Cross-shaped photoluminescence of excimers in perylene crystals

Cross-shaped excimer (self-trapped exciton) luminescence from α- and β-perylene single crystals of 50–100 μm was found when they were excited at the center of the crystals with a continuous-wave (cw) laser resonant with the exciton absorption. The cross shape is formed by the two lines which intersect at the excited position and are perpendicular to the sides of the crystals of parallelogram shape. Luminescence is emitted from the excited spot and 4 side edges in the cross shape. The most striking feature is that the luminescence intensity at the edges was as high as or higher than at the excited spot. The possibility of the exciton propagation or the waveguide effect is rejected both experimentally and theoretically. This phenomenon can be reasonably explained only when the radiative transition probability of excimers is significantly enhanced at the crystals side edges than at the center due to the lower symmetry.     

Effect of localized-exciton energy relaxation on the emission spectrum of ZnS-ZnSe single quantum wells

A study is reported on the dependence of exciton photoluminescence spectra of ZnS-ZnSe quantum wells with different localization-center concentrations on the excitation intensity and temperature. The shape of the experimental low-temperature photoluminescence band is shown to agree well with that calculated in a model of exciton hopping to the nearest localization center and in one that takes into account transitions of a localized exciton to all centers in its local environment. The parameters characterizing localized excitons in these quantum structures of a submonolayer thickness have been determined.     

Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.     

Exciton and biexciton fine structure in single elongated islands grown on a vicinal surface

We report a microphotoluminescence study of the exciton and the biexciton localized in very elongated islands formed by well-width fluctuations in a thin CdTe/CdMgTe quantum well grown on a vicinal surface. The electron-hole exchange interaction in a local reduced symmetry splits the exciton states. The resulting transitions are linearly polarized along the two orthogonal principal axes of the island. The valence band mixing induced by the elongated shape of the potential leads to a strong polarization anisotropy and to the observation of dark exciton states under magnetic field. The biexciton-exciton transition reproduces all the fine structure of the exciton state including the transition of the biexciton to the dark exciton state.     

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Exciton quantization in parabolic quantum wells

The exciton wavefunction in parabolic quantum wells is calculated using variational techniques and effective mass theory. The influences of the potential shape and of confinement on the exciton binding energies are studied. The results are in good agreement with previous calculations. The oscillator-strength of excitons in GaAs/Ga_1-xAl_xAS quantum wells has a maximum value very close to the cross-over from three to two dimensions.     

A model for exciton bound to a donor pair in silicon

A model that an exciton is bound to a pair of donors is proposed to explain the characteristic features of photoluminescence in phosphorus doped silicon, recently observed by Shiraki and Nakashima. In order to substantiate the model, the energy of the exciton bound to a pair of donors is estimated. It is found that calculated line shape for photoluminescence is quite satisfactory both in magnitude of its line width and dependence of its symmetry on the donor concentration.     

Near-field optical mapping of exciton wave functions in a GaAs quantum dot

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.     

Exciton localization at islands in quantum-well structures

Exciton localization at islands of cylindrical shape in quantum-well structures has been studied theoretically. It is shown that an exciton localizes at such islands, as a rule, as a whole. The validity regions of the adiabatic approximation in factorization of the exciton wave function have been established. The variation in the character of localization with increasing quantum-well barrier height is analyzed. Fiz. Tverd. Tela (St. Petersburg) 39, 1871–1874 (October 1997)     

Wannier exciton in microcrystals

A simple variational calculation is presented of the ground state properties of the electron-hole system confined in three-dimensional quantum wells with spherical shape. As the radius of the wall is reduced to a few times the effective Bohr radius of the bulk exciton, the character of the ground state changes abruptly but continuously from that of a Wannier exciton confined as a quasiparticle to that of the electron and the hole confined as individual particles. The optical experiments on CuCl microcrystals in alkali halides and in silicate glasses are briefly discussed.     

量子点单光子源及其制备方法研究进展

本文概述了量子保密通讯中面临的三大关键技术。并对关键技术之一的单光子源技术进行了重点介绍，如量子点作为单光子源的研究概况，单光子发射的实现、激子跃迁、双激子跃迁引起的聚束和反聚束效应和光子相关测量等。并介绍了国际上制备半导体自组装量子点在形貌、发光谱、极化以及近场光学用于量子点极化和激子发射等研究方面所取得的进展。     

Exciton absorption parameters in TlGaS2 crystals

It is shown that, because the shape of the exciton absorption curve in crystalline TlGaS₂ is described by the Fano antiresonance profile, the experimentally observed exciton peak corresponds to a modified state which is the result of the configuration interaction of a discrete state (exciton) with the quasi-continuum of conduction-band states. The oscillator strength for the transition to the discrete (“pure”) exciton state is calculated as F ₀=1.22×10^?2. The exciton transition selection rules are calculated for two assumed symmetry groups, D _2h and D _4h . An analysis of the selection rules for the dipole-allowed exciton transition permits one to conclude that the symmetry group for the TlGaS₂ crystal is D _2h .     

Exciton bound to an ionized donor impurity in GaAs/Ga1−xAlxAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga_1−xAl_xAs ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.     

Shape of recombination lines for exciton complexes in quantum dots with in-plane electric field

We study electron–hole recombination lines of exciton (X) and exciton complexes (X−, X+, 2X) in planar quantum dots with the electric field oriented within the plane of confinement. A model of a two-dimensional circular infinite quantum well is applied and the ground state of the complexes is found using an exact diagonalization method. We demonstrate that for each of the exciton complexes the recombination lines become non-monotonic for some material and sample parameters as a result of Coulomb interactions. A phase diagram for the line shape is presented. The relation of the exact results to the mean field approach is also discussed.     

Quantum algorithm for distributed clock synchronization

We study theoretically the coherent nonlinear optical response of doped quantum wells with several subbands. When the Fermi energy approaches the exciton level of an upper subband, the absorption spectrum acquires a characteristic double-peak shape originating from the interference between the Fermi-edge singularity and the exciton resonance. We demonstrate that, for off-resonant pump excitation, the pump-probe spectrum undergoes a striking transformation, with a time-dependent exchange of oscillator strength between the Fermi-edge singularity and exciton peaks. This effect originates from the many-body electron-hole correlations which determine the dynamical response of the Fermi sea.     

Excitons in tubular molecular aggregates

We present a brief overview of recent work on the optical properties of molecular aggregates with a tubular (cylindrical) shape. The exciton states responsible for these properties can be distinguished with regard to a transverse wave number, which directly relates to optical selection rules and polarization direction of the associated absorption line. We discuss two types of analytical solutions for the exciton wave functions and the associated absorption and dichroism spectra.     

Storage and reconstruction of quantum information using exciton resonance in a solid

The methods for storage and reconstruction of quantum information in a crystal with impurity ∧ atoms are studied in the case when the upper state is located in the exciton band. The upper level of the impurity atom forms a Fano resonant state similar to the autoionization atomic state due to the configuration interaction with the continuum of the exciton band. The slowing of light pulses is realized in the presence of a control field, so that the group velocity appears much lower than the velocity of light in vacuum. The processes are considered in the adiabatic limit of the field activation. The possibility for the storage and reconstruction of a quantum pulse is studied in the case of the instantaneous on-off switching of the control field. It is demonstrated that the shape and quantum state of the light pulse are preserved in the case of its propagation and reconstruction. The polariton states are obtained and their properties are studied for the case of transitions through the exciton resonance.     

Anisotropic exchange splitting of excitonic levels in small quantum systems

A theory of long-range exchange interaction between an electron and a hole in a zero-dimensional exciton has been developed taking into account the retardation effect. It has been shown that, for the anisotropic localizing potential in bulk semiconductors or semiconductor nanostructures, the long-range interaction leads to a splitting of radiative localized-exciton levels. Polarization of split sublevels is determined by the shape of the exciton envelope function. For excitons localized in quantum wells, typical splitting is expected to be tens of μ eV in agreement with recent experiments.     

Size distribution and concentration of islands of a condensed exciton phase in a quantum well

A theory describing the formation of condensed-phase regions at a high concentration of excitons in a quantum well is constructed. The condensed phase can be either an exciton or an electron-hole liquid. When the condensed phase and exciton gas coexist, islands of the condensed phase have the shape of disks. A simultaneous solution to the kinetic equation (determining the island sizes) and the diffusion equation for excitons outside the islands is obtained for stationary pumping. It is assumed that the exciton gas is nondegenerate outside the islands. The mutual influence of islands through the concentration fields of excitons is taken into account assuming that the mean radius of islands substantially exceeds the mean distance between them. The radius distribution and concentration of islands are determined as functions of the rate of exciton production and the parameters of the system. It is found that the radius distribution of islands is broadened near the threshold of formation of the condensed phase.