n-1/10. Results are derived first via simple intuitive arguments, and then verified with more careful analytic derivations and numeric calculations.

  17. 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. PMID:20449863

  18. 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.

  19. Exciton localization in solution-processed organolead trihalide perovskites

    Science.gov (United States)

    He, Haiping; Yu, Qianqian; Li, Hui; Li, Jing; Si, Junjie; Jin, Yizheng; Wang, Nana; Wang, Jianpu; He, Jingwen; Wang, Xinke; Zhang, Yan; Ye, Zhizhen

    2016-01-01

    Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices. PMID:26996605

  20. Radiative coupling and weak lasing of exciton-polariton condensates

    OpenAIRE

    Aleiner, I. L.; Altshuler, B. L.; Rubo, Y. G.

    2011-01-01

    In spite of having finite life-time exciton-polaritons in microcavities are known to condense at strong enough pumping of the reservoir. We present an analytical theory of such Bose-condensates on a set of localized one-particle states: condensation centers. To understand physics of these arrays one has to supplement the Josephson coupling by the radiative coupling caused by the interference of the light emitted by different centers. Combination of these couplings with the one-site interactio...

  1. The Nature of Singlet Exciton Fission in Carotenoid Aggregates

    OpenAIRE

    Musser, Andrew J.; Maiuri, Margherita; Brida, Daniele; Cerullo, Giulio; Friend, Richard H.; Clark, Jenny

    2015-01-01

    Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains...

  2. Exciton dephasing in single InGaAs quantum dots

    DEFF Research Database (Denmark)

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

    2000-01-01

    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. The ho...... 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....

  3. On the feasibility of a nuclear exciton laser

    OpenAIRE

    Brinke, Nicolai ten; Schützhold, Ralf; Habs, Dietrich

    2012-01-01

    Nuclear excitons known from M\\"ossbauer spectroscopy describe coherent excitations of a large number of nuclei -- analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately,...

  4. Exciton Relaxation Dynamics in Photo-Excited CsPbI3 Perovskite Nanocrystals

    Science.gov (United States)

    Liu, Qinghui; Wang, Yinghui; Sui, Ning; Wang, Yanting; Chi, Xiaochun; Wang, Qianqian; Chen, Ying; Ji, Wenyu; Zou, Lu; Zhang, Hanzhuang

    2016-01-01

    The exciton relaxation process of CsPbI3 perovskite nanocrystals (NCs) has been investigated by using transient absorption (TA) spectroscopy. The hot exciton relaxation process is confirmed to exist in the CsPbI3 NCs, through comparing the TA data of CsPbI3 NCs in low and high energy excitonic states. In addition, the Auger recombination and intrinsic decay paths also participate in the relaxation process of CsPbI3 NCs, even the number of exciton per NC is estimated to be less than 1. Excitation intensity-dependent TA data further confirms the existence of Auger recombination. Meanwhile, the spectral data also confirms that the weight of hot exciton also increase together with that of Auger recombination at high excitation intensity when CsPbI3 NCs in high energy excitonic states. PMID:27405786

  5. Parameters of exciton absorption in TlGaS2 crystal

    International Nuclear Information System (INIS)

    It is shown that because the form of the exciton absorption in the TlGaS2 crystal is described by the Fano anti-resonance circuit, the experimentally observed exciton peak corresponds to the modified state, originating as the result of the configurational interaction of the exciton discrete state with the quasi-uninterrupted continuum of the conductivity zone state. The oscillator power for transition into the discrete exciton state, which constitutes F0 = 1.22 x 10-2, is calculated. The rules for selecting the exciton transition for two supposed groups of the D2h and D4h symmetry are obtained. The conclusion is made on the basis of analysis of the rules for selecting exciton transition, allowed in the dipole approximation, that the D2h group should be considered as the symmetry group for the TlGaS2 crystal

  6. 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.

  7. Bleaching in the region of exciton resonance of layered GaSe crystals

    Science.gov (United States)

    Kyazym-zade, A. G.; Salmanov, V. M.; Guseinov, A. G.; Salmanova, A. A.; Mamedov, R. M.; Dzhavadzade, A. A.

    2014-09-01

    Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd3+ laser emitting 30-ps light pulses and a dye laser with a pulse width of ˜3 ns tunable within the range 594-643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.

  8. Ultrasensitive photoelectrochemical immunoassay through tag induced exciton trapping.

    Science.gov (United States)

    Wen, Guangming; Ju, Huangxian

    2015-03-01

    The development of photoelectrochemical (PEC) sensors with novel principles is of significance in realizing sensitive and low-cost detection. This work uses CuO NPs labeled antibody to construct a simple and sensitive sandwich-type immunobiosensor for the detection of protein. The detection signal is produced by dissolving the CuO NPs to release copper ions, which are then added on a quantum dots (QDs) modified F-doped tin oxide to quench the photocurrent of QDs via copper ion-induced formation of exciton trapping. The formed exciton trapping blocks the escape of photoelectron and thus leads to a "signal off" PEC method for sensitive immunoassay. The proposed method shows a detectable range from 0.05 to 500 ng/mL for α-fetoprotein (AFP) with a detection limit (LOD) of 0.038 ng/mL. This work further extends the application of exciton trapping-based PEC biosensing strategy in bioanalysis. The sensitive analytical performance of the designed route implies a promising potential of the PEC sensing in clinical diagnosis. PMID:25618699

  9. 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.

  10. Exciton-polariton state in nanocrystalline SiC films

    Science.gov (United States)

    Semenov, А. V.; Lopin, А. V.

    2016-05-01

    We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.

  11. Wannier-Mott excitons in isotope-disordered crystals

    International Nuclear Information System (INIS)

    Most of the physical properties of a solid depend to a greater or lesser degree on its isotopic composition. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor and insulator crystals. A systematic analysis is for the first time presented of isotopic and disorder effects observed in crystals of various isotopic composition via low-temperature large-radius exciton spectroscopy. Substituting a light isotope with a heavy one increases the interband transition energy and the binding energy of the Wannier-Mott exciton as well as the magnitude of the longitudinal-transverse splitting. The nonlinear variation of these quantities with the isotope concentration is due to the isotopic disordering of the crystal lattice and is consistent with the concentration dependence of line half-widths in exciton reflection and luminescence spectra. The common nature of the isotopic and disorder effects in the crystals of C, LiH, ZnO, ZnSe, CuCl, CdS, Cu2O, GaAs, Si and Ge is emphasized. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor and insulator crystals. (author)

  12. Multiple Energy Exciton Shelves in Quantum-Dot-DNA Nanobioelectronics.

    Science.gov (United States)

    Goodman, Samuel M; Singh, Vivek; Ribot, Josep Casamada; Chatterjee, Anushree; Nagpal, Prashant

    2014-11-01

    Quantum dots (QDs) are semiconductor nanocrystallites with multiple size-dependent quantum-confined states that are being explored for utilizing broadband radiation. While DNA has been used for the self-assembly of nanocrystals, it has not been investigated for the formation of simultaneous conduction pathways for transporting multiple energy charges or excitons. These exciton shelves can be formed by coupling the conduction band, valence band, and hot-carrier states in QDs with different HOMO-LUMO levels of DNA nucleobases, resulting from varying degrees of conjugation in the nucleobases. Here we present studies on the electronic density of states in four naturally occurring nucleobases (guanine, thymine, cytosine, and adenine), which energetically couple to quantized states in semiconductor QDs. Using scanning tunneling spectroscopy of single nanoparticle-DNA constructs, we demonstrate composite DOS of chemically coupled DNA oligonucleotides and cadmium chalcogenide QDs (CdS, CdSe, CdTe). While perfectly aligned CdTe QD-DNA states lead to exciton shelves for multiple energy charge transport, mismatched energy levels in CdSe QD-DNA introduce intrabandgap states that can lead to charge trapping and recombination. Although further investigations are required to study the rates of charge transfer, recombination, and back-electron transfer, these results can have important implications for the development of a new class of nanobioelectronics and biological transducers. PMID:26278768

  13. Exciton emission in PTCDA films and PTCDA/Alq3 multilayers

    OpenAIRE

    Wagner, H.; Desilva, A.; Kampen, T.

    2004-01-01

    We investigate the exciton emission in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) thin films and PTCDA/aluminium-tris-hydroxyqinoline (Alq3) multilayers by photoluminescence spectroscopy in the temperature range from 10 to 300 K. The films are grown by organic molecular beam deposition on Si(001) and PyrexTM substrates at high vacuum. The obtained temperature dependence of the different recombination channels arising from indirect Frenkel excitons, charge transfer excitons, exci...

  14. Exciton-optical-phonon coupling: comparison with experiments for ZnO quantum wells

    OpenAIRE

    Makino, T; Segawa, Y; Kawasaki, M.

    2005-01-01

    The temperature-dependent linewidths of excitons in ZnO quantum wells were studied by measuring absorption spectra from 5 K to room temperature. We deduced experimentally the exciton-longitudinal-optical (LO) phonon coupling strength, which showed reduction of coupling with decrease in well width. This reduction was explained in terms of confinement-induced enhancement of the excitonic binding energy by comparing the binding energy dependence of calculated coupling strength.

  15. Josephson current in a four terminal superconductor - exciton condensate - superconductor system

    OpenAIRE

    Peotta S.; Gibertini M.; Dolcini F.; Taddei F.; Polini M.; Ioffe L.B.; Fazio R.; MacDonald A.H.

    2011-01-01

    We investigate the transport properties of a bilayer exciton condensate that is contacted by four superconducting leads. We focus on the equilibrium regime and investigate how the Josephson currents induced in the bilayer by phase biases applied to the superconducting electrodes are affected by the presence of an exciton condensate in the bulk of the system. As long as the distance between the superconducting electrodes is much larger than the exciton coherence length, the Josephson current d...

  16. Exciton Transport in Thin-Film Cyanine Dye J-Aggregates

    OpenAIRE

    Valleau, Stephanie; Semion K. Saikin; Yung, Man-Hong; Aspuru-Guzik, Alan

    2012-01-01

    We present a theoretical model for the study of exciton dynamics in J-aggregated monolayers of fluorescent dyes. The excitonic evolution is described by a Monte-Carlo wave function approach which allows for a unified description of the quantum (ballistic) and classical (diffusive) propagation of an exciton on a lattice in different parameter regimes. The transition between the ballistic and diffusive regime is controlled by static and dynamic disorder. As an example, the model is applied t...

  17. Temperature dependence of excitonic transition in ZnSe/ZnCdSe quantum wells

    Institute of Scientific and Technical Information of China (English)

    GUO Zi-zheng; LIANG Xi-xia; BAN Shi-liang

    2005-01-01

    A theoretical calculation for the temperature dependence of the excitonic transition in ZnSe/ZnCdSe quantum wells is performed. The exciton binding energy is calculated with a variational technique by considering the temperature-dependence parameters. Our results show that the exciton binding energy reduces linearly with temperature increasing. We find that the strain due to lattice mismatch and differential thermal expansion decreases with the temperature increasing.

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

    DEFF Research Database (Denmark)

    Hoffmann, J.; Umlauff, M.; Kalt, H.; Langbein, Wolfgang Werner; Hvam, Jørn Märcher

    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...... are theoretically reproduced by explicitly calculating the relevant phonon-exciton scattering rates....

  19. Static configurations of an exciton field in interaction with a deformable two-dimensional hexagonal lattice

    OpenAIRE

    Watson, M.J.; Zakrzewski, W. J.

    2004-01-01

    A model describing the dynamics of crystal exciton states on a hexagonal lattice has been studied. The hamiltonian includes terms describing the movement of the exciton throughout the crystal. Here we generalize the hamiltonian by including first order couplings of such terms to the lattice phonon fields. A discussion of the ground state configuration of the exciton field as a function of the system couplings is given. Finally we analyse the stability of certain excited states under perturbat...

  20. 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.

  1. 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.

  2. Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites

    Science.gov (United States)

    Abdel-Baki, K.; Boitier, F.; Diab, H.; Lanty, G.; Jemli, K.; Lédée, F.; Garrot, D.; Deleporte, E.; Lauret, J. S.

    2016-02-01

    Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites has emerged for few years. When they are arranged in two dimensions, these materials can be considered as hybrid quantum wells. One consequence of the unique structure of 2D hybrid organic perovskites is a huge exciton binding energy that can be tailored through chemical engineering. We present experimental investigations of the exciton non-linearities by means of femtosecond pump-probe spectroscopy. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ˜100 ps. Moreover, an ultrafast intraband relaxation (energy.

  3. Optical Study of Exciton Localization Phenomena in Semimagnetic Semiconductors and Their Multiple Quantum Wells.

    Science.gov (United States)

    Zhang, Xi-Cheng

    1986-12-01

    The results of picosecond photomodulation and photoluminescence spectroscopies in novel II-VI semimagnetic semiconductors Cd(,1-x)Mn(,x)Te (x CdTe/Cd(,1 -x)Mn(,x)Te MQW samples at low temperature shows intense excitonic emission where their radiative quantum efficiencies are two or three orders of magnitude larger than that of the high quality CdTe bulk samples. Time-resolved photoluminescence shows that the excitons have relatively short lifetime (500 picosecond). High quantum efficiency and short exciton lifetime suggest that the radiative recombination is a dominating factor in the excitonic-decay processes in the MQW samples. In general, excitonic emission energies in CdMnTe MQW samples are lower than the free exciton energies (typically 20-40 meV lower as noted from the reflectance spectra). The behavior of these emissions under an external magnetic field (up to 36 tesla) shows that excitons prefer to be localized at the heterointerfaces rather than at the center of the wells in MQW samples. The kinetics of the free and the heterointerface localized excitons in the Cd(,1-x)Mn(,x)Te/Cd(,1-y)Mn(,y)Te MQW samples have been studied by using a transient photoluminescence technique. Exciton lifetimes have been measured in several samples with various quantum well widths. The trapping time of the free exciton localized at the interface has been observed in the wide quantum well samples. The average energy loss rate of localized excitons has been calculated. The resonance excitation spectra of steady-state and transient luminescence show that the exciton spectra are spatially inhomogeneously broadened. An external magnetic field accelerates the localized exciton recombination processes. The dynamics of the excitons in CdMnTe MQW samples suggests that quasi-2D bound magnetic polarons (BMP) exist. A discussion of this quasi-2D BMP and the influence of an external magnetic field on exciton lifetime and exciton dynamics in Cd(,1 -x)Mn(,x)Te MQW is included.

  4. Effects of Electric and Magnetic Fields on Pure Dephasing of Exciton Qubits

    Institute of Scientific and Technical Information of China (English)

    LIU Yun-Fei; XIAO Jing-Lin

    2009-01-01

    In a two-dimensional quantum dot (QD) with parabolic confinement potential, we investigate pure dephasing due to deformation potential exciton-bulk longitudinal acoustic phonons (LAP) interaction for exciton qubits under the influence of external static electric and magnetic fields by adopting the full quantum-mechanical method of Kunihiro Kojima and Akihisa Tomita. The wave function is found and the dependence of the pure dephusing factor on the confinement length of the QD and time and temperature is discussed. We find the external electric and magnetic fields have important effects on pure dephasing of exciton qubits because exciton-LAP interaction increases, leading to more pure dephasing.

  5. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy

    Science.gov (United States)

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-10-01

    Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

  6. 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.

  7. Electronic properties of exciton and biexciton in a CdTe/ZnTe nano-heterostructure

    International Nuclear Information System (INIS)

    Energy eigen values and the binding energies of exciton and biexciton in a CdTe/ZnTe quantum dot are studied with the geometrical confinement effect. The single exciton binding energy and the biexciton binding energy are calculated as a function of dot radius. Overlap integral of the exciton and the biexciton is found in the CdTe/ZnTe quantum dot. The electron and hole potentials are calculated from the Poisson equations. Self-consistent method is to compute the energy eigenvalues of the exciton and the biexciton. The Hartree potential is employed to obtain the Coulomb interaction energy

  8. Energetic disorder and exciton states of individual molecular rings

    International Nuclear Information System (INIS)

    Exciton states in molecular rings (resembling, e.g. the B850 ring from LH2 complexes of purple bacterium Rhodopseudomonas acidophila) with strong intermolecular interaction are still a question of interest [V. Sundstrom, T. Pullerits, R. van Grondelle, J. Phys. Chem. B 103 (1999) 2327]. In our theoretical model we use the ring of two-level systems, simulating, e.g., the bacteriochlorophylls B850. The dynamical aspects in ensemble of rings are reflected in optical line shapes of electronic transitions. The observed linewidths reflect the combined influence of different types of static and dynamic disorder. To avoid the broadening of lines due to ensemble averaging one uses the single-molecule spectroscopy technique to obtain a fluorescence-excitation spectrum. For zero disorder the exciton manifold features two non-degenerate and eight pairwise degenerate states. In the presence of energetic disorder the degeneracy of the exciton states is lifted and oscillator strength is redistributed among the exciton states. A satisfactory understanding of the nature of static disorder in light-harvesting systems has not been reached [S. Jang, S.F. Dempster, R.J. Silbey, J. Phys. Chem. B 105 (2001) 6655]. In the local site basis, there can be present static disorder in both diagonal and off-diagonal Hamiltonian matrix elements. Silbey et al. [J. Phys. Chem. B 105 (2001) 6655] pointed out several questions: is former enough or the latter should be included as well? If both are considered, then there remains a question about whether they are independent or correlated. The distribution of the energetic separation E(k=+/-1) and relative orientation of the transition-dipole moments has been recently investigated [S. Jang, et al., J. Phys. Chem. B 105 (2001) 6655; C. Hofmann, T.J. Aartsma, J. Koehler, Chem. Phys. Lett. 395 (2004) 373]. In our present contribution we have extended such a type of investigation to four models of noncorrelated static disorder: (A) Gaussian disorder in the

  9. On the kinetics and thermodynamics of excitons at the surface of semiconductor nanocrystals: Are there surface excitons?

    International Nuclear Information System (INIS)

    Highlights: • The surface of semiconductor nanocrystals is one of their defining features. • The kinetics of surface trapping can be monitored by pump/probe spectroscopy. • The thermodynamics of surface trapping is revealed by photoluminescence spectroscopy. • We produce the first microscopic picture of how excitons are coupled to the surface. • We discuss the possibility of surface excitons in nanocrystals. - Abstract: The surface of semiconductor nanocrystals is one of their defining features by virtue of their nanometer size. Yet the surface is presently among the most poorly understood aspects of nanocrystal science. This perspective provides an overview of spectroscopic work that has revealed the first insights into the nature of the surface, focusing upon CdSe nanocrystals. We focus on two aspects of surface processes in nanocrystals: the kinetics of surface trapping and the thermodynamics of core/surface equilibria. We describe femtosecond pump/probe spectroscopic experiments which reveal the signatures of carrier trapping at the surface. We also describe temperature dependent steady-state photoluminescence experiments which reveal new aspects of the surface. This work suggest that the surface emission is largely driven by homogeneous broadening via phonon progressions. The implications are that the surface electronic state bears similarity to the quantized excitonic core of the nanocrystal

  10. Recombination of free and bound excitons in GaN

    Energy Technology Data Exchange (ETDEWEB)

    Monemar, B.; Paskov, P.P.; Bergman, J.P. [Department of Physics, Chemistry and Biology, Linkoeping University (Sweden); Toropov, A.A.; Shubina, T.V. [Ioffe Physico-Technical Institute, St. Petersburg (Russian Federation); Malinauskas, T. [Institute of Materials Science and Applied Research, Vilnius University (Lithuania); Usui, A. [R and D Division, Furukawa Co., Ltd. Tsukuba, Ibaraki (Japan)

    2008-09-15

    We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <10{sup 16} cm{sup -3} in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 me

  11. 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

  12. PREFACE: International Conference on Optics of Excitons in Confined Systems

    Science.gov (United States)

    Viña, Luis; Tejedor, Carlos; Calleja, José M.

    2010-01-01

    The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43

  13. Photonics based on carbon nanotubes

    OpenAIRE

    Gu, Qingyuan; Gicquel-Guézo, Maud; Loualiche, Slimane; Pouliquen, Julie Le; Batte, Thomas; Folliot, Hervé; Dehaese, Olivier; Grillot, Frederic; Battie, Yann; Loiseau, Annick; Liang, Baolai; Huffaker, Diana

    2013-01-01

    Among direct-bandgap semiconducting nanomaterials, single-walled carbon nanotubes (SWCNT) exhibit strong quasi-one-dimensional excitonic optical properties, which confer them a great potential for their integration in future photonics devices as an alternative solution to conventional inorganic semiconductors. In this paper, we will highlight SWCNT optical properties for passive as well as active applications in future optical networking. For passive applications, we directly compare the effi...

  14. 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...

  15. Excitons, Biexcitons and Dephasing in GaAs T-shaped quantum wires

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Gislason, Hannes; Hvam, Jørn Märcher

    2000-01-01

    The binding energy of excitons and biexcitons and the exciton dephasing in T-shaped GaAs quantum wires is investigated by transient four-wave mixing. The T-shaped structure is fabricated by cleaved-edge overgrowth and its geometry is engineered to optimize the one-dimensional confinement. In this...

  16. Direct Imaging of Long-Range Exciton Transport in Quantum Dot Superlattices by Ultrafast Microscopy.

    Science.gov (United States)

    Yoon, Seog Joon; Guo, Zhi; Dos Santos Claro, Paula C; Shevchenko, Elena V; Huang, Libai

    2016-07-26

    Long-range charge and exciton transport in quantum dot (QD) solids is a crucial challenge in utilizing QDs for optoelectronic applications. Here, we present a direct visualization of exciton diffusion in highly ordered CdSe QDs superlattices by mapping exciton population using ultrafast transient absorption microscopy. A temporal resolution of ∼200 fs and a spatial precision of ∼50 nm of this technique provide a direct assessment of the upper limit for exciton transport in QD solids. An exciton diffusion length of ∼125 nm has been visualized in the 3 ns experimental time window and an exciton diffusion coefficient of (2.5 ± 0.2) × 10(-2) cm(2) s(-1) has been measured for superlattices constructed from 3.6 nm CdSe QDs with center-to-center distance of 6.7 nm. The measured exciton diffusion constant is in good agreement with Förster resonance energy transfer theory. We have found that exciton diffusion is greatly enhanced in the superlattices over the disordered films with an order of magnitude higher diffusion coefficient, pointing toward the role of disorder in limiting transport. This study provides important understandings on energy transport mechanisms in both the spatial and temporal domains in QD solids. PMID:27387010

  17. Exciton size and binding energy limitations in one-dimensional organic materials

    Energy Technology Data Exchange (ETDEWEB)

    Kraner, S., E-mail: stefan.kraner@iapp.de; Koerner, C.; Leo, K. [Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden (Germany); Scholz, R. [Institut für Angewandte Photophysik, Technische Universität Dresden, Dresden (Germany); Dresden Center of Computational Materials Science, Technische Universität Dresden, D-01062 Dresden (Germany); Plasser, F. [Institute for Theoretical Chemistry, University of Vienna, A-1090 Vienna (Austria)

    2015-12-28

    In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.

  18. Exciton size and binding energy limitations in one-dimensional organic materials

    International Nuclear Information System (INIS)

    In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy

  19. Transient hot-phonon-to-exciton spectroscopy in organic molecular semiconductors

    NARCIS (Netherlands)

    Cordella, F.; Orru, R.; Loi, M.A.; Mura, A.; Bongiovanni, G.

    2003-01-01

    The interaction dynamics between excitons and intramolecular vibrations is investigated in α-sexithiophene crystals using transient pump-probe spectroscopy. The ultrafast fission of vibrons, i.e., exciton states bound to intramolecular vibrations, earlier predicted, is observed. The fission process

  20. 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 ...

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

    DEFF Research Database (Denmark)

    Umlauff, M.; Hoffmann, J.; Kalt, H.; Langbein, Wolfgang Werner; Hvam, Jørn Märcher; Scholl, M.; Söllner, J.; Heuken, M.; Jobst, B.; Hommel, D.

    1998-01-01

    We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum stales with picosecond laser pulses. The subs...

  2. 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. B...

  3. Picosecond dynamics of internal exciton transitions in CdSe nanorods

    DEFF Research Database (Denmark)

    Cooke, D. G.; Jepsen, Peter Uhd; Lek, Jun Yan; Lam, Yeng Ming; Sy, F.; Dignam, M. M.

    2013-01-01

    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...

  4. Exciton storage by Mn2+ in colloidal Mn2+-doped CdSe quantum dots

    NARCIS (Netherlands)

    Beaulac, R.; Archer, P.I.; van Rijssel, J.; Meijerink, A.; Gamelin, D.R.

    2008-01-01

    Colloidal Mn2+-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to τexc = 15 μs at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by ∼103 and are shown to arise from the creation of excitons by back energy transfer

  5. Ultrafast dynamics of confined and localised excitons and biexcitons in low-dimensional semiconductors

    DEFF Research Database (Denmark)

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

    1999-01-01

    Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the...

  6. 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...

  7. Characterizing intra-exciton Coulomb scattering in terahertz excitations

    International Nuclear Information System (INIS)

    An intense terahertz field is applied to excite semiconductor quantum wells yielding strong non-equilibrium exciton distributions. Even though the relaxation channels involve a complicated quantum kinetics of Coulomb and phonon effects, distinct relaxation signatures of Coulomb scattering are identified within time-resolved photoluminescence by comparing the experiment with a reduced model that contains all relevant microscopic processes. The analysis uncovers a unique time scale for the Coulomb scattering directly from experiments and reveals the influence of phonon relaxation as well as radiative decay

  8. Higher order coherence of exciton-polariton condensates

    OpenAIRE

    Horikiri, Tomoyuki; Schwendimann, Paolo; Quattropani, Antonio; Hofling, Sven; Forchel, Alfred; Yamamoto, Yoshihisa

    2009-01-01

    The second and third order coherence functions $g^{(n)}(0) (n=2 {\\rm and} 3)$ of an exciton-polariton condensate is measured and compared to the theory. Contrary to an ideal photon laser, deviation from unity in the second and third order coherence functions is observed, thus showing a bunching effect, but not the characteristics of a standard thermal state with $g^{(n)}(0)=n!$. The increase of bunching with the order of the coherence function, $g^{(3)}(0) > g^{(2)}(0)>1$, indicates that the ...

  9. Hybrid exciton recombination dynamics in inorganic-organic materials

    Energy Technology Data Exchange (ETDEWEB)

    Mastour, N., E-mail: mastounouha@yahoo.fr; Bouchriha, H., E-mail: mastounouha@yahoo.fr [Laboratoire : Matériaux avancés et phénomènes quantiques, Faculté des Sciences de Tunis, Université de Tunis El Manar, Campus Universitaire, 2092 Tunis (Tunisia)

    2013-12-16

    A systematic analysis of hybrid Frenkel–Wannier–Mott excitons recombination dynamics in nanocomposite material (organic–inorganic) is performed. A theoretical model based on the rate equation is used in the calculation of the light intensity and relative quantum efficiency. Numerical results have been presented for low and high concentration of quantum dots (Qds). Our results show that the light emission and relative quantum efficiency are significantly enhanced by incorporation of Qds in polymer matrix. Moreover our calculations were found to be in good agreement with the experimental data.

  10. Excitons with charge transfer in sncl2-phthalocyanine films

    OpenAIRE

    Vertsimakha, Ya. I.; Lutsyk, P. M.

    2014-01-01

    The absorption, modulated photoreflectance, and photovoltage spectra of dichlorotin phthalocyanine (SnCl2Pc) films have been measured. These films are thermally deposited in vacuum at different substrate temperatures. The energies of charge-transfer-states (CT-states) in SnCl2Pc films (1.35, 1.52, and 2.05 eV) and the diffusion length of Frenkel excitons (130+/-30 nm) have been determined. The photosensitivity of SnCl2Pc films is comparable to that of n-type perylene derivative (MPP) layers a...

  11. Strong Exciton-photon Coupling in Semiconductor Microcavities

    DEFF Research Database (Denmark)

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

    1999-01-01

    directionality of the spontaneous emission in the diodes.At low temperatures, a strong coupling between the excitons in the quantum well and the electromagnetic field in the cavity may be achieved. This coupling leads to new quantum mechanical states, so-called polaritons, the properties of which are still not...... place. Due to the steep dispersion, the amount of phase space available for polariton scattering is reduced, yielding longer dephasing times and hence narrower lines. The possibility of tailoring the polariton dispersion in order to reduce the line width is very interesting, for instance for all...

  12. Quantum-well thickness dependence of spin polarization of excitons

    Directory of Open Access Journals (Sweden)

    M. Idrish Miah

    2011-07-01

    Full Text Available The optical orientation of exciton spins in semiconductor quantum wells (SQWs was investigated by observing the circular polarization of the photoluminescence (PL. The left/right circularly polarized PL in SQWs was measured. It was found that there is a difference between the two different polarization conditions, which is caused by spin-dependent phase-space filling. The PL polarization was estimated from the signals of the left and right circularly polarized PL and was found to depend on the well thickness of SQWs as well as on the sample temperature. The influence of an electric field on the PL polarization was studied.

  13. Further tests of the Weisskopf-Ewing exciton model

    International Nuclear Information System (INIS)

    A phenomenological model wherein the statistical contribution to the cross section is evaluated from the Weisskopf-Ewing theory and the pre-equilibrium part from the exciton model is tested for neutron, proton and α-particle induced reactions on several nuclei. Using global prescriptions for the model parameters, this model is shown to provide a simple and reliable method for a fast calculation of the excitation functions and emission spectra in a wide energy range. The spectra of emitted particles obtained within this model are shown to be consistent with the data and with the more fundamental Feshbach, Kerman and Koonin theory. (author)

  14. 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.

  15. Correlated fluctuations in the exciton dynamics and spectroscopy of DNA

    Energy Technology Data Exchange (ETDEWEB)

    Dijkstra, Arend G; Tanimura, Yoshitaka, E-mail: dijkstra@kuchem.kyoto-u.ac.j, E-mail: tanimura@kuchem.kyoto-u.ac.j [Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502 (Japan)

    2010-05-15

    The absorption of ultraviolet light creates excitations in DNA, which subsequently start moving in the helix. Their fate is important for an understanding of photodamage, and is determined by the interplay of electronic couplings between bases and the structure of the DNA environment. We model the effect of dynamical fluctuations in the environment and study correlation, which is present when multiple base pairs interact with the same mode in the environment. We find that the correlations strongly affect the exciton dynamics, and show how they are observed in the decay of the anisotropy as a function of coherence and population time in a nonlinear optical experiment.

  16. Correlated fluctuations in the exciton dynamics and spectroscopy of DNA

    International Nuclear Information System (INIS)

    The absorption of ultraviolet light creates excitations in DNA, which subsequently start moving in the helix. Their fate is important for an understanding of photodamage, and is determined by the interplay of electronic couplings between bases and the structure of the DNA environment. We model the effect of dynamical fluctuations in the environment and study correlation, which is present when multiple base pairs interact with the same mode in the environment. We find that the correlations strongly affect the exciton dynamics, and show how they are observed in the decay of the anisotropy as a function of coherence and population time in a nonlinear optical experiment.

  17. Spin polarization of excitons in organic multiferroic composites

    Science.gov (United States)

    Han, Shixuan; Yang, Liu; Gao, Kun; Xie, Shijie; Qin, Wei; Ren, Shenqiang

    2016-01-01

    Recently, the discovery of room temperature magnetoelectricity in organic charge transfer complexes has reignited interest in the multiferroic field. The solution processed, large-area and low cost organic semiconductor materials offer new possibilities for the functional all organic multiferroic devices. Here we report the spin polarization of excitons and charge transfer states in organic charge transfer composites by using extended Su-Schrieffer-Heeger model including Coulomb interaction and spin-flip effect. With the consideration of spin polarization, we suggest a possible mechanism for the origin of excited ferromagnetism. PMID:27334680

  18. Lateral excitonic switching in vertically stacked quantum dots

    Science.gov (United States)

    Jarzynka, Jarosław R.; McDonald, Peter G.; Shumway, John; Galbraith, Ian

    2016-06-01

    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.

  19. Erasing the exciton fine structure splitting in semiconductor quantum dots

    International Nuclear Information System (INIS)

    Full text: In semiconductor quantum dots (QDs), the anisotropic electron-hole exchange interaction induces a fine structure splitting (FSS) between the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. In this presentation, we will show that it is always possible to erase the FSS by the simultaneous application of large strain and electric fields. Our result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology. (author)

  20. Configuration space method for calculating binding energies of exciton complexes in quasi-1D/2D semiconductors

    Science.gov (United States)

    Bondarev, Igor

    A configuration space method, pioneered by Landau and Herring in studies of molecular binding and magnetism, is developed to obtain universal asymptotic relations for lowest energy exciton complexes (trion, biexciton) in confined semiconductor nanostructures such as nanowires and nanotubes, as well as coupled quantum wells. Trions are shown to be more stable (have greater binding energy) than biexcitons in strongly confined quasi-1D structures with small reduced electron-hole masses. Biexcitons are more stable in less confined quasi-1D structures with large reduced electron-hole masses. The theory predicts a crossover behavior, whereby trions become less stable than biexcitons as the transverse size of the quasi-1D nanostructure increases, which might be observed on semiconducting carbon nanotubes of increasing diameters. This method is also efficient in calculating binding energies for trion-type electron-hole complexes formed by indirect excitons in double coupled quantum wells, quasi-2D nanostructures that show new interesting electroabsorption/refraction phenomena. Supported by DOE-DE-SC0007117.

  1. Structure of luminescence spectrum of PbI2 in the region of strong light-exciton mixing

    International Nuclear Information System (INIS)

    The temperature dependence on the PbI(2H) monocrystal photoluminescence spectrum is studied in the range of the exciton transition n = 1. It is shown that a doublet nature of the luminescence band of free excitons is caused by optical characteristics of the crystals, associated with transverse-longitudinal splitting of light-exciton states

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-06-15

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

  3. Exciton effects on dipole-allowed optical absorptions in a two-dimensional parabolic quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Yuan, Jian-Hui, E-mail: jianhui831110@163.com [The Department of Physics, Guangxi Medical University, Nanning, Guangxi 530021 (China); Zhang, Yan; Huang, Daizheng [The Department of Physics, Guangxi Medical University, Nanning, Guangxi 530021 (China); Zhang, Jianjun [School of Physics, Huazhong University of Science and Technology, Wuhan 430074 (China); Zhang, Xin [The 34th Institute of China Electronics and Technology Group Corporation, Guilin, Guangxi 541004 (China)

    2013-11-15

    Exciton effects on the linear and nonlinear optical absorptions (the transition from the S state (L=0) to the P state (L=1)) in two-dimensional quantum dots are theoretically studied by using the configuration-integration methods (CI) and the compact density-matrix approach. The results show that the optical absorption coefficient, which can be controlled by the confinement potential strength and the incident optical intensity, is enhanced obviously when the exciton effect is taken into account. We find that both a trapped electron–hole pair and the incident optical intensity can bleach the exciton absorption and the appearance of the new absorption may be due to biexciton. -- Highlights: • This paper investigates exciton effects on dipole-allowed optical absorptions in quantum dots. • Both a trapped electron–hole pair and the incident optical intensity can bleach the exciton absorption. • The appearance of the new biexciton absorption may be due to biexciton.

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

    DEFF Research Database (Denmark)

    Johansen, Jeppe; Julsgaard, Brian; Stobbe, Søren;

    2010-01-01

    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...... 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...

  5. Theoretical investigation of nonthermal equilibrium exciton dynamics in GaN using hydrogen plasma model

    Science.gov (United States)

    Iwahori, Tomohiro; Ma, Bei; Morita, Ken; Ishitani, Yoshihiro

    2016-05-01

    As a basis of the study on exciton stability under a nonthermal equilibrium state, the excitation and deexcitation population fluxes and population densities of several states of the principal quantum number p are calculated using a hydrogen plasma model for various electron excitation densities and temperatures of the lattice, electron, and exciton. It is found that the balance of the excitation and deexcitation population fluxes depends on the p number. At a lower-lattice-temperature region, ladderlike deexcitation flux is dominant for low p states, while the quasi-Saha–Boltzmann relation holds for high p states. At temperatures higher than 150 K, the exciton formation and dissociation fluxes become dominant. Exciton dissociation is enhanced at temperatures higher than approximately 120 K. This process is triggered by the excitation between the states of p = 1 and 2. High- and low-order states sometimes exhibit different population flow characteristics, which reveal the exciton dissociation dynamics.

  6. Stark effect of excitons in corrugated lateral surface superlattices: effect of centre-of-mass quantization

    International Nuclear Information System (INIS)

    The quantum confined Stark effect (QCSE) of excitons in GaAs/AlAs corrugated lateral surface superlattices (CLSSLs) is calculated. Blue and red shifts in the exciton energies are predicted for the heavy- and light-excitons in the CLSSLs, respectively, comparing with those in the unmodulated quantum well due to the different effective hole masses in the parallel direction. Sensitive dependence of the QCSE on the hole effective mass in the parallel direction is expected because of the ''centre-of-mass'' quantization (CMQ) induced by the periodic corrugated interfaces of the CLSSLs. The effect of the CMQ on the exciton mini-bands and the localization of the excitons in the CLSSLs is discussed. (author)

  7. Rapid determination of memantine in human plasma by using nanoring carboxyl-functionalized paramagnetic molecularly imprinted polymer d-μ-SPE and UFLC-MS/MS.

    Science.gov (United States)

    Qiu, Hai-Wen; Xia, Lei; Gong, Li-Min; Ruan, Lie-Min; Zhao, Yong-Gang

    2015-06-01

    A novel, simple, and sensitive method based on the use of dispersive micro-solid-phase extraction (d-μ-SPE) procedure combined with ultra-fast liquid chromatography-tandem quadrupole mass spectrometry (UFLC-MS/MS) for the determination of memantine (ME) was developed and validated over the linearity range 0.05-10.0 µg/L with 100 μL of human plasma using memantine-D6 (ME-D6) as the internal standard. The novel nanoring carboxyl-functionalized paramagnetic molecularly imprinted polymer (NR-CF-Mag-MIP) was synthesized by ultrasound-assisted suspension polymerization, using ME as a template molecule, methacrylic acid as a functional monomer, and divinylbenzene as a cross-linking agent. The NR-CF-Mag-MIP was used as the d-μ-SPE sorbent to extract ME from human plasma samples. The obtained results demonstrated the higher extraction capacity of NR-CF-Mag-MIP with recoveries between 97.6 and 101%. The limits of quantification (LOQs) for ME was 0.015 µg/L. Validation results on linearity, specificity, accuracy, precision, and stability, as well as on application to the analysis of samples taken up to 480 h after oral administration of 20 mg (two 10 mg capsules) of ME in healthy volunteers demonstrated the applicability to bioequivalence studies. PMID:25209851

  8. Excellent microwave-absorbing properties of elliptical Fe3O4 nanorings made by a rapid microwave-assisted hydrothermal approach

    Science.gov (United States)

    Liu, Yun; Cui, Tingting; Wu, Tong; Li, Yana; Tong, Guoxiu

    2016-04-01

    High-quality elliptical polycrystalline Fe3O4 nanorings (NRs) with continuously tunable size have been synthesized in large amounts via a rapid microwave-assisted hydrothermal approach. The surface-protected glucose reducing/etching/Ostwald ripening mechanism is responsible for the formation of NRs. Ring size can be modulated by selecting iron glycolate nanosheets with various sizes as precursors. The size-dependent magnetic behavior of the NRs was observed. Our research gives insights into the understanding of the microwave absorption mechanism of elliptical Fe3O4 NRs. Owing to their large specific surface area, shape anisotropy, and closed ring-like configuration, elliptical polycrystalline Fe3O4 NRs exhibited significantly enhanced microwave absorption performance compared with Fe3O4 circular NRs, nanosheets, microspheres, nanospindles, and nanotubes. An optimal reflection loss value of -41.59 dB is achieved at 5.84 GHz and R L values (≤-20 dB) are observed at 3.2-10.4 GHz. Some new mechanisms including multiple scattering, oscillation resonance absorption, microantenna radiation, and interference are also crucial to the enhanced absorption properties of NRs. These findings indicate that ring-like nanostructures are a promising structure for devising new and effective microwave absorbers.

  9. CDW-Exciton Condensate Competition and a Condensate Driven Force

    Science.gov (United States)

    Özgün, Ege; Hakioğlu, Tuğrul

    2016-08-01

    We examine the competition between the charge-density wave (CDW) instability and the excitonic condensate (EC) in spatially separated layers of electrons and holes. The CDW and the EC order parameters (OPs), described by two different mechanisms and hence two different transition temperatures TcCDW and TcEC, are self-consistently coupled by a microscopic mean field theory. We discuss the results in our model specifically focusing on the transition-metal dichalcogenides which are considered as the most typical examples of strongly coupled CDW/EC systems with atomic layer separations where the electronic energy scales are large with the critical temperatures in the range TcEC ˜ TcCDW ˜ 100-200 K. An important consequence of this is that the excitonic energy gap, hence the condensed free energy, vary with the layer separation resulting in a new type of force FEC. We discuss the possibility of this force as the possible driver of the structural lattice deformation observed in some TMDCs with a particular attention on the 1T-TiSe2 below 200 K.

  10. Ovshinsky Sustainable Energy Fellowship: Excitonics for Transparent Photovoltaics

    Science.gov (United States)

    Lunt, Richard

    Room-­temperature excitonic materials offer new opportunities for low­-cost photovoltaic (PV) systems and provide prospects for unique solar harvesting science and applications. In the first part of this talk, I will introduce our pioneering work on developing transparent PVs that are creating a new paradigm for seamless solar harvesting around buildings, automobiles, and mobile electronics. These devices are enabled by the manipulation of excitonic semiconductor materials with selective harvesting in the near­infrared and ultraviolet components of the solar spectrum. I will describe key photophysical properties, outline the thermodynamic and practical limits to these new classes of materials and devices, and briefly discuss their commercial impact for a range of applications. In the second part, I will describe the development of a new series organic salts that allow tunable photoresponse from 900nm to 1600nm, an unprecedented range for small­molecule semiconductors. These organic salts also enable precise tuning of frontier orbital levels and heterojunction interface gaps through anion alloying that result in voltages near the thermodynamic limit. This design strategy can further enable rapid development of efficient and low­cost multijunction devices (both opaque and transparent) with complimentary response across the solar spectrum.

  11. 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.

  12. 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.

  13. Transport Gap and exciton binding energy determination in organic semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Krause, Stefan; Schoell, Achim; Reinert, Friedrich; Umbach, Eberhard [University of Wuerzburg (Germany). Experimental Physics II; Casu, Benedetta [Inst. f. Physik. u. Theor. Chemie, Tuebingen (Germany)

    2008-07-01

    The transport gap of an organic semiconductor is defined as the energy difference between the HOMO and LUMO levels in the presence of a hole or electron, respectively, after relaxation has occurred. Its knowledge is mandatory for the optimisation of electronic devices based on these materials. UV photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES) are routinely applied to measure these molecular levels. However, the precise determination of the transport gap on the basis of the respective data is not an easy task. It involves fundamental questions about the properties of organic molecules and their condensates, about their reaction on the experimental probe, and on the evaluation of the spectroscopic data. In particular electronic relaxation processes, which occur on the time scale of the photo excitation, have to be considered adequately. We determined the transport gap for the organic semiconductors PTCDA, Alq3, DIP, CuPc, and PBI-H4. After careful data analysis and comparison to the respective values for the optical gap we obtain values for the exciton binding energies between 0.1-0.5 eV. This is considerably smaller than commonly believed and indicates a significant delocalisation of the excitonic charge over various molecular units.

  14. Broadband Absorbing Exciton-Plasmon Metafluids with Narrow Transparency Windows.

    Science.gov (United States)

    Yang, Jihua; Kramer, Nicolaas J; Schramke, Katelyn S; Wheeler, Lance M; Besteiro, Lucas V; Hogan, Christopher J; Govorov, Alexander O; Kortshagen, Uwe R

    2016-02-10

    Optical metafluids that consist of colloidal solutions of plasmonic and/or excitonic nanomaterials may play important roles as functional working fluids or as means for producing solid metamaterial coatings. The concept of a metafluid employed here is based on the picture that a single ballistic photon, propagating through the metafluid, interacts with a large collection of specifically designed optically active nanocrystals. We demonstrate water-based metafluids that act as broadband electromagnetic absorbers in a spectral range of 200-3300 nm and feature a tunable narrow (∼100 nm) transparency window in the visible-to-near-infrared region. To define this transparency window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped silicon nanocrystals as opaque optical absorbers ("optical wall") in the UV and blue-green range of the spectrum. Water itself acts as an opaque "wall" in the near-infrared to infrared. We explore the limits of the concept of a "simple" metafluid by computationally testing and validating the effective medium approach based on the Beer-Lambert law. According to our simulations and experiments, particle aggregation and the associated decay of the window effect are one example of the failure of the simple metafluid concept due to strong interparticle interactions. PMID:26808215

  15. Excitonic entanglement of protected states in quantum dot molecules

    Science.gov (United States)

    Borges, H. S.; Sanz, L.; Alcalde, A. M.

    2016-09-01

    The entanglement of an optically generated electron-hole pair in artificial quantum dot molecules is calculated considering the effects of decoherence by interaction with environment. Since the system evolves into mixed states and due to the complexity of energy level structure, we use the negativity as entanglement quantifier, which is well defined in D ⊗D‧ composite vector spaces. By a numerical analysis of the non-unitary dynamics of the exciton states, we establish the feasibility of producing protected entangled superposition by an appropriate tuning of bias electric field, F. A stationary state with a high value of negativity (high degree of entanglement) is obtained by fine tuning of F close to a resonant condition between indirect excitons. We also found that when the optical excitation is approximately equal to the electron tunneling coupling, Ω /Te ∼ 1, the entanglement reaches a maximum value. In front of the experimental feasibility of the specific condition mentioned before, our proposal becomes an useful strategy to find robust entangled states in condensed matter systems.

  16. 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.

  17. Radiative and nonradiative exciton energy transfer in monolayers of two-dimensional group-VI transition metal dichalcogenides

    Science.gov (United States)

    Manolatou, Christina; Wang, Haining; Chan, Weimin; Tiwari, Sandip; Rana, Farhan

    2016-04-01

    We present results on the rates of interlayer energy transfer between excitons in monolayers of two-dimensional group-VI transition metal dichalcogenides (TMDs). We consider both radiative (mediated by real photons) and nonradiative (mediated by virtual photons) mechanisms of energy transfer using a unified Green's function approach that takes into account modification of the exciton energy dispersions as a result of interactions. The large optical oscillator strengths associated with excitons in TMDs result in very fast energy transfer rates. The energy transfer times depend on the exciton momentum, exciton linewidth, and the interlayer separation and can range from values less than 100 femtoseconds to more than tens of picoseconds. Whereas inside the light cone the energy transfer rates of longitudinal and transverse excitons are comparable, outside the light cone the energy transfer rates of longitudinal excitons far exceed those of transverse excitons. Average energy transfer times for a thermal ensemble of longitudinal and transverse excitons is temperature dependent and can be smaller than a picosecond at room temperature for interlayer separations smaller than 10 nm. Energy transfer times of localized excitons range from values less than a picosecond to several tens of picoseconds. When the exciton scattering and dephasing rates are small, energy transfer dynamics exhibit coherent oscillations. Our results show that electromagnetic interlayer energy transfer can be an efficient mechanism for energy exchange between TMD monolayers.

  18. Nonlinear optical spectra having characteristics of Fano interferences in coherently coupled lowest exciton biexciton states in semiconductor quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Gotoh, Hideki, E-mail: gotoh.hideki@lab.ntt.co.jp; Sanada, Haruki; Yamaguchi, Hiroshi; Sogawa, Tetsuomi [NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198 (Japan)

    2014-10-15

    Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

  19. Nonlinear optical spectra having characteristics of Fano interferences in coherently coupled lowest exciton biexciton states in semiconductor quantum dots

    International Nuclear Information System (INIS)

    Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics

  20. Resonant surface plasmon-exciton interaction in hybrid MoSe2@Au nanostructures.

    Science.gov (United States)

    Abid, I; Bohloul, A; Najmaei, S; Avendano, C; Liu, H-L; Péchou, R; Mlayah, A; Lou, J

    2016-04-14

    In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The work is focused on a resonant situation where the surface plasmon resonance is tuned to the excitonic transition. In that case, the near-field interaction between the surface plasmons and the confined excitons leads to interference between the plasmonic and excitonic resonances that manifests in the optical spectra as a transparency dip. The plasmonic-excitonic interaction regime is determined using quantitative analysis of the optical extinction spectra based on an analytical model supported by numerical simulations. We found that the plasmonic-excitonic resonances do interfere thus leading to a typical Fano lineshape of the optical extinction. The near-field nature of the plasmonic-excitonic interaction is pointed out experimentally from the dependence of the optical absorption on the number of monolayer stacks on the Au nanodisks. The results presented in this work contribute to the development of new concepts in the field of hybrid plasmonics. PMID:27029770

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

    Science.gov (United States)

    Olsen, Thomas; Latini, Simone; Rasmussen, Filip; Thygesen, Kristian S.

    2016-02-01

    We present a generalized hydrogen model for the binding energies (EB ) and radii of excitons in two-dimensional (2D) materials that sheds light on the fundamental differences between excitons in two and three dimensions. In contrast to the well-known hydrogen model of three-dimensional (3D) excitons, 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 that only depends on the excitonic mass and the 2D polarizability α . The model is shown to produce accurate results for 51 transition metal dichalcogenides. Remarkably, over a wide range of polarizabilities the binding energy becomes independent of the mass and we obtain EB2 D≈3 /(4 π α ), which explains the recently observed linear scaling of exciton binding energies with band gap. It is also shown that the model accurately reproduces the nonhydrogenic Rydberg series in WS2 and can account for screening from the environment.

  2. Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites

    Energy Technology Data Exchange (ETDEWEB)

    Abdel-Baki, K.; Boitier, F.; Diab, H.; Lanty, G.; Jemli, K.; Lédée, F.; Deleporte, E.; Lauret, J. S., E-mail: jean-sebastien.lauret@lac.u-psud.fr [Laboratoire Aimé Cotton, CNRS, Univ. Paris-Sud, ENS Cachan, Université Paris-Saclay, 91405 Orsay Cedex (France); Garrot, D. [GEMAC, CNRS, UVSQ, Université Paris-Saclay, 45 avenue des États Unis 78035 Versailles Cedex (France)

    2016-02-14

    Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites has emerged for few years. When they are arranged in two dimensions, these materials can be considered as hybrid quantum wells. One consequence of the unique structure of 2D hybrid organic perovskites is a huge exciton binding energy that can be tailored through chemical engineering. We present experimental investigations of the exciton non-linearities by means of femtosecond pump-probe spectroscopy. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. Moreover, an ultrafast intraband relaxation (<150 fs) is also reported. Finally, the transient modification of the excitonic line is analyzed through the moment analysis and described in terms of reduction of the oscillator strength and linewidth broadening. We show that excitonic non-linearities in 2D hybrid organic perovskites share some behaviours of inorganic semiconductors despite their high exciton binding energy.

  3. Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains.

    Science.gov (United States)

    Bednarz, M; Malyshev, V A; Knoester, J

    2004-02-22

    We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes. PMID:15268548

  4. Fine structure of a resonantly excited p -shell exciton in a CdTe quantum dot

    Science.gov (United States)

    Smoleński, T.; Kazimierczuk, T.; Goryca, M.; Wojnar, P.; Kossacki, P.

    2016-05-01

    We present a polarization-resolved photoluminescence excitation study of the absorption spectrum of a p -shell neutral exciton in a single CdTe/ZnTe quantum dot. We find that the fine structure of the p -shell exciton is completely analogous to the fine structure of the s -shell exciton, including the selection rules and the effects of a magnetic field applied in Faraday and Voigt configurations. The energy spectrum of the p -shell exciton is found to be well described by introducing respective isotropic and anisotropic constants of the exchange interaction between a p -shell electron and a p -shell hole. The typical values of these exchange constants averaged over several randomly selected quantum dots yield δ0p p=(0.92 ±0.16 ) meV and δ1p p=(0.58 ±0.25 ) meV. Additionally, we demonstrate that the nonresonant relaxation of the p -shell exciton conserves the exciton spin to a very high degree for both bright and dark exciton configurations.

  5. 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.

  6. Exciton dynamics and non-linearities in two-dimensional hybrid organic perovskites

    International Nuclear Information System (INIS)

    Due to their high potentiality for photovoltaic applications or coherent light sources, a renewed interest in hybrid organic perovskites has emerged for few years. When they are arranged in two dimensions, these materials can be considered as hybrid quantum wells. One consequence of the unique structure of 2D hybrid organic perovskites is a huge exciton binding energy that can be tailored through chemical engineering. We present experimental investigations of the exciton non-linearities by means of femtosecond pump-probe spectroscopy. The exciton dynamics is fitted with a bi-exponential decay with a free exciton life-time of ∼100 ps. Moreover, an ultrafast intraband relaxation (<150 fs) is also reported. Finally, the transient modification of the excitonic line is analyzed through the moment analysis and described in terms of reduction of the oscillator strength and linewidth broadening. We show that excitonic non-linearities in 2D hybrid organic perovskites share some behaviours of inorganic semiconductors despite their high exciton binding energy

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

    International Nuclear Information System (INIS)

    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

  8. Phosphor-doping enhanced efficiency in bilayer organic solar cells due to longer exciton diffusion length

    International Nuclear Information System (INIS)

    We fabricated bilayer organic solar cells (OSCs) in the structure glass/ITO/PEDOT:PSS/PtOEP:MEH-PPV/C70/Al, where MEH-PPV was doped with platinum octaethylporphyrin (PtOEP). Enhanced exciton diffusion length (LD) is realized via converting generated singlet excitons to triplet excitons. Investigation based on transfer matrix simulations reveals that it is the extended exciton LD of the doping donor layer that leads to the short-circuit current density (Jsc) and power conversion efficiency (PCE) improvement, when compared with those of the OSCs with a non-doping donor layer. As a result of the increased LD, Jsc and PCE increase by 30% and 42% respectively for a device with 5 wt% PtOEP-doped 25 nm-thick donor layer. Meanwhile, by doping with phosphorescent bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III), the reduction in open-circuit voltage and the comparable Jsc are shown due to its higher HOMO level and higher LUMO level, leading to the decrease of PCE. It demonstrates that doping a polymer with a suitable phosphorescent molecule is an important approach to be considered to increase the exciton LD. - Highlights: • Optical model based on transfer matrix method was used to study phosphor-doped organic planar hetero-junction solar cells. • The enhanced exciton diffusion length was experimentally investigated by absorption, PL, time-resolved transient PL, J–V and EQE curves. • Only suitable phosphor dyes can increase exciton diffusion length

  9. Exciton dissociation at organic small molecule donor-acceptor interfaces (Presentation Recording)

    Science.gov (United States)

    Robey, Steven W.

    2015-08-01

    Exciton dissociation at organic semiconductor donor-acceptor (D-A) heterojunctions is critical for the performance of organic photovoltaic (OPV) structures. Interfacial charge separation and recombination processes control device efficiency. We have investigated these fundamental interfacial issues using time-resolved two-photon photoemission (TR-2PPE), coupled with the formation of well-controlled D-A structures by organic molecular beam epitaxy. The interfacial electronic and molecular structure of these model interfaces was well-characterized using scanning tunneling microscopy and ultraviolet photoemission. Exciton dissociation dynamics were investigated by using a sub-picosecond pump pulse to create Pc π-->π* transitions, producing a population of singlet (S1) Pc excitons. The subsequent decay dynamics of this population was monitored via photoemission with a time-delayed UV pulse. For CuPcC60 interfaces, S1 exciton population decay in the interfacial CuPc layer was much faster than decay in the bulk due to interfacial charge separation. The rate constant for exciton dissociation was found to be ≍ 7 x 10 12 sec-1 (≍ 140 fs). Excitons that lose energy via intersystem crossing (ISC) to triplet levels dissociate approximately 500 to 1000 times slower. The dependence of exciton dissociation on separation was also studied. Exciton dissociation falls of rapidly with distance from the interface. Dissociation from the 2nd, and subsequent, layers of H2Pc is reduced by at least a factor of 10 from that in the interfacial layer. Finally, investigations of the relative efficiency for interfacial exciton dissociation by alternative acceptors based on perylene cores, (perylene tetracarboxylic dianhydride, or PTCDA) compared to fullerene-based acceptors such as C60 will also be discussed.

  10. Large Scale Integration of Carbon Nanotubes in Microsystems

    DEFF Research Database (Denmark)

    Gjerde, Kjetil

    2007-01-01

    Kulstof nanorør har mange egenskaber der kunne anvendes i kombination med traditionelle mikrosystemer, her især overlegne mekaniske og elektriske egenskaber. I dette arbejde bliver metoder til stor-skala integration av kulstof nanorør i mikrosystemer undersøgt, med henblik på anvendelse som mekan...

  11. Size dependent exciton dynamics in one-dimensional perylene bisimide aggregates

    CERN Document Server

    Wolter, Steffen; Fennel, Franziska; Seidel, Marcus; Würthner, Frank; Kühn, Oliver; Lochbrunner, Stefan

    2012-01-01

    The size dependent exciton dynamics of one-dimensional aggregates of substituted perylene bisimides are studied by ultrafast transient absorption spectroscopy and kinetic Monte-Carlo simulations in dependence on the temperature and the excitation density. For low temperatures the aggregates can be treated as infinite chains and the dynamics is dominated by diffusion driven exciton-exciton annihilation. With increasing temperature the aggregates decompose into small fragments consisting of very few monomers. This scenario is also supported by the time dependent anisotropy deduced from polarization dependent experiments.

  12. Entanglement Transfer Between Cavity Fields and Excitons in a Driven Quantum Dot System

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    We investigate entanglement transfer from two separate cavities to the excitons in two quantum dots separately placed in the two cavities. The cavity fields and the excitons are treated as two continuous-variable (CV) subsystems. The time-dependent characteristic functions in the Wigner representation for the two subsystems are analytically obtained. Under the conditions that one of the two CV subsystems is initially prepared in a two-mode squeezed vacuum state and the other in its lowest energy state, we show that the entanglement reciprocation between the cavity Reids and the excitons is realizable.

  13. 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.

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

    Institute of Scientific and Technical Information of China (English)

    李惠生; 朱卡的

    2003-01-01

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

  15. Statistics, synergy, and mechanism of multiple photogeneration of excitons in quantum dots: Fundamental and applied aspects

    International Nuclear Information System (INIS)

    The effect of multiple exciton generation is analyzed based on statistical physics, quantum mechanics, and synergetics. Statistical problems of the effect of multiple exciton generation (MEG) are broadened and take into account not only exciton generation, but also background excitation. The study of the role of surface states of quantum dots is based on the synergy of self-catalyzed electronic reactions. An analysis of the MEG mechanism is based on the idea of electronic shaking using the sudden perturbation method in quantum mechanics. All of the above-mentioned results are applied to the problem of calculating the limiting efficiency to transform solar energy into electric energy. (authors)

  16. 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.

  17. Tunable optical delay via carrier induced exciton dephasing in semiconductor quantum wells.

    Science.gov (United States)

    Sarkar, Susanta; Guo, Yan; Wang, Hailin

    2006-04-01

    We report the experimental realization of a tunable optical delay by exploiting unique incoherent nonlinear optical processes in semiconductors. The tunable optical delay takes advantage of the strong Coulomb interactions between excitons and free carriers and uses optical injection of free carriers to broaden and bleach an exciton absorption resonance. Fractional delay exceeding 200% has been obtained for an 8 ps optical pulse propagating near the heavy-hole excitonic transition in a GaAs quantum well structure. Tunable optical delay based on optical injection of free carriers avoids strong absorption of the pump beam and is also robust against variations in the frequency of the pump beam. PMID:19516421

  18. Excitons in a cylindrical GaAs Poeschl-Teller quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Mora-Ramos, M.E. [Facultad de Ciencias, Universidad Autonoma del Estado de Morelos, Av. Universidad 1001, CP 62209 Cuernavaca, Morelos (Mexico); Barseghyan, M.G. [Department of Solid State Physics, Yerevan State University, Al. Manookian 1, 0025 Yerevan (Armenia); Duque, C.A. [Instituto de Fisica, Universidad de Antioquia, AA 1226, Medellin (Colombia)

    2011-06-15

    The exciton binding energy in a cylindrical quantum dot (QD) with Poeschl-Teller confining potential is studied with the use of the effective mass approximation and a variational calculation procedure. A GaAs prototype structure is particularly considered and both the spatially-direct (DE) and spatially-indirect (IE) exciton regimes are taken into account. We show that there is a significant effect of the Poeschl-Teller quantum well (QW) symmetry profile on the exciton properties in both configurations. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  19. Exciton Mott transition and pair condensation in the electron-hole system

    Energy Technology Data Exchange (ETDEWEB)

    Ohashi, Takuma; Ueda, Tomoya; Asano, Kenichi; Ogawa, Tetsuo, E-mail: ohashi@acty.phys.sci.osaka-u.ac.j [Department of Physics, Osaka University, Toyonaka, Osaka 560-0043 (Japan)

    2009-02-01

    We investigate the exciton Mott transition and pair condensation in the spinless electron-hole Hubbard model by means of the dynamical mean field theory combined with the noncrossing approximation. By investigating the single-particle density of states, we find the crossover between the metallic electron-hole plasma and the exciton-like insulator. We also investigate the electron-hole pair condensation transition and the optical response, by calculation of the two-particle Green's function for the pair correlation with vertex corrections. It is shown that the excitonic peak in the optical response function gets strongly enhanced around the electron-hole pair condensation transition.

  20. Controlling Exciton Diffusion and Fullerene Distribution in Photovoltaic Blends by Side Chain Modification.

    Science.gov (United States)

    Sajjad, Muhammad T; Ward, Alexander J; Kästner, Christian; Ruseckas, Arvydas; Hoppe, Harald; Samuel, Ifor D W

    2015-08-01

    The influence of crystallinity on exciton diffusion and fullerene distribution was investigated by blending amorphous and semicrystalline copolymers. We measured exciton diffusion and fluorescence quenching in such blends by dispersing fullerene molecules into them. We find that the diffusion length is more than two times higher in the semicrystalline copolymer than in the amorphous copolymer. We also find that fullerene preferentially mixes into disordered regions of the polymer film. This shows that relatively small differences in molecular structure are important for exciton diffusion and fullerene distribution. PMID:26267202

  1. Excitons in van der Waals heterostructures:The important role of dielectric screening

    OpenAIRE

    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-dimensional (2D) excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model, which assumes a dielectric function of the form epsilon(q) = 1 + 2 pi ...

  2. Excitons in van der Waals heterostructures: The important role of dielectric screening

    OpenAIRE

    Latini, Simone; Olsen, Thomas; Thygesen, Kristian S.

    2015-01-01

    The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semi-conductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions (2D), a systematic investigation of the role of screening on 2D excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model which assumes a dielectric function of the form {\\epsilon}(q) = 1 + 2{\\pi}{\\alpha}q...

  3. Optical characterization of excitons in InSe crystals doped with rare earth ions

    International Nuclear Information System (INIS)

    Full text : Investigations of exciton luminescence and absorption spectra of InSe lamellar semiconductors doped with different kinds of compounds at T=1.8 K are carried out. it is found that changes in the luminescence intensity of free and bound excitons are due to screening of Coulomb interaction between electron and hole. Increase of free excitons luminescence intensity in InSe crystals activated by holmium ions is attributed to healing of metal and chalcogenide vacancies and also to interlaminar interaction increase due to the exchange interaction of local formations

  4. Determination of the Exciton Binding Energy Using Photothermal and Photoluminescence Spectroscopy

    Science.gov (United States)

    Strzałkowski, K.; Zakrzewski, J.; Maliński, M.

    2013-04-01

    In this paper, experimental photoluminescence (PL) and piezoelectric photothermal (PPT) spectra of selected II-VI binary crystals are presented and analyzed. The quantitative analysis of the photothermal spectra was performed using a modified and extended Jackson-Amer model. The values of the bandgap energies of investigated semiconductors were computed from the PT amplitude and phase spectra. From the temperature dependence of the exciton emission so-called "excitonic energy gaps" have been determined. It follows from the theory that the exciton binding energy is the difference of these two values of energy gaps derived from PPT and PL spectroscopy.

  5. Interaction between excitons determines the non-linear response of nanocrystals

    International Nuclear Information System (INIS)

    The non-linear response of semiconductor quantum dots is investigated using three-pulse photon echo peak shift (3PEPS) experiments and simulations. The third-order non-linear response is modeled by a three-level system, utilizing Brownian oscillators to model the line-broadening functions. Our results show that biexciton formation and exciton-exciton scattering significantly influence the non-linear response of quantum dots. The exciton to biexciton excited state absorption pathways are also investigated for quantum dots with different crystal structures. Our calculations suggest that the probability of excited state absorption to the biexcitonic state is higher for zinc-blende structured nanocrystals

  6. Excitonic fano resonance in free-standing graphene.

    Science.gov (United States)

    Chae, Dong-Hun; Utikal, Tobias; Weisenburger, Siegfried; Giessen, Harald; Klitzing, Klaus V; Lippitz, Markus; Smet, Jurgen

    2011-03-01

    We investigate the role of electron-hole correlations in the absorption of free-standing monolayer and bilayer graphene using optical transmission spectroscopy from 1.5 to 5.5 eV. Line shape analysis demonstrates that the ultraviolet region is dominated by an asymmetric Fano resonance. We attribute this to an excitonic resonance that forms near the van Hove singularity at the saddle point of the band structure and couples to the Dirac continuum. The Fano model quantitatively describes the experimental data all the way down to the infrared. In contrast, the common noninteracting particle picture cannot describe our data. These results suggest a profound connection between the absorption properties and the topology of the graphene band structure. PMID:21322607

  7. Nonradiative recombination of excitons in semimagnetic quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Chernenko, A. V., E-mail: chernen@issp.ac.ru [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation)

    2015-12-15

    The mechanisms of the nonradiative recombination of excitons in neutral and charged quantum dots based on II–VI semimagnetic semiconductors are investigated. It is shown that, along with the dipole–dipole and direct-exchange mechanisms, there is one more mechanism referred to as the indirect-exchange mechanism and related to sp–d mixing. The selection rules for nonradiative recombination by exchange mechanisms are subsequently derived. The dependence of the efficiency of all recombination mechanisms on the quantum-dot size is studied. The experimentally observed growth in the intracenter photoluminescence intensity with decreasing size of dots and nanocrystals is accounted for. Methods for experimental determination of the contributions of different mechanisms to nonradiative recombination are discussed.

  8. Locally Broken Crystal Symmetry Facilitates Singlet Exciton Fission.

    Science.gov (United States)

    Petelenz, Piotr; Snamina, Mateusz

    2016-05-19

    Photovoltaic yield is normally limited to at most two charge carriers per photon. In solid pentacene this limit may be potentially bypassed owing to singlet exciton fission into a pair of triplets. The process occurs via a superexchange mechanism mediated by charge-transfer (CT) configurations and is sensitive to their energies. As demonstrated recently, these strongly depend on the local environment of the two molecules on which the charges reside. Using a multiscale model, here we show that in the crystal bulk approximate local symmetry affects CT state energetics in a way unfavorable for fission, so that at the places where this symmetry is broken the fission probability is enhanced by up to an order of magnitude. These fission-favorable locations entail the vicinity of vacancies, specific impurities, and interfaces, such as crystallite boundaries. Hence, photovoltaic yield might be substantially increased by using nanoscopically disordered pentacene rather than highly ordered specimens. PMID:27152577

  9. 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.

  10. Exciton dynamics and device performance in polythiophene heterojunctions for photovoltaics

    Science.gov (United States)

    Chasteen, Stephanie V.; Carter, Sue A.; Rumbles, Garry

    2005-10-01

    We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO2 solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO2 solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO2.

  11. Exciton-LO phonon interaction in II-VI self-assembled quantum dots

    International Nuclear Information System (INIS)

    We study the exciton-LO phonon interaction in CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots (QDs) by means of resonantly excited PL spectroscopy. We find that in the case of large (∝8-10 nm in diameter) CdSe QDs the strength of the exciton-LO phonon coupling is nearly independent of the dot size (e.g. emission energy). In contrast, for CdTe dots (significantly smaller than the CdSe dots) the exciton-LO phonon coupling increases strongly for QDs with higher emission energy (smaller size). Finally, we show for annealed CdTe QDs that, the exciton-LO phonon interaction again becomes independent of emission energy, consistent with the larger dot sizes created by the annealing. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  12. Ab-initio calculation of excitons in conventional and anorganic semiconductors

    Science.gov (United States)

    Ambrosch-Draxl, Claudia; Laskowsky, Robert

    2005-03-01

    The excitonic effects on the optical absorption properties of organic as well as inorganic semiconductors are studied from first-principles. The Coulomb interaction between the electron and the hole is accounted for by solving the two-particle Bethe-Salpeter equation. In the organic semiconductors the exciton binding energies strongly depend on the molecular size, the crystalline packing, as well as the polarization direction of the incoming light. We show that the electron-hole interaction can lead to strongly bound excitons with binding energies of the order of 1eV or to a mere redistribution of oscillator strength. In several cases, the screening is efficient enough such that free charge carriers govern the optical absorption process. In the inorganic counterparts the sensitivity of the exciton binding energy is tested against the structural parameters and the screening of the electron-hole Coulomb interaction.

  13. Nano-optical imaging of WS e2 waveguide modes revealing light-exciton interactions

    Science.gov (United States)

    Fei, Z.; Scott, M. E.; Gosztola, D. J.; Foley, J. J.; Yan, J.; Mandrus, D. G.; Wen, H.; Zhou, P.; Zhang, D. W.; Sun, Y.; Guest, J. R.; Gray, S. K.; Bao, W.; Wiederrecht, G. P.; Xu, X.

    2016-08-01

    We report on a nano-optical imaging study of WS e2 thin flakes with scanning near-field optical microscopy (NSOM). The NSOM technique allows us to visualize in real space various waveguide photon modes inside WS e2 . By tuning the excitation laser energy, we are able to map the entire dispersion of these waveguide modes both above and below the A exciton energy of WS e2 . We found that all the modes interact strongly with WS e2 excitons. The outcome of the interaction is that the observed waveguide modes shift to higher momenta right below the A exciton energy. At higher energies, on the other hand, these modes are strongly damped due to adjacent B excitons or band-edge absorptions. The mode-shifting phenomena are consistent with polariton formation in WS e2 .

  14. Radiative recombination from dark excitons in nanocrystals: Activation mechanisms and polarization properties

    Science.gov (United States)

    Rodina, Anna V.; Efros, Alexander L.

    2016-04-01

    We analyze theoretically physical mechanisms responsible for the radiative recombination of the ground optically passive ("dark") exciton (DE), which dominates in photoluminescence (PL) of colloidal nanocrystals (NCs) at low temperatures. The DE becomes optically active due to its mixing with the bright excitons caused by an external magnetic field, dangling-bond spins or by acoustic and optical phonons. These activation mechanisms mix the DE with different bright excitons and, consequently, lead to different PL polarization properties, because they are determined by dipole orientations of the bright excitons, which the DE is coupled with. We show that the PL polarization properties of prolate and oblate shape NCs are different due to different activation mechanisms responsible for the DE recombination.

  15. Absorption spectrum of the excitons bound on neutral boron atoms in cubic silicon carbide

    International Nuclear Information System (INIS)

    Absorption spectrum of the excitons strongly bound on neutral deep acceptors (boron) is observed for the first time in silicon carbide 3C-SiC:B cubic crystals. The exciton binding energy with EBex acceptor equals 227 meV. The spectrum consists of three absorption lines at photon energies 2.1508; 2.1585 and 2.189 eV and a series of high energy maxima. The spectrum structure is explained by splitting of coupled exciton states by hole-hole exchange interaction in the local acceptor field. It is shown that because of extremely high probability of Auger-processes the decay of excitons bound on neutral boron in 3C-SiC is a radiationless one

  16. Influence of intra-pigment vibrations on dynamics of photosynthetic exciton

    International Nuclear Information System (INIS)

    We have numerically investigated the effect of an underdamped intra-pigment vibrational mode on an exciton's quantum coherence and energy transfer efficiency. Our model describes a bacteriochlorophyll a pigment-protein dimer under the conditions at which photosynthetic energy transfer occurs. The dimer is modeled using a theoretical treatment of a vibronic exciton, and its dynamics are numerically analyzed using a non-Markovian and non-perturbative method. We examined the system's response to various values of the Huang-Rhys factor, site energy difference, reorganization energy, and reorganization energy difference. We found that the inclusion of the intra-pigment vibronic mode allows for long-lived oscillatory quantum coherences to occur. This excitonic coherence is robust against static site-energy disorder. The vibrational mode also promotes exciton transfer along the site-energy landscape thus improving the overall energy transfer efficiency

  17. Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots

    International Nuclear Information System (INIS)

    It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot)–(insulating matrix) interface) in a nanosystem containing insulating Al2O3 quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al2O3 single crystal. It is established that, in the band gap of an Al2O3 nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al2O3 nanoparticles at room temperature from the absorption spectrum of the nanosystem

  18. Experimental investigation of quantum effects in time-resolved resonance Rayleigh scattering from quantum well excitons

    DEFF Research Database (Denmark)

    Birkedal, Dan; Shah, Jagdeep; Shchegrov, Andrei V.;

    2000-01-01

    Resonant Rayleigh scattering from quantum well excitons is investigated using ultrafast spectral interferometry. We isolate the coherent Rayleigh scattering from incoherent luminescence in a single speckle. Averaging the resonant Rayleigh intensity over several speckles allows us to identify...

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

    CERN Document Server

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

    2016-01-01

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

  20. 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...