Excitons in the rare gas solids
International Nuclear Information System (INIS)
1988-01-01
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
International Nuclear Information System (INIS)
Kozhushner, M.
1975-01-01
The theory of quasi particles is explained to layman readers and the significance of the discovery of excitons is pointed out. New possibilities of the study of electron-hole interactions and of superconductivity are indicated. (L.O.)
Energy Technology Data Exchange (ETDEWEB)
Kozhushner, M
1975-06-01
The theory of quasi particles is explained to layman readers and the significance of the discovery of excitons is pointed out. New possibilities of the study of electron-hole interactions and of superconductivity are indicated.
Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder
Energy Technology Data Exchange (ETDEWEB)
Kovalev, V. M.; Chaplik, A. V., E-mail: chaplik@isp.nsc.ru [Russian Academy of Sciences, Rzhanov Institute of Semiconductor Physics, Siberian Branch (Russian Federation)
2016-03-15
A theory for the linear and quadratic responses of a 2D gas of indirect dipolar excitons to an external surface acoustic wave perturbation in the presence of a static random potential is considered. The theory is constructed both for high temperatures, definitely greater than the exciton gas condensation temperature, and at zero temperature by taking into account the Bose–Einstein condensation effects. The particle Green functions, the density–density correlation function, and the quadratic response function are calculated by the “cross” diagram technique. The results obtained are used to calculate the absorption of Rayleigh surface waves and the acoustic exciton gas drag by a Rayleigh wave. The damping of Bogoliubov excitations in an exciton condensate due to theirs scattering by a random potential has also been determined.
Collective Behavior of a Spin-Aligned Gas of Interwell Excitons in Double Quantum Wells
DEFF Research Database (Denmark)
Larionov, A. V.; Bayer, M.; Hvam, Jørn Märcher
2005-01-01
The kinetics of a spin-aligned gas of interwell excitons in GaAs/AlGaAs double quantum wells (n–i–n heterostructure) is studied. The temperature dependence of the spin relaxation time for excitons, in which a photoexcited electron and hole are spatially separated between two adjacent quantum wells...
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...
Observation of surface excitons in rare gas solids
International Nuclear Information System (INIS)
Saile, V.; Skibowski, M.; Steinmann, W.; Guertler, P.; Koch, E.E.; Kozevnikov, A.
1976-04-01
Evidence is obtained for the excitation of surface excitons in solid Ar, Kr and Xe in optical transmission and reflection experiments using synchrotron radiation. They are located at photon energies ranging from 0.6 eV for Ar to 0.1 eV for Xe below the corresponding bulk excitons excited from the valence bands. Their halfwidths (20-50 MeV) is less than half the values found for the bulk excitons. Some are split by an amount considerably smaller than the spin orbit splitting of the valence bands. (orig.) [de
Term value/band-gap energy correlations for solid rare gas excitons
International Nuclear Information System (INIS)
Anon.
1987-01-01
Term value/ionization energy correlation algorithms have proven to be of considerable utility in the assignment of atomic and molecular Rydberg states. Many examples of empirical term value/ionization energy correlations are known for diverse classes of atoms and molecules. The purpose of this paper is to demonstrate that similar correlations are also obtained for excitons in rare gas solids
International Nuclear Information System (INIS)
Grasser, R.; Scharmann, A.
1983-01-01
This chapter investigates absorption, reflectivity, and intrinsic luminescence spectra of free and/or self-trapped (localized) excitons in alkali halides and rare gas solids. Introduces the concepts underlying the Wannier-Mott and Frenkel exciton models, two extreme pictures of an exciton in crystalline materials. Discusses the theoretical and experimental background; excitons in alkali halides; and excitons in rare gas solids. Shows that the intrinsic optical behavior of wide gap insulators in the range of the fundamental absorption edge is controlled by modified Wannier-Mott excitons. Finds that while that alkali halides only show free and relaxed molecular-like exciton emission, in rare gas crystals luminescence due to free, single and double centered localized excitons is observed. Indicates that the simultaneous existence of free and self-trapped excitons in these solid requires an energy barrier for self-trapping
Multicomponent exciton gas in cuprous oxide: cooling behaviour and the role of Auger decay
Semkat, D.; Sobkowiak, S.; Schöne, F.; Stolz, H.; Koch, Th; Fehske, H.
2017-10-01
In this paper we present a hydrodynamic model to describe the dynamics of para- and orthoexcitons in cuprous oxide at ultralow temperatures inside a stress induced potential trap. We take into account the finite lifetime of the excitons, the excitation process and exciton-phonon as well as exciton-exciton interaction. Furthermore, we model the two-body loss mechanism assuming an Auger-like effect and compare it to an alternative explanation which relies on the formation of biexcitons. We discuss in detail the influence on the numerical results and compare the predictions to experimental data.
Kovalev, Vadim M; Tse, Wang-Kong
2017-11-22
We develop a microscopic theory for the relaxation dynamics of an optically pumped two-level system (TLS) coupled to a bath of weakly interacting Bose gas. Using Keldysh formalism and diagrammatic perturbation theory, expressions for the relaxation times of the TLS Rabi oscillations are derived when the boson bath is in the normal state and the Bose-Einstein condensate (BEC) state. We apply our general theory to consider an irradiated quantum dot coupled with a boson bath consisting of a two-dimensional dipolar exciton gas. When the bath is in the BEC regime, relaxation of the Rabi oscillations is due to both condensate and non-condensate fractions of the bath bosons for weak TLS-light coupling and pre dominantly due to the non-condensate fraction for strong TLS-light coupling. Our theory also shows that a phase transition of the bath from the normal to the BEC state strongly influences the relaxation rate of the TLS Rabi oscillations. The TLS relaxation rate is approximately independent of the pump field frequency and monotonically dependent on the field strength when the bath is in the low-temperature regime of the normal phase. Phase transition of the dipolar exciton gas leads to a non-monotonic dependence of the TLS relaxation rate on both the pump field frequency and field strength, providing a characteristic signature for the detection of BEC phase transition of the coupled dipolar exciton gas.
International Nuclear Information System (INIS)
Doke, T.; Hitachi, A.; Hoshi, Y.; Masuda, K.; Hamada, T.
1977-01-01
Precise measurements of W-values, the average energy expended per electron-hole pair in liquid Ar and Xe, were made by the electron-pulse method, and that in liquid Kr by the steady conduction current method. The results showed that the W-values were clearly smaller than those in gaseous Ar, Xe and Kr as predicted by Doke. The results can be explained by the conduction bands which exist in these rare gas liquids as well as in the solid state. The enhanced ionization yield was observed for Xe-doped liquid Ar, and it was attributed to the ionizing excitation transfer process from Ar excitons to doped Xe. This is very similar to the Jesse effect in the gas phase. The saturated value of the enhanced ionization was in good agreement with the theoretical value, and it provides strong evidence for the existence of the exciton states in liquid Ar. Fano factors in liquid Ar, Kr, Xe and Xe-doped liquid Ar have been estimated from the Fano Formula, and they were smaller than those in the gas phase. The drift velocity of electrons in liquid Ar, liquid Ar-gas mixtures and liquid Xe have been measured with gridded ionization chambers. The average electron energy in liquid Ar has been measured. The electron-induced scintillations of liquid Xe and Ar have been studied. (Kato, T.)
International Nuclear Information System (INIS)
Torres, Clivia M.S.
1998-01-01
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
Exciton fission in monolayer transition metal dichalcogenide semiconductors.
Steinhoff, A; Florian, M; Rösner, M; Schönhoff, G; Wehling, T O; Jahnke, F
2017-10-27
When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission-fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles.
Excitonic processes at organic heterojunctions
He, ShouJie; Lu, ZhengHong
2018-02-01
Understanding excitonic processes at organic heterojunctions is crucial for development of organic semiconductor devices. This article reviews recent research on excitonic physics that involve intermolecular charge transfer (CT) excitons, and progress on understanding relationships between various interface energy levels and key parameters governing various competing interface excitonic processes. These interface excitonic processes include radiative exciplex emission, nonradiative recombination, Auger electron emission, and CT exciton dissociation. This article also reviews various device applications involving interface CT excitons, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, organic rectifying diodes, and ultralow-voltage Auger OLEDs.
Exciton emissions in alkali cyanides
International Nuclear Information System (INIS)
Weid, J.P. von der.
1979-10-01
The emissions of Alkali Cyanides X irradiated at low temperature were measured. In addition to the molecular (Frenkel Type) exciton emissions, another emitting centre was found and tentatively assigned to a charge transfer self trapped exciton. The nature of the molecular exciton emitting state is discussed. (Author) [pt
Sarritzu, Valerio; Sestu, Nicola; Marongiu, Daniela; Chang, Xueqing; Wang, Qingqian; Loi, Maria Antonietta; Quochi, Francesco; Saba, Michele; Mura, Andrea; Bongiovanni, Giovanni
2018-01-01
Understanding exciton formation is of fundamental importance for emerging optoelectronic materials, like hybrid organic-inorganic perovskites, as excitons are the lowest-energy photoexcitations in semiconductors, are electrically neutral, and do not directly contribute to charge transport, but can
International Nuclear Information System (INIS)
Strien, A.J. van.
1981-01-01
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.)
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...
Radiative recombination process of high density excitons in CdS crystals
Energy Technology Data Exchange (ETDEWEB)
Dneprovskij, V.S.; Klimov, V.I.; Martynenko, E.D.; Stadnik, V.A.
1983-11-01
The behaviour of the P-, E-, L- and Q-lines of luminescence in CdS is compared with calculated results for the processes of exciton-exciton scattering, exciton-electron scattering, annihilation of equilibrium electron-hole fluid (EHF), annihilation of electron-hole plasma (processes of amplification and reabsorption are taken into account). The comparison permitted to determine parameters of high density exciton gas and EHF. Spectral-kinetic properties of generation are investigated, and amplification factor in CdS is estimated.
Some evidence for the high density phase of excitons in CdS
International Nuclear Information System (INIS)
Rueckmann, I.; May, V.; Voigt, J.
1980-01-01
Reflection spectra without and with additional pumping are measured in CdS at 1.8 and 77 K, respectively, starting from very low pump intensities. At low pump intensities a special behaviour of the reflection minimum is found indicating the importance of an exciton dead layer. The experimental spectra up to the highest pump intensities can be fitted very well taking into account spatial dispersion, exciton dead layer, and density dependent excitonic parameters. Polarizability, damping, and layer thickness change continuously in the whole range of excitation intensities. On the other side, the transverse dielectric function of a high density gas of interacting 1s excitons is calculated. A comparison between theoretically and experimentally obtained excitonic parameters at different densities shows good agreement. Hence, the importance of many-exciton interaction for the reflection spectra up to vanishing at highest pump intensities is concluded. (author)
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...
Plasmon-exciton-polariton lasing
Ramezani, M.; Halpin, A.; Fernández-Dominguez, A.I.; Feist, J.; Rodriguez, S.R.K.; Gómez-Rivas, J.; Garcia-Vidal, F.J.
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
Bose-Einstein condensation of excitons in Cu2O
International Nuclear Information System (INIS)
Snoke, D.W.
1990-01-01
Free excitons provide the only experimental system other than helium in which the behavior of particles with mass is known to follow Bose-Einstein statistics. Experimental observations are presented of the kinetic energy distribution of excitons in the direct-gap semiconductor Cu 2 O, both the triplet orthoexciton state and the singlet paraexciton state. The density and temperature of the exciton gas closely follow the phase boundary for Bose-Einstein condensation. At the highest densities, the lower-lying paraexcitons take on an anomalous energy distribution with a sharp, high-energy edge. This odd distribution of particle energies may be associated with Bose-Einstein condensation into a state with nonzero momentum. Indeed, the excitons leave the region of their creation at supersonic velocities. In addition to the experimental observations, theoretical models are presented for several aspects of this nonequilibrium system. The equilibration of a nearly-ideal boson gas is modeled, finding that a significant time is required for the approach to condensation. The temperature and density of the excitons in steady state are modeled based on known classical kinetic effects in semiconductors, and the effects of Bose-Einstein statistics on these processes estimated
Models of coherent exciton condensation
International Nuclear Information System (INIS)
Littlewood, P B; Eastham, P R; Keeling, J M J; Marchetti, F M; Simons, B D; Szymanska, M H
2004-01-01
That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focusing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers
Models of coherent exciton condensation
Energy Technology Data Exchange (ETDEWEB)
Littlewood, P B [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Eastham, P R [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Keeling, J M J [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Marchetti, F M [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Simons, B D [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom); Szymanska, M H [Theory of Condensed Matter, Cavendish Laboratory, Cambridge CB3 0HE (United Kingdom)
2004-09-08
That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focusing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.
Scaling laws of Rydberg excitons
Heckötter, J.; Freitag, M.; Fröhlich, D.; Aßmann, M.; Bayer, M.; Semina, M. A.; Glazov, M. M.
2017-09-01
Rydberg atoms have attracted considerable interest due to their huge interaction among each other and with external fields. They demonstrate characteristic scaling laws in dependence on the principal quantum number n for features such as the magnetic field for level crossing or the electric field of dissociation. Recently, the observation of excitons in highly excited states has allowed studying Rydberg physics in cuprous oxide crystals. Fundamentally different insights may be expected for Rydberg excitons, as the crystal environment and associated symmetry reduction compared to vacuum give not only optical access to many more states within an exciton multiplet but also extend the Hamiltonian for describing the exciton beyond the hydrogen model. Here we study experimentally and theoretically the scaling of several parameters of Rydberg excitons with n , for some of which we indeed find laws different from those of atoms. For others we find identical scaling laws with n , even though their origin may be distinctly different from the atomic case. At zero field the energy splitting of a particular multiplet n scales as n-3 due to crystal-specific terms in the Hamiltonian, e.g., from the valence band structure. From absorption spectra in magnetic field we find for the first crossing of levels with adjacent principal quantum numbers a Br∝n-4 dependence of the resonance field strength, Br, due to the dominant paramagnetic term unlike for atoms for which the diamagnetic contribution is decisive, resulting in a Br∝n-6 dependence. By contrast, the resonance electric field strength shows a scaling as Er∝n-5 as for Rydberg atoms. Also similar to atoms with the exception of hydrogen we observe anticrossings between states belonging to multiplets with different principal quantum numbers at these resonances. The energy splittings at the avoided crossings scale roughly as n-4, again due to crystal specific features in the exciton Hamiltonian. The data also allow us to
Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites
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.
Detuning-Controlled Internal Oscillations in an Exciton-Polariton Condensate
Voronova, N. S.; Elistratov, A. A.; Lozovik, Yu. E.
2015-10-01
We theoretically analyze exciton-photon oscillatory dynamics within a homogenous polariton gas in the presence of energy detuning between the cavity and quantum well modes. Whereas pure Rabi oscillations consist of the particle exchange between the photon and exciton states in the polariton system without any oscillations of the phases of the two subcondensates, we demonstrate that any nonzero detuning results in oscillations of the relative phase of the photon and exciton macroscopic wave functions. Different initial conditions reveal a variety of behaviors of the relative phase between the two condensates, and a crossover from Rabi-like to Josephson-like oscillations is predicted.
Magnetic exciton dispersion in praseodymium
DEFF Research Database (Denmark)
Rainford, B. D.; Houmann, Jens Christian Gylden
1971-01-01
Measurements of the dispersion of magnetic excitons have been made in a single crystal of praseodymium metal using inelastic neutron scattering. A preliminary analysis of the data yields the first detailed information about the exchange interactions and the crystal field splittings in the light...... rare-earth metals....
Plasmon exciton-polariton lasing
Ramezani, M.; Halpin, H.A.; Feist, J.; Fernández-Dominguez, A.; Rodriguez, S.R.K.; Garcia-Vidal, F.J.; Gomez-Rivas, J.
2017-01-01
Strong light-matter interaction leads to the appearance of new states, i.e. exciton-polaritons, with photophysical properties rather distinct from their constituents. Recent developments in fabrication techniques allow us to make metallic structures with strong electric field confinement in
Spatially indirect excitons in coupled quantum wells
Energy Technology Data Exchange (ETDEWEB)
Lai, Chih-Wei Eddy [Univ. of California, Berkeley, CA (United States)
2004-03-01
Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer)^{2} were
Exciton Seebeck effect in molecular systems
Energy Technology Data Exchange (ETDEWEB)
Yan, Yun-An, E-mail: yunan@nano.gznc.edu.cn [Guizhou Provincial Key Laboratory of Computational Nanomaterial Science, Guizhou Normal College, Guiyang, Guizhou 550018 (China); Cai, Shaohong [Guizhou Key Laboratory of Economic System Simulation, Guizhou University of Finance and Economics, Guiyang 550004 (China)
2014-08-07
We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.
Exciton Binding Energy of Monolayer WS2
Zhu, Bairen; Chen, Xi; Cui, Xiaodong
2015-03-01
The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to 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 0.71 +/- 0.01 eV around K valley in the Brillouin zone.
Fractional Solitons in Excitonic Josephson Junctions
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 ? 0 applied. The system is mapped into a pseudospin ferromagnet then described numeric...
Density-dependent squeezing of excitons in highly excited semiconductors
International Nuclear Information System (INIS)
Nguyen Hong Quang.
1995-07-01
The time evolution from coherent states to squeezed states of high density excitons is studied theoretically based on the boson formalism and within the Random Phase Approximation. Both the mutual interaction between excitons and the anharmonic exciton-photon interaction due to phase-space filling of excitons are taken into account. It is shown that the exciton squeezing depends strongly on the exciton density in semiconductors and becomes smaller with increasing the latter. (author). 16 refs, 2 figs
Condensation of exciton polaritons
International Nuclear Information System (INIS)
Kasprzak, J.
2006-10-01
Because of their unique property of bringing pure quantum effects into the real world scale, phase transitions towards condensed phases - like Bose-Einstein condensation (BEC), superfluidity, and superconductivity - have always fascinated scientists. The BEC, appearing upon cooling a gas of bosons below a critical temperature, has been given a striking demonstration in dilute atomic gases of rubidium atoms at temperatures below 200 nK. By confining photons in a semiconductor micro-cavity, and strongly coupling them to electronic excitations, one may create polaritons. These bosonic quasi-particles are 10 9 times lighter than rubidium atoms, thus theoretically allowing a BEC at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for a BEC of polaritons. Above a critical density, we observe massive occupation of the ground state, developing from a thermalized and saturated distribution of the polariton population at (16-20) K. We demonstrate as well the existence of a critical temperature for this transition. The spontaneous onset of a coherent state is manifested by the increase of temporal coherence, the build-up of long-range spatial coherence and the reduction of the thermal noise observed in second order coherence experiments. The marked linear polarization of the emission from the condensate is also measured. All of these findings indicate the spontaneous onset of a macroscopic quantum phase. (author)
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.
International Nuclear Information System (INIS)
Nguyen Ba An; Hoang Ngoc Cam; Nguyen Trung Dan
1990-08-01
Analytical expressions of the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D structure materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass ratio and two-exciton state spin symmetry. Numerical calculations show that the character of the exciton-exciton interaction is determined by all of the above-mentioned dependences. (author). 32 refs, 7 figs
The interplay between excitons and trions in a monolayer of MoSe2
Lundt, N.; Cherotchenko, E.; Iff, O.; Fan, X.; Shen, Y.; Bigenwald, P.; Kavokin, A. V.; Höfling, S.; Schneider, C.
2018-01-01
The luminescence and absorption properties of transition metal dichalcogenide monolayers are widely determined by neutral and charged excitonic complexes. Here, we focus on the impact of a free carrier reservoir on the optical properties of excitonic and trionic complexes in a MoSe2 monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser, the electron density can be controlled in our sample, which is directly reflected in the contribution of excitons and trions to the luminescence signal. We find significant shifts of both the exciton and trion energies in the presence of an induced electron gas both in power- and in time evolution (on the second to minute scale) in our photoluminescence spectra. In particular, in the presence of the photo-doped carrier reservoir, we observe that the splitting between excitons and trions can be enhanced by up to 4 meV. This behaviour is phenomenologically explained by an interplay between an increased screening of excitons via electrons in our system and a modification of the Fermi level. We introduce a simple but still quantitative treatment of these effects within a variational approach that takes into account both screening and phase space filling effects.
Resonant transfer of excitons and quantum computation
International Nuclear Information System (INIS)
Lovett, Brendon W.; Reina, John H.; Nazir, Ahsan; Kothari, Beeneet; Briggs, G. Andrew D.
2003-01-01
Resonant energy transfer mechanisms have been observed in the sensitized luminescence of solids, and in quantum dots, molecular nanostructures, and photosynthetic organisms. We demonstrate that such mechanisms, together with the exciton-exciton binding energy shift typical of these nanostructures, can be used to perform universal quantum logic and generate quantum entanglement
Radiative recombination of excitons in amorphous semiconductors
Energy Technology Data Exchange (ETDEWEB)
Singh, Jai [School of Engineering and Logistics, Faculty Technology, B-41, Charles Darwin University, Darwin, NT 0909 (Australia)]. E-mail: jai.singh@cdu.edu.au
2005-04-15
A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments.
Two-photon transitions to exciton polaritons
International Nuclear Information System (INIS)
Hassan, A.R.
1979-08-01
A semiclassical theory for the creation of excitonic polariton states by two-photon absorption, via an intermediate exciton state, is given. A band model has been introduced which gives the dominant contribution to this process. A numerical calculation is found to be in good agreement with a recent observation in CuCl. (author)
Radiative recombination of excitons in amorphous semiconductors
International Nuclear Information System (INIS)
Singh, Jai
2005-01-01
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
Bondarev, Igor; Popescu, Adrian
We develop an analytical theory for the intra-intermolecular exciton intermixing in periodic 1D chains of planar organic molecules with two isolated low-lying Frenkel exciton states, typical of copper phthalocyanine (CuPc) and other transition metal phthalocyanine molecules. We formulate the Hamiltonian and use the exact Bogoliubov diagonalization procedure to derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer (CT) exciton state. By comparing our theoretical spectrum with available experimental CuPc absorption data, we obtain the parameters of the Frenkel-CT exciton intermixing in CuPc thin films. The two Frenkel exciton states here are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the CT exciton, showing the coupling constant 0.17 eV in agreement with earlier electron transport experiments. Our results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines. DOE-DE-SC0007117 (I.B.), UNC-GA ROI Grant (A.P.).
Exciton correlations and input–output relations in non-equilibrium exciton superfluids
International Nuclear Information System (INIS)
Ye, Jinwu; Sun, Fadi; Yu, Yi-Xiang; Liu, Wuming
2013-01-01
The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron–hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg–Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input–output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton–polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron–electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor ν T =1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: ► Establish the relations between photoluminescence and transport measurements. ► Stress the
Instantaneous Rayleigh scattering from excitons localized in monolayer islands
DEFF Research Database (Denmark)
Langbein, Wolfgang; Leosson, Kristjan; Jensen, Jacob Riis
2000-01-01
We show that the initial dynamics of Rayleigh scattering from excitons in quantum wells can be either instantaneous or delayed, depending on the exciton ensemble studied. For excitation of the entire exciton resonance, a finite rise time given by the inverse inhomogeneous broadening: of the exciton...
Dark excitons in transition metal dichalcogenides
Malic, Ermin; Selig, Malte; Feierabend, Maja; Brem, Samuel; Christiansen, Dominik; Wendler, Florian; Knorr, Andreas; Berghäuser, Gunnar
2018-01-01
Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a nondirect gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.
Excitonic Effects in Methylammonium Lead Halide Perovskites
Energy Technology Data Exchange (ETDEWEB)
Beard, Matthew C [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Chen, Xihan [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Lu, Haipeng [National Renewable Energy Laboratory (NREL), Golden, CO (United States); Yang, Ye [National Renewable Energy Laboratory (NREL), Golden, CO (United States)
2018-05-01
The exciton binding energy in methylammonium lead iodide (MAPbI3) is about 10 meV, around 1/3 of the available thermal energy (kBT ~ 26 meV) at room temperature. Thus, exciton populations are not stable at room temperature at moderate photoexcited carrier densities. However, excitonic resonances dominate the absorption onset. Furthermore, these resonances determine the transient absorbance and transient reflectance spectra. The exciton binding energy is a reflection of the Coulomb interaction energy between photoexcited electrons and holes. As such, it serves as a marker for the strength of electron/hole interactions and impacts a variety of phenomena, such as, absorption, radiative recombination, and Auger recombination. In this Perspective, we discuss the role of excitons and excitonic resonances in the optical properties of lead-halide perovskite semiconductors. Finally, we discuss how the strong light-matter interactions induce an optical stark effect splitting the doubly spin degenerate ground exciton states and are easily observed at room temperature.
Polariton condensation with localized excitons and propagating photons
International Nuclear Information System (INIS)
Keeling, Jonathan; Eastham, P.R.; Szymanska, M.H.; Littlewood, P.B.
2004-01-01
We estimate the condensation temperature for microcavity polaritons, allowing for their internal structure. We consider polaritons formed from localized excitons in a planar microcavity, using a generalized Dicke model. At low densities, we find a condensation temperature T c ∝ρ, as expected for a gas of structureless polaritons. However, as T c becomes of the order of the Rabi splitting, the structure of the polaritons becomes relevant, and the condensation temperature is that of a BCS-like mean-field theory. We also calculate the excitation spectrum, which is related to observable quantities such as the luminescence and absorption spectra
Excitons in the Fractional Quantum Hall Effect
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.
Switching Exciton Pulses Through Conical Intersections
Leonhardt, K.; Wüster, S.; Rost, J. M.
2014-11-01
Exciton pulses transport excitation and entanglement adiabatically through Rydberg aggregates, assemblies of highly excited light atoms, which are set into directed motion by resonant dipole-dipole interaction. Here, we demonstrate the coherent splitting of such pulses as well as the spatial segregation of electronic excitation and atomic motion. Both mechanisms exploit local nonadiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses. The fundamental ideas discussed here have general implications for excitons on a dynamic network.
Exciton-relaxation dynamics in lead halides
International Nuclear Information System (INIS)
Iwanaga, Masanobu; Hayashi, Tetsusuke
2003-01-01
We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide
Optical nutation in the exciton range of spectrum
International Nuclear Information System (INIS)
Khadzhi, P. I.; Vasiliev, V. V.
2013-01-01
Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level
Tailoring Quantum Dot Assemblies to Extend Exciton Coherence Times and Improve Exciton Transport
Seward, Kenton; Lin, Zhibin; Lusk, Mark
2012-02-01
The motion of excitons through nanostructured assemblies plays a central role in a wide range of physical phenomena including quantum computing, molecular electronics, photosynthetic processes, excitonic transistors and light emitting diodes. All of these technologies are severely handicapped, though, by quasi-particle lifetimes on the order of a nanosecond. The movement of excitons must therefore be as efficient as possible in order to move excitons meaningful distances. This is problematic for assemblies of small Si quantum dots (QDs), where excitons quickly localize and entangle with dot phonon modes. Ensuing exciton transport is then characterized by a classical random walk reduced to very short distances because of efficient recombination. We use a combination of master equation (Haken-Strobl) formalism and density functional theory to estimate the rate of decoherence in Si QD assemblies and its impact on exciton mobility. Exciton-phonon coupling and Coulomb interactions are calculated as a function of dot size, spacing and termination to minimize the rate of intra-dot phonon entanglement. This extends the time over which more efficient exciton transport, characterized by partial coherence, can be maintained.
PbSe Nanocrystal Excitonic Solar Cells
Choi, Joshua J.; Lim, Yee-Fun; Santiago-Berrios, Mitk’ El B.; Oh, Matthew; Hyun, Byung-Ryool; Sun, Liangfeng; Bartnik, Adam C.; Goedhart, Augusta; Malliaras, George G.; Abruña, Héctor D.; Wise, Frank W.; Hanrath, Tobias
2009-01-01
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
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......-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 alpha q, and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons...... exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model, and both are in good agreement with ab initio many-body calculations. On the other hand, for more complex structures...
Atomic lattice excitons: from condensates to crystals
International Nuclear Information System (INIS)
Kantian, A; Daley, A J; Toermae, P; Zoller, P
2007-01-01
We discuss atomic lattice excitons (ALEs), bound particle-hole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean set-up, with tunable masses and interactions, to study fundamental properties of excitons including exciton condensation. We also find that for a large effective mass ratio between particles and holes, effective long-range interactions can mediate the formation of an exciton crystal, for which superfluidity is suppressed. Using a combination of mean-field treatments, bosonized theory based on a Born-Oppenheimer approximation, and one-dimensional (1D) numerical computation, we discuss the properties of ALEs under varying conditions, and discuss in particular their preparation and measurement
Atomic lattice excitons: from condensates to crystals
Energy Technology Data Exchange (ETDEWEB)
Kantian, A [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria); Daley, A J [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria); Toermae, P [Nanoscience Center, Department of Physics, University of Jyvaeskylae, PO Box 35, FIN-40014 (Finland); Zoller, P [Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck (Austria)
2007-11-15
We discuss atomic lattice excitons (ALEs), bound particle-hole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean set-up, with tunable masses and interactions, to study fundamental properties of excitons including exciton condensation. We also find that for a large effective mass ratio between particles and holes, effective long-range interactions can mediate the formation of an exciton crystal, for which superfluidity is suppressed. Using a combination of mean-field treatments, bosonized theory based on a Born-Oppenheimer approximation, and one-dimensional (1D) numerical computation, we discuss the properties of ALEs under varying conditions, and discuss in particular their preparation and measurement.
Pentacene Excitons in Strong Electric Fields.
Kuhnke, Klaus; Turkowski, Volodymyr; Kabakchiev, Alexander; Lutz, Theresa; Rahman, Talat S; Kern, Klaus
2018-02-05
Electroluminescence spectroscopy of organic semiconductors in the junction of a scanning tunneling microscope (STM) provides access to the polarizability of neutral excited states in a well-characterized molecular geometry. We study the Stark shift of the self-trapped lowest singlet exciton at 1.6 eV in a pentacene nanocrystal. Combination of density functional theory (DFT) and time-dependent DFT (TDDFT) with experiment allows for assignment of the observation to a charge-transfer (CT) exciton. Its charge separation is perpendicular to the applied field, as the measured polarizability is moderate and the electric field in the STM junction is strong enough to dissociate a CT exciton polarized parallel to the applied field. The calculated electric-field-induced anisotropy of the exciton potential energy surface will also be of relevance to photovoltaic applications. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires
Yan, Jie-Yun
2018-06-01
Excitonic terahertz photoconductivity in intrinsic semiconductor nanowires is studied. Based on the excitonic theory, the numerical method to calculate the photoconductivity spectrum in the nanowires is developed, which can simulate optical pump terahertz-probe spectroscopy measurements on real nanowires and thereby calculate the typical photoconductivity spectrum. With the help of the energetic structure deduced from the calculated linear absorption spectrum, the numerically observed shift of the resonant peak in the photoconductivity spectrum is found to result from the dominant exciton transition between excited or continuum states to the ground state, and the quantitative analysis is in good agreement with the quantum plasmon model. Besides, the dependence of the photoconductivity on the polarization of the terahertz field is also discussed. The numerical method and supporting theoretical analysis provide a new tool for experimentalists to understand the terahertz photoconductivity in intrinsic semiconductor nanowires at low temperatures or for nanowires subjected to below bandgap photoexcitation, where excitonic effects dominate.
Triplet exciton diffusion in organic semiconductors
Energy Technology Data Exchange (ETDEWEB)
Koehler, Anna [Department of Physics, University of Bayreuth (Germany)
2010-07-01
Efficient triplet exciton emission has allowed improved operation of organic light-emitting diodes (LEDs). To enhance the device performance, it is necessary to understand what governs the motion of triplet excitons through the organic semiconductor. We use a series of poly(p-phenylene)-type conjugated polymers and oligomers of variable degree of molecular distortion (i.e. polaron formation) and energetic disorder as model systems to study the Dexter-type triplet exciton diffusion in thin films. We show that triplet diffusion can be quantitatively described in the framework of a Holstein small polaron model (Marcus theory) that is extended to include contributions from energetic disorder. The model predicts a tunnelling process at low temperatures followed by a thermally activated hopping process above a transition temperature. In contrast to charge transfer, the activation energy required for triplet exciton transfer can be deduced from the optical spectra. We discuss the implications for device architecture.
Exciton in type-II quantum dot
Energy Technology Data Exchange (ETDEWEB)
Sierra-Ortega, J; Escorcia, R A [Universidad del Magdalena, A. A. 731, Santa Marta (Colombia); Mikhailov, I D, E-mail: jsierraortega@gmail.co [Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)
2009-05-01
We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantum dot and wetting layer induced by trapped exciton strongly depends on the quantum dot morphology and the strength of the magnetic field.
Excitons in Single-Walled Carbon Nanotubes and Their Dynamics
Amori, Amanda R.; Hou, Zhentao; Krauss, Todd D.
2018-04-01
Understanding exciton dynamics in single-walled carbon nanotubes (SWCNTs) is essential to unlocking the many potential applications of these materials. This review summarizes recent progress in understanding exciton photophysics and, in particular, exciton dynamics in SWCNTs. We outline the basic physical and electronic properties of SWCNTs, as well as bright and dark transitions within the framework of a strongly bound one-dimensional excitonic model. We discuss the many facets of ultrafast carrier dynamics in SWCNTs, including both single-exciton states (bright and dark) and multiple-exciton states. Photophysical properties that directly relate to excitons and their dynamics, including exciton diffusion lengths, chemical and structural defects, environmental effects, and photoluminescence photon statistics as observed through photon antibunching measurements, are also discussed. Finally, we identify a few key areas for advancing further research in the field of SWCNT excitons and photonics.
Excitonic polaritons of zinc diarsenide single crystals
Energy Technology Data Exchange (ETDEWEB)
Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of); Stamov, I.G. [T.G. Shevchenko State University of Pridnestrovie, Tiraspol, Republic of Moldova (Moldova, Republic of); Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of); Dorogan, A. [Technical University of Moldova, Chisinau, Republic of Moldova (Moldova, Republic of)
2017-02-01
Excitonic polaritons of ZnAs{sub 2} single crystals had been investigated. Parameters of singlet excitons with Ð“{sub 2}¯(z) symmetry and orthoexcitons 2Ð“{sub 1}¯(y)+Ð“{sub 2}¯(x) had been determined. Spectral dependencies of ordinary and extraordinary dispersion of refractive index had been calculated using interferential reflection and transmittance spectra. It was shown, that A excitonic series were due to hole (V{sub 1}) and electron (C{sub 1}) bands. The values of effective masses of electrons (m{sub c}{sup *}=0.10 m{sub 0}) and holes (m{sub v1}{sup *}=0.89 m{sub 0}) had been estimated. It was revealed that the hole mass m{sub v1}{sup *} changes from 1.03 m{sub 0} to 0.55 m{sub 0} at temperature increasing from 10 K up to 230 K and that the electron mass m{sub c}{sup *} does not depend on temperature. The integral absorption A (eV cm{sup −1}) of the states n=1, 2 and 3 of Ð“{sub 2}¯(z) excitons depends on the A{sub n}≈n{sup −3} equality, which it is characteristic for S-type excitonic functions. Temperature dependences of the integral absorption of ground states for Ð“{sub 2}¯(z) and Ð“{sub 2}¯(Ñ…) excitons differ. The ground states of B and C excitons formed by V{sub 3} – C{sub 1} and V{sub 4} – C{sub 1} bands and its parameters had been determined.
Influence of excitonic effects on luminescence quantum yield in silicon
Energy Technology Data Exchange (ETDEWEB)
Sachenko, A.V.; Kostylyov, V.P.; Vlasiuk, V.M. [V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41 prospect Nauky, 03028 Kyiv (Ukraine); Sokolovskyi, I.O., E-mail: isokolovskyi@mun.ca [V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, 41 prospect Nauky, 03028 Kyiv (Ukraine); Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John' s, NL, A1B 3X7 Canada (Canada); Evstigneev, M. [Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John' s, NL, A1B 3X7 Canada (Canada)
2017-03-15
Nonradiative exciton lifetime in silicon is determined by comparison of the experimental and theoretical curves of bulk minority charge carriers lifetime on doping and excitation levels. This value is used to analyze the influence of excitonic effects on internal luminescence quantum yield at room temperature, taking into account both nonradiative and radiative exciton lifetimes. A range of Shockley-Hall-Reed lifetimes is found, where excitonic effects lead to an increase of internal luminescence quantum yield.
Bose-Einstein condensation and indirect excitons: a review.
Combescot, Monique; Combescot, Roland; Dubin, François
2017-06-01
We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath
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....
Exciton broadening in WS2 /graphene heterostructures
International Nuclear Information System (INIS)
Hill, Heather M.; Rigosi, Albert F.; Raja, Archana
2017-01-01
Here, we have used optical spectroscopy to observe spectral broadening of WS 2 exciton reflectance peaks in heterostructures of monolayer WS 2 capped with mono- to few-layer graphene. The broadening is found to be similar for the A and B excitons and on the order of 5–10 meV. No strong dependence on the number of graphene layers was observed within experimental uncertainty. The broadening can be attributed to charge- and energy-transfer processes between the two materials, providing an observed lower bound for the corresponding time scales of 65 fs.
Anatomy of an Exciton : Vibrational Distortion and Exciton Coherence in H- and J-Aggregates
Tempelaar, Roel; Stradomska, Anna; Knoester, Jasper; Spano, Frank C.
2013-01-01
In organic materials, coupling of electronic excitations to vibrational degrees of freedom results in polaronic excited states. Through numerical calculations, we demonstrate that the vibrational distortion field accompanying such a polaron scales as the product of the excitonic interaction field
Nonboson treatment of excitonic nonlinearity in optically excited media
International Nuclear Information System (INIS)
Nguyen Ba An.
1990-11-01
The present article shortly reviews some recent results in the study of excitonic nonlinearity in optically excited media using a nonboson treatment for many-exciton systems. After a brief discussion of the exciton nonbosonity the closed commutation relations are given for exciton operators which hold for any exciton density and type. The nonboson treatment is then applied to the problems of intrinsic optical bistability and nonlinear polariton yielding quite interesting and new effects, e.g. new shapes of hysteresis loops of intrinsic optical bistability or anomalies of polariton dispersion. (author). 71 refs, 4 figs
Interlayer excitons in a bulk van der Waals semiconductor
DEFF Research Database (Denmark)
Arora, Ashish; Drueppel, Matthias; Schmidt, Robert
2017-01-01
Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity......, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments...
Excitonic optical bistability in n-type doped semiconductors
International Nuclear Information System (INIS)
Nguyen Ba An; Le Thi Cat Tuong
1991-07-01
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
Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides
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.
Coherent quantum dynamics of excitons in monolayer transition metal dichalcogenides
Moody, Galan; Hao, Kai; Dass, Chandriker Kavir; Singh, Akshay; Xu, Lixiang; Tran, Kha; Chen, Chang-Hsiao; Li, Ming-yang; Li, Lain-Jong; Clark, Genevieve; Bergh ä user, Gunnar; Malic, Ermin; Knorr, Andreas; Xu, Xiaodong; Li, Xiaoqin
2016-01-01
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.
Properties of Excitons Bound to Ionized Donors
DEFF Research Database (Denmark)
Skettrup, Torben; Suffczynski, M.; Gorzkowski, W.
1971-01-01
Binding energies, interparticle distances, oscillator strengths, and exchange corrections are calculated for the three-particle complex corresponding to an exciton bound to an ionized donor. The results are given as functions of the mass ratio of the electron and hole. Binding of the complex is o...
Quantum-dot excitons in nanostructured environments
DEFF Research Database (Denmark)
Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter
2010-01-01
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...
Quantum-dot excitons in nanostructured environments
DEFF Research Database (Denmark)
Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter
2011-01-01
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...
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...
Triplet exciton formation in organic photovoltaics
Energy Technology Data Exchange (ETDEWEB)
Yang, Xudong; Westenhoff, Sebastian; Howard, Ian; Ford, Thomas; Friend, Richard; Hodgkiss, Justin; Greenham, Neil [Cavendish Laboratory, University of Cambridge (United Kingdom)
2009-07-01
We have recently found that the formation of triplet excitons can be an important loss mechanism in organic photovoltaics, particularly in donor-acceptor blends designed to have high open-circuit voltages. This can occur when the intrachain triplet state lies lower in energy than the charge-transfer state formed at the heterojunction. We find that in a blend based on the polyfluorene derivatives F8BT and PFB, triplet excitons are formed after photoexcitation with much higher efficiency than in the component polymers. We use transient absorption spectroscopy to study the dynamics of charges and triplet excitons on timescales from picoseconds to microseconds. This allows us to determine a characteristic time of {proportional_to} 40 ns for intersystem crossing in the charge-separated state, and to estimate that as many as 75% of photoexcitations lead to the formation of triplet states. To avoid losses to triplet excitons in photovoltaic devices, it is necessary to separate charge pairs before intersystem crossing can occur. We also present photophysical measurements of saturation and relaxation of the triplet excited state absorption used to quantify triplet populations.
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...
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...
Exciton diffusion length in narrow bandgap polymers
Mikhnenko, O.V.; Azimi, H.; Morana, M.; Blom, P.W.M.; Loi, M.A.
2012-01-01
We developed a new method to accurately extract the singlet exciton diffusion length in organic semiconductors by blending them with a low concentration of methanofullerene[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The dependence of photoluminescence (PL) decay time on the fullerene
Fractional Solitons in Excitonic Josephson Junctions
Su, Jung-Jung; Hsu, Ya-Fen
The Josephson effect is especially appealing because it reveals macroscopically the quantum order and phase. Here we study this effect in an excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. Such a junction is proposed to take place in the quantum Hall bilayer (QHB) that makes it subtler than in superconductor because of the counterflow of excitonic supercurrent and the interlayer tunneling in QHB. We treat the system theoretically by first mapping it into a pseudospin ferromagnet then describing it by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, the excitonic Josephson junction can possess a family of fractional sine-Gordon solitons that resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Interestingly, each fractional soliton carries a topological charge Q which is not necessarily a half/full integer but can vary continuously. The resultant current-phase relation (CPR) shows that solitons with Q =ϕ0 / 2 π are the lowest energy states for small ϕ0. When ϕ0 > π , solitons with Q =ϕ0 / 2 π - 1 take place - the polarity of CPR is then switched.
Fine structure of the exciton electroabsorption in semiconductor superlattices
Energy Technology Data Exchange (ETDEWEB)
Monozon, B.S., E-mail: borismonozon@mail.ru [Physics Department, Marine Technical University, 3 Lotsmanskaya Str., 190008 St.Petersburg (Russian Federation); Schmelcher, P. [Zentrum für Optische Quantentechnologien, The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg (Germany)
2017-02-15
Wannier-Mott excitons in a semiconductor layered superlattice (SL) are investigated analytically for the case that the period of the superlattice is much smaller than the 2D exciton Bohr radius. Additionally we assume the presence of a longitudinal external static electric field directed parallel to the SL axis. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the electric field strength are presented in an explicit form. We focus on the fine structure of the ground quasi-2D exciton level formed by the series of closely spaced energy levels adjacent from the high frequencies. These levels are related to the adiabatically slow relative exciton longitudinal motion governed by the potential formed by the in-plane exciton state. It is shown that the external electric fields compress the fine structure energy levels, decrease the intensities of the corresponding optical peaks and increase the exciton binding energy. A possible experimental study of the fine structure of the exciton electroabsorption is discussed.
Relaxation of nonthermal hh and lh excitons in ZnSe quantum wells
DEFF Research Database (Denmark)
Kalt, H.; Hoffmann, J.; Umlauff, M.
1998-01-01
The strong exciton-LO phonon coupling in ZnSe QWs gives a direct access to the relaxation dynamics of nonthermal, free heavy-hole and light-hole excitons. Narrow hot-exciton distributions can be generated by LO-phonon assisted exciton formation. The thermalization of these excitons is monitored b...
Excitonic and photonic processes in materials
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.
Energy relaxation and transfer in excitonic trimer
International Nuclear Information System (INIS)
Herman, Pavel; Barvik, Ivan; Urbanec, Martin
2004-01-01
Two models describing exciton relaxation and transfer (the Redfield model in the secular approximation and Capek's model) are compared for a simple example - a symmetric trimer coupled to a phonon bath. Energy transfer within the trimer occurs via resonance interactions and coupling between the trimer and the bath occurs via modulation of the monomer energies by phonons. Two initial conditions are adopted: (1) one of higher eigenstates of the trimer is initially occupied and (2) one local site of the trimer is initially occupied. The diagonal exciton density matrix elements in the representation of eigenstates are found to be the same for both models, but this is not so for the off-diagonal density matrix elements. Only if the off-diagonal density matrix elements vanish initially (initial condition (1)), they then vanish at arbitrary times in both models. If the initial excitation is local, the off-diagonal matrix elements essentially differ
Excitonic transitions in homoepitaxial GaN
Energy Technology Data Exchange (ETDEWEB)
Martinez-Criado, G.; Cros, A.; Cantarero, A. [Materials Science Inst. and Dept. of Applied Physics, Univ. of Valencia (Spain); Miskys, C.R.; Ambacher, O.; Stutzmann, M. [Technische Univ. Muenchen, Garching (Germany). Walter-Schottky-Inst. fuer Physikalische Grundlagen der Halbleiterelektronik
2001-11-08
The photoluminescence spectrum of a high quality homoepitaxial GaN film has been measured as a function of temperature. As temperature increases the recombination of free excitons dominates the spectra. Their energy shift has successfully fitted in that temperature range by means of the Bose-Einstein expression instead of Varshni's relationship. Values for the parameters of both semi-empirical relations describing the energy shift are reported and compared with the literature. (orig.)
Comments on exciton-phonon coupling. II
International Nuclear Information System (INIS)
Allen, J.W.; Silbey, R.
1979-01-01
Two variational calculations of the energy and correlation functions for a simple exciton-phonon coupled system are presented and contrasted to the adiabatic solution and the exact solution. The simpler variational solution leads to two minima and abrupt changes in the properties of the system; an asymmetric variational wavefunction, motivated by the form of perturbation theory for this problem, leads to smooth behavior in agreement with the exact result. (Auth.)
Chiral topological excitons in a Chern band insulator
Chen, Ke; Shindou, Ryuichi
2017-10-01
A family of semiconductors called Chern band insulators are shown to host exciton bands with nonzero topological Chern integers and chiral exciton edge modes. Using a prototypical two-band Chern insulator model, we calculate a cross-correlation function to obtain the exciton bands and their Chern integers. The lowest exciton band acquires Chern integers such as ±1 and ±2 in the electronic Chern insulator phase. The nontrivial topology can be experimentally observed both by a nonlocal optoelectronic response of exciton edge modes and by a phase shift in the cross-correlation response due to the bulk mode. Our result suggests that magnetically doped HgTe, InAs/GaSb quantum wells, and (Bi,Sb)2Te3 thin films are promising candidates for a platform of topological excitonics.
Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials
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.
Fluorescence Spectroscopy, Exciton Dynamics and Photochemistry of Single Allophycocyanin Trimers
International Nuclear Information System (INIS)
Ying, Liming; Xie, Xiaoliang
1998-01-01
We report a study of the spectroscopy and exciton dynamics of the allophycocyanin trimer (APC), a light harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed, because of generation of exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible ixciton trap formation within the three quasi-independent pairs of strong interacting a84 and B84 chromophores in APC, as well a photobleaching of individual chromophores. Comparison experiments under continuous wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into exciton dynamics and photochemistry of light-harvesting complexes
Excitons in atomically thin 2D semiconductors and their applications
Xiao, Jun; Zhao, Mervin; Wang, Yuan; Zhang, Xiang
2017-06-01
The research on emerging layered two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), reveals unique optical properties generating significant interest. Experimentally, these materials were observed to host extremely strong light-matter interactions as a result of the enhanced excitonic effect in two dimensions. Thus, understanding and manipulating the excitons are crucial to unlocking the potential of 2D materials for future photonic and optoelectronic devices. In this review, we unravel the physical origin of the strong excitonic effect and unique optical selection rules in 2D semiconductors. In addition, control of these excitons by optical, electrical, as well as mechanical means is examined. Finally, the resultant devices such as excitonic light emitting diodes, lasers, optical modulators, and coupling in an optical cavity are overviewed, demonstrating how excitons can shape future 2D optoelectronics.
New method for control over exciton states in quantum wells
International Nuclear Information System (INIS)
Maslov, A Yu; Proshina, O V
2010-01-01
The theoretical study of the exciton states in the quantum well is performed with regard to the distinctions of the dielectric properties of quantum well and barrier materials. The strong exciton-phonon interaction is shown to be possible in materials with high ionicity. This leads to the essential modification of the exciton states. The relationship between the exciton binding energy, along with oscillator strength and the barrier material dielectric properties is found. This suggests the feasibility of the exciton spectrum parameter control by the choice of the barrier material. It is shown that such exciton spectrum engineering also is possible in the quantum wells based on the materials with low ionicity. The reason is the dielectric confinement effect in the quantum wells.
Engineering and manipulating exciton wave packets
Zang, Xiaoning; Montangero, Simone; Carr, Lincoln D.; Lusk, Mark T.
2017-05-01
When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction, and spectral make-up that allows them to be selectively passed, rejected, or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage, and remove structured excitons comprises the foundation for optoexcitonic circuits with application to a wide range of quantum information, energy, and light-flow technologies. The paradigm is demonstrated using both tight-binding and time-domain density functional theory simulations.
Excitons in intact cells of photosynthetic bacteria.
Freiberg, Arvi; Pajusalu, Mihkel; Rätsep, Margus
2013-09-26
Live cells and regular crystals seem fundamentally incompatible. Still, effects characteristic to ideal crystals, such as coherent sharing of excitation, have been recently used in many studies to explain the behavior of several photosynthetic complexes, especially the inner workings of the light-harvesting apparatus of the oldest known photosynthetic organisms, the purple bacteria. To this date, there has been no concrete evidence that the same effects are instrumental in real living cells, leaving a possibility that this is an artifact of unnatural study conditions, not a real effect relevant to the biological operation of bacteria. Hereby, we demonstrate survival of collective coherent excitations (excitons) in intact cells of photosynthetic purple bacteria. This is done by using excitation anisotropy spectroscopy for tracking the temperature-dependent evolution of exciton bands in light-harvesting systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic organisms.
Organic-Inorganic Composites of Semiconductor Nanocrystals for Efficient Excitonics.
Guzelturk, Burak; Demir, Hilmi Volkan
2015-06-18
Nanocomposites of colloidal semiconductor nanocrystals integrated into conjugated polymers are the key to soft-material hybrid optoelectronics, combining advantages of both plastics and particles. Synergic combination of the favorable properties in the hybrids of colloidal nanocrystals and conjugated polymers offers enhanced performance and new functionalities in light-generation and light-harvesting applications, where controlling and mastering the excitonic interactions at the nanoscale are essential. In this Perspective, we highlight and critically consider the excitonic interactions in the organic-inorganic nanocomposites to achieve highly efficient exciton transfer through rational design of the nanocomposites. The use of strong excitonic interactions in optoelectronic devices can trigger efficiency breakthroughs in hybrid optoelectronics.
Excitonic bistabilities, instabilities and chaos in laser-pumped semiconductor
International Nuclear Information System (INIS)
Nguyen Ba An; Nguyen Trung Dan; Hoang Xuan Nguyen
1992-07-01
The Hurwitz criteria are used for a stability analysis of the steady state excitonic optical bistability curves in a semiconductor pumped by an external laser resonant with the exciton level. Besides the middle branch of the bistability curves which is unstable in the sense of the linear stability theory, we have found other domains of instability in the upper and lower branches of the steady state curves. Numerical results show that a possible route to chaos in the photon-exciton system is period-doubling self-oscillation process. The influence of the presence of free carriers that coexist with the excitons is also discussed. (author). 16 refs, 6 figs
Measurement of Exciton Binding Energy of Monolayer WS2
Chen, Xi; Zhu, Bairen; Cui, Xiaodong
Excitonic effects are prominent in monolayer crystal of transition metal dichalcogenides (TMDCs) because of spatial confinement and reduced Coulomb screening. Here we use linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE) to measure the exciton binding energy of monolayer WS2. Peaks for excitonic absorptions of the direct gap located at K valley of the Brillouin zone and transitions from multiple points near Γ point of the Brillouin zone, as well as trion side band are shown in the linear absorption spectra of WS2. But there is no gap between distinct excitons and the continuum of the interband transitions. Strong electron-phonon scattering, overlap of excitons around Γ point and the transfer of the oscillator strength from interband continuum to exciton states make it difficult to resolve the electronic interband transition edge even down to 10K. The gap between excited states of the band-edge exciton and the single-particle band is probed by TP-PLE measurements. And the energy difference between 1s exciton and the single-particle gap gives the exciton binding energy of monolayer WS2 to be about 0.71eV. The work is supported by Area of excellency (AoE/P-04/08), CRF of Hong Kong Research Grant Council (HKU9/CRF/13G) and SRT on New Materials of The University of Hong Kong.
Exciton management in organic photovoltaic multidonor energy cascades.
Griffith, Olga L; Forrest, Stephen R
2014-05-14
Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.
Exciton molecule in semiconductors by two-photon absorption
International Nuclear Information System (INIS)
Arya, K.; Hassan, A.R.
1976-07-01
Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect
Molecular weight dependence of exciton diffusion in poly(3-hexylthiophene)
DEFF Research Database (Denmark)
Masri, Zarifi; Ruseckas, Arvydas; Emelianova, Evguenia V.
2013-01-01
A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial π–π aggregates of polymer chromophores and about 100...... times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient...
Cavigli, Lucia; Gabrieli, Riccardo; Gurioli, Massimo; Bogani, Franco; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël; Grandjean, Nicolas; Vinattieri, Anna
2010-09-01
A detailed experimental investigation of the phonon-assisted emission in a high-quality c -plane GaN epilayer is presented up to 200 K. By performing photoluminescence and reflectivity measurements, we find important etaloning effects in the phonon-replica spectra, which have to be corrected before addressing the lineshape analysis. Direct experimental evidence for free exciton thermalization is found for the whole temperature range investigated. A close comparison with existing models for phonon replicas originating from a thermalized free exciton distribution shows that the simplified and commonly adopted description of the exciton-phonon interaction with a single excitonic band leads to a large discrepancy with experimental data. Only the consideration of the complex nature of the excitonic band in GaN, including A and B exciton contributions, allows accounting for the temperature dependence of the peak energy, intensity, and lineshape of the phonon replicas.
International Nuclear Information System (INIS)
1996-01-01
The French government has decided to modify the conditions of extension of local natural gas authorities to neighbouring districts. The European Union is studying the conditions of internal gas market with the objective of more open markets although considering public service requirements
Directing energy transport in organic photovoltaic cells using interfacial exciton gates.
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.
Coherent excitonic nonlinearity versus inhomogeneous broadening in single quantum wells
DEFF Research Database (Denmark)
Langbein, Wolfgang Werner; Borri, Paola; Hvam, Jørn Märcher
1998-01-01
The coherent response of excitons in semiconductor nanostructures, as measured in four wave mixing (FWM) experiments, depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs-AlGaAs single quantum wells (SQW) of 4 nm to 25 nm well width. Two main mechanisms...
Energy dissipation of free exciton polaritons in semiconducting films
International Nuclear Information System (INIS)
De Crescenzi, M.; Harbeke, G.; Tosatti, E.
1978-08-01
The effective (thickness-dependent) light absorption coefficient K(ω,d) is discussed for thin semiconducting films in the frequency range of free, spatially dispersive exciton polaritons. We find that (i) it oscillates strongly for small film thicknesses; (ii) it exhibits a slanted peak lineshape; (iii) its integrated strength also depends upon the exciton damping and extrapolates to zero for vanishing damping
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...
Bose Condensation of Interwell Excitons in Double Quantum Wells
DEFF Research Database (Denmark)
Larionov, A. V.; Timofeev, V. B.; Ni, P. A.
2002-01-01
The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n–i–n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring...
Exciton dephasing in ZnSe quantum wires
DEFF Research Database (Denmark)
Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jørn Märcher
1998-01-01
The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering...
Magnetic excitons in singlet-ground-state ferromagnets
DEFF Research Database (Denmark)
Birgeneau, R.J.; Als-Nielsen, Jens Aage; Bucher, E.
1971-01-01
The authors report measurements of the dispersion of singlet-triplet magnetic excitons as a function of temperature in the singlet-ground-state ferromagnets fcc Pr and Pr3Tl. Well-defined excitons are observed in both the ferromagnetic and paramagnetic regions, but with energies which are nearly...
Influence of structural defects on excitonic photoluminescence of pentacene
International Nuclear Information System (INIS)
Piryatins'kij, Yu.P.; Kurik, M.V.
2011-01-01
The exciton reflection, absorption, and photoluminescence spectra for single crystals and polycrystalline films have been studied in the temperature range of 4.2-296 K. A significant influence of structural defects arising during phase transitions on the exciton spectra of pentacene has been detected. The mechanisms of photoluminescence in single crystals and crystalline films of pentacene have been considered.
Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers
Andernach, Rolf; Utzat, Hendrik; Dimitrov, Stoichko; McCulloch, Iain; Heeney, Martin; Durrant, James; Bronstein, Hugo
2015-01-01
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
Bistable Topological Insulator with Exciton-Polaritons
Kartashov, Yaroslav V.; Skryabin, Dmitry V.
2017-12-01
The functionality of many nonlinear and quantum optical devices relies on the effect of optical bistability. Using microcavity exciton-polaritons in a honeycomb arrangement of microcavity pillars, we report the resonance response and bistability of topological edge states. A balance between the pump, loss, and nonlinearity ensures a broad range of dynamical stability and controls the distribution of power between counterpropagating states on the opposite edges of the honeycomb lattice stripe. Tuning energy and polarization of the pump photons, while keeping their momentum constant, we demonstrate control of the propagation direction of the dominant edge state. Our results facilitate the development of practical applications of topological photonics.
Entangled exciton states in quantum dot molecules
Bayer, Manfred
2002-03-01
Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For
Resolving ultrafast exciton migration in organic solids at the nanoscale
Ginsberg, Naomi
The migration of Frenkel excitons, tightly-bound electron-hole pairs, in photosynthesis and in organic semiconducting films is critical to the efficiency of natural and artificial light harvesting. While these materials exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton migration lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to uncover the correlations between the structural and energetic landscapes that the excitons explore. By combining the ultrafast super-resolved measurements with exciton hopping simulations we furthermore specify the nature (in addition to the extent) of exciton migration as a function of the intrinsic and ensemble chromophore energy scales that determine a spatio-energetic landscape for migration. In collaboration with: Samuel Penwell, Lucas Ginsberg, University of California, Berkeley and Rodrigo Noriega University of Utah.
Magneto-exciton dephasing in a single quantum dot
Rodriguez, F. J.; Reyes, A.; Olaya-Castro, A.; Quiroga, L.
2001-03-01
Ultrafast spectroscopy experiments on single quantum dot (SQD) in magnetic fields provide a variety of unexpected results, one of them being the recently reported entanglement of exciton states. In order to explore the entanglement robustness, dephasing mechanisms must be considered. By calculating the non-linear time resolved optical spectrum of a SQD, we present a theoretical study on the exciton-exciton scattering contribution to the magneto-exciton dephasing time. Our results show that the time evolution of \\chi^(3) presents, under non-steady-state condition, a beating between the bound biexciton and the first unbound biexciton state in the strong confinement regime. The contribution coming from both left and right polarized emitted photons allows us to predict the creation of exciton entanglement, in agreement with recent experimental results. Previous theoretical works have only addressed the stationary optical response. By contrast, our results based on a full time dependent calculation show new features specially for the fast dephasing case.
How to Draw Energy Level Diagrams in Excitonic Solar Cells.
Zhu, X-Y
2014-07-03
Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells.
Excitons in single-walled carbon nanotubes: environmental effect
International Nuclear Information System (INIS)
Smyrnov, O.A.
2010-01-01
The properties of excitons in semiconducting single-walled carbon nanotubes (SWCNTs) isolated in vacuum or a medium and their contributions to the optical spectra of nanotubes are studied within the elementary potential model, in which an exciton is represented as a bound state of two oppositely charged quasiparticles confined to the nanotube surface. The emphasis is given on the influence of the dielectric environment surrounding a nanotube on the exciton spectra. For nanotubes in the environment with a permittivity less than ∼ 1:8; the ground-state exciton binding energies exceed the respective energy gaps, whereas the obtained binding energies of excitons in nanotubes in a medium with permittivity greater than ∼ 4 are in good accordance with the corresponding experimental data and consistent with the known scaling relation for the environmental effect. The stabilization of a single-electron spectrum in SWCNTs in media with rather low permittivities is discussed.
PbSe Nanocrystal Excitonic Solar Cells
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.
Exciton coherence in semiconductor quantum dots
International Nuclear Information System (INIS)
Ishi-Hayase, Junko; Akahane, Kouichi; Yamamoto, Naokatsu; Sasaki, Masahide; Kujiraoka, Mamiko; Ema, Kazuhiro
2009-01-01
The coherent dynamics of excitons in InAs quantum dots (QDs) was investigated in the telecommunication wavelength range using a transient four-wave mixing technique. The sample was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for obtaining a high S/N in the four-wave mixing measurements, although no high-sensitive heterodyne detection was carried out. The dephasing time and transition dipole moment were precisely estimated from the polarization dependence of signals, taking into account their anisotropic properties. The population lifetimes of the excitons were also measured by using a polarization-dependent pumpprobe technique. A quantitative comparison of these anisotropies demonstrates that in our QDs, non-radiative population relaxation, polarization relaxation and pure dephasing are considerably smaller than the radiative relaxation. A comparison of the results of the four-wave mixing and pump-probe measurements revealed that the pure dephasing could be directly estimated with an accuracy of greater than 0.1 meV by comparing the results of four-wave mixing and pump-probe measurements. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
Energy and Information Transfer Via Coherent Exciton Wave Packets
Zang, Xiaoning
Electronic excitons are bound electron-hole states that are generated when light interacts with matter. Such excitations typically entangle with phonons and rapidly decohere; the resulting electronic state dynamics become diffusive as a result. However, if the exciton-phonon coupling can be reduced, it may be possible to construct excitonic wave packets that offer a means of efficiently transmitting information and energy. This thesis is a combined theory/computation investigation to design condensed matter systems which support the requisite coherent transport. Under the idealizing assumption that exciton-phonon entanglement could be completely suppressed, the majority of this thesis focuses on the creation and manipulation of exciton wave packets in quasi-one-dimensional systems. While each site could be a silicon quantum dot, the actual implementation focused on organic molecular assemblies for the sake of computational simplicity, ease of experimental implementation, potential for coherent transport, and promise because of reduced structural uncertainty. A laser design was derived to create exciton wave packets with tunable shape and speed. Quantum interference was then exploited to manipulate these packets to block, pass, and even dissociate excitons based on their energies. These developments allow exciton packets to be considered within the arena of quantum information science. The concept of controllable excitonic wave packets was subsequently extended to consider molecular designs that allow photons with orbital angular momentum to be absorbed to create excitons with a quasi-angular momentum of their own. It was shown that a well-defined measure of topological charge is conserved in such light-matter interactions. Significantly, it was also discovered that such molecules allow photon angular momenta to be combined and later emitted. This amounts to a new way of up/down converting photonic angular momentum without relying on nonlinear optical materials. The
Theoretical and computational studies of excitons in conjugated polymers
Barford, William; Bursill, Robert J.; Smith, Richard W.
2002-09-01
We present a theoretical and computational analysis of excitons in conjugated polymers. We use a tight-binding model of π-conjugated electrons, with 1/r interactions for large r. In both the weak-coupling limit (defined by W>>U) and the strong-coupling limit (defined by Wparticle models. We compare these to density matrix renormalization group (DMRG) calculations, and find good agreement in the extreme limits. We use these analytical results to interpret the DMRG calculations in the intermediate-coupling regime (defined by W~U), most applicable to conjugated polymers. We make the following conclusions. (1) In the weak-coupling limit the bound states are Mott-Wannier excitons, i.e., conduction-band electrons bound to valence-band holes. Singlet and triplet excitons whose relative wave functions are odd under a reflection of the relative coordinate are degenerate. Thus, the 2 1A+g and 1 3A-g states are degenerate in this limit. (2) In the strong-coupling limit the bound states are Mott-Hubbard excitons, i.e., particles in the upper Hubbard band bound to holes in the lower Hubbard band. These bound states occur in doublets of even and odd parity excitons. Triplet excitons are magnons bound to the singlet excitons, and hence are degenerate with their singlet counterparts. (3) In the intermediate-coupling regime Mott-Wannier excitons are the more appropriate description for large dimerization, while for the undimerized chain Mott-Hubbard excitons are the correct description. For dimerizations relevant to polyacetylene and polydiacetylene both Mott-Hubbard and Mott-Wannier excitons are present. (4) For all coupling strengths an infinite number of bound states exist for 1/r interactions for an infinite polymer. As a result of the discreteness of the lattice and the restrictions on the exciton wave functions in one dimension, the progression of states does not follow the Rydberg series. In practice, excitons whose particle-hole separation exceeds the length of the polymer
Padula, Daniele; Lee, Myeong H; Claridge, Kirsten; Troisi, Alessandro
2017-11-02
In this paper, we adopt an approach suitable for monitoring the time evolution of the intramolecular contribution to the spectral density of a set of identical chromophores embedded in their respective environments. We apply the proposed method to the Fenna-Matthews-Olson (FMO) complex, with the objective to quantify the differences among site-dependent spectral densities and the impact of such differences on the exciton dynamics of the system. Our approach takes advantage of the vertical gradient approximation to reduce the computational demands of the normal modes analysis. We show that the region of the spectral density that is believed to strongly influence the exciton dynamics changes significantly in the timescale of tens of nanoseconds. We then studied the impact of the intramolecular vibrations on the exciton dynamics by considering a model of FMO in a vibronic basis and neglecting the interaction with the environment to isolate the role of the intramolecular exciton-vibration coupling. In agreement with the assumptions in the literature, we demonstrate that high frequency modes at energy much larger than the excitonic energy splitting have negligible influence on exciton dynamics despite the large exciton-vibration coupling. We also find that the impact of including the site-dependent spectral densities on exciton dynamics is not very significant, indicating that it may be acceptable to apply the same spectral density on all sites. However, care needs to be taken for the description of the exciton-vibrational coupling in the low frequency part of intramolecular modes because exciton dynamics is more susceptible to low frequency modes despite their small Huang-Rhys factors.
Wu, Shudong; Cheng, Liwen; Wang, Qiang
2017-08-01
The size- and dimensionality-dependence of excitonic effects and related properties in semiconductor nanostructures are theoretically studied in detail within the effective-mass approximation. When nanostructure sizes become smaller than the bulk exciton Bohr radius, excitonic effects are significantly enhanced with reducing size or dimensionality. This is as a result of quantum confinement in more directions leading to larger exciton binding energies and normalized exciton oscillator strengths. These excitonic effects originate from electron-hole Coulombic interactions, which strongly enhance the oscillator strength between the electron and hole. It is also established that the universal scaling of exciton binding energy versus the inverse of the exciton Bohr radius follows a linear scaling law. Herein, we propose a stretched exponential law for the size scaling of optical gap, which is in good agreement with the calculated data. Due to differences in the confinement dimensionality, the radiative lifetime of low-dimensional excitons becomes shorter than that of bulk excitons. The size dependence of the exciton radiative lifetimes is in good agreement with available experimental data. This strongly enhanced electron-hole exchange interaction is expected in low-dimensional structures due to enriched excitonic effects. The main difference in nanostructures compared to the bulk can be interpreted in terms of the enhanced excitonic effects induced by exciton localization. The enhanced excitonic effects are expected to be of importance in developing stable and high-efficiency nanoscale excitonic optoelectronic devices.
Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers
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.
Interlayer excitons in a bulk van der Waals semiconductor.
Arora, Ashish; Drüppel, Matthias; Schmidt, Robert; Deilmann, Thorsten; Schneider, Robert; Molas, Maciej R; Marauhn, Philipp; Michaelis de Vasconcellos, Steffen; Potemski, Marek; Rohlfing, Michael; Bratschitsch, Rudolf
2017-09-21
Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments and ab initio calculations for 2H-MoTe 2 , we explain their salient features: the positive sign of the g-factor and the large diamagnetic shift. Our investigations solve the long-standing puzzle of positive g-factors in transition metal dichalcogenides, and pave the way for studying collective phenomena in these materials at elevated temperatures.Excitons, quasi-particles of bound electron-hole pairs, are at the core of the optoelectronic properties of layered transition metal dichalcogenides. Here, the authors unveil the presence of interlayer excitons in bulk van der Waals semiconductors, arising from strong localization and spin-valley coupling of charge carriers.
Excitons in InP/InAs inhomogeneous quantum dots
International Nuclear Information System (INIS)
Assaid, E; Feddi, E; Khamkhami, J El; Dujardin, F
2003-01-01
Wannier excitons confined in an InP/InAs inhomogeneous quantum dot (IQD) have been studied theoretically in the framework of the effective mass approximation. A finite-depth potential well has been used to describe the effect of the quantum confinement in the InAs layer. The exciton binding energy has been determined using the Ritz variational method. The spatial correlation between the electron and the hole has been taken into account in the expression for the wavefunction. It has been shown that for a fixed size b of the IQD, the exciton binding energy depends strongly on the core radius a. Moreover, it became apparent that there are two critical values of the core radius, a crit and a 2D , for which important changes of the exciton binding occur. The former critical value, a crit , corresponds to a minimum of the exciton binding energy and may be used to distinguish between tridimensional confinement and bidimensional confinement. The latter critical value, a 2D , corresponds to a maximum of the exciton binding energy and to the most pronounced bidimensional character of the exciton
Exciton binding energy in a pyramidal quantum dot
Anitha, A.; Arulmozhi, M.
2018-05-01
The effects of spatially dependent effective mass, non-parabolicity of the conduction band and dielectric screening function on exciton binding energy in a pyramid-shaped quantum dot of GaAs have been investigated by variational method as a function of base width of the pyramid. We have assumed that the pyramid has a square base with area a× a and height of the pyramid H=a/2. The trial wave function of the exciton has been chosen according to the even mirror boundary condition, i.e. the wave function of the exciton at the boundary could be non-zero. The results show that (i) the non-parabolicity of the conduction band affects the light hole (lh) and heavy hole (hh) excitons to be more bound than that with parabolicity of the conduction band, (ii) the dielectric screening function (DSF) affects the lh and hh excitons to be more bound than that without the DSF and (iii) the spatially dependent effective mass (SDEM) affects the lh and hh excitons to be less bound than that without the SDEM. The combined effects of DSF and SDEM on exciton binding energy have also been calculated. The results are compared with those available in the literature.
Bright triplet excitons in caesium lead halide perovskites
Becker, Michael A.; Vaxenburg, Roman; Nedelcu, Georgian; Sercel, Peter C.; Shabaev, Andrew; Mehl, Michael J.; Michopoulos, John G.; Lambrakos, Samuel G.; Bernstein, Noam; Lyons, John L.; Stöferle, Thilo; Mahrt, Rainer F.; Kovalenko, Maksym V.; Norris, David J.; Rainò, Gabriele; Efros, Alexander L.
2018-01-01
Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.
Exciton-dopant and exciton-charge interactions in electronically doped OLEDs
International Nuclear Information System (INIS)
Williams, Christopher; Lee, Sergey; Ferraris, John; Zakhidov, A. Anvar
2004-01-01
The electronic dopants, like tetrafluorocyanoquinodimethane (F 4 -TCNQ) molecules, used for p-doping of hole transport layers in organic light-emitting diodes (OLEDs) are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We observed EL quenching in OLED with F 4 -TCNQ doped N,N'-diphenyl-N'N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine hole transport layer at large dopant concentrations, >5%. To separate the effects of exciton-dopant quenching, from exciton-polaron quenching we have intentionally doped the emissive layer of (8-tris-hydroxyquinoline) with three acceptors (A) of different electron affinities: F 4 -TCNQ, TCNQ, and C 60 , and found that C 60 is the strongest EL-quencher, while F 4 -TCNQ is the weakest, contrary to intuitive expectations. The new effects of charge transfer and usually considered energy transfer from exciton to neutral (A) and charged acceptors (A - ) are compared as channels for non-radiative Ex-A decay. At high current loads the EL quenching is observed, which is due to decay of Ex on free charge carriers, hole polarons P + . We consider contributions to Ex-P + interaction by short-range charge transfer and describe the structure of microscopic charge transfer (CT)-processes responsible for it. The formation of metastable states of 'charged excitons' (predicted and studied by Agranovich et al. Chem. Phys. 272 (2001) 159) by electron transfer from a P to an Ex is pointed out, and ways to suppress non-radiative Ex-P decay are suggested
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.
Ultrafast electric phase control of a single exciton qubit
Widhalm, Alex; Mukherjee, Amlan; Krehs, Sebastian; Sharma, Nandlal; Kölling, Peter; Thiede, Andreas; Reuter, Dirk; Förstner, Jens; Zrenner, Artur
2018-03-01
We report on the coherent phase manipulation of quantum dot excitons by electric means. For our experiments, we use a low capacitance single quantum dot photodiode which is electrically controlled by a custom designed SiGe:C BiCMOS chip. The phase manipulation is performed and quantified in a Ramsey experiment, where ultrafast transient detuning of the exciton energy is performed synchronous to double pulse π/2 ps laser excitation. We are able to demonstrate electrically controlled phase manipulations with magnitudes up to 3π within 100 ps which is below the dephasing time of the quantum dot exciton.
Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies
Choi, Joshua J.; Luria, Justin; Hyun, Byung-Ryool; Bartnik, Adam C.; Sun, Liangfeng; Lim, Yee-Fun; Marohn, John A.; Wise, Frank W.; Hanrath, Tobias
2010-01-01
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.
Excitonic AND Logic Gates on DNA Brick Nanobreadboards
2015-01-01
A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems. PMID:25839049
Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies
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.
Multi-Excitonic Quantum Dot Molecules
Scheibner, M.; Stinaff, E. A.; Doty, M. F.; Ware, M. E.; Bracker, A. S.; Gammon, D.; Ponomarev, I. V.; Reinecke, T. L.; Korenev, V. L.
2006-03-01
With the ability to create coupled pairs of quantum dots, the next step towards the realization of semiconductor based quantum information processing devices can be taken. However, so far little knowledge has been gained on these artificial molecules. Our photoluminescence experiments on single InAs/GaAs quantum dot molecules provide the systematics of coupled quantum dots by delineating the spectroscopic features of several key charge configurations in such quantum systems, including X, X^+,X^2+, XX, XX^+ (with X being the neutral exciton). We extract general rules which determine the formation of molecular states of coupled quantum dots. These include the fact that quantum dot molecules provide the possibility to realize various spin configurations and to switch the electron hole exchange interaction on and off by shifting charges inside the molecule. This knowledge will be valuable in developing implementations for quantum information processing.
Exciton transfer under dichotomic noise: GME treatment
International Nuclear Information System (INIS)
Barvik, I.; Warns, C.; Reineker, P.
1995-08-01
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
Optical Selection Rule of Excitons in Gapped Chiral Fermion Systems
Zhang, Xiaoou; Shan, Wen-Yu; Xiao, Di
2018-02-01
We show that the exciton optical selection rule in gapped chiral fermion systems is governed by their winding number w , a topological quantity of the Bloch bands. Specifically, in a CN-invariant chiral fermion system, the angular momentum of bright exciton states is given by w ±1 +n N with n being an integer. We demonstrate our theory by proposing two chiral fermion systems capable of hosting dark s -like excitons: gapped surface states of a topological crystalline insulator with C4 rotational symmetry and biased 3 R -stacked MoS2 bilayers. In the latter case, we show that gating can be used to tune the s -like excitons from bright to dark by changing the winding number. Our theory thus provides a pathway to electrical control of optical transitions in two-dimensional material.
Hopping approach towards exciton dissociation in conjugated polymers
International Nuclear Information System (INIS)
Emelianova, E. V.; Auweraer, M. van der; Baessler, H.
2008-01-01
By employing random walk an analytic theory for the dissociation of singlet excitons in a random organic solid, for instance, a conjugated polymer, has been developed. At variance of conventional three-dimensional Onsager theory, it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager's concept Brownian motion. The results indicate that, depending of the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it
Excitons in van der Waals Heterostructures: A theoretical study
DEFF Research Database (Denmark)
Latini, Simone
)electronics devices, e.g. light emitting diodes, solar cells, ultra-fast photodetectors, transistors etc., have been successfully fabricated. It is well established that for isolated 2D semiconductors and vdWHs the optical response is governed by excitonic effects. While it is understood that the reduced amount...... of electronic screening in freestanding 2D materials is the main origin of extraordinarily strongly bound excitons, a theoretical understanding of excitonic effects and of how the electronic screening is affected for the more complex case of multi-layer structures is still lacking due to the computational...... in a generalized hydrogenic model to compute exciton binding energies in isolated, supported, or encapsulated 2D semiconductors. The non-locality of the dielectric screening is inherently included in our method and we can successfully describe the non-hydrogenic Rydberg series of low-dimensional systems...
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: 2.209, year: 2015
The effect of excitons on CdTe solar cells
International Nuclear Information System (INIS)
Karazhanov, S. Zh.; Zhang, Y.; Mascarenhas, A.; Deb, S.
2000-01-01
Temperature and doping-level dependence of CdTe solar cells is investigated, taking into account the involvement of excitons on photocurrent transport. We show that the density of excitons in CdTe is comparable with that of minority carriers at doping levels ≥10 15 cm -3 . From the investigation of the dark-saturation current, we show that the product of electron and hole concentrations at equilibrium is several orders of magnitude more than the square of the intrinsic carrier concentration. With this assumption, we have studied the effect of excitons on CdTe solar cells, and the effect is negative. CdTe solar cell performance with excitons included agrees well with existing experimental results. (c) 2000 American Institute of Physics
Decoherence suppression of excitons by bang-bang control
International Nuclear Information System (INIS)
Kishimoto, T.; Hasegawa, A.; Mitsumori, Y.; Ishi-Hayase, J.; Sasaki, M.; Minami, F.
2007-01-01
We report the demonstration of decoherence control of excitons on a layered compound semiconductor GaSe by using successive three femtosecond pulses, i.e., the six-wave mixing configuration. The second pulse acts as a π pulse which reverses the time evolution of non-Markovian dynamics. By changing the pulse interval conditions, we confirmed for the first time the suppression of exciton decoherence by π pulse irradiation
Excitonic quantum interference in a quantum dot chain with rings.
Hong, Suc-Kyoung; Nam, Seog Woo; Yeon, Kyu-Hwang
2008-04-16
We demonstrate excitonic quantum interference in a closely spaced quantum dot chain with nanorings. In the resonant dipole-dipole interaction model with direct diagonalization method, we have found a peculiar feature that the excitation of specified quantum dots in the chain is completely inhibited, depending on the orientational configuration of the transition dipole moments and specified initial preparation of the excitation. In practice, these excited states facilitating quantum interference can provide a conceptual basis for quantum interference devices of excitonic hopping.
Quantum condensation from a tailored exciton population in a microcavity
International Nuclear Information System (INIS)
Eastham, P. R.; Phillips, R. T.
2009-01-01
An experiment is proposed on the coherent quantum dynamics of a semiconductor microcavity containing quantum dots. Modeling the experiment using a generalized Dicke model, we show that a tailored excitation pulse can create an energy-dependent population of excitons, which subsequently evolves to a quantum condensate of excitons and photons. The population is created by a generalization of adiabatic rapid passage and then condenses due to a dynamical analog of the BCS instability.
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....
Exciton-plasmon coupling interactions: from principle to applications
Cao, En; Lin, Weihua; Sun, Mengtao; Liang, Wenjie; Song, Yuzhi
2018-01-01
The interaction of exciton-plasmon coupling and the conversion of exciton-plasmon-photon have been widely investigated experimentally and theoretically. In this review, we introduce the exciton-plasmon interaction from basic principle to applications. There are two kinds of exciton-plasmon coupling, which demonstrate different optical properties. The strong exciton-plasmon coupling results in two new mixed states of light and matter separated energetically by a Rabi splitting that exhibits a characteristic anticrossing behavior of the exciton-LSP energy tuning. Compared to strong coupling, such as surface-enhanced Raman scattering, surface plasmon (SP)-enhanced absorption, enhanced fluorescence, or fluorescence quenching, there is no perturbation between wave functions; the interaction here is called the weak coupling. SP resonance (SPR) arises from the collective oscillation induced by the electromagnetic field of light and can be used for investigating the interaction between light and matter beyond the diffraction limit. The study on the interaction between SPR and exaction has drawn wide attention since its discovery not only due to its contribution in deepening and broadening the understanding of SPR but also its contribution to its application in light-emitting diodes, solar cells, low threshold laser, biomedical detection, quantum information processing, and so on.
Excitonic effects in the luminescence of quantum wells
International Nuclear Information System (INIS)
Deveaud, B.; Kappei, L.; Berney, J.; Morier-Genoud, F.; Portella-Oberli, M.T.; Szczytko, J.; Piermarocchi, C.
2005-01-01
We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high-quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum wells: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We are also able to give the boundaries of the Mott transition in our system, and to show the absence of observable renormalization of the gap below the onset of this transition. We detail the characteristics of the trion formation and evidence the possible formation of both positive and negative trions in the absence of any resident free carrier populations
Effect of exciton polaritons of absorption edge of GaTe
International Nuclear Information System (INIS)
Kurbatov, L.N.; Dirochka, A.I.; Sosin, V.A.
1979-01-01
The experimental results, pointing to the dependence of spectral and integral coefficients of exciton absorption as well as to the exciton relaxation parameter γsub(0) over the exciton zone on the sample thickness, are presented. It is tried to explain the inverse dependences of absorption intensity in the maximum of αsub(max) and γsub(0) exciton line within the limits of polariton theory. The values of polariton free path length in GaTe at various temperatures, as well as the volume γsub(vol.) and surface γsub(surf.) parameters of exciton relaxation over the exciton zone are discussed
Impurity trapped excitons under high hydrostatic pressure
Grinberg, Marek
2013-09-01
Paper summarizes the results on pressure effect on energies of the 4fn → 4fn and 4fn-15d1 → 4fn transitions as well as influence of pressure on anomalous luminescence in Lnα+ doped oxides and fluorides. A model of impurity trapped exciton (ITE) was developed. Two types of ITE were considered. The first where a hole is localized at the Lnα+ ion (creation of Ln(α+1)+) and an electron is attracted by Coulomb potential at Rydberg-like states and the second where an electron captured at the Lnα+ ion (creation of Ln(α-1)+) and a hole is attracted by Coulomb potential at Rydberg-like states. Paper presents detailed analysis of nonlinear changes of energy of anomalous luminescence of BaxSr1-xF2:Eu2+ (x > 0.3) and LiBaF3:Eu2+, and relate them to ITE-4f65d1 states mixing.
The creation of defects in ammonium halides by excitons
International Nuclear Information System (INIS)
Kim, L.M.
2002-01-01
The ammonium halides crystals and alkali halides crystals are analogous by kind chemical bonds and crystalline lattices. The anionic sublattice is identical in this crystals. It is known the main mechanism of defect creation by irradiation is radiationless decay of excitons in alkali halides crystals. The F-, H-centers are formation in this processes. However, F, H-centres are not detected in ammonium halides. The goal of this work is investigation the creation of defects in ammonium halides by excitons. We established that excitons in ammonium chlorides and bromides are similar to excitons in alkali halides. It is known excitons are self-trapped and have identical parameters of the exciton-phonon interaction in both kind crystals. It is supposed, that processes of radiationless disintegration of excitons are identical in ammonium and alkali halides. It is necessary to understand why F-, H-centers are absent in ammonium halides. V k -centres are created by the excitation of the ammonium halides crystals in the absorption band of excitons. It was established by thermoluminescence and spectrums of absorption. The V k -centers begin to migrate at 110-120 K in ammonium chlorides and bromides. The curve of thermoluminescence have peak with maximum at this temperatures. It is known V k -centers in ammonium chlorides have the absorption band at 380 nm. We discovered this absorption band after irradiation of crystals by ultra-violet. In alkali halides F-center is anionic vacancy with electron. The wave function of electron are spread ed at the cations around anionic vacancy. We established the cation NH 4 + in ammonium halides can to capture electron. The ion NH 4 2+ is unsteady. It is disintegrated to NH 3 + and H + . We suppose that excitons in ammonium and alkali halides are disintegrated identically. When cation NH 4 + capture electron, in the anionic sublattice the configuration are created in a direction (100) The indicated configuration is unsteady in relation to a
Excitons in InP/InAs inhomogeneous quantum dots
Assaid, E; Khamkhami, J E; Dujardin, F
2003-01-01
Wannier excitons confined in an InP/InAs inhomogeneous quantum dot (IQD) have been studied theoretically in the framework of the effective mass approximation. A finite-depth potential well has been used to describe the effect of the quantum confinement in the InAs layer. The exciton binding energy has been determined using the Ritz variational method. The spatial correlation between the electron and the hole has been taken into account in the expression for the wavefunction. It has been shown that for a fixed size b of the IQD, the exciton binding energy depends strongly on the core radius a. Moreover, it became apparent that there are two critical values of the core radius, a sub c sub r sub i sub t and a sub 2 sub D , for which important changes of the exciton binding occur. The former critical value, a sub c sub r sub i sub t , corresponds to a minimum of the exciton binding energy and may be used to distinguish between tridimensional confinement and bidimensional confinement. The latter critical value, a ...
Spectral properties of excitons in the bilayer graphene
Apinyan, V.; Kopeć, T. K.
2018-01-01
In this paper, we consider the spectral properties of the bilayer graphene with the local excitonic pairing interaction between the electrons and holes. We consider the generalized Hubbard model, which includes both intralayer and interlayer Coulomb interaction parameters. The solution of the excitonic gap parameter is used to calculate the electronic band structure, single-particle spectral functions, the hybridization gap, and the excitonic coherence length in the bilayer graphene. We show that the local interlayer Coulomb interaction is responsible for the semimetal-semiconductor transition in the double layer system, and we calculate the hybridization gap in the band structure above the critical interaction value. The formation of the excitonic band gap is reported as the threshold process and the momentum distribution functions have been calculated numerically. We show that in the weak coupling limit the system is governed by the Bardeen-Cooper-Schrieffer (BCS)-like pairing state. Contrary, in the strong coupling limit the excitonic condensate states appear in the semiconducting phase, by forming the Dirac's pockets in the reciprocal space.
Tailorable Exciton Transport in Doped Peptide–Amphiphile Assemblies
Energy Technology Data Exchange (ETDEWEB)
Solomon, Lee A. [Center; Sykes, Matthew E. [Center; Wu, Yimin A. [Center; Schaller, Richard D. [Center; Department; Wiederrecht, Gary P. [Center; Fry, H. Christopher [Center
2017-08-29
Light-harvesting biomaterials are an attractive target in photovoltaics, photocatalysis, and artificial photosynthesis. Through peptide self-assembly, complex nanostructures can be engineered to study the role of chromophore organization during light absorption and energy transport. To this end, we demonstrate the one-dimensional transport of excitons along naturally occurring, light-harvesting, Zn-protoporphyrin IX chromophores within self-assembled peptide-amphiphile nanofibers. The internal structure of the nanofibers induces packing of the porphyrins into linear chains. We find that this peptide assembly can enable long-range exciton diffusion, yet it also induces the formation of excimers between adjacent molecules, which serve as exciton traps. Electronic coupling between neighboring porphyrin molecules is confirmed by various spectroscopic methods. The exciton diffusion process is then probed through transient photoluminescence and absorption measurements and fit to a model for one-dimensional hopping. Because excimer formation impedes exciton hopping, increasing the interchromophore spacing allows for improved diffusivity, which we control through porphyrin doping levels. We show that diffusion lengths of over 60 nm are possible at low porphyrin doping, representing an order of magnitude improvement over the highest doping fractions.
Angular momentum transport with twisted exciton wave packets
Zang, Xiaoning; Lusk, Mark T.
2017-10-01
A chain of cofacial molecules with CN or CN h symmetry supports excitonic states with a screwlike structure. These can be quantified with the combination of an axial wave number and an azimuthal winding number. Combinations of these states can be used to construct excitonic wave packets that spiral down the chain with well-determined linear and angular momenta. These twisted exciton wave packets can be created and annihilated using laser pulses, and their angular momentum can be optically modified during transit. This allows for the creation of optoexcitonic circuits in which information, encoded in the angular momentum of light, is converted into excitonic wave packets that can be manipulated, transported, and then reemitted. A tight-binding paradigm is used to demonstrate the key ideas. The approach is then extended to quantify the evolution of twisted exciton wave packets in a many-body, multilevel time-domain density functional theory setting. In both settings, numerical methods are developed that allow the site-to-site transfer of angular momentum to be quantified.
Effect of disorder on exciton dissociation in conjugated polymers
International Nuclear Information System (INIS)
Feng Yuwen; Zhao Hui; Chen Yuguang; Yan Yonghong
2017-01-01
By using a multi-configurational time-dependent Hartree–Fock (MCTDHF) method for the time-dependent Schrödinger equation and a Newtonian equation of motion for lattice, we investigate the disorder effects on the dissociation process of excitons in conjugated polymer chains. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include on-site disorder, off-diagonal, electron–electron interaction, and an external electric field. Our results show that Coulomb correlation effects play an important role in determining the exciton dissociation process. The electric field required to dissociate an exciton can practically impossibly occur in a pure polymer chain, especially in the case of triplet exciton. However, when the on-site disorder effects are taken into account, this leads to a reduction in mean dissociation electric fields. As the disorder strength increases, the dissociation field decreases effectively. On the contrary, the effects of off-diagonal disorder are negative in most cases. Moreover, the dependence of exciton dissociation on the conjugated length is also discussed. (paper)
Excitons in Core-Shell Nanowires with Polygonal Cross Sections.
Sitek, Anna; Urbaneja Torres, Miguel; Torfason, Kristinn; Gudmundsson, Vidar; Bertoni, Andrea; Manolescu, Andrei
2018-04-11
The distinctive prismatic geometry of semiconductor core-shell nanowires leads to complex localization patterns of carriers. Here, we describe the formation of optically active in-gap excitonic states induced by the interplay between localization of carriers in the corners and their mutual Coulomb interaction. To compute the energy spectra and configurations of excitons created in the conductive shell, we use a multielectron numerical approach based on the exact solution of the multiparticle Hamiltonian for electrons in the valence and conduction bands, which includes the Coulomb interaction in a nonperturbative manner. We expose the formation of well-separated quasidegenerate levels, and focus on the implications of the electron localization in the corners or on the sides of triangular, square, and hexagonal cross sections. We obtain excitonic in-gap states associated with symmetrically distributed electrons in the spin singlet configuration. They acquire large contributions due to Coulomb interaction, and thus are shifted to much higher energies than other states corresponding to the conduction electron and the vacancy localized in the same corner. We compare the results of the multielectron method with those of an electron-hole model, and we show that the latter does not reproduce the singlet excitonic states. We also obtain the exciton lifetime and explain selection rules which govern the recombination process.
Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers
Kim, Kwiseon; Erkan Kose, Muhammet; Graf, Peter; Kopidakis, Nikos; Rumbles, Garry; Shaheen, Sean E.
2009-03-01
Phenyl cored 3-arm and 4-arm thiophene dendrimers are promising materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules to guide the synthesis of novel dendrimers with improved efficiency. A method is developed to estimate the exciton diffusion lengths for the dendrimers and similar chromophores in amorphous films. The approach exploits Fermi's Golden Rule to estimate the energy transfer rates for an ensemble of bimolecular complexes in random orientations. Using Poisson's equation to evaluate Coulomb integrals led to efficient calculation of excitonic couplings between the transition densities. Monte-Carlo simulations revealed the dynamics of energy transport in the dendrimers. Experimental exciton diffusion lengths of the dendrimers range 10 ˜ 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with experiments. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors that determine the exciton diffusion length in amorphous films.
Modeling temperature dependent singlet exciton dynamics in multilayered organic nanofibers
de Sousa, Leonardo Evaristo; de Oliveira Neto, Pedro Henrique; Kjelstrup-Hansen, Jakob; da Silva Filho, Demétrio Antônio
2018-05-01
Organic nanofibers have shown potential for application in optoelectronic devices because of the tunability of their optical properties. These properties are influenced by the electronic structure of the molecules that compose the nanofibers and also by the behavior of the excitons generated in the material. Exciton diffusion by means of Förster resonance energy transfer is responsible, for instance, for the change with temperature of colors in the light emitted by systems composed of different types of nanofibers. To study in detail this mechanism, we model temperature dependent singlet exciton dynamics in multilayered organic nanofibers. By simulating absorption and emission spectra, the possible Förster transitions are identified. Then, a kinetic Monte Carlo model is employed in combination with a genetic algorithm to theoretically reproduce time-resolved photoluminescence measurements for several temperatures. This procedure allows for the obtainment of different information regarding exciton diffusion in such a system, including temperature effects on the Förster transfer efficiency and the activation energy of the Förster mechanism. The method is general and may be employed for different systems where exciton diffusion plays a role.
Inverse Funnel Effect of Excitons in Strained Black Phosphorus
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Pablo San-Jose
2016-09-01
Full Text Available We study the effects of strain on the properties and dynamics of Wannier excitons in monolayer (phosphorene and few-layer black phosphorus (BP, a promising two-dimensional material for optoelectronic applications due to its high mobility, mechanical strength, and strain-tunable direct band gap. We compare the results to the case of molybdenum disulphide (MoS_{2} monolayers. We find that the so-called funnel effect, i.e., the possibility of controlling exciton motion by means of inhomogeneous strains, is much stronger in few-layer BP than in MoS_{2} monolayers and, crucially, is of opposite sign. Instead of excitons accumulating isotropically around regions of high tensile strain like in MoS_{2}, excitons in BP are pushed away from said regions. This inverse funnel effect is moreover highly anisotropic, with much larger funnel distances along the armchair crystallographic direction, leading to a directional focusing of exciton flow. A strong inverse funnel effect could enable simpler designs of funnel solar cells and offer new possibilities for the manipulation and harvesting of light.
Dynamically controlling the emission of single excitons in photonic crystal cavities
Pagliano, F.; Cho, Y.; Xia, T.; Otten, van F.W.M.; Johne, R.; Fiore, A.
2014-01-01
Single excitons in semiconductor microcavities represent a solid state and scalable platform for cavity quantum electrodynamics, potentially enabling an interface between flying (photon) and static (exciton) quantum bits in future quantum networks. While both singlephoton emission and the strong
Moody, Galan; Kavir Dass, Chandriker; Hao, Kai; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berghä user, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin
2015-01-01
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
Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells
Dimitrov, Stoichko; Schroeder, Bob; Nielsen, Christian; Bronstein, Hugo; Fei, Zhuping; McCulloch, Iain; Heeney, Martin; Durrant, James
2016-01-01
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
Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure
Cadiz, F.; Robert, C.; Courtade, E.; Manca, M.; Martinelli, L.; Taniguchi, T.; Watanabe, K.; Amand, T.; Rowe, A. C. H.; Paget, D.; Urbaszek, B.; Marie, X.
2018-04-01
We have combined spatially resolved steady-state micro-photoluminescence with time-resolved photoluminescence to investigate the exciton diffusion in a WSe2 monolayer encapsulated with hexagonal boron nitride. At 300 K, we extract an exciton diffusion length of LX = 0.36 ± 0.02 μm and an exciton diffusion coefficient of DX = 14.5 ± 2 cm2/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species: bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of LXD=1.5 ±0.02 μ m.
Novel Quantum Condensates in Excitonic Matter
International Nuclear Information System (INIS)
Littlewood, P. B.; Keeling, J. M. J.; Simons, B. D.; Eastham, P. R.; Marchetti, F. M.; Szymanska, M. H.
2009-01-01
These lectures interleave discussion of a novel physical problem of a new kind of condensate with teaching of the fundamental theoretical tools of quantum condensed matter field theory. Polaritons and excitons are light mass composite bosons that can be made inside solids in a number of different ways. As bosonic particles, they are liable to make a phase coherent ground state - generically called a Bose-Einstein condensate (BEC) - and these lectures present some models to describe that problem, as well as general approaches to the theory. The focus is very much to explain how mean-field-like approximations that are often presented heuristically can be derived in a systematic fashion by path integral methods. Going beyond the mean field theory then produces a systematic approach to calculation of the excitation energies, and the derivation of effective low energy theories that can be generalised to more complex dynamical and spatial situations than is practicable for the full theory, as well as to study statistical properties beyond the semi-classical regime. in particular, for the polariton problem, it allows one to connect the regimes of equilibrium BEC and non-equilibrium laser. The lectures are self-sufficient, but not highly detailed. The methodological aspects are covered in standard quantum field theory texts and the presentation here is deliberately cursory: the approach will be closest to the book of Altland and Simons. Since these lectures concern a particular type of condensate, reference should also be made to texts on BEC, for example by Pitaevskii and Stringari. A recent theoretically focussed review of polariton systems covers many of the technical issues associated with the polariton problem in greater depth and provides many further references.
Bozio, Renato; Righetto, Marcello; Minotto, Alessandro
2017-08-01
Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.
Quantum confinement-induced tunable exciton states in graphene oxide.
Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin
2013-01-01
Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.
Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots.
Ardelt, P-L; Gawarecki, K; Müller, K; Waeber, A M; Bechtold, A; Oberhofer, K; Daniels, J M; Klotz, F; Bichler, M; Kuhn, T; Krenner, H J; Machnikowski, P; Finley, J J
2016-02-19
We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k·p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.
Signatures of exciton condensation in a transition metal dichalcogenide
Kogar, Anshul; Rak, Melinda S.; Vig, Sean; Husain, Ali A.; Flicker, Felix; Joe, Young Il; Venema, Luc; MacDougall, Greg J.; Chiang, Tai C.; Fradkin, Eduardo; van Wezel, Jasper; Abbamonte, Peter
2017-12-01
Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed technique of momentum-resolved electron energy-loss spectroscopy (M-EELS), we studied electronic collective modes in the transition metal dichalcogenide semimetal 1T-TiSe2. Near the phase-transition temperature (190 kelvin), the energy of the electronic mode fell to zero at nonzero momentum, indicating dynamical slowing of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. Our study provides compelling evidence for exciton condensation in a three-dimensional solid and establishes M-EELS as a versatile technique sensitive to valence band excitations in quantum materials.
Correlation effect of Rabi oscillations of excitons in quantum dots
International Nuclear Information System (INIS)
Ishi-Hayase, J.; Akahane, K.; Yamamoto, Y.; Kujiraoka, M.; Ema, K.; Sasaki, M.
2008-01-01
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
Preface to the SPECIAL ISSUE: Excitonic Solar Cells(II)
Institute of Scientific and Technical Information of China (English)
Jianjun Tian; Meicheng Li; Kaibo Zheng
2016-01-01
Among all the excitonic solar cells(ESCs)including dyesensitized solar cells(DSSCs),quantum solar cells(QDSCs),perovskites solar cells(PSCs),and organic photovoltaics(OPVs),PSCs attracted enormous research attention in the past 7 years and attained the highest power conversion efficiency(PCE)of over 20%with the biggest progress,from 3.8%to over 22.1%in 7 years.However,one can easily realize the fact that such a rapid progress achieved in PSCs was made possible is largely based on the fundamental knowledge,experimental skills,and characterization facilities obtained and accumulated through the multi-decade long endeavor in the study of other excitonic solar cells.Even though PSCs have attractedmuch research human resource and funding,the study on other excitonic solar cells has never stopped,and such persistent efforts
Exciton absorption of entangled photons in semiconductor quantum wells
Rodriguez, Ferney; Guzman, David; Salazar, Luis; Quiroga, Luis; Condensed Matter Physics Group Team
2013-03-01
The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers. Research funds from Facultad de Ciencias, Universidad de los Andes
Phonon-assisted two-photon exciton transitions in semiconductors
International Nuclear Information System (INIS)
Hassan, A.R.
1987-08-01
The theory of phonon-assisted two-photon transitions to excitonic states in semiconductors has been theoretically investigated. The effects of both the nonparabolicity of the band and the degeneracy of the valence band have been taken into account. Expressions for the absorption coefficient through different band models are calculated. The numerical applications to CdI 2 and GaP show that the 4-band model gives the dominant contribution which leads to a final s-exciton state. An exciton peak appears at an energy which is close to that recently observed in CdI 2 . The non-parabolic effect enhances the absorption coefficient by a two-order of magnitude. (author). 6 refs, 1 fig., 1 tab
Spectral signatures of x((5)) processes in four-wave mixing of homogeneously broadened excitons
DEFF Research Database (Denmark)
Langbein, W.; Meier, T.; Koch, S.W.
2001-01-01
-biexciton beating at the exciton resonance is observed that is vanishing for long negative delays owing to the faster dephasing in the two-exciton continuum compared with the bound biexciton state. These results are in qualitative agreement with microscopic model calculations that include the coherent dynamics...... of one- and two-exciton resonances up to the fifth order in the optical field....
Exciton dephasing and biexciton binding in CdSe/ZnSe islands
DEFF Research Database (Denmark)
Wagner, Hans Peter; Tranitz, H.-P.; Preis, H
1999-01-01
The dephasing of excitons and the formation of biexcitons in self-organized CdSe/ZnSe islands grown by molecular-beam epitaxy is investigated using spectrally resolved four-wave mixing. A distribution of exciton-exciton scattering efficiencies and dephasing times in the range of 0.5-10 ps...
Excitonic surface polaritons in luminescence from ZnTe crystals
International Nuclear Information System (INIS)
Brodin, M.S.; Bandura, V.M.; Matsko, M.G.
1984-01-01
The form and structure of reflection and exciton-polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal-transverse splitting magnitude ΔE/sub LT/ is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k-linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wavelength of the exciting light is investigated. (author)
Excitonic surface polaritons in luminescence from ZnTe crystals
Energy Technology Data Exchange (ETDEWEB)
Brodin, M.S.; Bandura, V.M.; Matsko, M.G. (AN Ukrainskoj SSR, Kiev. Inst. Fiziki)
1984-10-01
The form and structure of reflection and exciton-polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal-transverse splitting magnitude ..delta..E/sub LT/ is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k-linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wavelength of the exciting light is investigated.
Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells
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.
Defect Structure of Localized Excitons in a WSe2 Monolayer
Zhang, Shuai
2017-07-26
The atomic and electronic structure of intrinsic defects in a WSe2 monolayer grown on graphite was revealed by low temperature scanning tunneling microscopy and spectroscopy. Instead of chalcogen vacancies that prevail in other transition metal dichalcogenide materials, intrinsic defects in WSe2 arise surprisingly from single tungsten vacancies, leading to the hole (p-type) doping. Furthermore, we found these defects to dominate the excitonic emission of the WSe2 monolayer at low temperature. Our work provided the first atomic-scale understanding of defect excitons and paved the way toward deciphering the defect structure of single quantum emitters previously discovered in the WSe2 monolayer.
Exciton localization-delocalization transition in an extended dendrimer
Energy Technology Data Exchange (ETDEWEB)
Pouthier, Vincent, E-mail: vincent.pouthier@univ-fcomte.fr [Institut UTINAM, Université de Franche-Comté, CNRS UMR 6213, 25030 Besançon Cedex (France)
2013-12-21
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G{sub c} ≈ 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol.
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....
Incomplete Exciton Harvesting from Fullerenes in Bulk Heterojunction Solar Cells
Burkhard, George F.; Hoke, Eric T.; Scully, Shawn R.; McGehee, Michael D.
2009-01-01
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.
Exciton localization-delocalization transition in an extended dendrimer
International Nuclear Information System (INIS)
Pouthier, Vincent
2013-01-01
Exciton-mediated quantum state transfer between the periphery and the core of an extended dendrimer is investigated numerically. By mapping the dynamics onto that of a linear chain, it is shown that a localization-delocalization transition arises for a critical value of the generation number G c ≈ 5. This transition originates in the quantum interferences experienced by the excitonic wave due to the multiple scatterings that arise each time the wave tunnels from one generation to another. These results suggest that only small-size dendrimers could be used for designing an efficient quantum communication protocol
Neutron scattering on equilibrium and nonequilibrium phonons, excitons and polaritons
International Nuclear Information System (INIS)
Broude, V.L.; Sheka, E.F.
1978-01-01
A number of problems of solid-state physics representing interest for neutron spectroscopy of future is considered. The development of the neutron inelastic scattering spectroscopy (neutron spectroscopy of equilibrium phonons) is discussed with application to nuclear dynamics of crystals in the thermodynamic equilibrium. The results of high-flux neutron source experiments on molecular crystals are presented. The advantages of neutron inelastic scattering over optical spectroscopy are discussed. The spectroscopy of quasi-equilibrium and non-equilibrium quasi-particles is discussed. In particular, the neutron scattering on polaritons, excitons in thermal equilibrium and production of light-excitons are considered. The problem of the possibility of such experiments is elucidated
Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.
Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H
2013-06-18
Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons
Self-trapped excitons in LH2 bacteriochlorophyll-protein complexes under high pressure
International Nuclear Information System (INIS)
Timpmann, K.; Ellervee, Aleksandr; Kuznetsov, Anatoli; Laisaar, Arlentin; Trinkunas, Gediminas; Freiberg, Arvi
2003-01-01
The absorption and emission spectra of excitons in LH2 antenna complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been studied under hydrostatic pressure. The measurements made between ambient pressure and 6 kbar over a broad temperature range reveal largely different rates of the pressure-induced shifts for the absorption and emission bands. Numerical calculations based on exciton polaron model provide evidence for the exciton self-trapping at ambient pressure as well as for the pressure stabilization of the self-trapped exciton states responsible for the emission, whereas the light absorbing states belong to nearly free excitons over the whole pressure and temperature ranges studied
Correction of the exciton Bohr radius in monolayer transition metal dichalcogenides
Li, Run-Ze; Dong, Xi-Ying; Li, Zhi-Qing; Wang, Zi-Wu
2018-07-01
We theoretically investigate the correction of exciton Bohr radius in monolayer transition metal dichalcogenides (TMDCs) on different polar substrates arising from the exciton-optical phonon coupling, in which both the intrinsic longitudinal optical phonon and surface optical phonon modes couple with the exciton are taken into account. We find that the exciton Bohr radius is enlarged markedly due to these coupling. Moreover, it can be changed on a large scale by modulating the polarizability of polar substrate and the internal distance between the monolayer TMDCs and polar substrate. Theoretical result provides a potential explanation for the variation of the exciton Bohr radius in experimental measurement.
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......, which illustrates the important role of interfaces for quantum dot based nanophotonic structures....
Photoluminescence dynamics of weakly confined excitons in GaAs thin films
International Nuclear Information System (INIS)
Kanno, Atsushi; Katouf, Redouane; Kojima, Osamu; Ishi-Hayase, Junko; Sasaki, Masahide; Tsuchiya, Masahiro; Isu, Toshiro
2008-01-01
We investigate the dynamics of weakly confined excitons in GaAs thin films measured by time-resolved photoluminescence (PL) technique. When excitation energy was above the resonant energy of the exciton, a long PL rise time of about 200 ps was observed. It is considered that an exciton formation process from excited continuum energy states to discrete energy states of the exciton in the thin film causes the slow PL rise. The observed PL decay time constant was about 14 ns due to high quality fabricated samples. The observed population dynamics can be surely ascribed to the specific features of weakly confined excitons
Decay of orientational grating of weakly confined excitons in GaAs thin films
International Nuclear Information System (INIS)
Kojima, O.; Isu, T.; Ishi-Hayase, J.; Kanno, A.; Katouf, R.; Sasaki, M.; Tsuchiya, M.
2008-01-01
We report the dynamical properties of the exciton orientation in GaAs thin films using the orientational grating (OG) technique. From the results of excitation-power dependence of OG signal, we confirmed that the OG signal comes from the optical nonlinearity of weakly confined excitons. In addition, the OG-decay time decreases with an increase of excitation power due to exciton-exciton interaction, and the shortest decay time is below 1 ps. Our results may imply the potential application of optical nonlinearity of weakly confined exciton to ultrafast switching devices operating at 1 Tbit/s
Strongly Enhanced Free-Exciton Luminescence in Microcrystalline CsPbBr3 Films
Kondo, Shin-ichi; Kakuchi, Mitsugu; Masaki, Atsushi; Saito, Tadaaki
2003-07-01
The luminescence properties of CsPbBr3 films prepared via the amorphous phase by crystallization are dominated by free-exciton emission, and only a weak trace of emission due to trapped excitons was observed, in contrast to the case of bulk CsPbBr3 crystals. In particular, the films in the microcrystalline state show by more than an order of magnitude stronger free-exciton emission than in the polycrystalline state. The enhanced free-exciton emission is suggestive of excitonic superradiance.
Exciton Recombination in Formamidinium Lead Triiodide : Nanocrystals versus Thin Films
Fang, Hong-Hua; Protesescu, Loredana; Balazs, Daniel M.; Adjokatse, Sampson; Kovalenko, Maksym V.; Loi, Maria Antonietta
2017-01-01
The optical properties of the newly developed near-infrared emitting formamidinium lead triiodide (FAPbI(3)) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady-state and time-resolved photoluminescence. The excitonic emission is
Excitonic Behavior of Rhodamine Dimers: A Single-Molecule Study
Hernando Campos, J.; van der Schaaf, Martijn; van Dijk, E.M.H.P.; Sauer, Markus; Garcia Parajo, M.F.; van Hulst, N.F.
2003-01-01
The optical behavior of a dimer of tetramethylrhodamine-5-isothiocyanate has been investigated by means of single-molecule measurements. Bulk absorption and fluorescence spectra show the existence of two populations of the dimer molecule that exhibit distinct excitonic interactions (strong and weak
Nonmonotonic energy harvesting efficiency in biased exciton chains
Vlaming, S.M.; Malyshev, V.A.; Knoester, J.
2007-01-01
We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average
Finite life time effects in the coherent exciton transfer
International Nuclear Information System (INIS)
Barvik, I.; Herman, P.
1992-04-01
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
Wannier-Mott Excitons in Nanoscale Molecular Ices
Chen, Y.-J.; Muñoz Caro, G. M.; Aparicio, S.; Jiménez-Escobar, A.; Lasne, J.; Rosu-Finsen, A.; McCoustra, M. R. S.; Cassidy, A. M.; Field, D.
2017-10-01
The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8 eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4 ×107 V m-1 , are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.
Jointly Tuned Plasmonic–Excitonic Photovoltaics Using Nanoshells
Paz-Soldan, Daniel; Lee, Anna; Thon, Susanna M.; Adachi, Michael M.; Dong, Haopeng; Maraghechi, Pouya; Yuan, Mingjian; Labelle, André J.; Hoogland, Sjoerd; Liu, Kun; Kumacheva, Eugenia; Sargent, Edward H.
2013-01-01
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
Exciton binding energy in a pyramidal quantum dot
Indian Academy of Sciences (India)
A ANITHA
2018-03-27
Mar 27, 2018 ... screening function on exciton binding energy in a pyramid-shaped quantum dot of ... tures may generate unique properties and they show .... where Ee is the ground-state energy of the electron in ... Figure 1. The geometry of the pyramidal quantum dot. base and H is the height of the pyramid which is taken.
Magneto-exciton transitions in laterally coupled quantum dots
Barticevic, Zdenka; Pacheco, Monica; Duque, Carlos A.; Oliveira, Luiz E.
2008-03-01
We present a study of the electronic and optical properties of laterally coupled quantum dots. The excitonic spectra of this system under the effects of an external magnetic field applied perpendicular to the plane of the dots is obtained, with the potential of every individual dot taken as the superposition of a quantum well potential along the axial direction with a lateral parabolic confinement potential, and the coupled two- dot system then modeled by a superposition of the potentials of each dot, with their minima at different positions and truncated at the intersection plane. The wave functions and eigenvalues are obtained in the effective-mass approximation by using an extended variational approach in which the magneto- exciton states are simultaneously obtained [1]. The allowed magneto-exciton transitions are investigated by using circularly polarized radiation in the plane perpendicular to the magnetic field. We present results on the excitonic absorption coefficient as a function of the photon energy for different geometric quantum-dot confinement and magnetic-field values. Reference: [1] Z. Barticevic, M. Pacheco, C. A. Duque and L. E. Oliveira, Phys. Rev. B 68, 073312 (2003).
Synthetic Control of Exciton Behavior in Colloidal Quantum Dots.
Pu, Chaodan; Qin, Haiyan; Gao, Yuan; Zhou, Jianhai; Wang, Peng; Peng, Xiaogang
2017-03-08
Colloidal quantum dots are promising optical and optoelectronic materials for various applications, whose performance is dominated by their excited-state properties. This article illustrates synthetic control of their excited states. Description of the excited states of quantum-dot emitters can be centered around exciton. We shall discuss that, different from conventional molecular emitters, ground-state structures of quantum dots are not necessarily correlated with their excited states. Synthetic control of exciton behavior heavily relies on convenient and affordable monitoring tools. For synthetic development of ideal optical and optoelectronic emitters, the key process is decay of band-edge excitons, which renders transient photoluminescence as important monitoring tool. On the basis of extensive synthetic developments in the past 20-30 years, synthetic control of exciton behavior implies surface engineering of quantum dots, including surface cation/anion stoichiometry, organic ligands, inorganic epitaxial shells, etc. For phosphors based on quantum dots doped with transition metal ions, concentration and location of the dopant ions within a nanocrystal lattice are found to be as important as control of the surface states in order to obtain bright dopant emission with monoexponential yet tunable photoluminescence decay dynamics.
Optical absorption of charged excitons in semiconducting carbon nanotubes
DEFF Research Database (Denmark)
Rønnow, Troels Frimodt; Pedersen, Thomas Garm; Cornean, Horia
2012-01-01
In this article we examine the absorption coefficient of charged excitons in carbon nanotubes. We investigate the temperature and damping dependence of the absorption spectra. We show that the trion peak in the spectrum is asymmetric for temperatures greater than approximately 1 K whereas...
Direct measurement of exciton dissociation energy in polymers
Czech Academy of Sciences Publication Activity Database
Toušek, J.; Toušková, J.; Chomutová, R.; Paruzel, Bartosz; Pfleger, Jiří
2017-01-01
Roč. 7, č. 1 (2017), s. 1-6, č. článku 015113. ISSN 2158-3226 Institutional support: RVO:61389013 Keywords : exciton dissociation energy * polymers * SCR Subject RIV: CD - Macromolecular Chemistry OBOR OECD: Polymer science Impact factor: 1.568, year: 2016
Signatures of exciton condensation in a transition metal dichalcogenide
Kogar, A.; Rak, M.S.; Vig, S.; Husain, A.A.; Flicker, F.; Joe, Y.I.; Venema, L.; MacDougall, G.J.; Chiang, T.C.; Fradkin, E.; van Wezel, J.; Abbamonte, P.
2017-01-01
Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently
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), 1-6, č. článku 035129. 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
Colloquium: Excitons in atomically thin transition metal dichalcogenides
Wang, Gang; Chernikov, Alexey; Glazov, Mikhail M.; Heinz, Tony F.; Marie, Xavier; Amand, Thierry; Urbaszek, Bernhard
2018-04-01
Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.
Excitonic magnetism in d.sup.6./sup. perovskites
Czech Academy of Sciences Publication Activity Database
Afonso, J.F.; Kuneš, Jan
2017-01-01
Roč. 95, č. 11 (2017), s. 1-8, č. článku 115131. ISSN 2469-9950 EU Projects: European Commission(XE) 646807 - EXMAG Institutional support: RVO:68378271 Keywords : excitonic magnetism * cobaltites Subject RIV: BM - Solid Matter Physics ; Magnetism OBOR OECD: Condensed matter physics (including formerly solid state physics, supercond.) Impact factor: 3.836, year: 2016
Extension of the radiative lifetime of Wannier-Mott excitons in semiconductor nanoclusters
International Nuclear Information System (INIS)
Kukushkin, V. A.
2015-01-01
The purpose of the study is to calculate the radiative lifetime of Wannier-Mott excitons in three-dimensional potential wells formed of direct-gap narrow-gap semiconductor nanoclusters in wide-gap semiconductors and assumed to be large compared to the exciton radius. Calculations are carried out for the InAs/GaAs heterosystem. It is shown that, as the nanocluster dimensions are reduced to values on the order of the exciton radius, the exciton radiative lifetime becomes several times longer compared to that in a homogeneous semiconductor. The increase in the radiative lifetime is more pronounced at low temperatures. Thus, it is established that the placement of Wannier-Mott excitons into direct-gap semiconductor nanoclusters, whose dimensions are of the order of the exciton radius, can be used for considerable extension of the exciton radiative lifetime
Highly mobile charge-transfer excitons in two-dimensional WS2/tetracene heterostructures
Zhu, Tong; Yuan, Long; Zhao, Yan; Zhou, Mingwei; Wan, Yan; Mei, Jianguo; Huang, Libai
2018-01-01
Charge-transfer (CT) excitons at heterointerfaces play a critical role in light to electricity conversion using organic and nanostructured materials. However, how CT excitons migrate at these interfaces is poorly understood. We investigate the formation and transport of CT excitons in two-dimensional WS2/tetracene van der Waals heterostructures. Electron and hole transfer occurs on the time scale of a few picoseconds, and emission of interlayer CT excitons with a binding energy of ~0.3 eV has been observed. Transport of the CT excitons is directly measured by transient absorption microscopy, revealing coexistence of delocalized and localized states. Trapping-detrapping dynamics between the delocalized and localized states leads to stretched-exponential photoluminescence decay with an average lifetime of ~2 ns. The delocalized CT excitons are remarkably mobile with a diffusion constant of ~1 cm2 s−1. These highly mobile CT excitons could have important implications in achieving efficient charge separation. PMID:29340303
Theoretical study of excitonic complexes in semiconductors quantum wells
International Nuclear Information System (INIS)
Dacal, Luis Carlos Ogando
2001-08-01
A physical system where indistinguishable particles interact with each other creates the possibility of studying correlation and exchange effect. The simplest system is that one with only two indistinguishable particles. In condensed matter physics, these complexes are represented by charged excitons, donors and acceptors. In quantum wells, the valence band is not parabolic, therefore, the negatively charged excitons and donors are theoretically described in a simpler way. Despite the fact that the stability of charged excitons (trions) is known since the late 50s, the first experimental observation occurred only at the early 90s in quantum well samples, where their binding energies are one order of magnitude larger due to the one dimensional carriers confinement. After this, these complexes became the subject of an intense research because the intrinsic screening of electrical interactions in semiconductor materials allows that magnetic fields that are usual in laboratories have strong effects on the trion binding energy. Another rich possibility is the study of trions as an intermediate state between the neutral exciton and the Fermi edge singularity when the excess of doping carriers is increased. In this thesis, we present a theoretical study of charged excitons and negatively charged donors in GaAs/Al 0.3 Ga 0.7 As quantum wells considering the effects of external electric and magnetic fields. We use a simple, accurate and physically clear method to describe these systems in contrast with the few and complex treatments s available in the literature. Our results show that the QW interface defects have an important role in the trion dynamics. This is in agreement with some experimental works, but it disagrees with other ones. (author)
Exciton center-of-mass localization and dielectric environment effect in monolayer WS2
Hichri, Aïda; Ben Amara, Imen; Ayari, Sabrine; Jaziri, Sihem
2017-06-01
The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. In this paper, we study the strong localization of exciton center-of-mass motion within random potential fluctuations caused by the monolayer defects. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping, and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.
Exciton-polariton dynamics in quantum dot-cavity system
Energy Technology Data Exchange (ETDEWEB)
Neto, Antonio F.; Lima, William J.; Villas-Boas, Jose M. [Universidade Federal de Uberlandia (UFU), MG (Brazil). Inst. de Fisica
2012-07-01
Full text: One of the basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. This imply in know all sources of decoherence and elaborate ways to avoid them. In recent work, A. Laucht et al. [1] presented detailed theoretical and experimental investigations of electrically tunable single quantum dot (QD) - photonic crystal (PhC) nanocavity systems operating in the strong coupling regime of the light matter interaction. Unlike previous studies, where the exciton-cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, they employ the quantum confined Stark-effect to electro-optically control the exciton-cavity detuning. The new built device enabled them to systematically probe the emission spectrum of the strongly coupled system as a function of external control parameters, as for example the incoherent excitation power density or the lattice temperature. Those studies reveal for the first time insights in dephasing mechanisms of 0D exciton polaritons [1]. In another study [2], using a similar device, they investigate the coupling between two different QDs with a single cavity mode. In both works, incoherent pumping was used, but for quantum information, coherent and controlled excitations are necessary. Here, we theoretically investigate the dynamics a single quantum dot inside a cavity under coherent pulse excitation and explore a wide range of parameters, as for example, the exciton-cavity detunings, the excitation power, the spontaneous decay, and pure dephasing. We use density matrix formalism in the Lindblad form, and we solve it numerically. Our results show that coherent excitation can be used to probe strong coupling between exciton and cavity mode by monitoring the exciton Rabi oscillation as function of the cavity detuning. This can give new insights for future experimental measurement focusing on quantum
Self-trapped excitonic green emission from layered semiconductors
International Nuclear Information System (INIS)
Miah, M. Idrish
2009-01-01
Crystals of layered semiconductor are grown by Bridgman technique and are studied them under two-photon excitation by a Q-switched 20-ns pulse laser. The photoluminescence (PL) emission spectra of the crystals are measured at various pumping powers and temperatures. The PL spectra appear broad and structureless emissions with their peaks in the green spectral region. The characteristic emissions are from self-trapped excitons of the crystals. An analysis of the spectra measured at various pumping powers shows a quadratic dependence of the PL peak intensity on the power, confirming a biphotonic process of the two-photon pumping. The temperature dependence shows an enhancement of the nonlinear response at low temperatures. The activation energy is estimated and found to be 2.4 meV. The roles of the bound excitons in the observed PL are discussed briefly.
Self-trapped excitonic green emission from layered semiconductors
Energy Technology Data Exchange (ETDEWEB)
Miah, M. Idrish, E-mail: m.miah@griffith.edu.au [Nanoscale Science and Technology Centre, Griffith University, Nathan, Brisbane, QLD 4111 (Australia); School of Biomolecular and Physical Sciences, Griffith University, Nathan, Brisbane, QLD 4111 (Australia); Department of Physics, University of Chittagong, Chittagong 4331 (Bangladesh)
2009-08-15
Crystals of layered semiconductor are grown by Bridgman technique and are studied them under two-photon excitation by a Q-switched 20-ns pulse laser. The photoluminescence (PL) emission spectra of the crystals are measured at various pumping powers and temperatures. The PL spectra appear broad and structureless emissions with their peaks in the green spectral region. The characteristic emissions are from self-trapped excitons of the crystals. An analysis of the spectra measured at various pumping powers shows a quadratic dependence of the PL peak intensity on the power, confirming a biphotonic process of the two-photon pumping. The temperature dependence shows an enhancement of the nonlinear response at low temperatures. The activation energy is estimated and found to be 2.4 meV. The roles of the bound excitons in the observed PL are discussed briefly.
Jointly Tuned Plasmonic–Excitonic Photovoltaics Using Nanoshells
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.
Quasiparticle and excitonic gaps of one-dimensional carbon chains.
Mostaani, E; Monserrat, B; Drummond, N D; Lambert, C J
2016-06-01
We report diffusion quantum Monte Carlo (DMC) calculations of the quasiparticle and excitonic gaps of hydrogen-terminated oligoynes and extended polyyne. The electronic gaps are found to be very sensitive to the atomic structure in these systems. We have therefore optimised the geometry of polyyne by directly minimising the DMC energy with respect to the lattice constant and the Peierls-induced carbon-carbon bond-length alternation. We find the bond-length alternation of polyyne to be 0.136(2) Å and the excitonic and quasiparticle gaps to be 3.30(7) and 3.4(1) eV, respectively. The DMC zone-centre longitudinal optical phonon frequency of polyyne is 2084(5) cm(-1), which is consistent with Raman spectroscopic measurements for large oligoynes.
Excitonic condensation for the surface states of topological insulator bilayers
International Nuclear Information System (INIS)
Wang Zhigang; Fu Zhenguo; Zhang Ping; Hao Ningning
2012-01-01
We propose a generic topological insulator bilayer (TIB) system to study the excitonic condensation with self-consistent mean-field (SCMF) theory. We show that the TIB system presents the crossover behavior from the Bardeen-Cooper-Schrieffer (BCS) limit to the Bose-Einstein condensation (BEC) limit. Moreover, in comparison with traditional semiconductor systems, we find that for the present system the superfluid property in the BEC phase is more sensitive to electron-hole density imbalance and the BCS phase is more robust. Applying this TIB model to the Bi 2 Se 3 -family material, we find that the BEC phase is most likely to be observed in experiment. We also calculate the critical temperature for the Bi 2 Se 3 -family TIB system, which is ∼100 K. More interestingly, one can expect this relative high-temperature excitonic condensation, since our calculated SCMF critical temperature is approximately equal to the Kosterlitz-Thouless transition temperature. (paper)
Organic photovoltaic cell incorporating electron conducting exciton blocking layers
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.
Exciton distribution function and secondary radiation in polar semiconductors
International Nuclear Information System (INIS)
Trallero Giner, C.; Sotolongo Costa, O.
1985-07-01
An explicit non-equilibrium distribution function for excitons in the ground state n=1 in the case when the fundamental interaction is with acoustical phonons is calculated for polar semiconductors. Using it, a general expression for the secondary radiation cross-section (valid for Raman, hot and thermalized luminescence processes), is obtained. The results are applied to explain the temperature dependence of the 1LO and 2LO luminescence lines half-width in CdS single crystals. The relative contributions of 3LO Raman and luminescence intensities and the variation of the secondary emission spectrum as function of exciton life-time are studied. Comparison with experimental results yields quantitative agreement. (author)
Geometrical-confinement effects on excitons in quantum disks
International Nuclear Information System (INIS)
Song, J.; Ulloa, S.E.
1995-01-01
Excitons confined to flat semiconductor quantum dots with elliptical cross sections are considered as we study geometrical effects on exciton binding energy, electron-hole separation, and the resulting linear optical properties. We use numerical matrix diagonalization techniques with appropriately large and optimized basis sets in an effective-mass Hamiltonian approach. The linear optical susceptibilities of GaAs and InAs dots for several different size ratios are discussed and compared to experimental photoluminescence spectra obtained on GaAs/Al x Ga 1-x As and InAs/GaAs quantum dots. For quantum dots of several nm in size, there is a strong blueshift of the luminescence due to geometrical-confinement effects. Also, transition peaks are split and shifted towards higher energy, in comparison with dots with circular cross sections
Modeling Temperature Dependent Singlet Exciton Dynamics in Multilayered Organic Nanofibers
DEFF Research Database (Denmark)
de Sousa, Leonardo Evaristo; de Oliveira Neto, Pedro Henrique; Kjelstrup-Hansen, Jakob
2018-01-01
Organic nanofibers have shown potential for application in optoelectronic devices because of the tunability of their optical properties. These properties are influenced by the electronic structure of the molecules that compose the nanofibers, but also by the behavior of the excitons generated...... dynamics in multilayered organic nanofibers. By simulating absorption and emission spectra, the possible Förster transitions are identified. Then, a Kinetic Monte Carlo (KMC) model is employed in combination with a genetic algorithm to theoretically reproduce time resolved photoluminescence measurements...
Nonlinear spectroscopy of excitons and biexcitons in ZnS
International Nuclear Information System (INIS)
Pavlov, L.I.; Paskov, P.P.; Lalov, I.J.
1989-01-01
Four- photon spectroscopy on exciton and biexciton states in ZnS is reported at T = 10 K. The Nd:YAG laser is used as a fundamental source in the experimental setup. Second harmonic radiation ω 2 pumps the dye laser of ω 1 tunable frequency. The ZnS single crystal is placed in an optical cryostat for resonant spectroscopy at low temperature. Four-photon mixing ω 3 = 2ω 1 -ω 2 signal is separated by MDR-23 monochromator and is registered by a laser photometer. The hexagonal ZnS crystal is experimentally investigated when the waves ω 1 and ω 2 propagate colinear with the optical axis. The crystal is cut along the (1120) plane. The photon 2ℎω 1 energy scans over the range 3.895-3.940 eV. The dispersion of I 3 (ω 3 ) upon 2ℎω 1 is obtained. Three resonances are registered E M = 3.8964, E B 1 = 3.9010 and E B 2 = 3.9311 eV. The recorded low temperature resonance in dispersion of nonlinearity χ (3) are identified with B 1 s and B 2 s excitons as well as with biexciton in ZnS which is observed for the first time in this crystal. An experimental dependence of the signal I 3 (ω 3 ) intensity upon the pump I 1 (ω 1 ) is obtained. The E M resonance is saturated with the I 1 (ω 1 ) enhancement while the E B 1 resonance increases. Authors explain such a behaviour by the fact that the recombination probability of the biexcitons to excitons increases with the pump level growth. Estimations for the exciton density and the bounding energy are given. (author)
Exciton diffusion coefficient measurement in ZnO nanowires under electron beam irradiation
Donatini, Fabrice; Pernot, Julien
2018-03-01
In semiconductor nanowires (NWs) the exciton diffusion coefficient can be determined using a scanning electron microscope fitted with a cathodoluminescence system. High spatial and temporal resolution cathodoluminescence experiments are needed to measure independently the exciton diffusion length and lifetime in single NWs. However, both diffusion length and lifetime can be affected by the electron beam bombardment during observation and measurement. Thus, in this work the exciton lifetime in a ZnO NW is measured versus the electron beam dose (EBD) via a time-resolved cathodoluminescence experiment with a temporal resolution of 50 ps. The behavior of the measured exciton lifetime is consistent with our recent work on the EBD dependence of the exciton diffusion length in similar NWs investigated under comparable SEM conditions. Combining the two results, the exciton diffusion coefficient in ZnO is determined at room temperature and is found constant over the full span of EBD.
Exciton spectra of mixed LiH1-xDx crystals
International Nuclear Information System (INIS)
Plekhanov, V.G.
1989-01-01
The results of low-tempertaure experimental investigation of exciton spectra of pure surface of mixed crystals LiH 1-x d x forming the continuous series of a solved solution are presented. The long-wave reflection spectra is formed, as in pure crystals, by excitons of a large radius. The developed structure of spectra of exciton luminiscence consisting mainly of LO-lines, testifies to the intraband Frelich mechanism of free exciton scattering by LO-phonos, playing the considerable role in renormalization of the exciton Rydberg and the energy of interband transitions. Increase of the concentration of deuterium in mixed crystals causes a short-wave shift in the reflection spectrum and luminescence and continuous decrease of LO-phonon energy together with the increase of Rydberg exciton
Harmonic Quantum Coherence of Multiple Excitons in PbS/CdS Core-Shell Nanocrystals
Tahara, Hirokazu; Sakamoto, Masanori; Teranishi, Toshiharu; Kanemitsu, Yoshihiko
2017-12-01
The generation and recombination dynamics of multiple excitons in nanocrystals (NCs) have attracted much attention from the viewpoints of fundamental physics and device applications. However, the quantum coherence of multiple exciton states in NCs still remains unclear due to a lack of experimental support. Here, we report the first observation of harmonic dipole oscillations in PbS/CdS core-shell NCs using a phase-locked interference detection method for transient absorption. From the ultrafast coherent dynamics and excitation-photon-fluence dependence of the oscillations, we found that multiple excitons cause the harmonic dipole oscillations with ω , 2 ω , and 3 ω oscillations, even though the excitation pulse energy is set to the exciton resonance frequency, ω . This observation is closely related to the quantum coherence of multiple exciton states in NCs, providing important insights into multiple exciton generation mechanisms.
Ohnishi, Hiromasa; Tomita, Norikazu; Nasu, Keiichiro
2018-03-01
We study conceptional problems of a photo-electron emission (PEE) process from a free exciton in insulating crystals. In this PEE process, only the electron constituting the exciton is suddenly emitted out of the crystal, while the hole constituting the exciton is still left inside and forced to be recoiled back to its original valence band. This recoil on the hole is surely reflected in the spectrum of the PEE with a statistical distribution along the momentum-energy curve of the valence band. This distribution is nothing but the square of the exciton wavefunction amplitude, since it shows how the electron and the hole are originally bound together. Thus, the momentum-resolved PEE can directly determine the exciton wavefunction. These problems are clarified, taking the Γ and the saddle point excitons in GaAs, as typical examples. New PEE experiments are also suggested.
Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation
Kryjevski, Andrei; Mihaylov, Deyan; Kilina, Svetlana; Kilin, Dmitri
2017-10-01
We use a Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory based on density functional theory simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2) and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ˜1.6 at about 3Eg, where Eg is the electronic gap.
Theory for electric dipole superconductivity with an application for bilayer excitons.
Jiang, Qing-Dong; Bao, Zhi-qiang; Sun, Qing-Feng; Xie, X C
2015-07-08
Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we derive the London-type and Ginzburg-Landau-type equations for the electric dipole superconductors. By using these equations, we discover the Meissner-type effect and the electric dipole current Josephson effect. These effects can provide direct evidence for the formation of the exciton superfluid state in bilayer systems and pave new ways to drive an electric dipole current.
Excitons in conjugated polymers: Do we need a paradigma change?
Energy Technology Data Exchange (ETDEWEB)
Beenken, Wichard J.D. [Department of Theoretical Physics I, Ilmenau University of Thechnology (Germany)
2009-12-15
We have previously shown that both, polymer conformation and dynamics are crucial for the exciton transport in conjugated polymers. Thereby we found that the usual Foerster-type hopping transfer model - even if one applies the line-dipole approximation - falls short in one crucial aspect: the nature of the sites the excitons are transferred between is still unclear. We found that the simple model of spectroscopic units defined as segments of the polymer chains separated by structural defects breaking the {pi}-conjugation is only justified for chemical defects like hydrogenated double bonds, or extreme gauche (90 ) torsions between the monomers. Both defects are far too rare in a well-prepared conjugated polymer to explain the mean spectroscopic-unit length of typically 6-7 monomers. Meanwhile, also the concept of dynamical formation of the spectroscopic units, we had previously suggested, has also failed. Thus the question of a paradigma change concerning the exciton transport in conjugated polymers appears on the agenda. (Abstract Copyright [2009], Wiley Periodicals, Inc.)
Photoluminescence and Confinement of Excitons in Disordered Porous Films
Energy Technology Data Exchange (ETDEWEB)
Bondar, N. V., E-mail: jbond@iop.kiev.ua; Brodin, M. S. [National Academy of Sciences of Ukraine, Institute of Physics (Ukraine); Brodin, A. M. [National Technical University of Ukraine “KPI” (Ukraine); Matveevskaya, N. A. [National Academy of Sciences of Ukraine, Institute for Single Crystals (Ukraine)
2016-03-15
The exciton confinement effect in quantum dots at the surface of SiO{sub 2} spheres and the percolation phase transition in films based on a mixture of pure SiO{sub 2} spheres and spheres covered by CdS quantum dots (SiO{sub 2}/CdS nanoparticles) are studied. It is found that, due to the high surface energy of spheres, the quantum dots deposited onto their surface are distorted, which modifies the exciton confinement effect: the effect is retained only in one direction, the direction normal to the surface of the spheres. As a result, the energy of the exciton ground state exhibits a complex dependence on both the quantum-dot radius and sphere size. In the optical spectra of films based on this mixture, the clustering of small-sized nanoparticles and then, at a critical concentration of nanoparticles of ~60%, the formation of a percolation cluster are detected for the first time. The critical concentration is twice higher than the corresponding quantity given by the model of geometrical “colored percolation”, which is a consequence of interaction between submicrometer nanoparticles. The relation between the basic parameters of the percolation transition, such as the film porosity, coordination number, and the quantity defining the number of particles in the percolation cluster, is obtained and analyzed.
The nature of singlet excitons in oligoacene molecular crystals
Yamagata, H.; Norton, J.; Hontz, E.; Olivier, Y.; Beljonne, D.; Brédas, J. L.; Silbey, R. J.; Spano, F. C.
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.
Optical orientation and alignment of excitons in ensembles of inorganic perovskite nanocrystals
Nestoklon, M. O.; Goupalov, S. V.; Dzhioev, R. I.; Ken, O. S.; Korenev, V. L.; Kusrayev, Yu. G.; Sapega, V. F.; de Weerd, C.; Gomez, L.; Gregorkiewicz, T.; Lin, Junhao; Suenaga, Kazutomo; Fujiwara, Yasufumi; Matyushkin, L. B.; Yassievich, I. N.
2018-01-01
We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI$_3$ perovskite nanocrystals prepared by colloidal synthesis and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 120{\\mu}eV. Our calculations demonstrate that there is a significant contribution to the splitting due t...
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.
Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells
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.
Roslyak, Oleksiy; Cherqui, Charles; Dunlap, David H; Piryatinski, Andrei
2014-07-17
We report on a general theoretical approach to study exciton transport and emission in a single-walled carbon nanotube (SWNT) in the presence of a localized surface-plasmon (SP) mode within a metal nanoparticle interacting via near-field coupling. We derive a set of quantum mechanical equations of motion and approximate rate equations that account for the exciton, SP, and the environmental degrees of freedom. The material equations are complemented by an expression for the radiated power that depends on the exciton and SP populations and coherences, allowing for an examination of the angular distribution of the emitted radiation that would be measured in experiment. Numerical simulations for a (6,5) SWNT and cone-shaped Ag metal tip (MT) have been performed using this methodology. Comparison with physical parameters shows that the near-field interaction between the exciton-SP occurs in a weak coupling regime, with the diffusion processes being much faster than the exciton-SP population exchange. In such a case, the effect of the exciton population transfer to the MT with its subsequent dissipation (i.e., the Förster energy transfer) is to modify the exciton steady state distribution while reducing the equilibration time for excitons to reach a steady sate distribution. We find that the radiation distribution is dominated by SP emission for a SWNT-MT separation of a few tens of nanometers due to the fast SP emission rate, whereas the exciton-SP coherences can cause its rotation.
Phosphorescence as a probe of exciton formation and energy transfer in organic light emitting diodes
International Nuclear Information System (INIS)
Baldo, M.; Segal, M.
2004-01-01
The development of highly efficient phosphorescent molecules has approximately quadrupled the quantum efficiency of organic light emitting devices (OLEDs). By harnessing triplet as well as singlet excitons, efficient molecular phosphorescence has also enabled novel studies of exciton physics in organic semiconductors. In this review, we will summarize recent progress in understanding exciton formation and energy transfer using phosphorescent molecular probes. Particular emphasis is given to two topics of current interest: energy transfer in blue phosphorescent OLEDs, and quantifying the formation ratio of singlet to triplet excitons in small-molecular weight materials and polymers. (orig.)
Exciton binding energy in GaAsBiN spherical quantum dot heterostructures
Das, Subhasis; Dhar, S.
2017-03-01
The ground state exciton binding energies (EBE) of heavy hole excitons in GaAs1-x-yBixNy - GaAs spherical quantum dots (QD) are calculated using a variational approach under 1s hydrogenic wavefunctions within the framework of effective mass approximation. Both the nitrogen and the bismuth content in the material are found to affect the binding energy, in particular for larger nitrogen content and lower dot radii. Calculations also show that the ground state exciton binding energies of heavy holes increase more at smaller dot sizes as compared to that for the light hole excitons.
The excitonic insulator route through a dynamical phase transition induced by an optical pulse
Energy Technology Data Exchange (ETDEWEB)
Brazovskii, S., E-mail: brazov@lptms.u-psud.fr [Université Paris-Saclay, LPTMS, CNRS, Univ. Paris-sud (France); Kirova, N. [Université Paris-Saclay, LPS, CNRS, Univ. Paris-sud (France)
2016-03-15
We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor–acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh.
Coherent detection of THz-induced sideband emission from excitons in the nonperturbative regime
Uchida, K.; Otobe, T.; Mochizuki, T.; Kim, C.; Yoshita, M.; Tanaka, K.; Akiyama, H.; Pfeiffer, L. N.; West, K. W.; Hirori, H.
2018-04-01
Strong interaction of a terahertz (THz) wave with excitons induces nonperturbative optical effects such as Rabi splitting and high-order sideband generation. Here, we investigated coherent properties of THz-induced sideband emissions from GaAs/AlGaAs multiquantum wells. With increasing THz electric field, optical susceptibility of the THz-dressed exciton shows a redshift with spectral broadening and extraordinary phase shift. This implies that the field ionization of the 1 s exciton modifies the THz-dressed exciton in the nonperturbative regime.
Lin, Kuen-Feng; Chiang, Chien-Hung; Wu, Chun-Guey
2014-01-01
The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA) were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport. PMID:25295290
Direct observation of free-exciton thermalization in quantum-well structures
DEFF Research Database (Denmark)
Umlauff, M.; Hoffmann, J.; Kalt, H.
1998-01-01
We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum stales with picosecond laser pulses. The subs......We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum stales with picosecond laser pulses...
Energetic disorder and exciton states of individual molecular rings
International Nuclear Information System (INIS)
Herman, Pavel; Barvik, Ivan; Zapletal, David
2006-01-01
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
Jahan, K Luhluh; Boda, A; Shankar, I V; Raju, Ch Narasimha; Chatterjee, Ashok
2018-03-22
The problem of an exciton trapped in a Gaussian quantum dot (QD) of GaAs is studied in both two and three dimensions in the presence of an external magnetic field using the Ritz variational method, the 1/N expansion method and the shifted 1/N expansion method. The ground state energy and the binding energy of the exciton are obtained as a function of the quantum dot size, confinement strength and the magnetic field and compared with those available in the literature. While the variational method gives the upper bound to the ground state energy, the 1/N expansion method gives the lower bound. The results obtained from the shifted 1/N expansion method are shown to match very well with those obtained from the exact diagonalization technique. The variation of the exciton size and the oscillator strength of the exciton are also studied as a function of the size of the quantum dot. The excited states of the exciton are computed using the shifted 1/N expansion method and it is suggested that a given number of stable excitonic bound states can be realized in a quantum dot by tuning the quantum dot parameters. This can open up the possibility of having quantum dot lasers using excitonic states.
Energy Technology Data Exchange (ETDEWEB)
Ding, Baofu, E-mail: b.ding@ecu.edu.au; Alameh, Kamal, E-mail: k.alameh@ecu.edu.au [Electron Science Research Institute, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027 (Australia)
2014-07-07
The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.
International Nuclear Information System (INIS)
Ding, Baofu; Alameh, Kamal
2014-01-01
The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.
DEFF Research Database (Denmark)
Larionov, A. V.; Timofeev, V. B.; Hvam, Jørn Märcher
2002-01-01
, and a significant increase in the radiative decay rate of the condensed phase. The collective exciton phase arises at temperatures T properties of the collective phase of interwell excitons and experimental manifestations of this coherence...
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
Room-Temperature Exciton Lasing in Ultrathin Film of Coupled Nanocrystals
International Nuclear Information System (INIS)
Appavoo, Kannatassen; Xiaoze, Liu; Menon, Vinod; Sfeir, Matthew Y.
2015-01-01
We demonstrate exciton lasing in sub-wavelength coupled nanostructures at ultralow fluence threshold, as probed by femtosecond broadband emission and absorption spectroscopy. The complex spectrotemporal dynamics reveal for the first time an excitonic-to-electron-hole plasma lasing mechanism.
Excitonic and electron-hole mechanisms of the creation of Frenkel defect in alkali halides
International Nuclear Information System (INIS)
Lushchik, A.; Kirm, M.; Lushchik, Ch.; Vasil'chenko, E.
2000-01-01
Excitonic and electron-hole (e-h) mechanisms of stable F centre creation by VUV radiation in alkali halide crystals are discussed. In KCl at 4.2 K, the efficiency of stable F-H pair creation is especially high at the direct optical formation of triplet excitons with n=1. At 200-400 K, the creation processes of stable F centres in KCl are especially efficient at the formation of one-halide exciton in the Urbach tail of an exciton absorption. In KCl and KBr, the decay of a cation exciton (∼20 eV) causes the formation of two e-h pairs, while in NaCl a cation exciton (33.5 eV) decays into two e-h and an anion exciton. An elastic uniaxial stress of a crystal excited by VUV radiation decreases the mean free path of excitons before their self-trapping (KI) and increases the mean free path of hot holes before self-trapping (NaCl)
Fermi-edge exciton-polaritons in doped semiconductor microcavities with finite hole mass
Pimenov, Dimitri; von Delft, Jan; Glazman, Leonid; Goldstein, Moshe
2017-10-01
The coupling between a 2D semiconductor quantum well and an optical cavity gives rise to combined light-matter excitations, the exciton-polaritons. These were usually measured when the conduction band is empty, making the single polariton physics a simple single-body problem. The situation is dramatically different in the presence of a finite conduction-band population, where the creation or annihilation of a single exciton involves a many-body shakeup of the Fermi sea. Recent experiments in this regime revealed a strong modification of the exciton-polariton spectrum. Previous theoretical studies concerned with nonzero Fermi energy mostly relied on the approximation of an immobile valence-band hole with infinite mass, which is appropriate for low-mobility samples only; for high-mobility samples, one needs to consider a mobile hole with large but finite mass. To bridge this gap, we present an analytical diagrammatic approach and tackle a model with short-ranged (screened) electron-hole interaction, studying it in two complementary regimes. We find that the finite hole mass has opposite effects on the exciton-polariton spectra in the two regimes: in the first, where the Fermi energy is much smaller than the exciton binding energy, excitonic features are enhanced by the finite mass. In the second regime, where the Fermi energy is much larger than the exciton binding energy, finite mass effects cut off the excitonic features in the polariton spectra, in qualitative agreement with recent experiments.
Pool-Frenkel thermoelectric modulation of exciton photoluminescence in GaSe crystals
International Nuclear Information System (INIS)
Ertap, H.; Mamedov, G.M.; Karabulut, M.; Bacioglu, A.
2011-01-01
Effect of external field on the exciton photoluminescence of GaSe crystals has been investigated and it has been observed that the PL is quenched with the applied field. The changes observed in the PL spectra have been analyzed with impact exciton, Franz-Keldysh and Pool-Frenkel effects. From the analyses of the experimental data, it has been found that the intensity of direct free, indirect free and bound exciton peaks decreased exponentially with the square root of applied field as I∼exp-β√E. The energy positions of emission peaks were found to shift to longer wavelength with the applied field as ΔE∼β√E. From these findings, the Pool-Frenkel thermoelectric field effect is seen to be the dominant mechanism in the variation of exciton PL with the applied field even though the impact exciton and Franz-Keldysh effects contribute. - Highlights: → Exciton PL intensity varies with the applied field. It decreases with the square root of E in accordance with Pool-Frenkel effect. → In the intrinsic region of PL spectrum, lines belonging to direct and indirect free/bound excitons were observed. → Line positions shifted to longer wavelengths with the applied field. → It was shown that amplitude modulation of exciton PL with electric field was possible.
Thermalization of Hot Free Excitons in ZnSe-Based Quantum Wells
DEFF Research Database (Denmark)
Hoffmann, J.; Umlauff, M.; Kalt, H.
1997-01-01
Thermalization of hot-exciton populations in ZnSe quantum wells occurs on a time scale of 100 ps. Strong exciton-phonon coupling in II-VI semiconductors leads to a direct access to the thermalization dynamics via time-resolved spectroscopy of phonon-assisted luminescence. The experimental spectra...
Preequilibrium decay in the exciton model for nuclear potential with a finite depth
International Nuclear Information System (INIS)
Bogila, Ye.A.; Kolomiets, V.M.; Sanzhur, A.I.; Shlomo, S.
1995-01-01
The spectra of preequilibrium particles, taking into account the energy dependence of the single-particle level density, are calculated using the particle-hole (exciton) level density. We demonstrate the significant effect of the finite depth of the potential well (continuum effect) on partial emission spectra for configurations with a small exciton number
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...
Picosecond dynamics of internal exciton transitions in CdSe nanorods
DEFF Research Database (Denmark)
Cooke, D. G.; Jepsen, Peter Uhd; Lek, Jun Yan
2013-01-01
. The onset of exciton-LO phonon coupling appears as a bleach in the optical conductivity spectra at the LO phonon energy for times > 1 ps after excitation. Simulations show a suppressed exciton temperature due to thermally excited hole states being rapidly captured onto ligands or unpassivated surface states...
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...
Tretiak, Sergei
2014-03-01
The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.
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 spectrall...
Magneto-optical quantum interferences in a system of spinor excitons
Kuan, Wen-Hsuan; Gudmundsson, Vidar
2018-04-01
In this work we investigate magneto-optical properties of two-dimensional semiconductor quantum-ring excitons with Rashba and Dresselhaus spin-orbit interactions threaded by a magnetic flux perpendicular to the plane of the ring. By calculating the excitonic Aharonov-Bohm spectrum, we study the Coulomb and spin-orbit effects on the Aharonov-Bohm features. From the light-matter interactions of the excitons, we find that for scalar excitons, there are open channels for spontaneous recombination resulting in a bright photoluminescence spectrum, whereas the forbidden recombination of dipolar excitons results in a dark photoluminescence spectrum. We investigate the generation of persistent charge and spin currents. The exploration of spin orientations manifests that by adjusting the strength of the spin-orbit interactions, the exciton can be constructed as a squeezed complex with specific spin polarization. Moreover, a coherently moving dipolar exciton acquires a nontrivial dual Aharonov-Casher phase, creating the possibility to generate persistent dipole currents and spin dipole currents. Our study reveals that in the presence of certain spin-orbit generated fields, the manipulation of the magnetic field provides a potential application for quantum-ring spinor excitons to be utilized in nano-scaled magneto-optical switches.
Klimov, Victor I.; McBranch, Duncan W.; Barashkov, Nikolay N.; Ferraris, John P.
1997-10-01
We report femtosecond transient absorption results for solutions and thin films of a substituted oligomer of poly(para-phenylene vinylene) performed over wide spectral and pump-intensity ranges. Solutions and films exhibit a photoinduced absorption (PA) band with dynamics matching those of the stimulated emission, demonstrating unambiguously that these features originate from intrachain singlet excitons. Thin films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the formation of non-emissive interchain excitons. Correlations in the dynamics of the two PA features, as well as the intensity-dependence, provide strong evidence that the formation of interchain excitons is mediated by intrachain two-exciton states.
Coherent Exciton Dynamics in GaAs-Based Semiconductor Structures
Colocci, M.; Bogani, F.; Ceccherini, S.; Gurioli, M.
We show that a very powerful tool in the investigation of the coherent exciton dynamics in semiconductors is provided by the study of the emitted light after resonant excitation from pairs of phase-locked femtosecond pulses. Under these conditions, not only the full dynamics of the coherent transients (dephasing times, quantum beat periods, etc.) can be obtained from linear experiments, but it can also be obtained a straightforward discrimination between the coherent or incoherent character of the emission by means of spectral filtering.
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.
Defect production at exciton decay in ionic crystals
International Nuclear Information System (INIS)
Lushchik, Ch.B.
1984-01-01
On the example of alkali halide crystals experimentally detected phenomenon of structural point defect production in wide-gap nonmetallic solids at low-temperature radiationless decay of self-localizing excitons and recombination of electrons with self-localized holes is considered. Factors promoting radiationless transformation of electron excitations to not small oscillations of many atoms (heat release), but to separate ion large shifts, that determine one of the most important mechanisms of radiation instability of solids, used, in particular, for data recording, are discussed
Trapping effects in exciton motion in the framework of CTRW
International Nuclear Information System (INIS)
Barvik, I.; Herman, P.
1990-10-01
Influence of a trap (sink) on an exciton transfer in molecular aggregates is investigated. Pausing time distribution functions φ p m (t) and probability densities Q mn (t) on the continuous time random walk theory are calculated from memory functions entering the generalized master equations. The presence of the sink changes their analytical form. We used trimmer as example to show that only for large trapping rates pausing time distribution functions become nonnegative. Only in this case they could be used in Monte Carlo modelling. (author). 14 refs, 2 figs
DEFF Research Database (Denmark)
Tavares, Luciana; Cadelano, Michele; Quochi, Francesco
2015-01-01
Multi-layered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials due to their well-defined morphology, high luminescence efficiencies, and color tunability. We resort to temperature-dependent cw and picosecond photoluminescence (PL......) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multi-layered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T), serving as exciton donor and acceptor material, respectively. The high probability for RET processes...... is confirmed by Quantum Chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P...
Correlated lifetimes of free paraexcitons and excitons trapped at oxygen vacancies in cuprous oxide
International Nuclear Information System (INIS)
Koirala, Sandhaya; Naka, Nobuko; Tanaka, Koichiro
2013-01-01
We have studied transients of luminescence due to free excitons and excitons trapped at oxygen vacancies in cuprous oxide. We find that both trapped and free paraexcitons have lifetime dependent on temperature and on the oxygen concentration. By using samples containing much less copper vacancies relative to oxygen vacancies, we find out the direct correlation between the free paraexciton lifetime and trapped exciton lifetime. - Highlights: ► We have investigated trapping of free excitons at oxygen vacancies in cuprous oxide. ► Lifetimes of free and trapped excitons exhibit correlative temperature dependence. ► Four-level model with the activation energy of 33 meV well explains the observation. ► Comparison is made using the four samples with different vacancy concentrations. ► We clarified the crucial role of the oxygen vacancy in shortening the lifetimes.
Excitons in semiconducting quantum filaments of CdS and CdSe with dielectric barriers
Dneprovskij, V S; Shalygina, O A; Lyaskovskij, V L; Mulyarov, E A; Gavrilov, S A; Masumoto, I
2002-01-01
The peculiarities of the luminescence spectra obtained by different polarization and intensity of the pumping excitation and luminescence kinetics of the CdS and CdSe nanocrystals are explained by the exciton transitions in the semiconducting quantum threads with dielectric barriers. The exciton transition energies correspond to the calculated ones with an account of both their dimensional quantization and the effect of the excitons dielectric intensification. It is shown that the excitons transition energies do not change by the change in the quantum threads diameter within the wide range, while the increase in the one-dimensional forbidden zone width of quantum thread by the decrease in its diameter is compensated through the decrease in the excitons binding energy
International Nuclear Information System (INIS)
Hong Sun
1998-11-01
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)
Charge separation in excitonic and bipolar solar cells - A detailed balance approach
International Nuclear Information System (INIS)
Kirchartz, Thomas; Rau, Uwe
2008-01-01
A generalized solar cell model for excitonic and classical, bipolar solar cells is developed that describes the combined transport and interaction of electrons, holes and excitons. Both, conventional inorganic solar cells as well as organic solar cells, where excitons play a dominant role for energy transport, turn out to be special cases of this model. Due to the inclusion of photon recycling effects, the approach is compatible with the principle of detailed balance and the Shockley-Queisser limit. We show how varying the interaction between excitons and charge carriers as well as varying the respective mobilities of the different species changes the operation mode of the solar cell path between excitonic and bipolar
How exciton-vibrational coherences control charge separation in the photosystem II reaction center.
Novoderezhkin, Vladimir I; Romero, Elisabet; van Grondelle, Rienk
2015-12-14
In photosynthesis absorbed sun light produces collective excitations (excitons) that form a coherent superposition of electronic and vibrational states of the individual pigments. Two-dimensional (2D) electronic spectroscopy allows a visualization of how these coherences are involved in the primary processes of energy and charge transfer. Based on quantitative modeling we identify the exciton-vibrational coherences observed in 2D photon echo of the photosystem II reaction center (PSII-RC). We find that the vibrations resonant with the exciton splittings can modify the delocalization of the exciton states and produce additional states, thus promoting directed energy transfer and allowing a switch between the two charge separation pathways. We conclude that the coincidence of the frequencies of the most intense vibrations with the splittings within the manifold of exciton and charge-transfer states in the PSII-RC is not occurring by chance, but reflects a fundamental principle of how energy conversion in photosynthesis was optimized.
Anisotropic Exciton Rabi Oscillation in Single Telecommunication-Band Quantum Dot
Toshiyuki Miyazawa,; Toshihiro Nakaoka,; Katsuyuki Watanabe,; Naoto Kumagai,; Naoki Yokoyama,; Yasuhiko Arakawa,
2010-06-01
Anisotropic Rabi oscillation in the exciton state in a single InAs/GaAs quantum dot (QD) was demonstrated in the telecommunication-band by selecting two orthogonal polarization angles of the excitation laser. Our InAs QDs were embedded in an intrinsic layer of an n-i-Schottky diode, which provides an electric field to extract photoexcited carriers from QDs. Owing to the potential anisotropy of QDs, the fine structure splitting (FSS) energy in the exciton state in single InAs QDs was ˜110 μeV, measured by polarization-resolved photocurrent spectroscopy. The ratio between two different Rabi frequencies, which reflect anisotropic dipole moments of two orthogonal exciton states, was estimated to be ˜1.2. This demonstrates that the selective control of two orthogonal polarized exciton states is a promising technique for exciton-based-quantum information devices compatible with fiber optics.
Intense coherent longitudinal optical phonons in CuI thin films under exciton-excitation conditions
International Nuclear Information System (INIS)
Kojima, O.; Mizoguchi, K.; Nakayama, M..
2005-01-01
We have investigated the dynamical properties of the coherent longitudinal optical (LO) phonon in CuI thin films grown on a NaCl substrate by vacuum deposition. The intense coherent LO phonon in the CuI thin film is observed under the exciton-excitation conditions. Moreover, the pump-energy dependence of the amplitude of the coherent LO phonon shows peaks at the heavy-hole and light-hole exciton energies. The enhancement of the coherent LO phonon under the exciton-resonance condition is much larger than that in an ordinary semiconductor quantum well system such as a GaAs/AlAs one. These facts demonstrate that the intense coherent LO phonon is generated under the exciton-excitation condition in a material with a strong exciton-phonon interaction such as CuI
Exciton fine structure in CdSe nanoclusters
International Nuclear Information System (INIS)
Leung, K.; Pokrant, S.; Whaley, K.B.
1998-01-01
The fine structure in the CdSe nanocrystal absorption spectrum is computed by incorporating two-particle electron-hole interactions and spin-orbit coupling into a tight-binding model, with an expansion in electron-hole single-particle states. The exchange interaction and spin-orbit coupling give rise to dark, low-lying states that are predominantly triplet in character, as well as to a manifold of exciton states that are sensitive to the nanocrystal shape. Near the band gap, the exciton degeneracies are in qualitative agreement with the effective mass approximation (EMA). However, instead of the infinite lifetimes for dark states characteristic of the EMA, we obtain finite radiative lifetimes for the dark states. In particular, for the lowest, predominantly triplet, states we obtain radiative lifetimes of microseconds, in qualitative agreement with the experimental measured lifetimes. The resonant Stokes shifts obtained from the splitting between the lowest dark and bright states are also in good agreement with experimental values for larger crystallites. Higher-lying states exhibit significantly more complex behavior than predicted by EMA, due to extensive mixing of electron-hole pair states. copyright 1998 The American Physical Society
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.
Detailed balance theory of excitonic and bulk heterojunction solar cells
Kirchartz, Thomas; Mattheis, Julian; Rau, Uwe
2008-12-01
A generalized solar cell model for excitonic and classical bipolar solar cells describes the combined transport and interaction of electrons, holes, and excitons in accordance with the principle of detailed balance. Conventional inorganic solar cells, single-phase organic solar cells and bulk heterojunction solar cells, i.e., nanoscale mixtures of two organic materials, are special cases of this model. For high mobilities, the compatibility with the principle of detailed balance ensures that our model reproduces the Shockley-Queisser limit irrespective of how the energy transport is achieved. For less ideal devices distinct differences become visible between devices that are described by linear differential equations and those with nonlinear effects, such as a voltage-dependent collection in bipolar p-i-n -type devices. These differences in current-voltage characteristics are also decisive for the validity of the reciprocity theorem between photovoltaic quantum efficiency and electroluminescent emission. Finally, we discuss the effect of band offset at the heterointerface in a bulk heterojunction cell and the effect of the average distances between these heterointerfaces on the performance of a solar cell in order to show how our detailed balance model includes also these empirically important quantities.
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.
Direct measurement of exciton valley coherence in monolayer WSe2
Hao, Kai
2016-02-29
In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time—a crucial quantity for valley pseudospin manipulation—is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron–hole pairs) in monolayer WSe_{2} (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.
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.
Superposition of the luminescence spectra of free and bound excitons in ZnP2-D48
International Nuclear Information System (INIS)
Stamov, Ion; Nemerenco, Lucretia; Ivanenco, Iurii; Syrbu, Nicolae
2011-01-01
The luminescence spectra of ZnP 2 tetragonal crystals doped Mn, Sn, Cd, Sb at 10 K emission lines of bound excitons is detected. In the spectra non-phonon emission lines of bound and free excitons and their phonon replicas is isolated. The emission lines by the levels of the axial center are described. The composition of the luminescence of free and bound excitons at the axial center is investigated. In the region of phonon replicas of free excitons observed enhancement of lines due to forbidden transitions involving the recombination of excitons. A model of optic recombination transitions of the axial centre is proposed
Ji, Minbiao; Park, Sungnam; Connor, Stephen T.; Mokari, Taleb; Cui, Yi; Gaffney, Kelly J.
2009-01-01
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.
Spin transport dynamics of excitons in CdTe/Cd1-xMnxTe quantum wells
International Nuclear Information System (INIS)
Kayanuma, Kentaro; Shirado, Eiji; Debnath, Mukul C.; Souma, Izuru; Chen, Zhanghai; Oka, Yasuo
2001-01-01
Transport properties of spin-polarized excitons were studied in the double quantum well system composed of Cd 0.95 Mn 0.05 Te and CdTe wells. Circular polarization degrees of the time resolved exciton photoluminescence in magnetic field showed that the spin-polarized excitons diffused from the magnetic quantum well and injected to the non-magnetic quantum well by conserving their spins. The spin-polarized excitons injected into the nonmagnetic well reaches 18% of the nonmagnetic well excitons. From the circular polarization degree and the lifetime of the magnetic quantum well excitons, the spin relaxation time of the excitons in the Cd 0.95 Mn 0.05 Te well was determined as 275 - 10 ps depending on the magnetic field strength. [copyright] 2001 American Institute of Physics
A study of polaritonic transparency in couplers made from excitonic materials
Energy Technology Data Exchange (ETDEWEB)
Singh, Mahi R.; Racknor, Chris [Department of Physics and Astronomy, Western University, London, Ontario N6A 3K7 (Canada)
2015-03-14
We have studied light matter interaction in quantum dot and exciton-polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is also found that transparent states can be switched on and off by applying an external control laser field.
Robust tunable excitonic features in monolayer transition metal dichalcogenide quantum dots
Fouladi-Oskouei, J.; Shojaei, S.; Liu, Z.
2018-04-01
The effects of quantum confinement on excitons in parabolic quantum dots of monolayer transition metal dichalcogenides (TMDC QDs) are investigated within a massive Dirac fermion model. A giant spin-valley coupling of the TMDC QDs is obtained, larger than that of monolayer TMDC sheets and consistent with recent experimental measurements. The exciton transition energy and the binding energy are calculated, and it is found that the strong quantum confinement results in extremely high exciton binding energies. The enormously large exciton binding energy in TMDC QDs (({{E}{{B2D}}}∼ 500 meV)different kinds of TMDC QDs) ensures that the many body interactions play a significant role in the investigation of the optical properties of these novel nanostructures. The estimated oscillator strength and radiative lifetime of excitons are strongly size-dependent and indicate a giant oscillator strength enhancement and ultrafast radiative annihilation of excitons, varying from a few tens of femtoseconds to a few picoseconds. We found that the spin-dependent band gap, spin-valley coupling, binding energy and excitonic effects can be tuned by quantum confinements, leading to tunable quantum dots in monolayer TMDCs. This finding offers new functionality in engineering the interaction of a 2D material with light and creates promise for the quantum manipulation of spin and valley degrees of freedom in TMDC nanostructures, enabling versatile novel 2D quantum photonic and optoelectronic nanodevices.
Creation of free excitons in solid krypton investigated by time-resolved luminescence spectroscopy
International Nuclear Information System (INIS)
Kisand, Vambola; Kirm, Marco; Negodin, Evgeni; Sombrowski, Elke; Steeg, Barbara; Vielhauer, Sebastian; Zimmerer, Georg
2003-01-01
The creation and relaxation of secondary excitons in solid Kr was investigated using energy-and time-resolved luminescence spectroscopy in the vacuum ultraviolet region. The spectrally selected emission of the free exciton (FE) was used as a probe for an investigation of the different exciton creation processes. Delayed FE creation via electron-hole recombination and 'prompt' (in terms of the time-resolution of the experiment) creation of excitons were separated. The 'prompt' creation of a FE appears in the region above threshold energy E th , which is equal to the sum of the band gap energy and the free exciton energy. 'Prompt' creation of excitons above E th is ascribed to a superposition of two processes: (i) creation of the electronic polaron complex (one-step process) and (ii) inelastic scattering of photoelectrons described in the framework of the multiple-parabolic-branch band model (two-step process). In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton (STE) emission was analysed. The behaviour of the ratio spectrum is a proof that electron-hole recombination leads to STE states through FE states as precursors
Effects of excitation spectral width on decay profile of weakly confined excitons
International Nuclear Information System (INIS)
Kojima, O.; Isu, T.; Ishi-Hayase, J.; Kanno, A.; Katouf, R.; Sasaki, M.; Tsuchiya, M.
2008-01-01
We report the effect due to a simultaneous excitation of several exciton states on the radiative decay profiles on the basis of the nonlocal response of weakly confined excitons in GaAs thin films. In the case of excitation of single exciton state, the transient grating signal has two decay components. The fast decay component comes from nonlocal response, and the long-lived component is attributed to free exciton decay. With an increase of excitation spectral width, the nonlocal component becomes small in comparison with the long-lived component, and disappears under irradiation of a femtosecond-pulse laser with broader spectral width. The transient grating spectra clearly indicates the contribution of the weakly confined excitons to the signal, and the exciton line width hardly changes by excitation spectral width. From these results, we concluded that the change of decay profile is attributed not to the many-body effect but to the effect of simultaneous excitation of several exciton states
Large-k exciton dynamics in GaN epilayers: Nonthermal and thermal regimes
Vinattieri, Anna; Bogani, Franco; Cavigli, Lucia; Manzi, Donatella; Gurioli, Massimo; Feltin, Eric; Carlin, Jean-François; Martin, Denis; Butté, Raphaël; Grandjean, Nicolas
2013-02-01
We present a detailed investigation performed at low temperature (T<50 K) concerning the exciton dynamics in GaN epilayers grown on c-plane sapphire substrates, focusing on the exciton formation and the transition from the nonthermal to the thermal regime. The time-resolved kinetics of longitudinal-optical-phonon replicas is used to address the energy relaxation in the excitonic band. From picosecond time-resolved spectra, we bring evidence for a long lasting nonthermal excitonic distribution, which accounts for the first 50 ps. Such a behavior is confirmed in different experimental conditions when both nonresonant and resonant excitations are used. At low excitation power density, the exciton formation and their subsequent thermalization are dominated by impurity scattering rather than by acoustic phonon scattering. The estimate of the average energy of the excitons as a function of delay after the excitation pulse provides information on the relaxation time, which describes the evolution of the exciton population to the thermal regime.
Strong excitonic interactions in the oxygen K-edge of perovskite oxides
Energy Technology Data Exchange (ETDEWEB)
Tomita, Kota; Miyata, Tomohiro [Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan); Olovsson, Weine [Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping (Sweden); Mizoguchi, Teruyasu, E-mail: teru@iis.u-tokyo.ac.jp [Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan)
2017-07-15
Excitonic interactions of the oxygen K-edge electron energy-loss near-edge structure (ELNES) of perovskite oxides, CaTiO{sub 3}, SrTiO{sub 3}, and BaTiO{sub 3}, together with reference oxides, MgO, CaO, SrO, BaO, and TiO{sub 2}, were investigated using a first-principles Bethe–Salpeter equation calculation. Although the transition energy of oxygen K-edge is high, strong excitonic interactions were present in the oxygen K-edge ELNES of the perovskite oxides, whereas the excitonic interactions were negligible in the oxygen K-edge ELNES of the reference compounds. Detailed investigation of the electronic structure suggests that the strong excitonic interaction in the oxygen K-edge ELNES of the perovskite oxides is caused by the directionally confined, low-dimensional electronic structure at the Ti–O–Ti bonds. - Highlights: • Excitonic interaction in oxygen-K edge is investigated. • Strong excitonic interaction is found in the oxygen-K edge of perovskite oxides. • The strong excitonic interaction is ascribed to the low-dimensional and confined electronic structure.
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.
Li, Huanhuan; Bi, Ran; Chen, Ting; Yuan, Kai; Chen, Runfeng; Tao, Ye; Zhang, Hongmei; Zheng, Chao; Huang, Wei
2016-03-23
The concept of limiting the triplet exciton formation to fundamentally alleviate triplet-involved quenching effects is introduced to construct host materials for highly efficient and stable blue phosphorescent organic light-emitting diodes (PhOLEDs). The low triplet exciton formation is realized by small triplet exciton formation fraction and rate with high binding energy and high reorganization energy of triplet exciton. Demonstrated in two analogue molecules in conventional donor-acceptor molecule structure for bipolar charge injection and transport with nearly the same frontier orbital energy levels and triplet excited energies, the new concept host material shows significantly suppressed triplet exciton formation in the host to avoid quenching effects, leading to much improved device efficiencies and stabilities. The low-voltage-driving blue PhOLED devices exhibit maximum efficiencies of 43.7 cd A(-1) for current efficiency, 32.7 lm W(-1) for power efficiency, and 20.7% for external quantum efficiency with low roll-off and remarkable relative quenching effect reduction ratio up to 41%. Our fundamental solution for preventing quenching effects of long-lived triplet excitons provides exciting opportunities for fabricating high-performance devices using the advanced host materials with intrinsically small triplet exciton formation cross section.
Exciton diamagnetic shift and optical properties in CdSe nanocrystal quantum dots in magnetic fields
Wu, Shudong; Cheng, Liwen
2018-04-01
The magnetic field dependence of the optical properties of CdSe nanocrystal quantum dots (NQDs) is investigated theoretically using a perturbation method within the effective-mass approximation. The results show that the magnetic field lifts the degeneracy of the electron (hole) states. A blue-shift in the absorption spectra of m ≥ 0 exciton states is observed while the absorption peak of m attributed to the interplay of the orbital Zeeman effect and the additive confinement induced by the magnetic field. The excitonic absorption coefficient is almost independent of B in the strong confinement regime. The applied magnetic field causes the splitting of degenerated exciton states, resulting in the new absorption peaks. Based on the first-order perturbation theory, we propose the analytical expressions for the exciton binding energy, exciton transition energy and exciton diamagnetic shift of 1s, 1p-1, 1p0, 1p1, 1d-2, 1d-1, 1d0, 1d1, 1d2 and 2s exciton states on the applied magnetic field in the strong confinement regime.
Exciton spectrum of surface-corrugated quantum wells: the adiabatic self-consistent approach
International Nuclear Information System (INIS)
Atenco A, N.; Perez R, F.; Makarov, N.M.
2005-01-01
A theory for calculating the relaxation frequency ν and the shift δ ω of exciton resonances in quantum wells with finite potential barriers and adiabatic surface disorder is developed. The adiabaticity implies that the correlation length R C for the well width fluctuations is much larger than the exciton radius a 0 (R C >> a 0 ). Our theory is based on the self-consistent Green's function method, and therefore takes into account the inherent action of the exciton scattering on itself. The self-consistent approach is shown to describe quantitatively the sharp exciton resonance. It also gives the qualitatively correct resonance picture for the transition to the classical limit, as well as within the domain of the classical limit itself. We present and analyze results for h h-exciton in a GaAs quantum well with Al 0.3 Ga 0.7 As barriers. It is established that the self-consistency and finite height of potential barriers significantly influence on the line-shape of exciton resonances, and make the values of ν and δ ω be quite realistic. In particular, the relaxation frequency ν for the ground-state resonance has a broad, almost symmetric maximum near the resonance frequency ω 0 , while the surface-induced resonance shift δ ω vanishes near ω 0 , and has different signs on the sides of the exciton resonance. (Author) 43 refs., 4 figs
International Nuclear Information System (INIS)
Ahmad, Shahab; Vijaya Prakash, G.; Baumberg, Jeremy J.
2013-01-01
Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C 12 H 25 NH 3 ) 2 PbI 4(1−y) Br 4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices
Ahmad, Shahab; Baumberg, Jeremy J.; Vijaya Prakash, G.
2013-12-01
Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H25NH3)2PbI4(1-y)Br4y (y = 0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510 nm to 350 nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices.
Energy transfer of excitons between quantum wells separated by a wide barrier
International Nuclear Information System (INIS)
Lyo, S. K.
2000-01-01
We present a microscopic theory of the excitonic Stokes and anti-Stokes energy-transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch (Δ) at low temperatures (T). Several important intrinsic energy-transfer mechanisms have been examined, including dipolar coupling, real and virtual photon-exchange coupling, and over-barrier ionization of the excitons via exciton-exciton Auger processes. The transfer rate is calculated as a function of T and the center-to-center distance d between the wells. The rates depend sensitively on T for plane-wave excitons. For localized excitons, the rates depend on T only through the T dependence of the exciton localization radius. For Stokes energy transfer, the dominant energy transfer occurs through a photon-exchange interaction, which enables the excitons from the higher-energy wells to decay into free electrons and holes in the lower-energy wells. The rate has a slow dependence on d, yielding reasonable agreement with recent data from GaAs/Al x Ga 1-x As quantum wells. The dipolar rate is about an order of magnitude smaller for large d (e.g., d=175Aa) with a stronger range dependence proportional to d -4 . However, the latter can be comparable to the radiative rate for small d (e.g., d≤80Aa). For anti-Stokes transfer through exchange-type (e.g., dipolar and photon-exchange) interactions, we show that thermal activation proportional to exp(-Δ/k B T) is essential for the transfer, contradicting a recent nonactivated result based on the Fo''rster-Dexter's spectral-overlap theory. Phonon-assisted transfer yields a negligibly small rate. On the other hand, energy transfer through over-barrier ionization of excitons via Auger processes yields a significantly larger nonactivated rate which is independent of d. The result is compared with recent data
Superior Valley Polarization and Coherence of 2s Excitons in Monolayer WSe_{2}.
Chen, Shao-Yu; Goldstein, Thomas; Tong, Jiayue; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jun
2018-01-26
We report the experimental observation of 2s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2s luminescence is highly robust and persists up to 150 K, offering a new quantum entity for manipulating the valley degree of freedom. Remarkably, the 2s exciton displays superior valley polarization and coherence than 1s under similar experimental conditions. This observation provides evidence that the Coulomb-exchange-interaction-driven valley-depolarization process, the Maialle-Silva-Sham mechanism, plays an important role in valley excitons of monolayer transition metal dichalcogenides.
Superior Valley Polarization and Coherence of 2 s Excitons in Monolayer WSe2
Chen, Shao-Yu; Goldstein, Thomas; Tong, Jiayue; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jun
2018-01-01
We report the experimental observation of 2 s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2 s luminescence is highly robust and persists up to 150 K, offering a new quantum entity for manipulating the valley degree of freedom. Remarkably, the 2 s exciton displays superior valley polarization and coherence than 1 s under similar experimental conditions. This observation provides evidence that the Coulomb-exchange-interaction-driven valley-depolarization process, the Maialle-Silva-Sham mechanism, plays an important role in valley excitons of monolayer transition metal dichalcogenides.
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.
Dynamic Control of Plasmon-Exciton Coupling in Au Nanodisk–J-Aggregate Hybrid Nanostructure Arrays
Zheng, Yue Bing; Juluri, Bala Krishna; Jensen, Linlin; Jensen, Lasse; Huang, Tony Jun
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.
Ginsberg, Naomi
2015-03-01
The migration of Frenkel excitons, tightly-bound electron-hole pairs, in polymeric organic semiconducting films is critical to the efficiency of bulk heterojunction solar cells. While these materials exhibit a high degree of structural heterogeneity on the nanoscale, traditional measurements of exciton diffusion lengths are performed on bulk samples. Since both the characteristic length scales of structural heterogeneity and the reported bulk diffusion lengths are smaller than the optical diffraction limit, we adapt far-field super-resolution fluorescence imaging to uncover the correlations between the structural and energetic landscapes that the excitons explore.
Energy Technology Data Exchange (ETDEWEB)
Slavcheva, G., E-mail: gsk23@bath.ac.uk [Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ (United Kingdom); Kavokin, A.V., E-mail: A.Kavokin@soton.ac.uk [School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ (United Kingdom); Spin Optics Laboratory, St. Petersburg State University, 1, Ulyanovskaya 198504 (Russian Federation)
2014-11-15
Optical pumping of excited exciton states in a semiconductor quantum well embedded in a microcavity is a tool for realisation of ultra-compact terahertz (THz) lasers based on stimulated optical transition between excited (2p) and ground (1s) exciton state. We show that the probability of two-photon absorption by a 2p-exciton is strongly dependent on the polarisation of both pumping photons. Five-fold variation of the threshold power for terahertz lasing by switching from circular to co-linear pumping is predicted. We identify photon polarisation configurations for achieving maximum THz photon generation quantum efficiency.
Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon
International Nuclear Information System (INIS)
Thompson, Nicholas J.; Congreve, Daniel N.; Baldo, Marc A.; Goldberg, David; Menon, Vinod M.
2013-01-01
Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency
Control of quantum interference of an excitonic wave in a chlorophyll chain with a chlorophyll ring
International Nuclear Information System (INIS)
Hong, Suc-Kyoung; Nam, Seog-Woo; Yeon, Kyu-Hwang
2010-01-01
The quantum interference of an excitonic wave and its coherent control in a nanochain with a nanoring are studied. The nanochain is comprised of six chlorophylls, where four chlorophylls compose the nanoring and two chlorophylls are attached at two opposite sites on the nanoring. The exciton dynamics and the correlation of the excitation between chlorophylls are analyzed for a given configurational arrangement and dipolar orientation of the chlorophylls. The results of this study show that the excitation at specified chlorophylls is suppressed or enhanced by destructive or constructive interference of the excitonic wave in the chlorophyll nanochain.
Exciton trapping in interface defects/quantum dots in narrow quantum wells: magnetic-field effects
International Nuclear Information System (INIS)
Barticevic, Z.; Pacheco, M.; Duque, C.A.; Oliveira, L.E.
2003-01-01
The effects of applied magnetic fields on excitons trapped in quantum dots/interface defects in narrow GaAs/Ga 1-x Al x As quantum wells are studied within the effective-mass approximation. The magnetic fields are applied in the growth direction of the quantum wells, and exciton trapping is modeled through a quantum dot formed by monolayer fluctuations in the z-direction, together with lateral confinement via a truncated or infinite parabolic potential in the exciton in-plane coordinate. Theoretical results are found in overall agreement with available experimental measurements
Resonant Rayleigh scattering of exciton-polaritons in multiple quantum wells
DEFF Research Database (Denmark)
Malpuech, Guillaume; Kavokin, Alexey; Langbein, Wolfgang Werner
2000-01-01
A theoretical concept of resonant Rayleigh scattering (RRS) of exciton-polaritons in multiple quantum wells (QWs) is presented. The optical coupling between excitons in different QWs can strongly affect the RRS dynamics, giving rise to characteristic temporal oscillations on a picosecond scale....... Bragg and anti-Bragg arranged QW structures with the same excitonic parameters are predicted to have drastically different RRS spectra. Experimental data on the RRS from multiple QWs show the predicted strong temporal oscillations at small scattering angles, which are well explained by the presented...
International Nuclear Information System (INIS)
Oksengendler, B. L.; Turaeva, N. N.; Uralov, I.; Marasulov, M. B.
2012-01-01
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)
Optical nonlinearity and bistability in the bound exciton energy range of CdS
International Nuclear Information System (INIS)
Hoenig, T.; Gutowski, J.
1988-01-01
Under high excitation conditions thick CdS samples show pronounced broad-band nonlinear transmission in the bound exciton region and up to a wavelength of about 515 nm at cryo-temperatures. This behavior is only explainable in a model based on impurity neutralization and bound exciton creation. The suitability of these nonlinearities to yield optical bistability will be shown. Bistable operation is investigated in dependence of crystal thickness, impurity concentration, excitation density, wavelength, and temperature. A strong correlation to acceptor-bound exciton generation is obtained, and the explanation of this bistable operation fits well with that of the above mentioned transmission behavior. (author)
Deschler, Felix; da Como, Enrico; Limmer, Thomas; Tautz, Raphael; Godde, Tillmann; Bayer, Manfred; von Hauff, Elizabeth; Yilmaz, Seyfullah; Allard, Sybille; Scherf, Ullrich; Feldmann, Jochen
2011-09-01
We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer-fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and results in a favored formation of separated carriers. This is observed by the ultrafast quenching of photoluminescence from charge transfer excitons and the increase in photoinduced polaron density by ˜70%. The results are consistent with a reduced formation of emissive charge transfer excitons, induced by state filling of tail states.
Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon
Energy Technology Data Exchange (ETDEWEB)
Thompson, Nicholas J.; Congreve, Daniel N.; Baldo, Marc A., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Energy Frontier Research Center for Excitonics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Goldberg, David; Menon, Vinod M., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Department of Physics, Queens College and Graduate Center, The City University of New York, Flushing, New York 11367 (United States)
2013-12-23
Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.
Dynamics of indirect exciton transport by moving acoustic fields
International Nuclear Information System (INIS)
Violante, A; Lazić, S; Hey, R; Santos, P V; Cohen, K; Rapaport, R
2014-01-01
We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of μm. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport. (paper)
Multiple exciton generation in quantum dot-based solar cells
Goodwin, Heather; Jellicoe, Tom C.; Davis, Nathaniel J. L. K.; Böhm, Marcus L.
2018-01-01
Multiple exciton generation (MEG) in quantum-confined semiconductors is the process by which multiple bound charge-carrier pairs are generated after absorption of a single high-energy photon. Such charge-carrier multiplication effects have been highlighted as particularly beneficial for solar cells where they have the potential to increase the photocurrent significantly. Indeed, recent research efforts have proved that more than one charge-carrier pair per incident solar photon can be extracted in photovoltaic devices incorporating quantum-confined semiconductors. While these proof-of-concept applications underline the potential of MEG in solar cells, the impact of the carrier multiplication effect on the device performance remains rather low. This review covers recent advancements in the understanding and application of MEG as a photocurrent-enhancing mechanism in quantum dot-based photovoltaics.
Exciton interaction: its possible role in high temperature superconductivity
International Nuclear Information System (INIS)
Little, W.A.
1987-01-01
The recent remarkable developments in superconductivity has forced the group of physicists in the main stream of superconductivity research to re-examine the possible role of what has been referred to in the conference as novel mechanisms of superconductivity. The exciton mechanism is one such. While the many studies and developments in this subject are relatively well known to those involved in studies of organic superconductors and superconductors of reduced dimension, it appears that it is not well known to that large body of physicists involved in the more conventional mainstream of superconductivity. The salient features of the mechanism are reviewed and what it can and cannot do is discussed. Remarks are based on the most recent and most comprehensive review of the subject published in 1979, plus a few key papers since that time
Influence of fluctuating strain on exciton reflection spectra
DEFF Research Database (Denmark)
Skettrup, Torben
1982-01-01
The influence of an internal distribution of strain on the exciton reflection spectra is investigated. The resulting fluctuating optical constants give rise to a fluctuating phase of reflectivity. The standard deviation σ of these phase fluctuations is the quantity which can be observed...... to derive the dependence of the phase of reflectivity on the direction of the fluctuating optical axis. The results obtained for σ are compared with the experimental depolarization spectra of ZnO. The only fitting parameter is the common standard deviation of the strain components. It is found......, for example, between crossed polarizers or from ellipsometric measurements. Assuming the phase fluctuations to obey a Gaussian distribution, σ can be expressed in a simple way in terms of the degree of polarization or the depolarization of the reflected light. σ is then derived in terms of the standard...
Microscopic theory of singlet exciton fission. III. Crystalline pentacene
International Nuclear Information System (INIS)
Berkelbach, Timothy C.; Reichman, David R.; Hybertsen, Mark S.
2014-01-01
We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems
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 homogeneous and inhomogeneous broadening of InGaAs quantum dot luminescence is of central importance for the potential application of this material system in optoelectronic devices. Recent measurements of MOCVD-grown InAs/InGaAs quantum dots indicate a large homogeneous broadening at room temperature due...... to fast dephasing. We present an investigation of the low-temperature homogeneous linewidth of individual PL lines from MBE-grown In0.5Ga0.5As/GaAs quantum dots....
Complete quantum control of exciton qubits bound to isoelectronic centres.
Éthier-Majcher, G; St-Jean, P; Boso, G; Tosi, A; Klem, J F; Francoeur, S
2014-05-30
In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing.
Microscopic theory of singlet exciton fission. III. Crystalline pentacene
Energy Technology Data Exchange (ETDEWEB)
Berkelbach, Timothy C., E-mail: tcb2112@columbia.edu; Reichman, David R., E-mail: drr2103@columbia.edu [Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027 (United States); Hybertsen, Mark S., E-mail: mhyberts@bnl.gov [Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000 (United States)
2014-08-21
We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems.
Vacuum ultraviolet synchrotron measurements of excitons in NaMgF{sub 3}:Yb{sup 2+}
Energy Technology Data Exchange (ETDEWEB)
Hughes-Currie, Rosa B. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Ivanovskikh, Konstantin V. [ANK Service Ltd., PB 58, Novouralsk 624131, Sverdlovsk Region (Russian Federation); Ural Federal University, 19 Mira st., Ekaterinburg 620002 (Russian Federation); Reid, Michael F., E-mail: mike.reid@canterbury.ac.nz [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); MacDiarmid Institute for Advanced Materials and Nanotechnology (New Zealand); Wells, Jon-Paul R. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Dodd-Walls Centre for Quantum and Photonic Technologies (New Zealand); Reeves, Roger J. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); MacDiarmid Institute for Advanced Materials and Nanotechnology (New Zealand); Meijerink, Andries [Debye Institute, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht (Netherlands)
2016-01-15
Results of a vacuum ultraviolet spectroscopic characterization of NaMgF{sub 3}:Yb{sup 2+} are presented. The material demonstrates emission features associated with self-trapped excitons and impurity-trapped excitons. The emission features noticeably overlap giving rise to a broad emission band from 17 000 to 35 000 cm{sup −1} at a sample temperature of 8 K. To identify the true profiles of the emission features we have used a deconvolution procedure. The deconvolution was possible due to the thermal quenching of self-trapped excitons at room temperature that allowed for direct observations of the impurity trapped exciton emission band. Energy transfer between host electronic excitations (excitons and e–h pairs) and Yb{sup 2+} ions leading to the formation of impurity-trapped excitons is evident from excitation spectra. - Highlights: • We present VUV emission and excitation spectra of NaMgF{sub 3}:Yb{sup 2+}. • Formation of free excitons leads to emission from intrinsic and extrinsic excitons. • We deconvolute the emission to separate the two overlapping exciton bands. • The excitation spectra show two mechanisms for forming impurity-trapped excitons.
Chowdhury, Mithun; Sajjad, Muhammad T; Savikhin, Victoria; Hergué, Noémie; Sutija, Karina B; Oosterhout, Stefan D; Toney, Michael F; Dubois, Philippe; Ruseckas, Arvydas; Samuel, Ifor D W
2017-05-17
The influence of various processing conditions on the singlet exciton diffusion is explored in films of a conjugated random copolymer poly-(3-hexylthiophene-co-3-dodecylthiophene) (P3HT-co-P3DDT) and correlated with the degree of crystallinity probed by grazing incidence X-ray scattering and with exciton bandwidth determined from absorption spectra. The exciton diffusion coefficient is deduced from exciton-exciton annihilation measurements and is found to increase by more than a factor of three when thin films are annealed using CS 2 solvent vapour. A doubling of exciton diffusion coefficient is observed upon melt annealing at 200 °C and the corresponding films show about 50% enhancement in the degree of crystallinity. In contrast, films fabricated from polymer solutions containing a small amount of either solvent additive or nucleating agent show a decrease in exciton diffusion coefficient possibly due to formation of traps for excitons. Our results suggest that the enhancement of exciton diffusivity occurs because of increased crystallinity of alkyl-stacking and longer conjugation of aggregated chains which reduces the exciton bandwidth.
Exciton localization and interface roughness in growth-interrupted GaAs/AlAs quantum wells
DEFF Research Database (Denmark)
Leosson, Kristjan; Jensen, Jacob Riis; Langbein, Wolfgang Werner
2000-01-01
of the in-plane disorder potential and the exciton localization length determines the spectral shape of the exciton luminescence. When the correlation length of the in-plane disorder potential is larger than the exciton localization length, the excitonic spectrum splits up into discrete peaks, stemming from...... regions differing in effective thickness by an integral number of monolayers. The energies of monolayers peaks, taking into account the in-plane localization energy, are found to be reproducible in wafers grown under similar conditions. We conclude that atomically smooth growth islands are formed on both...... AlAs and GaAs surfaces after growth interruption. During overgrowth, surface segregation leads to the generation of an atomic-scale disorder in the first overgrown monolayers. This results in an additional in-plane disorder potential with a much shorter correlation length than the original surface...
Ayzner, Alexander L.
2015-12-30
© 2015 American Chemical Society. Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.
Theory of Excitonic Delocalization for Robust Vibronic Dynamics in LH2.
Caycedo-Soler, Felipe; Lim, James; Oviedo-Casado, Santiago; van Hulst, Niek F; Huelga, Susana F; Plenio, Martin B
2018-06-11
Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of a variety of photosynthetic complexes. Different theoretical models that involve the coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom have been put forward to explain these observations. The ensuing debate concerning the relevance of either mechanism may have obscured their complementarity. To illustrate this balance, we quantify how the excitonic delocalization in the LH2 unit of Rhodopseudomonas acidophila purple bacterium leads to correlations of excitonic energy fluctuations, relevant coherent vibronic coupling, and importantly, a decrease in the excitonic dephasing rates. Combining these effects, we identify a feasible origin for the long-lasting oscillations observed in fluorescent traces from time-delayed two-pulse single-molecule experiments performed on this photosynthetic complex and use this approach to discuss the role of this complementarity in other photosynthetic systems.
Ayzner, Alexander L.; Mei, Jianguo; Appleton, Anthony; DeLongchamp, Dean; Nardes, Alexandre; Benight, Stephanie; Kopidakis, Nikos; Toney, Michael F.; Bao, Zhenan
2015-01-01
© 2015 American Chemical Society. Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.
Influence of intra-pigment vibrations on dynamics of photosynthetic exciton.
Sato, Yoshihiro; Doolittle, Brian
2014-11-14
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.
Influence of intra-pigment vibrations on dynamics of photosynthetic exciton
International Nuclear Information System (INIS)
Sato, Yoshihiro; Doolittle, Brian
2014-01-01
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
Influence of intra-pigment vibrations on dynamics of photosynthetic exciton
Energy Technology Data Exchange (ETDEWEB)
Sato, Yoshihiro, E-mail: sato.yoshihiro77@nihon-u.ac.jp, E-mail: ysato.colby@gmail.com; Doolittle, Brian [Department of Physics and Astronomy, Colby College, Waterville, Maine 04901 (United States)
2014-11-14
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.
Effect of interface disorder on quantum well excitons and microcavity polaritons
International Nuclear Information System (INIS)
Savona, Vincenzo
2007-01-01
The theory of the linear optical response of excitons in quantum wells and polaritons in planar semiconductor microcavities is reviewed, in the light of the existing experiments. For quantum well excitons, it is shown that disorder mainly affects the exciton centre-of-mass motion and is modelled by an effective Schroedinger equation in two dimensions. For polaritons, a unified model accounting for quantum well roughness and fluctuations of the microcavity thickness is developed. Numerical results confirm that polaritons are mostly affected by disorder acting on the photon component, thus confirming existing studies on the influence of exciton disorder. The polariton localization length is estimated to be in the few-micrometres range, depending on the amplitude of disorder, in agreement with recent experimental findings
The transition equation of the state intensities for exciton model and the calculation program
International Nuclear Information System (INIS)
Yu Xian; Zheng Jiwen; Liu Guoxing; Chen Keliang
1995-01-01
An equation set of the exciton model is given and calculation program is developed. The process of approaching to equilibrium state has been investigated with the program for 12 C + 64 Ni reaction at energy 72 MeV
Radiationless decay, fission and fusion of excitons in irradiated molecular crystals
International Nuclear Information System (INIS)
Klein, Gerard.
1977-01-01
The creation and evolution of excited states in ionizing particle tracks were investigated. The passage of high energy ionizing particles in molecular crystals results in the formation of highly excited states which energy is generally above the molecular ionization potential. The theory of non radiative transitions, which describes the transitions from the highly excited states to the lowest singlet and triplet excitons S 1 and T 1 is developed. Among these non radiative transitions, the fission of singlet excitons into two singlet or triplet excitons of lower energies is studied experimentally. These results and a kinematics study of the S 1 and T 1 excitons in ionizing particle tracks were used to get a complete description of the scintillation. These results are in good agreement with the experimental measurements on the scintillation [fr
Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites
Yin, Jun; Li, Hong; Cortecchia, Daniele; Soci, Cesare; Bredas, Jean-Luc
2017-01-01
calculations including corrections due to spin orbit couplings and electron hole interactions, a computationally intensive molecular cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level
Radiative and non-radiative relaxation of excitons in strain-compensated quantum dots
International Nuclear Information System (INIS)
Kujiraoka, M.; Ishi-Hayase, J.; Akahane, K.; Yamamoto, Y.; Ema, K.; Sasaki, M.
2008-01-01
We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump-probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2 ns for orthogonally polarized exciton ground states
Temperature-dependent polarized luminescence of exciton polaritons in a ZnO film
Energy Technology Data Exchange (ETDEWEB)
Toropov, A.A.; Nekrutkina, O.V.; Shubina, T.V. [Ioffe Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021 (Russian Federation); Gruber, Th.; Kirchner, C. [Department of Semiconductor Physics, Ulm University, 89081 Ulm (Germany); Waag, A. [Institute of Semiconductor Technology, Braunschweig Technical University, 38106 Braunschweig (Germany); Karlsson, K.F.; Monemar, B. [Linkoeping University, 581 83 Linkoeping (Sweden)
2005-02-01
We report on the studies of linearly polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film, grown by metal organic chemical vapor deposition on a GaN template. The emission of mixed longitudinal-transverse exciton polariton modes was observed up to 130 K that evidences polaritonic nature of the excitonic spectrum up to this elevated temperature. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
On nonlinear dynamics of a dipolar exciton BEC in two-layer graphene
International Nuclear Information System (INIS)
Berman, O.L.; Kezerashvili, R.Ya.; Kolmakov, G.V.
2012-01-01
The nonlinear dynamics of a Bose–Einstein condensate (BEC) of dipolar excitons in two-layer graphene is studied. It is demonstrated that a steady turbulent state is formed in this system. A comparison between the dynamics of the exciton BEC in two-layer graphene and those in GaAs/AlGaAs coupled quantum wells shows that turbulence is a general effect in a BEC.
Skopelitis, Petros; Cherotchenko, Evgenia D.; Kavokin, Alexey V.; Posazhennikova, Anna
2018-03-01
We predict a strong enhancement of the critical temperature in a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor in the presence of a bosonic condensate of exciton polaritons. The effect depends strongly on the ratio of the cutoff frequencies for phonon and exciton-polariton mediated BCS superconductivity, respectively. We also discuss a possible design of hybrid semiconductor-superconductor structures suitable for the experimental observation of such an effect.
Bistable behaviour of biexciton population in a dense exciton-biexciton system in semiconductors
International Nuclear Information System (INIS)
Nguyen Ba An.
1986-05-01
The steady state bistable behaviour of biexciton population in a dense exciton-biexciton semiconductor is considered. The intrinsic optical feedback is provided by the recombination mechanism. The exciton-biexciton and biexciton-biexciton interactions play the role of non-linearity responsible for biexciton bistability to occur. The conditions leading to the effect of bistability are obtained and two-parameter phase transition diagrams are drawn for both intensity and frequency bistable phenomena. (author)
Particle-in-a-box model of exciton absorption and electroabsorption in conjugated polymers
Pedersen, Thomas G.
2000-12-01
The recently proposed particle-in-a-box model of one-dimensional excitons in conjugated polymers is applied in calculations of optical absorption and electroabsorption spectra. It is demonstrated that for polymers of long conjugation length a superposition of single exciton resonances produces a line shape characterized by a square-root singularity in agreement with experimental spectra near the absorption edge. The effects of finite conjugation length on both absorption and electroabsorption spectra are analyzed.
Disturbing the coherent dynamics of an excitonic polarization with strong terahertz fields
Drexler, M. J.; Woscholski, R.; Lippert, S.; Stolz, W.; Rahimi-Iman, A.; Koch, M.
2014-11-01
We present a paper based on combining four-wave mixing and strong fields in the terahertz frequency range to monitor the time evolution of a disturbed excitonic polarization in a multiple quantum well system. Our findings not only confirm a lower field-dependent ionization threshold for higher excitonic states, but furthermore provide experimental evidence for intraexcitonic Rabi flopping in the time domain. These measurements correspond to the picture of a reversible and irreversible transfer as previously predicted by a microscopic theory.
Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals
Bawendi M.G.; Strasfeld D.; Roitblat A.; Sachs H.; Gdor I.; Ruhman S.
2013-01-01
Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times tha...
Effect of the dielectric constant of mesoscopic particle on the exciton binding energy
International Nuclear Information System (INIS)
Lai Zuyou; Gu Shiwei
1991-09-01
For materials with big exciton reduced mass and big dielectric constant, such as TiO 2 , the variation of dielectric constant with the radius of an ultrafine particle (UFP) is important for determining the exciton binding energy. For the first time a phenomenological formula of the dielectric constant of a UFP with its radius in mesoscopic range is put forward in order to explain the optical properties of TiO 2 UFP. (author). 22 refs, 3 figs, 1 tab
Disorder effects on free excitons in CdSsub(1-x)Sesub(x) mixed crystals
International Nuclear Information System (INIS)
Goede, O.; Hennig, D.; John, L.
1979-01-01
In CdSsub(1-x)Sesub(x) mixed crystals the energies and transition probability ratios for free A, B, and C excitons are obtained as a function of the composition x by reflection and emission measurements at 77 K. The observation of the disorder-allowed A(GAMMA 6 ) exciton transition, the bowing of the energy difference between A and C exciton, and the broadening of the exciton lines clearly demonstrate the importance of disorder effects in these mixed crystals. This conclusion is further supported by the stress-induced enhancement of the A(GAMMA 6 ) exciton transition probability by uniaxial stress parallel to the c-axis of the mixed crystals. The experimental results are discussed on the basis of an effective exciton Hamiltonian consisting of a quasi-cubic VCA-Hamiltonian for wurtzite-type mixed crystals and an additional lower-symmetric term which describes the disorder effects phenomenologically by two fluctuating crystal fields parallel and perpendicular to the c-axis, respectively. (author)
Nonlinear spectroscopy of the bound exciton states in CdSe single crystals
International Nuclear Information System (INIS)
Lisitsa, M.P.; Onishchenko, N.A.; Stolyarenko, A.V.; Ananchenko, V.V.; Polishchuk, S.V.
1989-01-01
The study is devoted to the pulsed laser radiation effect on the time-resolved variations of free and bound exciton bands region at the helium temperature. A gradual disappearance of the bound I 2 exciton state is observed with increase of the excitation intensity I in CdSe transmission spectra. This phenomenon is explained by the fact that despite of the shorter life of I 2 excitons as compared to the free ones, the concentration of the centres on which they localize is rather low (≤10 16 cm -3 ) while the evolution of the light-generated electron-hole pairs is such as the most probable recombination through the bound excitons. The transmission spectrum kinetics is studied. The intensity limitation of the laser pulse transmitted through the crystal in the region of the exciton ground state region is shown to be related with two-photon absorption (TPA) in which the exciton state is an intermediate level. The calculation results are in good agreement with the experiment. The estimations show the giant TPA coefficient of ∼10 3 cm/MW. The evolution of photoexcited nonequilibrium electron-hole pairs is studied. The possibility of using CdSe single crystals as spectrum-selective limiters of the laser pulses is shown. (author)
The Dual Role of Disorder on the Dissociation of Interfacial Charge Transfer Excitons
Shi, Liang; Lee, Chee-Kong; Willard, Adam
In organic-based photovoltaics (OPV), dissociation of neutral photo-excitations (i.e., Frenkel excitons) into free charge carriers requires the excitons to overcome binding energy that can significantly exceed thermal energies. The inability of bound charges to overcome this large binding energy has been implicated as a primary source of efficiency loss in OPVs. Despite the potential impact on the performance of organic solar cells much remains to be understood about the microscopic mechanism of exciton dissociation in OPV materials. Here we explore the role of static molecular disorder in mediating this charge dissociation process. Using a simple lattice model of exciton dynamics we demonstrate that random spatial variations in the energetic landscape can mitigate the effects of the exciton binding energy by lowering the free energy barrier. By considering the competition between this thermodynamic effect and the disorder-induced slowing of dissociation kinetics we demonstrate that exciton dissociation yields are expected to depend non-monotonically on the degree of static disorder. We conclude that a certain amount of molecular-scale disorder is desirable in order to optimize the performance of organic photovoltaic materials.
Atomistic theory of excitonic fine structure in InAs/InP nanowire quantum dot molecules
Świderski, M.; Zieliński, M.
2017-03-01
Nanowire quantum dots have peculiar electronic and optical properties. In this work we use atomistic tight binding to study excitonic spectra of artificial molecules formed by a double nanowire quantum dot. We demonstrate a key role of atomistic symmetry and nanowire substrate orientation rather than cylindrical shape symmetry of a nanowire and a molecule. In particular for [001 ] nanowire orientation we observe a nonvanishing bright exciton splitting for a quasimolecule formed by two cylindrical quantum dots of different heights. This effect is due to interdot coupling that effectively reduces the overall symmetry, whereas single uncoupled [001 ] quantum dots have zero fine structure splitting. We found that the same double quantum dot system grown on [111 ] nanowire reveals no excitonic fine structure for all considered quantum dot distances and individual quantum dot heights. Further we demonstrate a pronounced, by several orders of magnitude, increase of the dark exciton optical activity in a quantum dot molecule as compared to a single quantum dot. For [111 ] systems we also show spontaneous localization of single particle states in one of nominally identical quantum dots forming a molecule, which is mediated by strain and origins from the lack of the vertical inversion symmetry in [111 ] nanostructures of overall C3 v symmetry. Finally, we study lowering of symmetry due to alloy randomness that triggers nonzero excitonic fine structure and the dark exciton optical activity in realistic nanowire quantum dot molecules of intermixed composition.
Energy Migration in Organic Thin Films--From Excitons to Polarons
Mullenbach, Tyler K.
The rise of organic photovoltaic devices (OPVs) and organic light-emitting devices has generated interest in the physics governing exciton and polaron dynamics in thin films. Energy transfer has been well studied in dilute solutions, but there are emergent properties in thin films and greater complications due to complex morphologies which must be better understood. Despite the intense interest in energy transport in thin films, experimental limitations have slowed discoveries. Here, a new perspective of OPV operation is presented where photovoltage, instead of photocurrent, plays the fundamental role. By exploiting this new vantage point the first method of measuring the diffusion length (LD) of dark (non-luminescent) excitons is developed, a novel photodetector is invented, and the ability to watch exciton arrival, in real-time, at the donor-acceptor heterojunction is presented. Using an enhanced understanding of exciton migration in thin films, paradigms for enhancing LD by molecular modifications are discovered, and the first exciton gate is experimentally and theoretically demonstrated. Generation of polarons from exciton dissociation represents a second phase of energy migration in OPVs that remains understudied. Current approaches are capable of measuring the rate of charge carrier recombination only at open-circuit. To enable a better understanding of polaron dynamics in thin films, two new approaches are presented which are capable of measuring both the charge carrier recombination and transit rates at any OPV operating voltage. These techniques pave the way for a more complete understanding of charge carrier kinetics in molecular thin films.
A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes
Sahin Tiras, Kevser; Wang, Yifei; Harmon, Nicholas J.; Wohlgenannt, Markus; Flatte, Michael E.
In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.
Excitons in tunnel coupled CdTe and (Cd,Mn)Te quantum wells
Energy Technology Data Exchange (ETDEWEB)
Terletskii, Oleg; Ryabchenko, Sergiy; Tereshchenko, Oleksandr [Institute of Physics NASU, pr. Nauki 46, 03680 Kyiv (Ukraine); Sugakov, Volodymyr; Vertsimakha, Ganna [Institute for Nuclear Research NASU, pr. Nauki 47, 03680 Kyiv (Ukraine); Karczewski, Grzegorz [Institute of Physics PAS, Al. Lotnikow 32/46, PL-02-668 Warsaw (Poland)
2017-05-15
The photoluminescence (PL) from structures containing Cd{sub 0.95}Mn{sub 0.05}Te and CdTe quantum wells (QWs) separated by a narrow (1.94 nm) barrier was studied. The PL lines of comparable intensities from several possible exciton states were observed simultaneously at energy distances substantially exceeding kT. This means that the energy transfer in the studied systems is slower than the radiative recombination of the confined excitons. For the CdTe QW width of about 8.7-9 nm, indirect excitons with the electron and heavy hole chiefly localized in the CdTe and Cd{sub 1-x}Mn{sub x}Te QWs, respectively, were detected in the magnetic field. These indirect excitons have PL energy of about 10-20 meV above the PL line of the direct excitons in the CdTe QW. The observation of the PL from the indirect excitons which are not the lowest excitations in the structure is a distinctive feature of the system. Photoluminescence intensity dependence on the energy and the magnetic field. (copyright 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)
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.
Hamid, Tasnuva; Yambem, Soniya D.; Crawford, Ross; Roberts, Jonathan; Pandey, Ajay K.
2017-08-01
Singlet exciton fission is a process where an excited singlet state splits into two triplets, thus leading to generation of multiple excitons per absorbed photon in organic semiconductors. Herein, we report a detailed exciton management approach for multiexciton harvesting over a broadband region of the solar spectrum in singlet fission sensitized organic photodiodes. Through systematic studies on the model cascade of pentacene/rubrene/C60, we found that efficient photocurrent generation from pentacene can still occur despite the presence of a >10nm thick interlayer of rubrene in between the pentacene/C60 heterojunction. Our results show that thin rubrene interlayers of thickness pentacene despite having a reasonably thick rubrene interlayer, that too with higher triplet energy (T1=1.12 eV) than pentacene (T1= 0.86 eV), makes its operation a rather interesting result. We discuss the role of rubrene interlayer film discontinuity, triplet exciton reflection from rubrene interlayer and triplet energy transfer from rubrene to pentacene layer followed by diffusion of triplet excitons through rubrene as plausible mechanisms that would enable triplet excitons from pentacene to generate significant photocurrent in a multilayer organic heterojunction.
International Nuclear Information System (INIS)
Luo Jian; Lu Di; Du Chaoling; Liu Youwen; Shi Daning; Lai Wei; Guo Chunlei; Gong Shangqing
2012-01-01
We theoretically investigate how to control the Rabi oscillation of excitons of the coupling quantum dots by manipulating static electric fields. Our results show that, for a single-photon process, when direct excitons change into indirect excitons with a bias applied on the sample, the Rabi oscillation rarely alters. However, for the two-photon process, a pronounced enhancement of Rabi oscillation is observed, which can be utilized as the logic gate in quantum information. (paper)
Dzhioev, R. I.; Korenev, V. L.; Lazarev, M. V.; Sapega, V. F.; Gammon, D.; Bracker, A. S.
2007-01-01
We report electric field induced increase of spin orientation of negatively charged excitons (trions) localized in n -type GaAs/AlGaAs quantum well. Under resonant excitation of free neutral heavy-hole excitons, the polarization of trions increases dramatically with electrical injection of electrons. The polarization enhancement correlates strongly with trion/exciton luminescence intensity ratio. This effect results from a very efficient trapping of free neutral excitons by the quantum well interfacial fluctuations (“natural” quantum dots) containing resident electrons.
International Nuclear Information System (INIS)
Senatore, G.; Rapisarda, F.; Conti, S.
1998-01-01
We review recent progress on the physics of electrons in the bilayered electron gas, relevant to coupled quantum wells in GaAs/AIGaAs heterostructures. First we focus on the phase diagram of a symmetric bilayer at T = B = 0, obtained by diffusion Monte Carlo (DMC) simulations. It is found that inter-layer correlations stabilize crystalline structures at intermediate inter-layer separation, while favoring a liquid phase at smaller distance. Also, the available DMC evidence is in contrast with the recently (Hartree-Fock) predicted total charge transfer (TCT), whereby all the electron spontaneously jump in one layer. In fact, one can show that such a TCT state is never stable in the ideal bilayer with no tunneling. We finally comment on ongoing DMC investigations on the electron-hole bilayer, where excitonic condensation is expected to take place. (Copyright (1998) World Scientific Publishing Co. Pte. Ltd)
Femtosecond study of exciton dynamics in 9,9-di-n-hexylfluorene/anthracene random copolymers
International Nuclear Information System (INIS)
Kreger, M. A.; Cherepy, N. J.; Zhang, J. Z.; Scott, J. C.; Klaerner, G.; Miller, R. D.; McBranch, D. W.; Kraabel, B.; Xu, S.
2000-01-01
Exciton dynamics of 9,9-di-n-hexylfluorene/anthracene (DHF/ANT) statistical copolymers have been measured using femtosecond transient absorption spectroscopy. An investigation of the excitation intensity dependence over the range of 0.1-1.0 mJ/(pulse cm2) for solutions and 1.0-17 μJ/(pulse cm2) for thin films has been conducted to explore exciton relaxation mechanisms below excitation densities where exciton-exciton interaction is important. Intrachain relaxation of photoexcited singlet excitons is observed in dilute solutions. In contrast, interchain relaxation mechanisms become predominant in thin films. Decay dynamics are independent of excitation intensity for dilute solutions and thin films of DHF/ANT when probed at 790 and 750 nm. In addition, time-resolved measurements for a DHF homopolymer and two copolymer thin films have been carried out as a function of probe wavelength. A stimulated emission (SE) feature and a photoinduced absorption (PA) feature are observed in the visible region. The SE and PA dynamics are similar for the copolymers, suggesting that the same excited state species, the singlet exciton, is responsible for both the SE and PA. There is a significant difference between the SE and PA dynamics for DHF thin films on the 0-3-ps timescale. The SE dynamics show a pulse-width limited rise and a subsequent decay. In contrast, both the 600 and 750 nm PA dynamics show a ''double'' rise that represents contributions from two separate photophysical processes. These results, in combination with the steady-state photoluminescence spectrum, which indicates excimer emission, lead to the conclusion that interchain species, such as excimers, are formed in <1 ps in DHF homopolymer films following photoexcitation. That the copolymer dynamics show no evidence of excited state species other than the singlet, emissive exciton, is consistent with the interpretation that anthracene substituents in the polymer backbone prevent interchain interactions in films. (c
Exciton spectrum of surface-corrugated quantum wells: the adiabatic self-consistent approach
Energy Technology Data Exchange (ETDEWEB)
Atenco A, N.; Perez R, F. [lnstituto de Fisica, Universidad Autonoma de Puebla, A.P. J-48, 72570 Puebla (Mexico); Makarov, N.M. [lnstituto de Ciencias, Universidad Autonoma de Puebla, Priv. 17 Norte No 3417, Col. San Miguel Hueyotlipan, 72050 Puebla (Mexico)
2005-07-01
A theory for calculating the relaxation frequency {nu} and the shift {delta} {omega} of exciton resonances in quantum wells with finite potential barriers and adiabatic surface disorder is developed. The adiabaticity implies that the correlation length R{sub C} for the well width fluctuations is much larger than the exciton radius a{sub 0} (R{sub C} >> a{sub 0}). Our theory is based on the self-consistent Green's function method, and therefore takes into account the inherent action of the exciton scattering on itself. The self-consistent approach is shown to describe quantitatively the sharp exciton resonance. It also gives the qualitatively correct resonance picture for the transition to the classical limit, as well as within the domain of the classical limit itself. We present and analyze results for h h-exciton in a GaAs quantum well with Al{sub 0.3} Ga{sub 0.7}As barriers. It is established that the self-consistency and finite height of potential barriers significantly influence on the line-shape of exciton resonances, and make the values of {nu} and {delta} {omega} be quite realistic. In particular, the relaxation frequency {nu} for the ground-state resonance has a broad, almost symmetric maximum near the resonance frequency {omega}{sub 0}, while the surface-induced resonance shift {delta} {omega} vanishes near {omega}{sub 0}, and has different signs on the sides of the exciton resonance. (Author) 43 refs., 4 figs.
Spin polarized semimagnetic exciton-polariton condensate in magnetic field.
Król, Mateusz; Mirek, Rafał; Lekenta, Katarzyna; Rousset, Jean-Guy; Stephan, Daniel; Nawrocki, Michał; Matuszewski, Michał; Szczytko, Jacek; Pacuski, Wojciech; Piętka, Barbara
2018-04-27
Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to the quantum wells, and consequently forming semimagnetic polaritons. In this system, increasing magnetic field can induce polariton condensation at constant excitation power. Here we evidence the spin polarization of a semimagnetic polaritons condensate exhibiting a circularly polarized emission over 95% even in a moderate magnetic field of about 3 T. Furthermore, we show that unlike nonmagnetic polaritons, an increase on excitation power results in an increase of the semimagnetic polaritons condensate spin polarization. These properties open new possibilities for testing theoretically predicted phenomena of spin polarized condensate.
Exciton Dynamics of 2D Hybrid Perovskite Nanocrystal
Guo, Rui; Zhu, Zhuan; Boulesbaa, Abdelaziz; Venkatesan, Swaminathan; Xiao, Kai; Bao, Jiming; Yao, Yan; Li, Wenzhi
Organic-inorganic hybrid perovskites have emerged as promising materials for applications in photovoltaic and optoelectronic devices. Among the perovskites, two dimensional (2D) perovskites are of great interests due to their remarkable optical and electrical properties as well as the flexibility of material selection for the organic and inorganic moieties. In this study, we demonstrate the solution-phase growth of large square-shaped single-crystalline 2D hybrid perovskites of (C6H5C2H4 NH3) 2 PbBr4 with a few unit cells thickness. Compared to the bulk crystal, a band gap shift and new photoluminescence (PL) peak are observed from the hybrid perovskite sheets. Color of the 2D crystals can be tuned by adjusting the sheet thickness. Pump-probe spectroscopy is used to investigate the exciton dynamics and exhibits a biexponential decay with an amplitude-weighted lifetime of 16.7 ps. Such high-quality (C6H5C2H4 NH3) 2 PbBr4 sheets are expected to have high PL quantum efficiency which can be adopted for light-emitting devices. National Science Foundation (Grant No. CMMI-1334417 and DMR-1506640).
Implicit and explicit host effects on excitons in pentacene derivatives
Charlton, R. J.; Fogarty, R. M.; Bogatko, S.; Zuehlsdorff, T. J.; Hine, N. D. M.; Heeney, M.; Horsfield, A. P.; Haynes, P. D.
2018-03-01
An ab initio study of the effects of implicit and explicit hosts on the excited state properties of pentacene and its nitrogen-based derivatives has been performed using ground state density functional theory (DFT), time-dependent DFT, and ΔSCF. We observe a significant solvatochromic redshift in the excitation energy of the lowest singlet state (S1) of pentacene from inclusion in a p-terphenyl host compared to vacuum; for an explicit host consisting of six nearest neighbour p-terphenyls, we obtain a redshift of 65 meV while a conductor-like polarisable continuum model (CPCM) yields a 78 meV redshift. Comparison is made between the excitonic properties of pentacene and four of its nitrogen-based analogs, 1,8-, 2,9-, 5,12-, and 6,13-diazapentacene with the latter found to be the most distinct due to local distortions in the ground state electronic structure. We observe that a CPCM is insufficient to fully understand the impact of the host due to the presence of a mild charge-transfer (CT) coupling between the chromophore and neighbouring p-terphenyls, a phenomenon which can only be captured using an explicit model. The strength of this CT interaction increases as the nitrogens are brought closer to the central acene ring of pentacene.
Exciton-polaritons in cuprous oxide: Theory and comparison with experiment
Schweiner, Frank; Ertl, Jan; Main, Jörg; Wunner, Günter; Uihlein, Christoph
2017-12-01
The observation of giant Rydberg excitons in cuprous oxide (Cu2O ) up to a principal quantum number of n =25 by T. Kazimierczuk et al. [Nature (London) 514, 343 (2014), 10.1038/nature13832] inevitably raises the question whether these quasiparticles must be described within a multipolariton framework since excitons and photons are always coupled in the solid. In this paper we present the theory of exciton-polaritons in Cu2O . To this end we extend the Hamiltonian which includes the complete valence-band structure, the exchange interaction, and the central-cell corrections effects, and which has been recently deduced by F. Schweiner et al. [Phys. Rev. B 95, 195201 (2017), 10.1103/PhysRevB.95.195201], for finite values of the exciton momentum ℏ K . We derive formulas to calculate not only dipole but also quadrupole oscillator strengths when using the complete basis of F. Schweiner et al., which has recently been proven as a powerful tool to calculate exciton spectra. Very complex polariton spectra for the three orientations of K along the axes [001 ] , [110 ] , and [111 ] of high symmetry are obtained and a strong mixing of exciton states is reported. The main focus is on the 1 S ortho-exciton-polariton, for which pronounced polariton effects have been measured in experiments. We set up a 5 ×5 matrix model, which accounts for both the polariton effect and the K -dependent splitting, and which allows treating the anisotropic polariton dispersion for any direction of K . We especially discuss the dispersions for K being oriented in the planes perpendicular to [1 1 ¯0 ] and [111 ] , for which experimental transmission spectra have been measured. Furthermore, we compare our results with experimental values of the K -dependent splitting, the group velocity, and the oscillator strengths of this exciton-polariton. The results are in good agreement. This proves the validity of the 5 ×5 matrix model as a useful theoretical model for further investigations on the 1 S
Numerical modeling of exciton-polariton Bose-Einstein condensate in a microcavity
Voronych, Oksana; Buraczewski, Adam; Matuszewski, Michał; Stobińska, Magdalena
2017-06-01
A novel, optimized numerical method of modeling of an exciton-polariton superfluid in a semiconductor microcavity was proposed. Exciton-polaritons are spin-carrying quasiparticles formed from photons strongly coupled to excitons. They possess unique properties, interesting from the point of view of fundamental research as well as numerous potential applications. However, their numerical modeling is challenging due to the structure of nonlinear differential equations describing their evolution. In this paper, we propose to solve the equations with a modified Runge-Kutta method of 4th order, further optimized for efficient computations. The algorithms were implemented in form of C++ programs fitted for parallel environments and utilizing vector instructions. The programs form the EPCGP suite which has been used for theoretical investigation of exciton-polaritons. Catalogue identifier: AFBQ_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFBQ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: BSD-3 No. of lines in distributed program, including test data, etc.: 2157 No. of bytes in distributed program, including test data, etc.: 498994 Distribution format: tar.gz Programming language: C++ with OpenMP extensions (main numerical program), Python (helper scripts). Computer: Modern PC (tested on AMD and Intel processors), HP BL2x220. Operating system: Unix/Linux and Windows. Has the code been vectorized or parallelized?: Yes (OpenMP) RAM: 200 MB for single run Classification: 7, 7.7. Nature of problem: An exciton-polariton superfluid is a novel, interesting physical system allowing investigation of high temperature Bose-Einstein condensation of exciton-polaritons-quasiparticles carrying spin. They have brought a lot of attention due to their unique properties and potential applications in polariton-based optoelectronic integrated circuits. This is an out-of-equilibrium quantum system confined
Halogenation of SiC for band-gap engineering and excitonic functionalization
Drissi, L. B.; Ramadan, F. Z.; Lounis, S.
2017-11-01
The optical excitation spectra and excitonic resonances are investigated in systematically functionalized SiC with Fluorine and/or Chlorine utilizing density functional theory in combination with many-body perturbation theory. The latter is required for a realistic description of the energy band-gaps as well as for the theoretical realization of excitons. Structural, electronic and optical properties are scrutinized and show the high stability of the predicted two-dimensional materials. Their realization in laboratory is thus possible. Large band-gaps of the order of 4 eV are found in the so-called GW approximation, with the occurrence of bright excitons, optically active in the four investigated materials. Their binding energies vary from 0.9 eV to 1.75 eV depending on the decoration choice and in one case, a dark exciton is foreseen to exist in the fully chlorinated SiC. The wide variety of opto-electronic properties suggest halogenated SiC as interesting materials with potential not only for solar cell applications, anti-reflection coatings or high-reflective systems but also for a possible realization of excitonic Bose-Einstein condensation.
Exciton Rydberg series in mono- and few-layer WS2
Chernikov, Alexey; Berkelbach, Timothy C.; Hill, Heather M.; Rigosi, Albert; Li, Yilei; Aslan, Özgur B.; Hybertsen, Mark S.; Reichman, David R.; Heinz, Tony F.
2014-03-01
Considered a long-awaited semiconducting analogue to graphene, the family of atomically thin transition metal dichalcogenides (TMDs) attracted intense interest in the scientific community due to their remarkable physical properties resulting from the reduced dimensionality. A fundamental manifestation of the two-dimensional nature is a strong increase in the Coulomb interaction. The resulting formation of tightly bound excitons plays a crucial role for a majority of optical and transport phenomena. In our work, we investigate the excitons in atomically thin TMDs by optical micro-spectroscopy and apply a microscopic, ab-initio theoretical approach. We observe a full sequence of excited exciton states, i.e., the Rydberg series, in the monolayer WS2, identifying tightly bound excitons with energies exceeding 0.3 eV - almost an order of magnitude higher than in the corresponding, three-dimensional crystal. We also find significant deviations of the excitonic properties from the conventional hydrogenic physics - a direct evidence of a non-uniform dielectric environment. Finally, an excellent quantitative agreement is obtained between the experimental findings and the developed theoretical approach.
Splitting of the excitonic peak in quantum wells with interfacial roughness
International Nuclear Information System (INIS)
Castella, H.; Wilkins, J.W.
1998-01-01
Excitons in a quantum well depend on the interfacial roughness resulting from its growth. The interface is characterized by islands of size ξ separated by one monolayer steps across which the confining potential decreases by V 0 for wider wells. A natural length is the localization length ξ 0 =πℎ/√ (2MV 0 ) characterizing the minimum size island to confine an exciton. For small islands (ξ 0 ), the absorption spectrum has a single exciton peak. As the island size ξ exceeds the localization length ξ 0 , the peak gradually splits into a doublet. Generally the spectra exhibit the following features: (1) the shape is very sensitive to ξ/ξ 0 and depends only weakly on the ratio of island size to exciton radius; (2) in the small island regime ξ 0 , the asymmetric shape of the exciton peak is correctly described by a model of white-noise potential, except for the position of the peak which still depends on the correlation length of the disorder. copyright 1998 The American Physical Society
Impact of the glass transition on exciton dynamics in polymer thin films
Ehrenreich, Philipp; Proepper, Daniel; Graf, Alexander; Jores, Stefan; Boris, Alexander V.; Schmidt-Mende, Lukas
2017-11-01
In the development of organic electronics, unlimited design possibilities of conjugated polymers offer a wide variety of mechanical and electronic properties. Thereby, it is crucially important to reveal universal physical characteristics that allow efficient and forward developments of new chemical compounds. In particular for organic solar cells, a deeper understanding of exciton dynamics in polymer films can help to improve the charge generation process further. For this purpose, poly(3-hexylthiophene) (P3HT) is commonly used as a model system, although exciton decay kinetics have found different interpretations. Using temperature-dependent time-resolved photoluminescence spectroscopy in combination with low-temperature spectroscopic ellipsometry, we can show that P3HT is indeed a model system in which excitons follow a simple diffusion/hopping model. Based on our results we can exclude the relevance of hot-exciton emission as well as a dynamic torsional relaxation upon photoexcitation on a ps time scale. Instead, we depict the glass transition temperature of polymers to strongly affect exciton dynamics.
Excitonic spectrum of the ZnO/ZnMgO quantum wells
Energy Technology Data Exchange (ETDEWEB)
Bobrov, M. A., E-mail: largaseal@gmail.com; Toropov, A. A.; Ivanov, S. V. [Russian Academy of Sciences, Ioffe Physical Technical Institute (Russian Federation); El-Shaer, A.; Bakin, A.; Waag, A. [TU Braunschweig, Institute of Semiconductor Technology (Germany)
2011-06-15
Excitonic spectrum of the wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schroedinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.
Well separated trion and neutral excitons on superacid treated MoS{sub 2} monolayers
Energy Technology Data Exchange (ETDEWEB)
Cadiz, Fabian, E-mail: cadiz@insa-toulouse.fr; Tricard, Simon; Gay, Maxime; Lagarde, Delphine; Wang, Gang; Robert, Cedric; Renucci, Pierre; Urbaszek, Bernhard; Marie, Xavier [Université de Toulouse, INSA-CNRS-UPS, LPCNO, 135 Av. Rangueil, 31077 Toulouse (France)
2016-06-20
Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS{sub 2} MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS{sub 2} MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.
Two exciton states in discrete and continuum alpha-helical proteins
International Nuclear Information System (INIS)
Latha, M.M.; Merlin, G.
2012-01-01
The dynamics of alpha-helical proteins is described by proposing a model Hamiltonian representing two exciton bound states. The dynamics is studied by constructing the equations of motion using a two exciton eigen-function in the discrete level. A numerical analysis shows the existence of two excitons in alpha-helical proteins and its propagation as solitons along the hydrogen bonding spines. The lattice model is also treated in the continuum limit which is a valid approximation in the low temperature, long wavelength limit. The resulting equation is studied using the multiple scale perturbation analysis which also shows the transfer of two exciton energy through alpha-helical proteins in the form of solitons with no change in velocity and amplitude. -- Highlights: ► The dynamics of alpha-helical proteins with two exciton states is studied. ► The dynamics is studied both in the discrete and continuum levels. ► The resulting equations are solved numerically and analytically. ► The solution supports the propagation of the energy in the form of solitons.
Exciton shelves for charge and energy transport in third-generation quantum-dot devices
Goodman, Samuel; Singh, Vivek; Noh, Hyunwoo; Casamada, Josep; Chatterjee, Anushree; Cha, Jennifer; Nagpal, Prashant
2014-03-01
Quantum dots are semiconductor nanocrystallites with size-dependent quantum-confined energy levels. While they have been intensively investigated to utilize hot-carriers for photovoltaic applications, to bridge the mismatch between incident solar photons and finite bandgap of semiconductor photocells, efficient charge or exciton transport in quantum-dot films has proven challenging. Here we show development of new coupled conjugated molecular wires with ``exciton shelves'', or different energy levels, matched with the multiple energy levels of quantum dots. Using single nanoparticle and ensemble device measurements we show successful extraction and transport of both bandedge and high-energy charge carriers, and energy transport of excitons. We demonstrate using measurements of electronic density of states, that careful matching of energy states of quantum-dot with molecular wires is important, and any mismatch can generate midgap states leading to charge recombination and reduced efficiency. Therefore, these exciton-shelves and quantum dots can lead to development of next-generation photovoltaic and photodetection devices using simultaneous transport of bandedge and hot-carriers or energy transport of excitons in these nanostructured solution-processed films.
Shi, Jinwei; Lin, Meng-Hsien; Chen, Yi-Tong; Estakhri, Nasim Mohammadi; Tseng, Guo-Wei; Wang, Yanrong; Chen, Hung-Ying; Chen, Chun-An; Shih, Chih-Kang; Alã¹, Andrea; Li, Xiaoqin; Lee, Yi-Hsien; Gwo, Shangjr
Recently, two-dimensional (2D) semiconductor heterostructures, i.e., atomically thin lateral heterostructures (LHSs) based on transition metal dichalcogenides (TMDs) have been demonstrated. In an optically excited LHS, exciton transport is typically limited to a rather short spatial range ( 1 micron). Furthermore, additional losses may occur at the lateral interfacial regions. Here, to overcome these challenges, we experimentally implement a planar metal-oxide-semiconductor (MOS) structure by placing a monolayer of WS2/MoS2 LHS on top of an Al2O3 capped Ag single-crystalline plate. We found that the exciton transport range can be extended to tens of microns. The process of long-range exciton transport in the MOS structure is confirmed to be mediated by an exciton-surface plasmon polariton-exciton conversion mechanism, which allows a cascaded energy transfer process. Thus, the planar MOS structure provides a platform seamlessly combining 2D light-emitting materials with plasmonic planar waveguides, offering great potential for developing integrated photonic/plasmonic functionalities.
Accessing the dark exciton spin in deterministic quantum-dot microlenses
Heindel, Tobias; Thoma, Alexander; Schwartz, Ido; Schmidgall, Emma R.; Gantz, Liron; Cogan, Dan; Strauß, Max; Schnauber, Peter; Gschrey, Manuel; Schulze, Jan-Hindrik; Strittmatter, Andre; Rodt, Sven; Gershoni, David; Reitzenstein, Stephan
2017-12-01
The dark exciton state in semiconductor quantum dots (QDs) constitutes a long-lived solid-state qubit which has the potential to play an important role in implementations of solid-state-based quantum information architectures. In this work, we exploit deterministically fabricated QD microlenses which promise enhanced photon extraction, to optically prepare and read out the dark exciton spin and observe its coherent precession. The optical access to the dark exciton is provided via spin-blockaded metastable biexciton states acting as heralding states, which are identified by deploying polarization-sensitive spectroscopy as well as time-resolved photon cross-correlation experiments. Our experiments reveal a spin-precession period of the dark exciton of (0.82 ± 0.01) ns corresponding to a fine-structure splitting of (5.0 ± 0.7) μeV between its eigenstates |↑ ⇑ ±↓ ⇓ ⟩. By exploiting microlenses deterministically fabricated above pre-selected QDs, our work demonstrates the possibility to scale up implementations of quantum information processing schemes using the QD-confined dark exciton spin qubit, such as the generation of photonic cluster states or the realization of a solid-state-based quantum memory.
Exciton and biexciton signatures in femtosecond transient absorption of π-conjugated oligomers
Klimov, Victor I.; McBranch, Duncan W.; Barashkov, Nikolay N.; Ferraris, John P.
1997-12-01
We report femtosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photochemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long- wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, we see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. We attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.
Exciton and biexciton signatures in femotosecond transient absorption of {pi}-conjugated oligomers
Energy Technology Data Exchange (ETDEWEB)
Klimov, V.; McBranch, D. [Los Alamos National Lab., NM (United States); Barashkov, N.; Ferraris, J. [Univ. of Texas, Dallas, TX (United States)
1997-10-01
The authors report femotosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene (PPV) prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photo-chemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long-wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, the authors see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. They attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.
Wang, Zi Shuai; Sha, Wei E. I.; Choy, Wallace C. H.
2016-12-01
Modeling the charge-generation process is highly important to understand device physics and optimize power conversion efficiency of bulk-heterojunction organic solar cells (OSCs). Free carriers are generated by both ultrafast exciton delocalization and slow exciton diffusion and dissociation at the heterojunction interface. In this work, we developed a systematic numerical simulation to describe the charge-generation process by a modified drift-diffusion model. The transport, recombination, and collection of free carriers are incorporated to fully capture the device response. The theoretical results match well with the state-of-the-art high-performance organic solar cells. It is demonstrated that the increase of exciton delocalization ratio reduces the energy loss in the exciton diffusion-dissociation process, and thus, significantly improves the device efficiency, especially for the short-circuit current. By changing the exciton delocalization ratio, OSC performances are comprehensively investigated under the conditions of short-circuit and open-circuit. Particularly, bulk recombination dependent fill factor saturation is unveiled and understood. As a fundamental electrical analysis of the delocalization mechanism, our work is important to understand and optimize the high-performance OSCs.
International Nuclear Information System (INIS)
Lu, Yan; Wei, Sheng; Jin, Jing; Wang, Li; Lu, Wengang
2016-01-01
We explore the electronic properties and exciton effects in short graphene nanoribbons (SGNRs), which have two armchair edges and two zigzag edges. Our results show that both of these two types of edges have profound effects on the electronic properties and exciton effects. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states are alternatively changed between the bulk and the edge states as the lengths of the zigzag edges increase, due to the competition between the states of the two types of edges. The energy gaps, as a function of the lengths of the armchair edges, will then induce two kinds of trends. Furthermore, two kinds of exciton energies and exciton binding energies are found, which can be understood through the two kinds of HOMO and LUMO states in SGNRs. In addition, we find that the three triplet exciton states are not totally energy degenerate in SGNRs due to the spin-polarized states on the zigzag edges. (paper)
Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities
Graf, Arko; Tropf, Laura; Zakharko, Yuriy; Zaumseil, Jana; Gather, Malte C.
2016-10-01
Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix and a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (>110 meV), efficient polariton relaxation and narrow band emission (<15 meV). Given their high charge carrier mobility and excellent photostability, SWCNTs represent a promising new avenue towards practical exciton-polariton devices operating at telecommunication wavelengths.
Exciton binding energy in wurtzite InGaN/GaN quantum wells
International Nuclear Information System (INIS)
Park, Seoung-Hwan; Kim, Jong-Jae; Kim, Hwa-Min
2004-01-01
The internal field and carrier density effects on the exciton binding energies in wurtzite (WZ) InGaN/GaN quantum-well (QW) structures are investigated using the multiband effective-mass theory, and are compared with those obtained from the at-band model and with those of GaN/AlGaN QW structures. The exciton binding energy is significantly reduced with increasing sheet carrier density, suggesting that excitons are nearly bleached at densities around 10 12 cm -2 for both InGaN/GaN and GaN/AlGaN QW structures. With the inclusion of the internal field, the exciton binding energy is substantialy reduced compared to that of the at-band model in the investigated region of the wells. This can be explained by a decrease in the momentum matrix element and an increase in the inverse screening length due to the internal field. The exciton binding energy of the InGaN/GaN structure is smaller than that of the GaN/AlGaN structure because InGaN/GaN structures have a smaller momentum matrix element and a larger inverse screening length than GaN/AlGaN structures.
The confinement effect in spherical inhomogeneous quantum dots and stability of excitons
Directory of Open Access Journals (Sweden)
F. Benhaddou
2017-06-01
Full Text Available We investigate in this work the quantum confinement effect of exciton in spherical inhomogeneous quantum dots IQDs. The spherical core is enveloped by two shells. The inner shell is a semiconductor characterized by a small band-gap. The core and the outer shell are the same semiconductor characterized by a large band-gap. So there is a significant gap-offset creating a deep potential well where the excitons are localized and strongly confined. We have adopted the Ritz variational method to calculate numerically the excitonic ground state energy and its binding energy in the strong, moderate and low confinement regimes. The results show that the Ritz variational method is in good agreement with the perturbation method in strong confinement. There is a double confinement effect and dual control. The calculation checks the effective Rydberg R* at the asymptotic limit of bulk semiconductor when the thickness takes very large values. The excitonic binding energy increases, Thus giving the excitons a high stability even at ambient temperature. These nanosystems are promising in several applications: lighting, detection, biological labeling and quantum computing.
Ground-state energy of an exciton-(LO) phonon system in a parabolic quantum well
Gerlach, B.; Wüsthoff, J.; Smondyrev, M. A.
1999-12-01
This paper presents a variational study of the ground-state energy of an exciton-(LO) phonon system, which is spatially confined to a quantum well. The exciton-phonon interaction is of Fröhlich type, the confinement potentials are assumed to be parabolic functions of the coordinates. Making use of functional integral techniques, the phonon part of the problem can be eliminated exactly, leading us to an effective two-particle system, which has the same spectral properties as the original one. Subsequently, Jensen's inequality is applied to obtain an upper bound on the ground-state energy. The main intention of this paper is to analyze the influence of the quantum-well-induced localization of the exciton on its ground-state energy (or its binding energy, respectively). To do so, we neglect any mismatch of the masses or the dielectric constants, but admit an arbitrary strength of the confinement potentials. Our approach allows for a smooth interpolation of the ultimate limits of vanishing and infinite confinement, corresponding to the cases of a free three-dimensional and a free two-dimensional exciton-phonon system. The interpolation formula for the ground-state energy bound corresponds to similar formulas for the free polaron or the free exciton-phonon system. These bounds in turn are known to compare favorably with all previous ones, which we are aware of.
Yi, Yuanping; Coropceanu, Veaceslav; Brédas, Jean-Luc
2009-01-01
The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C60 heterojunctions are investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation
Controlling excitons. Concepts for phosphorescent organic LEDs at high brightness
Energy Technology Data Exchange (ETDEWEB)
Reineke, Sebastian
2009-11-15
This work focusses on the high brightness performance of phosphorescent organic light-emitting diodes (OLEDs). The use of phosphorescent emitter molecules in OLEDs is essential to realize internal electron-photon conversion efficiencies of 100 %. However, due to their molecular nature, the excited triplet states have orders of magnitude longer time constants compared to their fluorescent counterparts which, in turn, strongly increases the probability of bimolecular annihilation. As a consequence, the efficiencies of phosphorescent OLEDs decline at high brightness - an effect known as efficiency roll-off, for which it has been shown to be dominated by triplet-triplet annihilation (TTA). In this work, TTA of the archetype phosphorescent emitter Ir(ppy){sub 3} is investigated in time-resolved photoluminescence experiments. For the widely used mixed system CBP:Ir(ppy){sub 3}, host-guest TTA - an additional unwanted TTA channel - is experimentally observed at high excitation levels. By using matrix materials with higher triplet energies, this effect is efficiently suppressed, however further studies show that the efficiency roll-off of Ir(ppy)3 is much more pronounced than predicted by a model based on Foerster-type energy transfer, which marks the intrinsic limit for TTA. These results suggest that the emitter molecules show a strong tendency to form aggregates in the mixed film as the origin for enhanced TTA. Transmission electron microscopy images of Ir(ppy){sub 3} doped mixed films give direct proof of emitter aggregates. Based on these results, two concepts are developed that improve the high brightness performance of OLEDs. In a first approach, thin intrinsic matrix interlayers are incorporated in the emission layer leading to a one-dimensional exciton confinement that suppresses exciton migration and, consequently, TTA. The second concept reduces the efficiency roll-off by using an emitter molecule with slightly different chemical structure, i.e. Ir(ppy){sub 2
Coherent dynamics of exciton and biexciton resonances in InGaAs/GaAs single quantum wells
DEFF Research Database (Denmark)
Borri, Paola; Langbein, Wolfgang; Hvam, Jørn Märcher
1999-01-01
The coherent dynamics of both exciton and biexciton resonances have been investigated in In0.18Ga0.82As/GaAs single quantum wells with thicknesses ranging from 1 to 4 nm, using time-integrated and spectrally-resolved transient four-wave mixing. From the temperature dependence of the exciton...
DEFF Research Database (Denmark)
Birkedal, Dan; Vadim, Lyssenko; Pantke, Karl-Heinz
1995-01-01
The interface roughness on a nanometer scale plays a decisive role in dephasing of excitons in GaAs multiple quantum wells. The excitonic four-wave mixing signal shows a free polarization decay and a corresponding homogeneously broadened line from areas with interface roughness on a scale larger...
Binding Energy and Lifetime of Excitons in InxGa1-xAs/GaAs Quantum Wells
DEFF Research Database (Denmark)
Orani, D.; Polimeni, A.; Patane, A.
1997-01-01
We report a systematic study of exciton binding energies and lifetimes in InGaAs/GaAs quantum wells. The experimental binding energies have been deduced from photoluminescence excitation measurements taking into account the contribution of the 2s state of the exciton and the line broadening...
Watanabe, Sadayuki; Furube, Akihiro; Katoh, Ryuzi
2006-08-31
We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.
International Nuclear Information System (INIS)
Nguyen Toan Thang; Nguyen Ai Viet; Nguyen Hong Quang
1987-06-01
Decay probabilities of light and heavy excitons interacting with acoustic phonons in cubic semiconductors with a degenerate valence band are calculated. The numerical results for GaAs showed that the decay probability of the light exciton is much greater than that of the heavy one. (author). 10 refs, 1 fig
Optical spectroscopy and imaging of the higher energy excitons and bandgap of monolayer MoS2
Borys, Nicholas; Bao, Wei; Barnard, Edward; Ko, Changhyun; Tongay, Sefaatin; Wu, Junqiao; Yang, Li; Schuck, P. James
Monolayer MoS2 (ML-MoS2) exhibits a rich manifold of excitons that dictate optoelectronic performance and functionality. Disentangling these states, which include the quasi-particle bandgap, is critical for developing 2D optoelectronic devices that operate beyond the optical bandgap. Whereas photoluminescence (PL) spectroscopy only probes the lowest-energy radiative state and absorption spectroscopy fails to discriminate energetically degenerate states, photoluminescence excitation (PLE) spectroscopy selectively probes only the excited states that thermalize to the emissive ground state exciton. Using PLE spectroscopy of ML-MoS2, we identify the Rydberg series of the exciton A and exciton B states as well as signatures of the quasi-particle bandgap and coupling between the indirect C exciton and the lowest-energy A exciton, which have eluded previous PLE studies. The assignment of these states is confirmed with density functional theory. Mapping the PLE spectrum reveals spatial variations of the higher-energy exciton manifold and quasi-particle bandgap which mirror the heterogeneity in the PL but also indicate variations in local exciton thermalization processes and chemical potentials.
Semenova, L. E.
2018-04-01
The treatment of the two-photon transitions to the An=1 exciton level and the resonant Raman scattering of light by LO-phonons is given for the hexagonal semiconductors A2B6, taking into account the influence of the complex top valence band and anisotropy of the exciton effective mass.
... to produce gas. Often, relatively simple changes in eating habits can lessen bothersome gas. Certain digestive system disorders, ... such as soda and beer, increase stomach gas. Eating habits, such as eating too quickly, drinking through a ...
Specific features of the temperature dependence of the exciton absorption integral in CdS crystals
International Nuclear Information System (INIS)
Novikov, A.B.; Solov'ev, L.E.; Talalaev, V.G.
1986-01-01
Cadmium sulfide crystals 0.4-2 μm thick in the 4.2-120 K temperature range are investigated experimentally. The shape of the first exciton absorption line in CdS and dependence of integral exciton absorption factor (IEAF) on the quenching constant j are calculated. Rapid growth of the absorption factor in the maximum of the absorption line and decrease of halfwidth of the factor are shown to take place with j increase. The calculation has disclosed that the Bouguer law is observed excluding negligible IEAF oscillations at variation of crystal thickness. Non-monotonous temperature dependence of IEAF is disclosed in some investigated samples; it, obviously, testifies to non-monotonous temperature dependence of j. Depolarization of the absorption line of high-energy exciton states with n=2 and n=3 is discovered in some samples for the first time
Self-localization of excitons in a periodically modulated molecular medium
International Nuclear Information System (INIS)
Zabolotskii, A. A.
2006-01-01
Electromagnetic field propagation is analyzed in a one-dimensional Bragg grating consisting of periodically arranged linear molecules making up a resonant medium. Dye J-aggregates and conjugated polymers are considered as examples of the medium. Both adiabatic and nonadiabatic dynamics of the acoustic waves generated by electromagnetic field in the system are examined. The effects of exciton-phonon and exciton-phonon-photon interactions on the band structure and formation of self-localized excitations are examined on various time scales. A new mechanism for controlling bandgap parameters in a bistable regime is described. Some effects of electromagnetic-field nonuniformity on generation of phonons in molecules and exciton self-localization are investigated
Identification of excitons, trions and biexcitons in single-layer WS{sub 2}
Energy Technology Data Exchange (ETDEWEB)
Plechinger, Gerd; Nagler, Philipp; Kraus, Julia; Paradiso, Nicola; Strunk, Christoph; Schueller, Christian; Korn, Tobias [Institut fuer Experimentelle und Angewandte Physik, Universitaet Regensburg, 93040, Regensburg (Germany)
2015-08-15
Single-layer WS{sub 2} is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS{sub 2}, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion dominates the photoluminescence, which we identify as biexciton emission. (copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)
Excitons and Cooper pairs two composite bosons in many-body physics
Combescot, Monique
2015-01-01
This book bridges a gap between two major communities of Condensed Matter Physics, Semiconductors and Superconductors, that have thrived independently. Through an original perspective that their key particles, excitons and Cooper pairs, are composite bosons, the authors raise fundamental questions of current interest: how does the Pauli exclusion principle wield its power on the fermionic components of bosonic particles at a microscopic level and how this affects the macroscopic physics? What can we learn from Wannier and Frenkel excitons and from Cooper pairs that helps us understand "bosonic condensation" of composite bosons and its difference from Bose-Einstein condensation of elementary bosons? The authors start from solid mathematical and physical foundation to derive excitons and Cooper pairs. They further introduce Shiva diagrams as a graphic support to grasp the many-body physics induced by fermion exchange - a novel mechanism not visualized by standard Feynman diagrams. Advanced undergraduate or grad...
Fine structure of spectra of a bound exciton in tetragonal zinc diphosphide
International Nuclear Information System (INIS)
Syrbu, N.N.; Morozova, V.I.; Stratan, G.I.
1989-01-01
Investigation into the low-temperature luminescence spectra recorded in different crystal geometry relative to the direction of incident radiation wave vector, has demonstrated the existence of saddle-shaped valent zone ceiling near k=0 in zinc tetragonal diphosphide monocrystals. Binding energies of free (2.2085 eV) and bound (A(2.1943eV)B(2.1765eV), C(2.1447eV)) excitons as well as the phonon energy value are determined by investigations into absorption spectrum and radiative recombination. Phonon-free lines of bound A 0 and C 0 excitons are splitted into 2.2 and 0.3 MeV respectively. The band exciton singlet-triplet state splittings in a magnetic field and their field dependences are obtained. The spectroscopic splitting factor g=1.9 is determined
Decay dynamics of neutral and charged excitonic complexes in single InAs/GaAs QDs
International Nuclear Information System (INIS)
Feucker, Max; Seguin, Robert; Rodt, Sven; Poetschke, Konstantin; Bimberg, Dieter
2008-01-01
Across the inhomogeneously broadened lineshape of a quantum dot (QD) ensemble the decay times are expected to vary since the wavefunctions and the oscillator strengths are sensitive to the actual geometry of the QD. We performed time-resolved cathodoluminescence spectroscopy of 26 different single InAs/GaAs QDs to investigate the decay dynamics of neutral and charged excitonic complexes. The largest decay rate was found for the XX + , followed by XX, X + and finally the X. We will show that the ratios of lifetimes of the different excitonic complexes are mainly governed by the number of involved recombination channels. There is excellent agreement between the measured and predicted values for the lifetime ratios of the neutral (X/XX) and the positively charged (X + /XX + ) complexes. Surprisingly the lifetime of the exciton (X) shows a much larger yet unexplained scatter than that of all the other complexes
Microscopic description of exciton polaritons in direct two-band semiconductors
Nguyen, Van Trong; Mahler, Günter
1999-07-01
Based on a quantum electrodynamical formulation, a microscopic description of exciton polaritons in a two-band semiconductor is presented. We show that the interband exchange Coulomb interaction, responsible for the coupling of the exciton with the longitudinal part of the induced field, should be treated on equal footing together with the coupling to the transverse part of the induced field (the photon field). The constitutive relation is established to connect the current density with the total electric field of polaritons. The classical Maxwell equations are derived from the quantum representation of photons to get a closed system of equations. The temporal evolution for an initial excited exciton state is studied in detail and an anisotropic polariton vacuum Rabi splitting is shown to occur. A number of up-to-now unresolved discrepancies in the literature are clarified.
Strong exciton-photon coupling in organic single crystal microcavity with high molecular orientation
Energy Technology Data Exchange (ETDEWEB)
Goto, Kaname [Department of Electronics, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan); Yamashita, Kenichi, E-mail: yamasita@kit.ac.jp [Faculty of Electrical Engineering and Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan); Yanagi, Hisao [Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan); Yamao, Takeshi; Hotta, Shu [Faculty of Materials Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
2016-08-08
Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ∼100 meV even in the “half-vertical cavity surface emitting lasing” microcavity structure.
Strong exciton-photon coupling in organic single crystal microcavity with high molecular orientation
Goto, Kaname; Yamashita, Kenichi; Yanagi, Hisao; Yamao, Takeshi; Hotta, Shu
2016-08-01
Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ˜100 meV even in the "half-vertical cavity surface emitting lasing" microcavity structure.
Strong exciton-photon coupling in organic single crystal microcavity with high molecular orientation
International Nuclear Information System (INIS)
Goto, Kaname; Yamashita, Kenichi; Yanagi, Hisao; Yamao, Takeshi; Hotta, Shu
2016-01-01
Strong exciton-photon coupling has been observed in a highly oriented organic single crystal microcavity. This microcavity consists of a thiophene/phenylene co-oligomer (TPCO) single crystal laminated on a high-reflection distributed Bragg reflector. In the TPCO crystal, molecular transition dipole was strongly polarized along a certain horizontal directions with respect to the main crystal plane. This dipole polarization causes significantly large anisotropies in the exciton transition and optical constants. Especially the anisotropic exciton transition was found to provide the strong enhancement in the coupling with the cavity mode, which was demonstrated by a Rabi splitting energy as large as ∼100 meV even in the “half-vertical cavity surface emitting lasing” microcavity structure.
Strong coupling of two interacting excitons confined in a nanocavity-quantum dot system
International Nuclear Information System (INIS)
Cardenas, Paulo C; RodrIguez, Boris A; Quesada, Nicolas; Vinck-Posada, Herbert
2011-01-01
We present a study of the strong coupling between radiation and matter, considering a system of two quantum dots, which are in mutual interaction and interact with a single mode of light confined in a semiconductor nanocavity. We take into account dissipative mechanisms such as the escape of the cavity photons, decay of the quantum dot excitons by spontaneous emission, and independent exciton pumping. It is shown that the mutual interaction between the dots can be measured off-resonance only if the strong coupling condition is reached. Using the quantum regression theorem, a reasonable definition of the dynamical coupling regimes is introduced in terms of the complex Rabi frequency. Finally, the emission spectrum for relevant conditions is presented and compared with the above definition, demonstrating that the interaction between the excitons does not affect the strong coupling.
Phonon effects on the radiative recombination of excitons in double quantum dots
Karwat, Paweł; Sitek, Anna; Machnikowski, Paweł
2011-11-01
We study theoretically the radiative recombination of excitons in double quantum dots in the presence of carrier-phonon coupling. We show that the phonon-induced pure dephasing effects and transitions between the exciton states strongly modify the spontaneous emission process and make it sensitive to temperature, which may lead to nonmonotonic temperature dependence of the time-resolved luminescence. We show also that, under specific resonance conditions, the biexcitonic interband polarization can be coherently transferred to the excitonic one, leading to an extended lifetime of the total coherent polarization, which is reflected in the nonlinear optical spectrum of the system. We study the stability of this effect against phonon-induced decoherence.
Effect of uniaxial stress on free and bismuth-bound excitons in InP
International Nuclear Information System (INIS)
Weber, G.; Ruehle, W.
1979-01-01
The reduction of the shear deformation potentials of holes bound to the isoelectronic impurity Bi in InP is determined by piezoluminescence. It is compared with the corresponding reduction for holes bound to the Coulomb-type acceptors C and Zn. The theory for an effective mass acceptor describes well the cases of C and Zn. However, additional effects as local strain and Stark fields must be involved in the case of Bi leading to an extremely large reduction of the deformation potentials. No change in binding energy with applied stress as well as no exchange splitting of the Bi-bound exciton can be detected within experimental accuracy. The stress dependence of the free exciton reflectance reveals values for the band deformation potentials and a value of 0.07 meV for the exchange splitting of the free exciton in InP. (author)
Excitonic Order and Superconductivity in the Two-Orbital Hubbard Model: Variational Cluster Approach
Fujiuchi, Ryo; Sugimoto, Koudai; Ohta, Yukinori
2018-06-01
Using the variational cluster approach based on the self-energy functional theory, we study the possible occurrence of excitonic order and superconductivity in the two-orbital Hubbard model with intra- and inter-orbital Coulomb interactions. It is known that an antiferromagnetic Mott insulator state appears in the regime of strong intra-orbital interaction, a band insulator state appears in the regime of strong inter-orbital interaction, and an excitonic insulator state appears between them. In addition to these states, we find that the s±-wave superconducting state appears in the small-correlation regime, and the dx2 - y2-wave superconducting state appears on the boundary of the antiferromagnetic Mott insulator state. We calculate the single-particle spectral function of the model and compare the band gap formation due to the superconducting and excitonic orders.
Exciton absorption spectrum of thin Ag sub 2 ZnI sub 4
Yunakova, O N; Kovalenko, E N
2002-01-01
In Ag sub 2 ZnI sub 4 compound thin films one investigated into the electron spectrum of absorption within 3-6 eV photon energy range. The boundary of interband absorption is determined to correspond to the direct permitted transitions with E sub g = 3.7 eV forbidden gap width. A strong exciton band at E = 3.625 eV (80 K) GAMMA half width temperature run of which within 80-390 K range is governed by exciton-phonon interaction typical for quasi-single-dimensional excitons, is adjacent to the absorption boundary. At T <= 390 K one observes a bend in E(T) and GAMMA(T) dependences associated with generation of the Frenkel defects and followed by transfer of Ag ions to the interstices and vacancies of the compound crystalline lattice
Production of excitons in grazing collisions of protons with LiF surfaces: An onion model
Energy Technology Data Exchange (ETDEWEB)
Miraglia, J. E.; Gravielle, M. S. [Instituto de Astronomia y Fisica del Espacio, Consejo Nacional de Investigaciones Cientificas y Tecnicas and Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Casilla de Correo 67, Sucursal 28, (C1428EGA) Buenos Aires (Argentina)
2011-12-15
In this work we evaluate the production of excitons of a lithium fluoride crystal induced by proton impact in the intermediate and high energy regime (from 100 keV to 1 MeV). A simple model is proposed to account for the influence of the Coulomb grid of the target by dressing crystal ions to transform them in what we call onions. The excited states of these onions can be interpreted as excitons. Within this model, total cross section and stopping power are calculated by using the first Born and the continuum distorted-wave (CDW) eikonal initial-state (EIS) approximations. We found that between 7 and 30 excitons per incident proton are produced in grazing collisions with LiF surfaces, becoming a relevant mechanism of inelastic transitions.
Wannier-Frenkel hybrid exciton in organic-semiconductor quantum dot heterostructures
International Nuclear Information System (INIS)
Birman, Joseph L.; Huong, Nguyen Que
2007-01-01
The formation of a hybridization state of Wannier Mott exciton and Frenkel exciton in different hetero-structure configurations involving quantum dots is investigated. The hybrid excitons exist at the interfaces of the semiconductors quantum dots and the organic medium, having unique properties and a large optical non-linearity. The coupling at resonance is very strong and tunable by changing the parameters of the systems (dot radius, dot-dot distance, generation of the organic dendrites and the materials of the system etc...). Different semiconductor quantum dot-organic material combination systems have been considered such as a semiconductor quantum dot lattice embedded in an organic host, a semiconductor quantum dot at the center of an organic dendrite, a semiconductor quantum dot coated by an organic shell
Thermal dependence of free exciton emission in ultraviolet cathodoluminescence of colloidal ZnS
Energy Technology Data Exchange (ETDEWEB)
Bui, Hong Van; Pham, Van Ben [Faculty of Physics, VNU-Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi (Viet Nam); Le, Si Dang [Institut Néel, CNRS, 25 rue des Martyrs, BP 166, F-38042 Grenoble Cedex 9 (France); Hoang, Nam Nhat, E-mail: namnhat@gmail.com [Faculty of Engineering Physics and Nanotechnology, VNU-University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi (Viet Nam)
2016-10-15
Cathodoluminescence properties of the colloidal ZnS nanopowders synthesized by using hydrothermal process, a large scale production method, are reported. The cathodoluminescence spectra were obtained for temperature from 5 to 300 K, where an intensive free exciton originated 326 nm emission was observed. This band did not split under the increase of excitation beam current density and prevailed even at room temperature. The weaker emissions appeared at 331, 333, 337 and 343 nm which were related to excitons bound to neutral acceptor (A{sup o}, X), transition from conduction band to acceptor levels (e, A) and their corresponding (e, A)−1LO, (e, A)−2LO phonon replicas. With increasing temperature the free exciton band shifted towards lower energy and its intensity decreased at 36.5 meV thermal quenching threshold. The dependence of band gap on temperature was also determined.
Migration of CT triplet excitons in TCNB-biphenyl and TCNB-HMB crystals
Kozankiewicz, BolesAw
1994-01-01
Delayed fluorescence decay curves of charge transfer (CT) crystals of tetracyanobenzene with biphenyl (TCNB-B) and with hexamethylbenzene (TCNB-HMB) have been studied over a wide temperature range (5-200 K). The decay curves have been adequately described by decay expressions derived for different mechanisms of triplet-triplet annihilation. This analysis points to one-dimensional, thermally activated motion of CT triplet excitons. The estimated activation energies for the exciton hopping are 360±60 and 650±100 cm -1 (or 550±150 cm -1 depending on the applied model) for the TCNB-B and TCNB-HMB crystals, respectively. The results seem to confirm the self-trapping of triplet CT excitons.
Quantum-correlated two-photon transitions to excitons in semiconductor quantum wells.
Salazar, L J; Guzmán, D A; Rodríguez, F J; Quiroga, L
2012-02-13
The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers.
Particle-in-a-box model of one-dimensional excitons in conjugated polymers
Pedersen, Thomas G.; Johansen, Per M.; Pedersen, Henrik C.
2000-04-01
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 backbone by rigid walls. Furthermore, by integrating out the transverse part, the two-particle equation is reduced to one-dimensional form. It is demonstrated how essentially exact solutions are obtained in the cases of short and long conjugation length, respectively. From a linear combination of these cases an approximate solution for the general case is obtained. As an application of the model the influence of a static electric field on the electron-hole overlap integral and exciton energy is considered.
Excitons in undoped AlGaAs/GaAs wide parabolic quantum wells
Energy Technology Data Exchange (ETDEWEB)
Tabata, A; Oliveira, J B B [Departamento de Fisica, Universidade Estadual Paulista, 17033-360, Bauru (Brazil); Silva, E C F da; Lamas, T E; Duarte, C A; Gusev, G M, E-mail: tabata@fc.unesp.b [Instituto de Fisica, Universidade de Sao Paulo, 05315-970, Sao Paulo (Brazil)
2010-02-01
In this work the electronic structure of undoped AlGaAs/GaAs wide parabolic quantum wells (PQWs) with different well widths (1000 A and 3000 A) were investigated by means of photoluminescence (PL) measurements. Due to the particular potential shape, the sample structure confines photocreated carriers with almost three-dimensional characteristics. Our data show that depending on the well width thickness it is possible to observe very narrow structures in the PL spectra, which were ascribed to emissions associated to the recombination of confined 1s-excitons of the parabolic potential wells. From our measurements, the exciton binding energies (of a few meV) were estimated. Besides the exciton emission, we have also observed PL emissions associated to electrons in the excited subbands of the PQWs.
H-point exciton transitions in bulk MoS2
International Nuclear Information System (INIS)
Saigal, Nihit; Ghosh, Sandip
2015-01-01
Reflectance and photoreflectance spectrum of bulk MoS 2 around its direct bandgap energy have been measured at 12 K. Apart from spectral features due to the A and B ground state exciton transitions at the K-point of the Brillouin zone, one observes additional features at nearby energies. Through lineshape analysis the character of two prominent additional features are shown to be quite different from that of A and B. By comparing with reported electronic band structure calculations, these two additional features are identified as ground state exciton transitions at the H-point of the Brillouin zone involving two spin-orbit split valance bands. The excitonic energy gap at the H-point is 1.965 eV with a valance bands splitting of 185 meV. While at the K-point, the corresponding values are 1.920 eV and 205 meV, respectively
Fundamental principles of nanostructures and multiple exciton generation effect in quantum dots
International Nuclear Information System (INIS)
Turaeva, N.; Oksengendler, B.; Rashidova, S.
2011-01-01
In this work the theoretical aspects of the effect of multiple exciton generation in QDs has been studied. The statistic theory of multiple exciton generation in quantum dots is presented based on the Fermi approach to the problem of multiple generation of elementary particles at nucleon-nucleon collisions. Our calculations show that the quantum efficiencies of multiple exciton generation in various quantum dots at absorption of single photon are in a good agreement with the experimental data. The microscopic mechanism of this effect is based on the theory of electronic 'shaking'. In the work the deviation of averaged multiplicity of MEG effect from the Poisson law of fluctuations has been investigated. Besides, the role of interface electronic states of quantum dot and ligand has been considered by means of quantum mechanics. The size optimization of quantum dot has been arranged to receive the maximum multiplicity of MEG effect. (authors)
International Nuclear Information System (INIS)
Zhao, Dongxing; Wu, Jiarui; Gu, Ying; Gong, Qihuang
2014-01-01
We propose tailoring of the double Fano profiles via plasmon-assisted quantum interference in a hybrid exciton-plasmon system. Tailoring is performed by the interference between two exciton channels interacting with a common localized surface plasmon. Using an applied field of low intensity, the absorption spectrum of the hybrid system reveals a double Fano lineshape with four peaks. For relatively large field intensity, a broad flat window in the absorption spectrum appears which results from the destructive interference between excitons. Because of strong constructive interference, this window vanishes as intensity is further increased. We have designed a nanometer bandpass optical filter for visible light based on tailoring of the optical spectrum. This study provides a platform for quantum interference that may have potential applications in ultracompact tunable quantum devices.
Chen, Lan; Li, Bin; Zhang, Chunfeng; Huang, Xinyu; Wang, Xiaoyong; Xiao, Min
2018-03-14
Perovskite semiconductor nanocrystals with different compositions have shown promise for applications in light-emitting devices. Dark excitonic states may suppress light emission from such nanocrystals by providing an additional nonradiative recombination channel. Here, we study the composition dependence of dark exciton dynamics in nanocrystals of lead halides by time-resolved photoluminescence spectroscopy at cryogenic temperatures. The presence of a spin-related dark state is revealed by magneto-optical spectroscopy. The energy splitting between bright and dark states is found to be highly sensitive to both halide elements and organic cations, which is explained by considering the effects of size confinement and charge screening, respectively, on the exchange interaction. These findings suggest the possibility of manipulating dark exciton dynamics in perovskite semiconductor nanocrystals by composition engineering, which will be instrumental in the design of highly efficient light-emitting devices.
Electrons, holes, and excitons in GaAs polytype quantum dots
Energy Technology Data Exchange (ETDEWEB)
Climente, Juan I.; Segarra, Carlos; Rajadell, Fernando; Planelles, Josep, E-mail: josep.planelles@uji.es [Departament de Química Física i Analítica, Universitat Jaume I, E-12080 Castelló (Spain)
2016-03-28
Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.
Exciton induced directed motion of unconstrained atoms in an ultracold gas
Leonhardt, K.; Wüster, S.; Rost, J. M.
2017-03-01
We demonstrate that through localised Rydberg excitation in a three-dimensional cold atom cloud atomic motion can be rendered directed and nearly confined to a plane, without spatial constraints for the motion of individual atoms. This enables creation and observation of non-adiabatic electronic Rydberg dynamics in atoms accelerated by dipole-dipole interactions under natural conditions. Using the full l = 0, 1 m=0,+/- 1 angular momentum state space, our simulations show that conical intersection crossings are clearly evident, both in atomic position information and excited state spectra of the Rydberg system. Hence, flexible Rydberg aggregates suggest themselves for probing quantum chemical effects in experiments on length scales much inflated as compared to a standard molecular situation.
Exploring ultrafast dynamics of excitons and multiexcitons in "giant" nanocrystal quantum dots
Sampat, Siddharth
In this work, we have performed extensive time resolved photoluminescence (PL) studies to further the understanding of charge dynamics in semiconductor nanocrystal quantum dots (QDs). Recent developments in QD synthesis have introduced a new set of QD known as "giant" quantum dots (gQDs) that consist of a CdSe core coated with up to 19 monolayers of a CdS shell. The thick shell layer is grown using a SILAR method resulting in a defect free, alloyed CdSe/CdS interface. This has been attributed to gQDs exhibiting excellent optical properties such as high excitonic quantum yield (QY), prolonged photostability and inhibition of flourescence intermittency ("blinking"), which is regularly observed in conventional QDs. In gQDs, however, owing to unique fabrication methods and material selection, the Auger process is strongly suppressed resulting in efficient radiative recombination of photogenerated excitons as well as high PL QY of charged excitonic and multiexcitonic species. We perform extensive single gQDs studies that establish the role played by gQD shell thickness and core size in governing their optical properties. It is found that both the core and shell dimensions can be tuned in order to achieve the smallest gQDs with the highest vii Auger suppression resulting in photostable dots with high QYs. Next, we perform a study of multiexcitonic species in gQDs that are encapsulated in an insulating SiO2shell. These silica-coated gQDs exhibit strong PL from charged excitons, biexcitons as well as triexcitons. This observation has led to an accurate description of excitonic and multiexcitonic behavior which is modeled using a statistical scaling approach. As a demonstration of the practical applicability of gQDs, energy transfer of excitons as well as multiexcitons to different substrates is studied. Finally, a back gated silicon nanomembrane FET device is discussed that exhibits a large photocurrent increase when sensitized with QDs.
Shin, S H; Lee, E H; Chae, K M; Park, S H; Kim, U
1998-01-01
We have investigated the influence of carrier generation on the absorption bleaching of the n=2 and n=3 excitons in GaAs/AlGaAs multiple quantum wells (MQWs). With the excitation near the resonance of the n=1 exciton absorption, the long range coulomb screening and collision broadening had significant effects on the exciton bleaching. At low excitation intensity, the absorption bleaching of the n=2 exciton in 75 A-thick MQWs and that of the n=3 exciton in 150 A-thick MQWs were due to linewidth broadening by the collision broadening effect only. At high excitation intensity, however, the reduction of oscillator strength due to the long range coulomb screening contributed dominantly to absorption bleaching.
Pressure-induced increase of exciton-LO-phonon coupling in a ZnCdSe/ZnSe quantum well
Guo, Z. Z.; Liang, X. X.; Ban, S. L.
2003-07-01
The possibility of pressure-induced increase of exciton-LO-phonon coupling in ZnCdSe/ZnSe quantum wells is studied. The ground state binding energies of the heavy hole excitons are calculated using a variational method with consideration of the electron-phonon interaction and the pressure dependence of the parameters. The results show that for quantum wells with intermediate well width, the exciton binding energy and the LO-phonon energy may coincide in the course of pressure increasing, resulting in the increase of exciton-LO-phonon coupling. It is also found that among the pressure-dependent parameters, the influence of the lattice constant is the most important one. The changes of both the effective masses and the dielectric constants have obvious effects on the exciton binding energy, but their influences are counterbalanced.
Energy-level repulsion by spin-orbit coupling in two-dimensional Rydberg excitons
Stephanovich, V. A.; Sherman, E. Ya.; Zinner, N. T.; Marchukov, O. V.
2018-05-01
We study the effects of Rashba spin-orbit coupling on two-dimensional Rydberg exciton systems. Using analytical and numerical arguments we demonstrate that this coupling considerably modifies the wave functions and leads to a level repulsion that results in a deviation from the Poissonian statistics of the adjacent level distance distribution. This signifies the crossover to nonintegrability of the system and hints at the possibility of quantum chaos emerging. Such behavior strongly differs from the classical realization, where spin-orbit coupling produces highly entangled, chaotic electron trajectories in an exciton. We also calculate the oscillator strengths and show that randomization appears in the transitions between states with different total momenta.
International Nuclear Information System (INIS)
Ganesan, Vellayappan Dheivanayagam; Shen, Lei; Linghu, Jiajun; Zhang, Chun; Feng, Yuan Ping
2016-01-01
Using the many-body perturbation GW theory, we study the quasiparticle conduction-band offsets of phosphorene, a two-dimensional atomic layer of black phosphorus, and transition-metal dichalcogenides (TMDs). The calculated large exciton binding energies of phosphorene and TMDs indicate that their type-II heterostructures are suitable for excitonic thin-film solar cell applications. Our results show that these heterojunctions have a potential maximum power conversion efficiency of up to 12%, which can be further enhanced up to 20% by strain engineering.
Exciton luminescence in CdxMn1-xTe compounds
International Nuclear Information System (INIS)
Caraman, M.; Gashin, P.; Metelitsa, Snejana; Nicorici, Valentina; Nicorici, A.
2002-01-01
The Cd x Mn 1-x Te (0.5 7 W/cm 2 . The luminescence spectra were observed at 78 K. The results of the study had shown that the presence of relatively narrow luminescence peaks localized in the region of the fundamental absorption edge is characteristic for these spectra and for the majority of the crystals a wide maximum in the long wavelength region is observed. The luminescence maxima with an accuracy of ∼ 5 meV correspond to the resonance energy of the excitons of the state with n=1 determined from the absorption spectra. Hence, these maxima can be considered as exciton luminescence stimulated either by the excitons of the state n=1 or bounded to the exciton ionization centers. From the analysis of the absorption and exciton luminescence spectra one can make a conclusion about the fact that the homogeneity extent of the crystals decreases from CdTe to the compounds with x= 0.8 - 0.7 and slightly increases at the x decrease to 0.5. The exciton luminescence lines in CdTe and Cd 0.99 Mn 0.01 Te crystals is shifting by 7 - 10 meV relatively to the lines of free excitons absorption. This fact is explained by the fact that in these crystals, probably, excitons bounding to the lattice inherited defects with the binding energy of 7 - 10 meV participate in the luminescence. In the long wavelength region a wide peak is observed on which the impurity lines are not displayed. In the luminescence spectra of CdTe with 0.1%. As crystals three maxima at 1.51 eV, 1.46 eV and 1.42 eV are revealed. For pure CdTe the maximum at 1.4 eV is also revealed. These maxima are explained by the luminescence through the recombination levels localized at 0.46 eV. (authors)
Nuclear Exciton Echo Produced by Ultrasound in Forward Scattering of Synchrotron Radiation
International Nuclear Information System (INIS)
Smirnov, G.V.; van Buerck, U.; Arthur, J.; Popov, S.L.; Baron, A.Q.; Chumakov, A.I.; Ruby, S.L.; Potzel, W.; Brown, G.S.
1996-01-01
The time evolution of the coherent decay of a collective nuclear excitation (nuclear exciton), induced by 14.4 keV synchrotron radiation, was studied in a system of two stainless steel foils, where one was vibrated by ultrasound (US). The US vibrations disrupted the coherent nuclear emission from the two targets. However, at times corresponding to multiples of the US period, spike increases in the decay were observed. The spikes are interpreted as echoes of the initial nuclear exciton. copyright 1996 The American Physical Society
Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology
Energy Technology Data Exchange (ETDEWEB)
Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B., E-mail: benito.alen@csic.es [IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid (Spain); Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A. [Institute for Systems based on Optoelectronics and Microtechnology (ISOM), Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid (Spain)
2015-11-02
External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.
International Nuclear Information System (INIS)
Nkoma, J.S.
1982-08-01
The effects of three additional boundary conditions (ABC's) on the reflection and transmission spectra for exciton polaritons propagating in a spatially dispersive media are studied for both p and s configurations. An investigation of the ratios of the electric field amplitudes associated with the normal modes in these media is carried out. There is qualitative agreement among the predictions of the different ABC's, but there are significant quantitative differences, especially in the longitudinal polariton spike excited only in the p-geometry. Contact with formulations not using the ABC approach is made. The results are illustrated by parameters modelling the 1s exciton of PbI 2 . (author)
Energy Technology Data Exchange (ETDEWEB)
Ganesan, Vellayappan Dheivanayagam; Shen, Lei, E-mail: shenlei@nus.edu.sg [Engineering Science Programme, National University of Singapore, 9 Engineering Drive 1, Singapore 117575 (Singapore); Linghu, Jiajun; Zhang, Chun; Feng, Yuan Ping [Department of Physics, National University of Singapore, Singapore 117542 (Singapore)
2016-03-21
Using the many-body perturbation GW theory, we study the quasiparticle conduction-band offsets of phosphorene, a two-dimensional atomic layer of black phosphorus, and transition-metal dichalcogenides (TMDs). The calculated large exciton binding energies of phosphorene and TMDs indicate that their type-II heterostructures are suitable for excitonic thin-film solar cell applications. Our results show that these heterojunctions have a potential maximum power conversion efficiency of up to 12%, which can be further enhanced up to 20% by strain engineering.
Exciton-polariton dynamics in a GaAs bulk microcavity
Ceccherini, S.; Gurioli, M.; Bogani, F.; Colocci, M.; Tredicucci, A.; Bassani, F.; Beltram, F.; Sorba, L.
1998-01-01
We present a full analysis of exciton dynamics in a GaAs λ/2 bulk microcavity following excitation by ultrafast laser pulses. Coherent dynamics was probed by means of an interferometric technique; beating and dephasing times were studied for various excitation intensities. At high incident power, population effects begin to show up reducing exciton oscillator strength and suppressing Rabi splitting. This feature produces marked non-linearities in the input-output characteristic of the optical functions, which were studied in view of reaching bistable operation. Theoretical calculations performed within the transfer-matrix framework show good agreement with experimental results.
Optical nonlinearities of excitonic states in atomically thin 2D transition metal dichalcogenides
Energy Technology Data Exchange (ETDEWEB)
Soh, Daniel Beom Soo [Sandia National Lab. (SNL-CA), Livermore, CA (United States). Proliferation Signatures Discovery and Exploitation Department
2017-08-01
We calculated the optical nonlinearities of the atomically thin monolayer transition metal dichalcogenide material (particularly MoS_{2}), particularly for those linear and nonlinear transition processes that utilize the bound exciton states. We adopted the bound and the unbound exciton states as the basis for the Hilbert space, and derived all the dynamical density matrices that provides the induced current density, from which the nonlinear susceptibilities can be drawn order-by-order via perturbative calculations. We provide the nonlinear susceptibilities for the linear, the second-harmonic, the third-harmonic, and the kerr-type two-photon processes.
The self-trapping of anion excitons in alkali halides at elastic deformation
International Nuclear Information System (INIS)
Tulepbergenov, S.K.; Dzhumanov, S.; Spivak-Lavrov, I.F.; Shunkeev, K.Sh.
2001-01-01
The self-trapping of electronic excitations (EE) (excitons, holes and electrons) in alkali halides (AH), fluorides and oxides plays an important roles in luminescence and defect formation. Therein the specific features of self-trapping of EE in various materials are essentially different. In particular, the self-trapping of excitons in some AH (i.e. alkali iodides and bromides) occurs with overcoming of the potential barrier and in other AH (e.g. alkali fluorides and chlorides) such a barrier is absent. Here we develop the continuum theory of self-trapping of within the adiabatic approximation elastically stressed AH. In the continuum model of solids the functional of the total energy of are interacting exciton-phonon system in the deformed ionic crystal just as in the undeformed crystal depends on the dilation Δ(r) described by the deformation potential of acoustic phonon, the electrostatic potential φ[r) due to the lattice polarization at optical lattice vibrations and the wave function of exciton chosen for hydro statically and uniaxially stressed 3D crystals. The functionals of the total energy of the interfacing exciton-phonon system E{Δ(r),φ(r),ψ(r)} are minimized relative to Δ, φ and ψ for the cases of isotropic and anisotropic 3D crystals. As a result, we obtained the functionals depending on μ and determined their possible extremum. We have show that the linear deformations under the hydrostatic and uniaxial stress at 80 K lead to the decreasing of the self trapping barrier for exciton and to the increasing of the luminescence of self-trapped excitons (STE). While the nonlinear deformations under the such stress at 80 K lead to the increasing of the self-trapping barrier for excitons and to the decreasing at the STE luminescence in AH. At T=0 K the small hydrostatic and uniaxial pressures lead to the same effects. Further at hydrostatic and uniaxial compressions of AH the minimums of the adiabatic potentials of quasifree and STE are shifted to
International Nuclear Information System (INIS)
Wen, P; Nelson, Keith A; Christmann, G; Baumberg, J J
2013-01-01
Using two-dimensional spectroscopy, we resolve multi-polariton coherences in quantum wells embedded inside a semiconductor microcavity and elucidate how multi-exciton correlations mediate polariton nonlinear dynamics. We find that polariton correlation strengths depend on spectral overlap with the biexciton resonance and that up to at least four polaritons can be correlated, a higher-order correlation than observed to date among excitons in bare quantum wells. The high-order correlations can be attributed to coupling through the cavity mode, although the role of high-order Coulomb correlations cannot be excluded. (paper)
Amplification of electromagnetic radiation in the exciton region of the spectrum of a semiconductor
International Nuclear Information System (INIS)
Nerkararyan, Kh.V.
1989-01-01
The problem of amplification of electromagnetic radiation in the exciton region of the spectrum of a semiconductor was first discussed by Haken. The possibility of amplification of an electromagnetic wave under conditions of Bose condensation of biexcitons was considered in Ref. 2. However, the difficulties encountered in the creation of a Bose condensed state of biexcitons complicate greatly the performance of an experiment of this kind. The authors shall show that amplification is possible also in a gaseous mixture of excitons and biexcitons which is in thermal equilibrium and can be described by the Maxwellian distribution function of the velocities
Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals
Gdor, I.; Sachs, H.; Roitblat, A.; Strasfeld, D.; Bawendi, M. G.; Ruhman, S.
2013-03-01
Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times that of the band gap.
Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals
Directory of Open Access Journals (Sweden)
Bawendi M.G.
2013-03-01
Full Text Available Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times that of the band gap.
Characteristics of exciton photoluminescence kinetics in low-dimensional silicon structures
Sachenko, A V; Manojlov, E G; Svechnikov, S V
2001-01-01
The time-resolved visible photoluminescence of porous nanocrystalline silicon films obtained by laser ablation have been measured within the temperature range 90-300 K. A study has been made of the interrelationship between photoluminescence characteristics (intensity, emission spectra, relaxation times, their temperature dependencies and structural and dielectric properties (size and shapes of Si nanocrystals, oxide phase of nanocrystal coating, porosity). A photoluminescence model is proposed that describes photon absorption and emission occurring in quantum-size Si nanocrystals while coupled subsystems of electron-hole pairs and excitons take part in the recombination. Possible excitonic Auger recombination mechanism in low-dimensional silicon structures is considered
Co-existence of free and self-trapped excitons in J-aggregates
International Nuclear Information System (INIS)
Malyukin, Yu.V.; Lebedenko, A.N.; Sorokin, A.V.; Yefimova, S.L.
2005-01-01
Nature of excited electronic states of amphi-PIC J-aggregates, which are the source of the self-trapping states, have been investigated using low-temperature site-selective, time-resolved spectroscopy techniques. The self-trapping states are shown to evolve from the delocalized exciton states within the J-band. The strongly localized electronic states located on the low-frequency edge of the J-band, are not able to form polaronic states and, hence, the polaronic relaxation process is particularly collective one. The exciton self-trapping is more effective in J-aggregates with strong disorder, requires overcoming a self-trapping barrier
Crossover between the dense electron-hole phase and the BCS excitonic phase in quantum dots
International Nuclear Information System (INIS)
Rodriguez, B.A.; Gonzalez, A.; Quiroga, L.; Capote, R.; Rodriguez, F.J.
1999-09-01
Second order perturbation theory and a Lipkin-Nogami scheme combined with an exact Monte Carlo projection after variation are applied to compute the ground-state energy of 6 ≤ N ≤ 210 electron-hole pairs confined in a parabolic two-dimensional quantum dot. The energy shows nice scaling properties as N or the confinement strength is varied. A crossover from the high-density electron-hole phase to the BCS excitonic phase is found at a density which is roughly four times the close-packing density of excitons. (author)
Stark effect of optical properties of excitons in a quantum nanorod with parabolic confinement
Energy Technology Data Exchange (ETDEWEB)
Lyo, S.K., E-mail: sklyo@uci.edu
2014-01-15
We study the quantum Stark effect of optical properties of a quasi-one-dimensional quantum rod with parabolic confinement. Interplays between the competing/cooperative forces from confinement, electron–hole (e–h) attraction, and an external field are examined by studying the binding energy, the oscillator strength, and the root-mean-square (RMS) average of the e–h separation in a nonlinear electric field. In a long rod with weak confinement, the e–h interaction dominates over the confinement effect, yielding an abrupt drop of the exciton binding energy, oscillator strength, and a sudden increase of the RMS average e–h separation as the excitons are dissociated at the threshold field as the field increases. The exciton-dissociation transition is gradual in a short rod, where the confinement force dominates over the e–h attraction. We show that a DC field can induce an optically active excited exciton state in a narrow field range, causing a sharp peak in the oscillator strength and a dip in the RMS average of the e–h separation as the field increases. The Stark effects are also investigated as a function of the linear confinement length (i.e., rod length) at fixed fields. -- Highlights: • Study the dependence of optical properties of nanorods on the rod size and field. • Study the interplay between forces of confinement, Coulomb attraction, and field. • A strong field induces an optically active excited state observed in quantum dots.
Dynamic dipole-dipole interactions between excitons in quantum dots of different sizes
DEFF Research Database (Denmark)
Matsueda, Hideaki; Leosson, Kristjan; Xu, Zhangcheng
2004-01-01
A model of the resonance dynamic dipole-dipole interaction between excitons confined in quantum dots (QDs) of different sizes at close enough distance is given in terms of parity inheritance and exchange of virtual photons. Microphotoluminescence spectra of GaAs-AlGaAs coupled QDs are proposed to...
Simulating Excitons in MoS2 with Time-Dependent Density Functional Theory
Flamant, Cedric; Kolesov, Grigory; Kaxiras, Efthimios
Monolayer molybdenum disulfide, owing to its graphene-like two-dimensional geometry whilst still having a finite bandgap, is a material of great interest in condensed matter physics and for potential application in electronic devices. In particular, MoS2 exhibits significant excitonic effects, a desirable quality for fundamental many-body research. Time-dependent density functional theory (TD-DFT) allows us to simulate dynamical effects as well as temperature-based effects in a natural way given the direct treatment of the time evolution of the system. We present a TD-DFT study of monolayer MoS2 exciton dynamics, examining various qualitative and quantitative predictions in pure samples and in the presence of defects. In particular, we generate an absorption spectrum through simulated pulse excitation for comparison to experiment and also analyze the response of the exciton in an external electric field.In this work we also discuss the electronic structure of the exciton in MoS2 with and without vacancies.
Programmed coherent coupling in a synthetic DNA-based excitonic circuit
Boulais, Étienne; Sawaya, Nicolas P. D.; Veneziano, Rémi; Andreoni, Alessio; Banal, James L.; Kondo, Toru; Mandal, Sarthak; Lin, Su; Schlau-Cohen, Gabriela S.; Woodbury, Neal W.; Yan, Hao; Aspuru-Guzik, Alán; Bathe, Mark
2018-02-01
Natural light-harvesting systems spatially organize densely packed chromophore aggregates using rigid protein scaffolds to achieve highly efficient, directed energy transfer. Here, we report a synthetic strategy using rigid DNA scaffolds to similarly program the spatial organization of densely packed, discrete clusters of cyanine dye aggregates with tunable absorption spectra and strongly coupled exciton dynamics present in natural light-harvesting systems. We first characterize the range of dye-aggregate sizes that can be templated spatially by A-tracts of B-form DNA while retaining coherent energy transfer. We then use structure-based modelling and quantum dynamics to guide the rational design of higher-order synthetic circuits consisting of multiple discrete dye aggregates within a DX-tile. These programmed circuits exhibit excitonic transport properties with prominent circular dichroism, superradiance, and fast delocalized exciton transfer, consistent with our quantum dynamics predictions. This bottom-up strategy offers a versatile approach to the rational design of strongly coupled excitonic circuits using spatially organized dye aggregates for use in coherent nanoscale energy transport, artificial light-harvesting, and nanophotonics.
Anomalous behavior of the excited state of the A exciton in bulk WS2
DEFF Research Database (Denmark)
Jindal, Vishwas; Bhuyan, Sumi; Deilmann, Thorsten
2018-01-01
Results of optical spectroscopy studies on bulk 2H-WS2 at energies close to its direct band gap are presented. Reflectance and absorption measurements at low temperature show only one dominant feature due to the A exciton of bulk WS2 at similar to 2.02 eV. However, a laser-modulated photoreflecta......Results of optical spectroscopy studies on bulk 2H-WS2 at energies close to its direct band gap are presented. Reflectance and absorption measurements at low temperature show only one dominant feature due to the A exciton of bulk WS2 at similar to 2.02 eV. However, a laser....... The experimental results are analyzed by comparison with many-body perturbation theory calculations, including the solutions of the Bethe-Salpeter equation. A* is identified as the first excited state of the A exciton, that is, A(n = 2). The anomalous behavior of A* is explained by its distinct wave function...... spread along the c axis, the direction of weak van der Waals bonding, which makes it more susceptible to perturbations. Our ab initio calculations suggest that the A exciton in the ground state has a two-dimensional (2D) nature with a large binding energy E-b, in fair agreement with E-b similar to 90...
Strong excitonic interactions in the oxygen K-edge of perovskite oxides.
Tomita, Kota; Miyata, Tomohiro; Olovsson, Weine; Mizoguchi, Teruyasu
2017-07-01
Excitonic interactions of the oxygen K-edge electron energy-loss near-edge structure (ELNES) of perovskite oxides, CaTiO 3 , SrTiO 3 , and BaTiO 3 , together with reference oxides, MgO, CaO, SrO, BaO, and TiO 2 , were investigated using a first-principles Bethe-Salpeter equation calculation. Although the transition energy of oxygen K-edge is high, strong excitonic interactions were present in the oxygen K-edge ELNES of the perovskite oxides, whereas the excitonic interactions were negligible in the oxygen K-edge ELNES of the reference compounds. Detailed investigation of the electronic structure suggests that the strong excitonic interaction in the oxygen K-edge ELNES of the perovskite oxides is caused by the directionally confined, low-dimensional electronic structure at the Ti-O-Ti bonds. Copyright © 2016 Elsevier B.V. All rights reserved.
Efficient Generation of Long-Lived Triplet Excitons in 2D Hybrid Perovskite.
Younts, Robert; Duan, Hsin-Sheng; Gautam, Bhoj; Saparov, Bayrammurad; Liu, Jie; Mongin, Cedric; Castellano, Felix N; Mitzi, David B; Gundogdu, Kenan
2017-03-01
Triplet excitons form in quasi-2D hybrid inorganic-organic perovskites and diffuse over 100 nm before radiating with >11% photoluminescence quantum efficiency (PLQE) at low temperatures. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Field-induced exciton condensation in LaCoO.sub.3./sub.
Czech Academy of Sciences Publication Activity Database
Sotnikov, A.; Kuneš, Jan
2016-01-01
Roč. 6, Jul (2016), 1-6, č. článku 30510. ISSN 2045-2322 EU Projects: European Commission(XE) 646807 - EXMAG Institutional support: RVO:68378271 Keywords : exciton condensation * LaCoO 3 * dynamical mean-field theory Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 4.259, year: 2016
Collective Behavior of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells
DEFF Research Database (Denmark)
Larionov, A. V.; Timofeev, V. B.; Hvam, Jørn Märcher
2000-01-01
Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell excition in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circular polarized light...
Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin
Mikhnenko, O.V.; Blom, P.W.M.; Loi, M.A.
2011-01-01
We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal-organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent
International Nuclear Information System (INIS)
Kagan, V.D.; Karpenko, S.L.; Katilyus, R.
1989-01-01
Quenching of exciton luminescence in the constant electric field in cadmium sulfide at 1.8K, caused by impact ionization of free and delocalization of bound excitons by hot electrons is observed. When the field is increase up to 1 kW/cm continuous transfer from the Taundsen-Shockley law to the Davydov-Wolf one takes place. Among the samples studied pure samples are distinguished by the exciton spectrum, where, as it is shown in the work, the high-energy electrons lose quasipulse, radiating spontaneously piezophonons; in other samples scattering on impurities prevails. Theoretical processing of data on the bound exciton radiation line quenching in the moderate field region presents about 10 -4 values in pure and about 5x10 -6 cm ones in other samples for the 4 MeV energy electron free flight length. So, the optical methods used allowed one to determine high-energy electron relaxation mechanisms, prevailing in CdS at low temperature
An exciton approach to the excited states of two electron atoms. I Formalism and interpretation
International Nuclear Information System (INIS)
Schipper, P.E.
1985-01-01
The exciton model is formally applied to a description of the excited states of two electron atoms with the explicit inclusion of exchange. The model leads to a conceptually simple framework for the discussion of the electronic properties of the archetypical atomic electron pair
Field-induced exciton dissociation in PTB7-based organic solar cells
Gerhard, Marina; Arndt, Andreas P.; Bilal, Mühenad; Lemmer, Uli; Koch, Martin; Howard, Ian A.
2017-05-01
The physics of charge separation in organic semiconductors is a topic of ongoing research of relevance to material and device engineering. Herein, we present experimental observations of the field and temperature dependence of charge separation from singlet excitons in PTB7 and PC71BM , and from charge-transfer states created across interfaces in PTB 7 /PC71BM bulk heterojunction solar cells. We obtain this experimental data by time-resolving the near infrared emission of the states from 10 K to room temperature and electric fields from 0 to 2.5 MVcm -1 . Examining how the luminescence is quenched by field and temperature gives direct insight into the underlying physics. We observe that singlet excitons can be split by high fields, and that disorder broadens the high threshold fields needed to split the excitons. Charge-transfer (CT) states, on the other hand, can be separated by both field and temperature. Also, the data imply a strong reduction of the activation barrier for charge splitting from the CT state relative to the exciton state. The observations provided herein of the field-dependent separation of CT states as a function of temperature offer a rich data set against which theoretical models of charge separation can be rigorously tested; it should be useful for developing the more advanced theoretical models of charge separation.
Rabi like angular splitting in Surface Plasmon Polariton - Exciton interaction in ATR configuration
Hassan, Heba; Abdallah, T.; Negm, S.; Talaat, H.
2018-05-01
We have studied the coupling of propagating Surface Plasmon Polaritons (SPP) on silver films and excitons in CdS quantum dots (QDs). We employed the Kretschmann-Raether configuration of the attenuated total reflection (ATR) to propagate the SPP on silver film of thickness 47.5 nm at three different wavelengths. The CdS QD have been chemically synthesized with particular size such that its exciton of energy would resonate with SPP. High resolution transmission electron microscopy (HRTEM) and scan tunneling microscopy (STM) were used to measure the corresponding QDs size and confirm its shape. Further confirmation of the size has been performed by the effective mass approximation (EMA) model utilizing the band gap of the prepared QDs. The band gaps have been measured through UV-vis absorption spectra as well as scan tunneling spectroscopy (STS). The coupling has been observed as two branching dips in the ATR spectra indicating Rabi like splitting. To the best of our knowledge, this is the first time that Rabi interaction is directly observed in an ATR angular spectra. This observation is attributed to the use a high resolution angular scan (±0.005°), in addition to the Doppler width of the laser line as well as the energy distribution of the excitons. The effect of three different linker molecules (TOPO, HDA), (Pyridine) and (Tri-butylamine) as surface ligands, on SPP-Exciton interaction has been examined.
Electrical control of charged carriers and excitons in atomically thin materials
Wang, Ke; De Greve, Kristiaan; Jauregui, Luis A.; Sushko, Andrey; High, Alexander; Zhou, You; Scuri, Giovanni; Taniguchi, Takashi; Watanabe, Kenji; Lukin, Mikhail D.; Park, Hongkun; Kim, Philip
2018-02-01
Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices1-3. The unique band structure4-7 of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley-spin qubits8. 2D TMDs also provide a platform for novel quantum optoelectronic devices9-11 due to their large exciton binding energy12,13. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings14-16. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin-valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons.
Localized excitons in quantum wells show spin relaxation without coherence loss
DEFF Research Database (Denmark)
Zimmermann, R.; Langbein, W.; Runge, E.
2001-01-01
The coherence in the secondary emission from quantum well excitons is studied using the speckle method. Analysing the different polarization channels allows to conclude that (i) no coherence loss occurs in the cross-polarized emission, favouring spin beating instead of spin dephasing, and that (i...
Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes
International Nuclear Information System (INIS)
Wang, Jigang; Graham, Matt W.; Ma, Yingzhong; Fleming, Graham R.; Kaindl, Robert A.
2009-01-01
The quasi-1D confinement and reduced screening of photoexcited charges in single-walled carbon nanotubes (SWNTs) entails strongly-enhanced Coulomb interactions and exciton binding energies. Such amplified electron-hole (e-h) correlations have important implications for both fundamental physics and optoelectronic applications of nanotubes. The availability of 'individualized' SWNT ensembles with bright and structured luminescence has rendered specific tube chiralities experimentally accessible. In these samples, evidence for excitonic behavior was found in absorption-luminescence maps, two-photon excited luminescence, or ultrafast carrier dynamics. Here, we report ultrafast mid-infrared (mid-IR) studies of individualized SWNTs, evidencing strong photoinduced absorption around 200 meV in semiconducting tubes of (6,5) and (7,5) chiralities. This manifests the observation of quasi-1D intra-excitonic transitions between different relative-momentum states, in agreement with the binding energy and calculated oscillator strength. Our measurements further reveal a saturation of the photoinduced absorption with increasing phase-space filling of the correlated e-h pairs. The transient mid-IR response represents a new tool, unhindered by restrictions of momentum or interband dipole moment, to investigate the density and dynamics of SWNT excitons.
How exciton-vibrational coherences control charge separation in the photosystem II reaction center
Novoderezhkin, V.I.; Romero Mesa, E.; van Grondelle, R.
2015-01-01
In photosynthesis absorbed sun light produces collective excitations (excitons) that form a coherent superposition of electronic and vibrational states of the individual pigments. Two-dimensional (2D) electronic spectroscopy allows a visualization of how these coherences are involved in the primary
Biexciton formation and exciton coherent coupling in layered GaSe
Dey, P.; Paul, J.; Moody, G.; Stevens, C. E.; Glikin, N.; Kovalyuk, Z. D.; Kudrynskyi, Z. R.; Romero, A. H.; Cantarero, A.; Hilton, D. J.; Karaiskaj, D.
2015-06-01
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ˜2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with "ab initio" theoretical calculations of the phonon spectra, indicate strong interaction with the A1 ' phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.
Biexciton formation and exciton coherent coupling in layered GaSe
International Nuclear Information System (INIS)
Dey, P.; Paul, J.; Stevens, C. E.; Glikin, N.; Karaiskaj, D.; Moody, G.; Kovalyuk, Z. D.; Kudrynskyi, Z. R.; Romero, A. H.; Cantarero, A.; Hilton, D. J.
2015-01-01
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ∼2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with “ab initio” theoretical calculations of the phonon spectra, indicate strong interaction with the A 1 ′ phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal
Biexciton formation and exciton coherent coupling in layered GaSe
Energy Technology Data Exchange (ETDEWEB)
Dey, P.; Paul, J.; Stevens, C. E.; Glikin, N.; Karaiskaj, D., E-mail: karaiskaj@usf.edu [Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620 (United States); Moody, G. [National Institute of Standards and Technology, 325 Broadway, Boulder, Colarado 80305 (United States); Kovalyuk, Z. D.; Kudrynskyi, Z. R. [Chernivtsi Department, Frantsevich Institute of Material Sciences Problems, The National Academy of Sciences of Ukraine, 5, Iryna Vilde St., 58001 Chernivtsi (Ukraine); Romero, A. H. [Physics Department, West Virginia University, Morgantown, West Virginia 26506-6315 (United States); Cantarero, A. [Materials Science Institute, University of Valencia, P.O. Box 2205, 46071 Valencia (Spain); Hilton, D. J. [Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294 (United States)
2015-06-07
Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ∼2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with “ab initio” theoretical calculations of the phonon spectra, indicate strong interaction with the A{sub 1}{sup ′} phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.
Density of states and excitonic condensation in the double layer correlated systems
Energy Technology Data Exchange (ETDEWEB)
Apinyan, V., E-mail: v.apinyan@int.pan.wroc.pl; Kopeć, T.K.
2016-01-15
We consider the single-particle density of states (DOS) in the strongly correlated double layer (DL) system, without applied external fields. We demonstrate an unusual collapse effect in the spectrum of the normal single-particle spectral function at the particular high-symmetry point corresponding to the specific bunching-point solution of the chemical potential in the Frenkel channel. We show that at the low-temperature limit the anomalous spectral function obeys a concave like structure, which is directly related to the interlayer pair formation and condensation. We calculate the normal DOS functions, and we find their temperature dependence for different values of the interlayer Coulomb interaction parameter. We show that the normal electron and hole DOS functions demonstrate typical condensates double peak structures on the background of the excitonic pair formation quasiparticle spectra and we have found the evidence of the hybridization gap in the case of high-temperature limit, and small interlayer coupling parameter. Meanwhile, we show a possible crossover from the excitonic condensate regime into the band insulator state. The structure of the normal DOS spectra, in the Frenkel channel and for the strong interlayer coupling regime, is found gapless for all temperature limits, which clearly indicates the strong coherence effects in the DL structure, and the excitonic condensates therein. We have shown that the excitonic pair formation and pair condensation occur simultaneously in the DL system, in contrast with the purely three-dimensional (3D) or two-dimensional cases (2D), discussed previously.
Visualising Berry phase and diabolical points in a quantum exciton-polariton billiard.
Estrecho, E; Gao, T; Brodbeck, S; Kamp, M; Schneider, C; Höfling, S; Truscott, A G; Ostrovskaya, E A
2016-11-25
Diabolical points (spectral degeneracies) can naturally occur in spectra of two-dimensional quantum systems and classical wave resonators due to simple symmetries. Geometric Berry phase is associated with these spectral degeneracies. Here, we demonstrate a diabolical point and the corresponding Berry phase in the spectrum of hybrid light-matter quasiparticles-exciton-polaritons in semiconductor microcavities. It is well known that sufficiently strong optical pumping can drive exciton-polaritons to quantum degeneracy, whereby they form a macroscopically populated quantum coherent state similar to a Bose-Einstein condensate. By pumping a microcavity with a spatially structured light beam, we create a two-dimensional quantum billiard for the exciton-polariton condensate and demonstrate a diabolical point in the spectrum of the billiard eigenstates. The fully reconfigurable geometry of the potential walls controlled by the optical pump enables a striking experimental visualization of the Berry phase associated with the diabolical point. The Berry phase is observed and measured by direct imaging of the macroscopic exciton-polariton probability densities.
Exciton Emission from Bare and Alq3/Gold Coated GaN Nanorods
Mohammadi, Fatemesadat; Kuhnert, Gerd; Hommel, Detlef; Schmitzer, Heidrun; Wagner, Hans-Peter
We study the excitonic and impurity related emission in bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). The GaN nanorods were grown by molecular beam epitaxy. Alq3 as well as Alq3/gold covered nanorods were synthesized by organic molecular beam deposition. In the near-band edge region a donor-bound-exciton (D0X) emission is observed at 3.473 eV. Another emission band at 3.275 eV reveals LO-phonon replica and is attributed to a donor-acceptor-pair (DAP) luminescence. TR PL traces at 20 K show a nearly biexponential decay for the D0X with lifetimes of approximately 180 and 800 ps for both bare and Alq3 coated nanorods. In GaN nanorods which were coated with an Alq3 film and subsequently with a 10 nm thick gold layer we observe a PL quenching of D0X and DAP band and the lifetimes of the D0X transition shorten. The quenching behaviour is partially attributed to the energy-transfer from free excitons and donor-bound-excitons to plasmon oscillations in the gold layer.
Exciton states in GaAs δ-doped systems under magnetic fields and hydrostatic pressure
Energy Technology Data Exchange (ETDEWEB)
Mora-Ramos, M.E. [Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, Ave. Universidad 1001, CP 62209 Cuernavaca, Morelos (Mexico); Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia); Duque, C.A., E-mail: cduque@fisica.udea.edu.co [Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia)
2013-04-15
Excitons in GaAs n-type δ-doped quantum wells are studied taking into account the effects of externally applied magnetic fields as well as of hydrostatic pressure. The one-dimensional potential profile in both the conduction and valence bands is described including Hartree effects via a Thomas–Fermi-based local density approximation. The allowed uncorrelated energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions and a variational method is used to obtain the exciton states. The results are presented as functions of the two-dimensional doping concentration and the magnetic field strength for zero and finite values of the hydrostatic pressure. In general, it is found that the exciton binding energy is a decreasing function of the doping-density and an increasing function of the magnetic field intensity. A comparison with recent experiments on exciton-related photoluminescence in n-type δ-doped GaAs is made.
Ultrafast interfeometric investigation of resonant secondary emission from quantum well excitons
DEFF Research Database (Denmark)
Birkedal, Dan; Shah, Jagdeep; Pfeiffer, L. N.
1999-01-01
Coherent Rayleigh scattering and incoherent luminescence comprise the secondary emission from quantum well exciton following ultrafast resonant excitation. We show that coherent Rayleigh scattering forms a time-dependent speckle pattern and isolate in a single speckle the Rayleigh component from...
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...... features in support of quantum corrections to the classical description of the underlying scattering process....
Sheng, ChuanXiang; Zhang, Chuang; Zhai, Yaxin; Mielczarek, Kamil; Wang, Weiwei; Ma, Wanli; Zakhidov, Anvar; Vardeny, Z. Valy
2015-03-01
We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH3NH3PbI3 and CH3NH3PbI1.1Br1.9 using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH3NH3PbI3 with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH3NH3PbI1.1Br1.9 . Surprisingly, we also discovered in CH3NH3PbI3 , but not in CH3NH3PbI1.1Br1.9 , transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH3NH3PbI3 , D ≈0.01 cm2 s-1 .
International Nuclear Information System (INIS)
Itskovich, I.F.; Shekhter, R.I.
1983-01-01
The effect of exciton pairing of charge carriers in a semimetal on the stationary Josephson current in superconductor-semimetal-superconductor junctions is considered. It is shown that the phase transition of the semimetal interlayer into an exciton dielectric state for T/sub γ/< T/sub c/ (T/sub γ/, T/sub c/ are the superconducting and exciton transition temperatures, respectively) is accompanied by a kink on the critical current j/sub c/ versus temperature curve at the point T = T/sub γ/. A sharp nonmonotonic temperature dependence of the reduced current j/sub c//j/sub c/0 (j/sub c/0 is the critical current at T/sub γ/ = 0) is also possible in the range T< T/sub γ/. At low temperatures T<< v/sub 1,2//d<< T/sub γ/ (v/sub 1,2/ are the Fermi velocities of the carriers in the semimetal, d is the thickness of the interlayer) the critical current of the superconductor-semimetal-superconductor junction is exponentially smaller than the current in the absence of exciton pairing
Coherence properties of exciton-polariton OPO condensates in one and two dimensions
DEFF Research Database (Denmark)
Spano, R.; Cuadra, J.; Anton, C.
2012-01-01
We give an overview of the coherence properties of exciton-polariton condensates generated by optical parametric scattering. Different aspects of the first-order coherence (g((1))) have been investigated. The spatial coherence extension of a two-dimensional (2D) polariton system, below and at the...
Deschler, Felix; Da Como, Enrico; Limmer, Thomas; Tautz, Raphael; Godde, Tillmann; Bayer, Manfred; von Hauff, Elizabeth; Yilmaz, Seyfullah; Allard, Sybille; Scherf, Ullrich; Feldmann, Jochen
2011-01-01
We investigate the effect of molecular doping on the recombination of electrons and holes localized at conjugated-polymer–fullerene interfaces. We demonstrate that a low concentration of p-type dopant molecules (<4% weight) reduces the interfacial recombination via charge transfer excitons and
Exciton dynamics in GaAs/AlxGa1-xAs quantum wells
DEFF Research Database (Denmark)
Litvinenko, K.; Birkedal, Dan; Lyssenko, V. G.
1999-01-01
The changes induced in the optical absorption spectrum of a GaAs/AlxGa1-xAs multiple quantum well due to a photoexcited carrier distribution are reexamined. We use a femtosecond pump-probe technique to excite excitons and free electron-hole pairs. We find that for densities up to 10(11) cm(-2...
Hershey, Kyle W.; Suddard-Bangsund, John; Qian, Gang; Holmes, Russell J.
2017-09-01
The analysis of organic light-emitting device degradation is typically restricted to fitting the overall luminance loss as a function of time or the characterization of fully degraded devices. To develop a more complete understanding of degradation, additional specific data are needed as a function of luminance loss. The overall degradation in luminance during testing can be decoupled into a loss in emitter photoluminescence efficiency and a reduction in the exciton formation efficiency. Here, we demonstrate a method that permits separation of these component efficiencies, yielding the time evolution of two additional specific device parameters that can be used in interpreting and modeling degradation without modification to the device architecture or introduction of any additional post-degradation characterization steps. Here, devices based on the phosphor tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) are characterized as a function of initial luminance and emissive layer thickness. The overall loss in device luminance is found to originate primarily from a reduction in the exciton formation efficiency which is exacerbated in devices with thinner emissive layers. Interestingly, the contribution to overall degradation from a reduction in the efficiency of exciton recombination (i.e., photoluminescence) is unaffected by thickness, suggesting a fixed exciton recombination zone width and degradation at an interface.
Communication: Strong excitonic and vibronic effects determine the optical properties of Li₂O₂
DEFF Research Database (Denmark)
García Lastra, Juan Maria; Bass, J. D.; Thygesen, Kristian Sommer
2011-01-01
The band structure and optical absorption spectrum of lithium peroxide (Li2O2) is calculated from first-principles using the G0W0 approximation and the Bethe-Salpeter equation, respectively. A strongly localized (Frenkel type) exciton corresponding to the π*→σ* transition on the O2 −2 peroxide ion...
Dispersion anisotropy of plasmon-exciton-polaritons in lattices of metallic nanoparticles
Ramezani, Mohammad; Halpin, Alexei; Feist, Johannes; Van Hoof, Niels; Fernández-Domínguez, Antonio I.; Garcia-Vidal, Francisco J.; Gómez Rivas, Jaime
2018-01-01
When the electromagnetic modes supported by plasmonic-based cavities interact strongly with molecules located within the cavity, new hybrid states known as plasmon-exciton-polaritons (PEPs) are formed. The properties of PEPs, such as group velocity, effective mass, and lifetime, depend on the
International Nuclear Information System (INIS)
Nguyen Ai Viet; Nguyen Toan Thang.
1987-06-01
The coefficient of the absorption of two polarized photons is calculated for direct band gap semiconductors with degenerate valence bands. Wannier-Mott exciton states are included in both the intermediate and final states. Numerical calculations are performed for ZnSe and are compared with Sondergeld's experimental and theoretical results. (author). 11 refs, 2 tabs
Critical tunnel currents and dissipation of Quantum-Hall bilayers in the excitonic condensate state
International Nuclear Information System (INIS)
Yoon, Y; Huang, X; Yarar, E; Dietsche, W; Tiemann, L; Schmult, S; Klitzing, K v
2011-01-01
Transport and tunneling is studied in the regime of the excitonic condensate at total filling factor one using the counterflow geometry. At small currents the coupling between the layers is large making the two layers virtually electrically inseparable. Above a critical current the tunneling becomes negligible. An onset of dissipation in the longitudinal transport is observed in the same current range.
Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero-Bilayers.
Luong, Dinh Hoa; Lee, Hyun Seok; Neupane, Guru Prakash; Roy, Shrawan; Ghimire, Ganesh; Lee, Jin Hee; Vu, Quoc An; Lee, Young Hee
2017-09-01
Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe 2 /MoS 2 hetero-bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band-to-band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Polarized excitons and optical activity in single-wall carbon nanotubes
Chang, Yao-Wen; Jin, Bih-Yaw
2018-05-01
The polarized excitons and optical activity of single-wall carbon nanotubes (SWNTs) are studied theoretically by π -electron Hamiltonian and helical-rotational symmetry. By taking advantage of the symmetrization, the single-particle energy and properties of a SWNT are characterized with the corresponding helical band structure. The dipole-moment matrix elements, magnetic-moment matrix elements, and the selection rules can also be derived. Based on different selection rules, the optical transitions can be assigned as the parallel-polarized, left-handed circularly-polarized, and right-handed circularly-polarized transitions, where the combination of the last two gives the cross-polarized transition. The absorption and circular dichroism (CD) spectra are simulated by exciton calculation. The calculated results are well comparable with the reported measurements. Built on the foundation, magnetic-field effects on the polarized excitons and optical activity of SWNTs are studied. Dark-bright exciton splitting and interband Faraday effect in the CD spectrum of SWNTs under an axial magnetic field are predicted. The Faraday rotation dispersion can be analyzed according to the selection rules of circular polarizations and the helical band structure.
Ni, Limeng; Huynh, Uyen; Cheminal, Alexandre; Thomas, Tudor H; Shivanna, Ravichandran; Hinrichsen, Ture F; Ahmad, Shahab; Sadhanala, Aditya; Rao, Akshay
2017-11-28
Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH 3 (CH 2 ) 3 NH 3 ) 2 PbI 4 and hexylammonium lead iodide (CH 3 (CH 2 ) 5 NH 3 ) 2 PbI 4 , both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton-phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm -1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm -1 . Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.
Directory of Open Access Journals (Sweden)
Hideki Gotoh
2014-10-01
Full Text Available 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.
Terahertz field-induced ionization and perturbed free induction decay of excitons in bulk GaAs
Murotani, Yuta; Takayama, Masayuki; Sekiguchi, Fumiya; Kim, Changsu; Akiyama, Hidefumi; Shimano, Ryo
2018-03-01
We investigated the interaction between an intense terahertz (THz) pulse and excitons in bulk GaAs by using THz pump near-infrared (NIR) optical probe spectroscopy. We observed a clear spectral oscillation in the NIR transient absorption spectra at low temperature, which is interpreted as the THz pump-induced perturbed free induction decay (PFID) of the excitonic interband polarization. We performed a numerical simulation based on a microscopic theory and identified that the observed PFID signal originates from the THz field-induced ionization of excitons. Using a real-space representation of the excitonic wave function, we visualized how the ionization of an exciton proceeds under the intense single-cycle THz electric field. We also calculated the nonlinear susceptibility with the lowest-order perturbation theory assuming a weak THz pump, which showed a similar spectral feature with that obtained by the full treatment to field-induced ionization process. This coincidence is attributed to the fact that 1s-excitonic interband polarization is modified predominantly through interactions with the p-wave component of the excitonic wave function. A simple phenomenological expression of the PFID signal is presented to discuss effects of the THz pump pulse duration on the spectral oscillation.
Exciton scattering approach for optical spectra calculations in branched conjugated macromolecules
International Nuclear Information System (INIS)
Li, Hao; Wu, Chao; Malinin, Sergey V.; Tretiak, Sergei; Chernyak, Vladimir Y.
2016-01-01
The exciton scattering (ES) technique is a multiscale approach based on the concept of a particle in a box and developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, electronic excitations in molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph whose edges and nodes stand for the molecular linear segments and vertices, respectively. Exciton propagation on the linear segments is characterized by the exciton dispersion, whereas exciton scattering at the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized “particle in a box” problems on the graph that represents the molecule. Similarly, unique energy-dependent ES dipolar parameters permit calculations of the corresponding oscillator strengths, thus, completing optical spectra modeling. Both the energetic and dipolar parameters can be extracted from quantum-chemical computations in small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within a considered molecular family could be performed with negligible numerical effort. We demonstrate the ES method application to molecular families of branched conjugated phenylacetylenes and ladder poly-para-phenylenes, as well as structures with electron donor and acceptor chemical substituents. Time-dependent density functional theory (TD-DFT) is used as a reference model for electronic structure. The ES calculations accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.
Exciton scattering approach for optical spectra calculations in branched conjugated macromolecules
Li, Hao; Wu, Chao; Malinin, Sergey V.; Tretiak, Sergei; Chernyak, Vladimir Y.
2016-12-01
The exciton scattering (ES) technique is a multiscale approach based on the concept of a particle in a box and developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, electronic excitations in molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph whose edges and nodes stand for the molecular linear segments and vertices, respectively. Exciton propagation on the linear segments is characterized by the exciton dispersion, whereas exciton scattering at the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized "particle in a box" problems on the graph that represents the molecule. Similarly, unique energy-dependent ES dipolar parameters permit calculations of the corresponding oscillator strengths, thus, completing optical spectra modeling. Both the energetic and dipolar parameters can be extracted from quantum-chemical computations in small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within a considered molecular family could be performed with negligible numerical effort. We demonstrate the ES method application to molecular families of branched conjugated phenylacetylenes and ladder poly-para-phenylenes, as well as structures with electron donor and acceptor chemical substituents. Time-dependent density functional theory (TD-DFT) is used as a reference model for electronic structure. The ES calculations accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.
Exciton scattering approach for optical spectra calculations in branched conjugated macromolecules
Energy Technology Data Exchange (ETDEWEB)
Li, Hao [Department of Chemistry, University of Houston, Houston, TX 77204 (United States); Wu, Chao [Electronic Structure Lab, Center of Microscopic Theory and Simulation, Frontier Institute of Science and Technology, Xian Jiaotong University, Xian 710054 (China); Malinin, Sergey V. [Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202 (United States); Tretiak, Sergei, E-mail: serg@lanl.gov [Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Chernyak, Vladimir Y., E-mail: chernyak@chem.wayne.edu [Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202 (United States)
2016-12-20
The exciton scattering (ES) technique is a multiscale approach based on the concept of a particle in a box and developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, electronic excitations in molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph whose edges and nodes stand for the molecular linear segments and vertices, respectively. Exciton propagation on the linear segments is characterized by the exciton dispersion, whereas exciton scattering at the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized “particle in a box” problems on the graph that represents the molecule. Similarly, unique energy-dependent ES dipolar parameters permit calculations of the corresponding oscillator strengths, thus, completing optical spectra modeling. Both the energetic and dipolar parameters can be extracted from quantum-chemical computations in small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within a considered molecular family could be performed with negligible numerical effort. We demonstrate the ES method application to molecular families of branched conjugated phenylacetylenes and ladder poly-para-phenylenes, as well as structures with electron donor and acceptor chemical substituents. Time-dependent density functional theory (TD-DFT) is used as a reference model for electronic structure. The ES calculations accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.
Role of many-body effects in the coherent dynamics of excitons in low-temperature-grown GaAs
Energy Technology Data Exchange (ETDEWEB)
Webber, D.; Hacquebard, L.; Hall, K. C. [Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2 (Canada); Liu, X.; Dobrowolska, M.; Furdyna, J. K. [Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556 (United States)
2015-10-05
Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs indicate a polarization-dependent nonlinear optical response at the exciton, which we attribute to Coulomb-mediated coupling between excitons and electron-hole pairs simultaneously excited by the broad-bandwidth laser pulses. Strong suppression of the exciton response through screening by carriers injected by a third pump pulse was observed, an effect that is transient due to rapid carrier trapping. Our findings highlight the need to account for the complex interplay of disorder and many-body effects in the design of ultrafast optoelectronic devices using this material.
Khadzhi, P. I.; Lyakhomskaya, K. D.; Nadkin, L. Y.; Markov, D. A.
2002-05-01
The characteristic peculiarities of the self-reflection of a strong electromagnetic wave in a system of coherent excitons and biexcitons due to the exciton-photon interaction and optical exciton-biexciton conversion in semiconductors were investigated as one of the manifestations of nonlinear optical Stark-effect. It was found that a monotonously decreasing standing wave with an exponential decreasing spatial tail is formed in the semiconductor. Under the action of the field of a strong pulse, an optically homogeneous medium is converted, into the medium with distributed feedback. The appearance of the spatially separated narrow pears of the reflective index, extinction and reflection coefficients is predicted.
Attosecond Electron Processes in Materials: Excitons, Plasmons, and Charge Dynamics
2015-05-19
focused using a f=1.5 m lens into a 250 micron hollow core fiber (HCF) filled with neon gas at atmospheric pressure to stretch the pulse spectrum from... insulator to metal transition. Introduction: The goal of this work was to understand the generation, transport, and manipulation of electronic charge...chemically sensitive probe pulse utilizing specific core level transitions in atoms that are part of a material under study. The measurements follow
Haas, Simon; Matsui, Hiroyuki; Hasegawa, Tatsuo
2010-10-01
We report pure electric-field effects on the excitonic absorbance of pentacene thin films as measured by unipolar field-effect devices that allowed us to separate the charge accumulation effects. The field-modulated spectra between 1.8 and 2.6 eV can be well fitted with the first derivative curve of Frenkel exciton absorption and its vibronic progression, and at higher energy a field-induced feature appears at around 2.95 eV. The results are in sharp contrast to the electroabsorption spectra reported by Sebastian in previous studies [Chem. Phys. 61, 125 (1981)10.1016/0301-0104(81)85055-0], and leads us to reconsider the excitonic structure including the location of charge-transfer excitons. Nonlinear π -electronic response is discussed based on second-order electro-optic (Kerr) spectra.
Exciton localization in (11-22)-oriented semi-polar InGaN multiple quantum wells
Monavarian, Morteza; Rosales, Daniel; Gil, Bernard; Izyumskaya, Natalia; Das, Saikat; Özgür, Ümit; Morkoç, Hadis; Avrutin, Vitaliy
2016-02-01
Excitonic recombination dynamics in (11-22) -oriented semipolar In0.2Ga0.8N/In0.06Ga0.94N multiquantum wells (MQWs) grown on GaN/m-sapphire templates have been investigated by temperature-dependent time-resolved photoluminescence (TRPL). The radiative and nonradiative recombination contributions to the PL intensity at different temperatures were evaluated by analysing temperature dependences of PL peak intensity and decay times. The obtained data indicate the existence of exciton localization with a localization energy of Eloc(15K) =7meV and delocalization temperature of Tdeloc = 200K in the semipolar InGaN MQWs. Presence of such exciton localization in semipolar (11-22) -oriented structures could lead to improvement of excitonic emission and internal quantum efficiency.
Radiative recombination of free and bound excitons in CdMnTe/CdMgTe quantum wells
Energy Technology Data Exchange (ETDEWEB)
Gubarev, S.I. [Rossijskaya Akademiya Nauk, Chernogolovka (Russian Federation). Inst. Fiziki Tverdogo Tela; Kulakovskii, V.D. [Rossijskaya Akademiya Nauk, Chernogolovka (Russian Federation). Inst. Fiziki Tverdogo Tela; Tyazhlov, M.G. [Rossijskaya Akademiya Nauk, Chernogolovka (Russian Federation). Inst. Fiziki Tverdogo Tela; Yakovlev, D.R. [Wuerzburg Univ. (Germany). Physikalisches Inst.; Waag, A. [Wuerzburg Univ. (Germany). Physikalisches Inst.; Landwehr, G. [Wuerzburg Univ. (Germany). Physikalisches Inst.
1995-06-01
The exchange induced dissociation of bound excitons (BE) has been studied in CdMnTe/CdMgTe quantum wells (QWs). It was found that value of the dissociation critical field does not depend on the field direction with respect to QW axis. This indicates that BE states in investigated structure are connected with excitons bound to neutral donors (D{sub 0}X states). The dependence of the critical field on the QW width has nonmonotonic character: the dissociation occurs at first in 60 A, then in 45 A, and at the end in 100 A QW. Such a behavior can be explained by transformation of bound exciton complex from quasi-3D to quasi-2D state with following increase of Coulomb correlations in confined exciton system. (orig.).
Band-Edge Exciton Fine Structure and Recombination Dynamics in InP/ZnS Colloidal Nanocrystals.
Biadala, Louis; Siebers, Benjamin; Beyazit, Yasin; Tessier, Mickaël D; Dupont, Dorian; Hens, Zeger; Yakovlev, Dmitri R; Bayer, Manfred
2016-03-22
We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. In addition to the typical bright-dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright-dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures.
International Nuclear Information System (INIS)
Pokutnyj, S.I.
2016-01-01
The effect of a significant increase in the exciton binding energy of space-separated electrons and holes (hole moves in the volume of the quantum dot, and the electron is localized on a spherical surface section quantum dot-matrix) in nanosystems containing germanium quantum dots grown in a matrix of silicon by compared with the binding energy of an exciton in a silicon single crystal. It was found that in such nanosystems in the conduction band silicon matrix is first a zone of states of electron-hole pairs, which with increasing radius of the quantum dot becomes a zone of exciton states, located in the band gap of silicon matrix. It is shown that the mechanism of light absorption in nanosystems due to transitions between quantum-electron levels of the electron-hole pairs, as well as the electron transitions between quantum-exciton levels.
Stuhldreier, Mayra C.; Röttger, Katharina; Temps, Friedrich
We report the observation by transient absorption spectroscopy of distinctive spectro-temporal signatures of delocalized exciton versus relaxed, weakly bound excimer states in the ultrafast electronic deactivation after UV photoexcitation of the adenine dinucleotide.
Energy Technology Data Exchange (ETDEWEB)
Takagi, Hidetsugu; Kunugita, Hideyuki; Ema, Kazuhiro [Department of Physics, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 (Japan); Sato, Mikio; Takeoka, Yuko [Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 (Japan)
2013-12-04
We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C{sub 4}H{sub 9}NH{sub 3}){sub 2}PbBr{sub 4} and (C{sub 6}H{sub 5}−C{sub 2}H{sub 4}NH{sub 3}){sub 2}PbBr{sub 4}, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.
Phase Diagram of the Bose Condensation of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells
DEFF Research Database (Denmark)
Dremin, A. A.; Timofeev, V. B.; Larionov, A. V.
2002-01-01
The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n–i–n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied at low temperatures down to 0.5 K. The properties of excitons whose photoexcited electron and hole...... was coated with a metal mask containing special openings (windows) of a micron size or smaller. Both photoexcitation and observation of luminescence were performed through these windows by the fiber optic technique. At low pumping powers, the interwell excitons were strongly localized because of the residual...... charged impurities, and the corresponding photoluminescence line was nonuniformly broadened. As the laser excitation power increased, a narrow line due to delocalized excitons arose in a threshold-like manner, after which its intensity rapidly increased with growing pumping and the line itself narrowed...
Exciton properties in zincblende InGaN-GaN quantum wells under the effects of intense laser fields.
Duque, Carlos M; Mora-Ramos, Miguel E; Duque, Carlos A
2012-08-31
: In this work, we study the exciton states in a zincblende InGaN/GaN quantum well using a variational technique. The system is considered under the action of intense laser fields with the incorporation of a direct current electric field as an additional external probe. The effects of these external influences as well as of the changes in the geometry of the heterostructure on the exciton binding energy are discussed in detail.
Schöne, Wolf-Dieter; Ekardt, Walter
2000-01-01
Experiments determining the lifetime of excited electrons in crystalline copper reveal states which cannot be interpreted as Bloch states [S. Ogawa {\\it et al.}, Phys. Rev. B {\\bf 55}, 10869 (1997)]. In this article we propose a model which explains these states as transient excitonic states in metals. The physical background of transient excitons is the finite time a system needs to react to an external perturbation, in other words, the time which is needed to build up a polarization cloud. ...
International Nuclear Information System (INIS)
Nguyen Toan Thang; Nguyen Ai Viet; Nguyen Que Huong
1987-06-01
The influence of the cubic structure on the energy spectrum of direct exciton is investigated, using the new method suggested by Nguyen Van Hieu and co-workers. Explicit expressions of the exciton energy levels 1S, 2S and 2P are derived. A comparison with the experiments and the other theory is done for ZnSe. (author). 10 refs, 1 fig., 2 tabs
Karima, H. R.; Majidi, M. A.
2018-04-01
Excitons, quasiparticles associated with bound states between an electron and a hole and are typically created when photons with a suitable energy are absorbed in a solid-state material. We propose to study a possible emergence of excitons, created not by photon absorption but the effect of strong electronic correlations. This study is motivated by a recent experimental study of a substrate material SrTiO3 (STO) that reveals strong exitonic signals in its optical conductivity. Here we conjecture that some excitons may already exist in the ground state as a result of the electronic correlations before the additional excitons being created later by photon absorption. To investigate the existence of excitons in the ground state, we propose to study a simple 4-energy-level model that mimics a situation in strongly-correlated semiconductors. The four levels are divided into two groups, lower and upper groups separated by an energy gap, Eg , mimicking the valence and the conduction bands, respectively. Further, we incorporate repulsive Coulomb interactions between the electrons. The model is then solved by exact diagonalization method. Our result shows that the toy model can demonstrate band gap widening or narrowing and the existence of exciton in the ground state depending on interaction parameter values.
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.
Sun, Jingya; Yu, Weili; Usman, Anwar; Isimjan, Tayirjan T.; Del Gobbo, Silvano; Alarousu, Erkki; Takanabe, Kazuhiro; Mohammed, Omar F.
2014-01-01
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.
Khoirunnisa, Humaira; Aziz Majidi, Muhammad
2018-04-01
The emergence of exitonic signal in the optical response of a wide band-gap semiconductor has been a common knowledge in physics. There have been numerous experimental studies exploring the important role of excitons on influencing both the transport and optical properties of the materials. Despite the existence of much information on excitonic effects, there has not been much literature that explores detailed theoretical explanation on how the exitonic signal appears and how it evolves with temperature. Here, we propose a theoretical study on the optical conductivity of ZnO, a well-known wide band-gap semiconductor that we choose as a case study. ZnO has been known to exhibit excitonic states in its optical spectra in the energy range of ∼3.13-3.41 eV, with a high exciton binding energy of ∼60 meV. An experimental study on ZnO in 2014 revealed such a signal in its optical conductivity spectrum. We present a theoretical investigation on the appearance of excitonic signal in optical conductivity of ZnO. We model the wurtzite ZnO within an 8-band k.p approximation. We calculate the optical conductivity by incorporating the first-order vertex correction derived from the Feynman diagrams. Our calculation up to the first-order correction spectrum qualitatively confirms the existence of excitons in wurtzite ZnO.
Radiative control of dark excitons at room temperature by nano-optical antenna-tip Purcell effect
Park, Kyoung-Duck; Jiang, Tao; Clark, Genevieve; Xu, Xiaodong; Raschke, Markus B.
2018-01-01
Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (XD) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of XD states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the XD emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe2 on a gold substrate, we demonstrate 6 × 105-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 103 of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.
Pressure-Dependent Light Emission of Charged and Neutral Excitons in Monolayer MoSe _{2}
Energy Technology Data Exchange (ETDEWEB)
Fu, Xinpeng [State; Li, Fangfei [State; Lin, Jung-Fu [Department; Gong, Yuanbo [State; Huang, Xiaoli [State; Huang, Yanping [State; Han, Bo [State; Zhou, Qiang [State; Cui, Tian [State
2017-07-19
Tailoring the excitonic properties in two-dimensional monolayer transition metal dichalcogenides (TMDs) through strain engineering is an effective means to explore their potential applications in optoelectronics and nanoelectronics. Here we report pressure-tuned photon emission of trions and excitons in monolayer MoSe2 via a diamond anvil cell (DAC) through photoluminescence measurements and theoretical calculations. Under quasi-hydrostatic compressive strain, our results show neutral (X0) and charged (X–) exciton emission of monolayer MoSe2 can be effectively tuned by alcohol mixture vs inert argon pressure transmitting media (PTM). During this process, X– emission undergoes a continuous blue shift until reaching saturation, while X0 emission turns up splitting. The pressure-dependent charging effect observed in alcohol mixture PTM results in the increase of the X– exciton component and facilitates the pressure-tuned emission of X– excitons. This substantial tunability of X– and X0 excitons in MoSe2 can be extended to other 2D TMDs, which holds potential for developing strained and optical sensing devices.
Yeboah, Douglas; Singh, Jai
2017-11-01
Recently, the dependence of exciton diffusion length (LD ) on some photophysical parameters of organic solids has been experimentally demonstrated, however no systematic theoretical analysis of this phenomenon has been carried out. We have conducted a theoretical study by using the Förster resonance energy transfer and Dexter carrier transfer mechanisms together with the Einstein-Smoluchowski diffusion equation to derive analytical models for the diffusion lengths (LD ) and diffusion coefficients (D) of singlet (S) and triplet (T) excitons in organic solids as functions of spectral overlap integral (J) , photoluminescence (PL) quantum yield (φD ) , dipole moment (μT ) and refractive index (n) of the photoactive material. The exciton diffusion lengths and diffusion coefficients in some selected organic solids were calculated, and we found that the singlet exciton diffusion length (LDS ) increases with φD and J, and decreases with n. Also, the triplet exciton diffusion length (LDT ) increases with φD and decreases with μT . These may be achieved through doping the organic solids into broad optical energy gap host materials as observed in previous experiments. The calculated exciton diffusion lengths are compared with experimental values and a reasonably good agreement is found between them. The results presented are expected to provide insight relevant to the synthesis of new organic solids for fabrication of bulk heterojunction organic solar cells characterized by better power conversion efficiency.