Instabilities and Self-Organization Phenomena in High Density Exciton Systems

The influence of final value of exciton lifetime on exciton condensed phase formation is investigated in deterministic and stochastic approach. It is shown that in some region of pumping, the periodical distribution of exciton density takes place. The numerical solution for periodical distribution of exciton density is obtained for different values of pumping. There are critical values of exciton lifetime for appearance of exciton condensed phase. The distribution density function becomes narrower with increasing pumping.     

Exciton condensation in semiconductor quantum wells with a periodical modulation of potential

The appearance and properties of a structure in the density distribution of indirect excitons in coupled quantum wells in semiconductor alloys upon periodical field modulation are studied. Calculations showed that besides periodical dependence of density distribution, caused by the potential modulation, a stratification of the exciton density shafts into separate islands in the cross direction arises. Appearance of islands is the result of exciton condensed phase formation and a nonequilibrium state of the system due to the finite exciton lifetime and pumping presence. The dependence of the structure on system parameters (the pumping value, the modulated field depth and period) is investigated. Also the influence of the exciton–exciton annihilation is taken into account.     

Exciton band structure of crystalline anthracene

The density of states in the first exciton band of crystalline anthracene has been obtained at room, dry-ice-alcohol, and liquid nitrogen temperatures by applying a thermal modulation technique to observe the hot band-first exciton band optical transition (1-0 transition). The density of states function obtained has been interpreted in terms of the exciton band structure and the Davydov splitting. The K dependent selection rule for the 1-0 transition is discussed. The bandwidth of the first exciton band is 0.09 eV.     

Entropy ionization of an exciton gas

Experimental evidence is presented demonstrating ionization of an exciton gas as its density is lowered at constant temperature. The measurements are made by monitoring the luminescence from a photo-excited sample of high-purity, single-crystal germanium in which the exciton density is nearly uniform.     

Exciton-plasma transition in GaAs

In this work, we study the stability of excitons at high density, i.e. we calculate the reduction of the exciton binding energy due to exciton-exciton interactions in a high-density exciton gas. We derive first the effective electron-hole interaction in the presence of free carriers and excitons. We use the static approximation. The exciton binding energy is calculated by the variational technique. The computations are specialized to GaAs. We investigate the critical density when the exciton binding disappears, which corresponds to the exciton plasma transition. We conclude that this transition occurs at higher density than the reverse plasma exciton transition, determined by the standard criteria a₀q_D =1.19 [Rogers F. J., Graboske H. C., Jr. and Harword D. J., Phys. Rev.A1, 1577 (1970)].     

Quantum-Statistics Behavior of the Exciton-Biexciton System In GaAs/AlAs Type-II Superlattices

We have investigated the quantum-statistics behavior of the exciton-biexciton system from the photoluminescence properties in (GaAs) m /(AlAs) m type-II superlattices with m = 12 and 13 monolayers, where the lowest-energy type-II exciton consists of the n = 1 X electron of AlAs and n = 1 o heavy hole of GaAs. The long exciton lifetime of the order of w s due to the indirect transition nature enables us to obtain precisely the density relation between the exciton and biexciton from the line-shape analysis of time-resolved photoluminescence spectra. In a relatively low exciton-density region, the biexciton density obeys a well-known square law. At an exciton density around 1 2 10 10 cm m 2 , the biexciton density suddenly increases with a threshold-like nature. This behavior, which is realized at a bath temperature up to 8 K under an excitation power of the order of 100 mW/cm 2 , results from the characteristics of Bose-Einstein statistics of the exciton-biexciton system.     

Influence of acoustic phonons on dephasing of free excitons in quantum wells

A theory of the dephasing rate of quasi-2D free excitons due to acoustic phonon interaction at low exciton densities is presented. Both deformation potential and piezoelectric couplings are considered for the exciton–phonon interaction in quantum wells. Using the derived interaction Hamiltonian obtained recently by us, exciton linewidth and dephasing rate are calculated as a function of the exciton density, exciton temperature, exciton momentum and lattice temperature.     

Movement and amplification of exciton condensed phase pulses and interaction between pulses in semiconductor quantum wells

Formation and movement of an exciton pulse in an inhomogeneous potential are studied. It is shown that the pulse does not blur and the maximum of the exciton density in the pulse remains constant during the exciton lifetime if the pulse is formed from the condensed phase. The path, traversed by the excitons, can be increased by imposing an additional laser pulse on the system. Thereby, such a system can be used for information transmission over the exciton condensed phase.     

Bose condensation of interwell excitons in lateral traps: A phase diagram

The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructures) containing large-scale random-potential fluctuations was studied. The study dealt with the properties of an exciton whose photoexcited electron and hole are spatially divided between the neighboring quantum wells under density variation and at temperatures of down to 0.5 K. We investigated domains ∼1 μm in size, which act as macroscopic exciton traps. Once the resonance laser pump power reaches a certain threshold, a very narrow delocalized exciton line appears (with a width less than 0.3 meV), which grows strongly in intensity with increasing pump power and shifts toward lower energies (by approximately 0.5 meV) in accordance with the exciton buildup in the lowest state in the domain. As the temperature increases, this spectral line disappears in a nonactivated manner. This phenomenon is assigned to Bose condensation occurring in the quasi-two-dimensional system of interwell excitons. The critical exciton density and temperature were determined within the temperature interval studied (0.5 to 3.6 K), and a phase diagram specifying the exciton condensate region was constructed. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 168–170. Original Russian Text Copyright ? 2004 by Dremin, Larionov, Timofeev.     

Vibronic structure and the anisotropy of excitons in cubic LiH single crystals

The shape of one-phonon sidebands of exciton in LiH is studied both in reflection and edge luminescence spectra. The main structure of the shape is found to reflect the phonon density of states. This fact is interpreted as a result of a high anisotropy of the exciton band.     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Resonant Photon Drag of Dipolar Excitons

The theory of the photon drag of dipolar excitons in double-quantum-well nanostructures is presented. It is shown that the exciton-drag flux density features a resonant behavior if the photon frequency is close to some transition frequency in the discrete exciton spectrum. When the structure is irradiated with polarized light, the resonant enhancement of the drag current occurs when the photon energy coincides with the energy of an excited level of the exciton internal motion and the components of the angular momentum of internal motion in the initial and final states differ by one. The proposed effect can be used to control exciton transport in nanostructures based on a two-dimensional exciton gas.     

Spin-flip limited exciton dephasing in CdSe/ZnS colloidal quantum dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.     

Acousto-exciton interaction in a gas of 2D indirect dipolar excitons in the presence of disorder

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.     

Self-induced transparency of excitons in semiconductors

Self-induced transparency of excitons is analysed in a model where intraband processes are described by density matrices and interband processes by coherent pair amplitudes. We determine the dispersion law of the carrier wave and the exciton wave function. The theory predicts: (a) a forbidden energy gap centered at the exciton line which broadens with increasing intensity, (b) a critical dependence of the effect on intensity, (c) a strong influence of intensity and carrier frequency on the exciton wave function.     

Will a bose-condensed exciton gas be superfluid?

For a system of Bose-condensed Wannier excitons, the second reduced density matrix for electrons and holes is shown to have the property of off-diagonal long-range order (ODLRO). From the equation of motion for this density matrix, we derive the conservation laws which are relevant for a condensed exciton system. These equations are obtained by projecting the electron-hole density matrices into the exciton space. From the conservation laws, a two-fluid model follows, which describes the superfluid flow of the excitation energy.     

Ultrafast nonlinear optical response and high density excitation effects of the stacking fault exciton in BiI3

On the exciton states localized at a two-dimensional stacking fault interface in a layered crystal BiI₃, some nonlinear optical phenomena clearly appear reflecting large transition probability. The optical Stark shift and other high density exciton effect on the energy-shift and the spectral broadening are observed with clear separation under intense laser pumping by time-resolved measurements. The optical Stark shift is analyzed based on the dressed exciton model. The ultrafast optical response faster than 3 ps for the pump-laser field is confirmed on the Stark shift. Degenerate four-wave-mixing signals show fairly long dephasing time of 40 ps in this system. The dephasing probability depends linearly on the pump-laser intensity in the same manner as that of the spectral line-broadening reflecting the relaxation process. The dephasing mechanisms are understood by the exciton scattering at high density in parallel with the spectral changes. The blue-shift due to the high density excitons are discussed on the basis of exciton-exciton interaction in connection with a phase-space filling theory in two-dimensional systems.     

Exciton linewidth due to scattering from free carriers in semiconducting quantum well structures

The contribution to the exciton linewidth in semiconducting quantum well structures due to the scattering of excitons by free carriers is calculated. It is found that this contribution becomes very important in limiting the exciton linewidth when a high density of free carriers is present or at low temperatures where the scattering of the excitons by optical and acoustic phonons is reduced. This contribution to the linewidth in quantum well structures is found to increase with the free carrier concentration and to extremely broaden and exciton peak at high carrier concentrations. At lower carrier concentrations, where the carriers behave as a nondegenerate gas of particles, the contribution to the exciton linewidth due to scattering by free carriers increases with temperature.     

A unified model of preequilibrium decay

A microscopic exciton model has been developed which describes the time evolution of a highly excited nucleus through a series of binary collisions creating additional particle-hole pairs. Starting from an arbitrary exciton density distribution, the spectral shapes of the succeeding stages are generated through recursion relations which keep track of the “exciton flux” explicitely. The computer code BEEFALO provides particle spectra and average life times of the successive stages. The results are compared with the predictions of other preequilibrium models using “equal probability” density distributions. The controversy about the use of single particle orn-exciton state life times has been resolved by showing that both approaches lead to the same particle spectra if proper book-keeping is made through appropriate recursion relations.     

半导体中光的相干传播理论(Ⅱ)

本文是关于半导体中光的相干传播理论的第二部分。考虑到电子空穴间的相互作用,我们讨论了光波与半导体的相互作用,得到了描述光激发电子空穴极化波的方程。我们指出只有在激发比较弱时,这组方程才可近似为一组线性方程,电子空穴极化波才可以看作玻色场。本文还着重讨论了分立的激子谱线的相干激发,证明了它可以近似等效于一个二能级系统的激发,等效的二能级系统的能级差和激发的程度有关,等效的二能级“原子”的“浓度”由激子波函数的性质决定。 关键词：