The neutron scattering and exciton spectrum due to impurities in the molecular crystals with the resonance interaction

The differential cross-section of neutron scattering by excitons and the frequency equation for exciton chain with point impurity are obtained by using the exact solution, received in our previous papers [1, 2] for the case of ideal exciton chain problem. All results are compared with those given by the known approximate solution of the same problem [4] based on the approximation of Pauli's operators of creation and annihilation of excitons by Bose's ones.     

Probing Bose-Einstein condensation of excitons with electromagnetic radiation

We examine the absorption spectrum of electromagnetic radiation from excitons, where an exciton in the 1s state absorbs a photon and makes a transition to the 2p state. We demonstrate that the absorption spectrum depends strongly on the quantum degeneracy of the exciton gas, and that it will generally manifest many-body effects. Based on our results we propose that absorption of infrared radiation could resolve recent contradictory experimental results on excitons in Cu(2)O.     

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.     

The trion as an exciton interacting with a carrier

The X⁻ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X⁻ trion creation operator in terms of excitons and electrons. We can also derive the X⁻ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.     

Strong Exciton-Plasmon Coupling and Hybridization of Organic-Inorganic Exciton-Polaritons in Plasmonic Nanocavity

We investigate strong exciton-plasmon coupling and plasmon-mediated hybridization between the Frenkel(F)and Wannier-Mott(WM)excitons of an organic-inorganic hybrid system consisting of a silver ring separated from a monolayer WS_2 by J-aggregates.The extinction spectra of the hybrid system calculated by employing the coupled oscillator model are consistent with the results simulated by the finite-difference time-domain method.The calculation results show that strong couplings among F excitons,WM excitons,and localized surface plasmon resonances(LSPRs) lead to the appearance of three plexciton branches in the extinction spectra.The weighting efficiencies of the F exciton,WM exciton and LSPR modes in three plexciton branches are used to analyze the exciton-polaritons in the system.Furthermore,the strong coupling between two different excitons and LSPRs is manipulated by tuning F or WM exciton resonances.     

Excitons in GaAs quantum wells

This paper attempts to summarize some of the salient properties of excitons in GaAs quantum wells and in doing so it will emphasize work at AT&T Bell Labs with which the authors have been associated. Although the text relies heavily on published material, an effort has been made to stress new material, and where feasible, unpublished aspects, e.g., figures, related to earlier work. Topics discussed on the quasi-2D excitons in GaAs quantum well include: their inherent tendency for intrinsic free-exciton emission, exciton binding energies, bound and localized excitons including biexcitons and excitons bound to neutral impurities, effects of n- and p-type modulation and antimodulation doping, and the developments leading to a proposed set of quantum well parameters that results in acceptable fits to the observed exciton transitions for GaAs quantum wells with both square and parabolic potential profiles.     

多组分纠缠在光子晶体中的存储

研究了激子的多组分纠缠相干态保真度在各向异性光子晶体中的演化行为.结果表明,当激子的跃迁频率处于光子晶体带隙时,保真度随时间变化作周期振荡,这与激子处于真空环境时,保真度振荡衰减的演化行为不同.此外,当激子跃迁频率离光子晶体带边较远时,其多组分纠缠相干态越容易被保存.     

Energy transport by excitons: an application of Mori-Zwanzig theory

The problem of energy transport by excitons is studied by evaluating the memory function starting from the Hamiltonian of the exciton-phonon system and applying the Mori-Zwanzig projection formalism. We thus provide a microscopic derivation of the time-dependent memory function for exciton motion on a linear chain which in the literature has been introduced so far only phenomenologically.     

Lasing threshold in traps for Bose-condensation of dipolar excitons

We consider exciton recombination lasing in heterostructure traps for Bose–Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose–Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose–Einstein condensation of excitons.     

Optically detected magnetic resonance studies of luminescence‐quenching processes in π‐conjugated materials and organic light‐emitting devices

It is widely recognized that nonradiative quenching of excitons by other excitons and polarons become the dominant decay mechanism of these excitons at high excitation densities. These quenching processes cause the roll‐off in the efficiency of organic light‐emitting devices (OLEDs) and prevent lasing at high injection current densities. This review presents the optically‐detected magnetic resonance (ODMR) evidence for these photoluminescence‐ and electroluminescence‐quenching processes. And while it provides such evidence for quenching of singlet excitons by polarons and triplet excitons, it reveals the central role of the strongly spin‐dependent annihilation of triplet excitons by polarons, since under normal excitation conditions the steady‐state polaron and triplet exciton populations are 100–10⁴ times the singlet exciton population. In addition, it also suggests that quenching of singlet excitons by bipolarons, likely stabilized by a counterpolaron or countercharge at specific sites, may also be a significant quenching mechanism that also affects the charge transport properties.     

Effects of excitation spectral width on decay profile of weakly confined excitons

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.     

Exciton interaction potential in polar crystals

The problem whether polaron effects can lead to a repulsive barrier in the exciton interaction potential is reinvestigated, because this effect is crucial for the possibility of a Bose-Einstein condensation of excitons (rather than of excitonic molecules). The energy for two excitons is calculated accurately as a function of their mean distance in the case of a heavy hole mass by using an integral expansion method of molecular physics. The resulting interaction potential shows for certain polaron radii a very weak repulsive barrier. Taking into account corrections to the Haken potential which have been introduced by Pollmann and Büttner, it is shown that the interaction potential between two excitons in the relative singlet state is always attractive, so that the exciton system is also in polar materials unstable against exciton molecule formation.This is a project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft     

Effects of uniaxial stress on luminescence of GaP(S)

The effect of a uniaxial stress on the luminescence spectra of the neutral-donor bound exciton complex is investigate and the deformation potentials determined. The analysis of the uniaxial-stress data shows that the shift and splitting of the bound excitons are well accounted for by a simple model where the deformation potentials of the bound excitons directly reflect the free exciton behaviour.     

Momentum redistribution times of 2D excitons measured by transient resonantly induced intersubband absorption

We apply a transient interband-pump–intersubband-probe technique, to directly measure the time it takes for resonantly photoexcited excitons in GaAs/AlGaAs superlattices to redistribute in momentum space. We determine the redistribution time and its excitation density and superlattice periodicity dependence from the temporal evolution of the conduction intersubband absorption spectrum.We find that resonantly excited heavy-hole excitons, at moderate densities, redistribute slowly and reach thermal distribution within a few tens of ps after the pulsed excitation. This redistribution time is nearly inversely proportional to the square root of the initial density of the photoexcited excitons and it depends on the periodicity of the superlattice structure. The smaller the periodicity in direct space is, the longer is the redistribution time. This is due to the relatively inefficient exciton–exciton scattering, and the small momentum that each resonantly excited exciton carries. From measurements performed on three samples of different periodicity we find that the redistribution time increases faster than the superlattice Brillouin-zone length squared.     

共轭高分子中激子演化过程的非绝热动力学研究

有机共轭高分子受光激发或被电荷掺杂后可能会产生各种激发状态的激子,激子的演化过程对有机发光光谱有着至关重要的影响.通过非绝热动力学演化的方法模拟了受光激发后有机高分子中激子驰豫的动力学过程,结果表明高激发态激子不稳定,由于电声耦合作用,高激发态激子会持续向低激发态激子演化,同时,低激发态激子的复合发光会发生红移.稳定的激子复合发光光谱中,基态激子发光强度最大,可高达70-80%;第一激发态激子及其它激发态激子发光强度的总和不超过20%.     

Exciton states in alkali halides

Recent experiments based on modulation spectroscopy have shown that it is possible to detect exciton levels in alkali halides up to n = 4. Therefore we worked out numerical calculations in order to predict the whole exciton series in KI and RbI. In our calculations the deep exciton levels are treated by considering the actual hole-electron interaction, whereas the effective mass approximation is used for the shallow exciton levels. The direct and exchange terms of the hole-electron interaction have been evaluated by performing three and four center integrals, the Wannier wave functions appearing in such integrals being approximated by suitable gaussian expansions of atomic orbitals.It is shown that by allowing the exciton state to extend up to 42 shells of neighbors it is possible to predict the exciton levels up to n = 2, the n = 3, 4 excitons being accounted for by the effective mass approximation. Similar computations performed for excitons in solid rare gases were found in excellent agreement with the experimental data and confirmed the reliability of our method.     

Effect of disorder on exciton dissociation in conjugated polymers

By using a multi-configurational time-dependent Hartree–Fock(MCTDHF) method for the time-dependent Schr ?dinger 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.     

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×10¹⁷/cm³. The upper bound lifetime is determined by the excitonic relative motion.     

Coherence length of cold exciton gases in coupled quantum wells

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.     

Decay of excitonic gratings: Effect of exciton diffusion and reabsorption of exciton fluorescence

A phenomenological theory of the decay time of an excitonic grating (a grating of exciton densities) formed at some initial moment in a plane-parallel plate of a molecular crystal has been developed. Along with the diffusion of excitons reabsorption of the fluorescence light radiated by excitons is also taken into account. It is shown that in crystals with large overlapping of absorption and emission spectra the life time of the exciton grating is diminished appreciably by reabsorption. Numerical estimations for this life for anthracene at room temperature were made. In the above conditions it is the reabsorption which is important, whereas the diffusion of excitons hardly affects the decay time of the excitonic grating. The available experimental data and possible trends of further investigations are discussed.