Interference of polariton waves in structures with wide GaAs/AlGaAs quantum wells

The optical reflection spectra of semiconductor GaAs/AlGaAs structures with wide quantum wells are studied experimentally. A theoretical analysis of the spectra is performed in terms of the exciton-polariton model in the approximation of quantum confinement of the exciton center of mass with regard to the contributions of both heavy and light excitons to the crystal polarization. The applicability range of the theory of the center-of-mass confinement for GaAs/AlGaAs heterostructures is estimated. It is established that, for quantum wells more than 180 nm wide, the interference effects observed in the reflection spectra of polariton waves are reproduced, to a good accuracy, by theoretical calculations based on the quantum confinement of the exciton center of mass. For quantum-well widths less than 150 nm, the experimental results are described better by the model of quantum confinement of electrons and holes.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Temperature properties of exciton luminescence from CdTe quantum wells with different thicknesses in the CdTe/CdMnTe structure

The optical spectra of the CdTe/Cd_0.7Mn_0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.     

Hybrid excitons in organic–inorganic semiconducting quantum wells in a microcavity

We investigate theoretically the optical properties of composite organic–inorganic semiconductor quantum wells. These properties are dominated by hybrid Frenkel (or charge-transfer) and Wannier–Mott excitonic states. An important effect is the possibility of using the Stark shift to tune the resonance between Frenkel and Wannier–Mott excitons. This fact is very important from a practical point of view because it may be difficult to grow such a structure exactly at resonance. We also discussed the coupling of Frenkel or charge transfer and Wannier–Mott exciton through a microcavity photon. We evaluate the hybrid exciton-polariton Rabi splitting. In the strong coupling regime the Rabi splitting depends essentially on the oscillator strength of the Frenkel or charge-transfer exciton.     

Polaron effects on excitons in parabolic quantum wells: fractional-dimension variational approach

Polaron effects on excitons in parabolic quantum wells are studied theoretically by using a variational approach with the so-called fractional dimension model. The numerical results for the exciton binding energies and longitudinal-optical phonon contributions in GaAs/Al_0.3Ga_0.7As parabolic quantum well structures are obtained as functions of the well width. It is shown that the exciton binding energies are obviously reduced by the electron (hole)-phonon interaction and the polaron effects are un-negligible. The results demonstrate that the fractional-dimension variational theory is effectual in the investigations of excitonic polaron problems in parabolic quantum wells.     

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.     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Femtosecond dynamics of secondary radiation formation from quantum well excitons

The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum wells is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theory for Rayleigh scattering from two-dimensional excitons subjected to disorder. These rise times are compared directly with times measured by time-integrated four-wave mixing (FWM). The comparison of the dynamics displayed in time-resolved secondary radiation and time-integrated FWM provide a clear understanding of the coherence properties of QW excitons in the first few picoseconds after excitation. High-contrast oscillations that are due to quantum beats between the heavy- and light-hole 1s-states are seen. The visibility decay at very low densities is long ps and is related to the action of potential fluctuations on the scattering of heavy-hole and light-hole excitons.     

Absorption measurements at high pressure (0–10kbar) on strained superlattices

Absorption data on strained GaAs_1?xP_x-GaAs superlattices (SL, 128-period, barrier size L_B≈75 Å, quantum-well size L_z≈75 Å, alloy composition x≈0.25) are presented in the range 0–10 kbars. The absorption curves obtained show no exciton show no exciton peaks such as seen in lattice- matched Al_xGa_1?xAs-GaAs SL's, and the pressure coefficient decreases from 11.5 meV/kbar to ≈ 10.5 meV/kbar in the wells and ≈6.5 meV/kbar at energies approaching and above the barrier energies. This behavior is attributed to the fluctuations in strain caused by the alloy disorder, and clustering, of the barriers.     

Crossover from exciton to biexciton polaritons in semiconductor microcavities

Pump-probe measurements in a microcavity containing a quantum well show that a population of circularly polarized ( sigma(+)) excitons can completely inhibit the transition to sigma(-) one-exciton states by transferring the oscillator strength to the biexcitonic resonance. With increasing pump intensity the linear exciton-polariton doublet evolves into a triplet polariton structure and finally into a shifted biexciton-polariton doublet. A theoretical model of interacting excitons demonstrates that the crossover from exciton to biexciton polaritons is driven by three-exciton Coulomb correlation.     

Resonance Bragg structure with double InGaN quantum wells

The effect of exciton-polariton resonance on the optical properties of periodic heterostructures with double InGaN quantum wells in a GaN matrix has been studied. It has been found that the light reflection is amplified at the frequency corresponding to the exciton energy when it coincides with the frequency of the Bragg resonance. This effect is observed to be twice as large as that in a similar system of single quantum wells.     

Collective behavior of interwell excitons in GaAs/AlGaAs double quantum wells

Photoluminescence spectra of interwell excitons in double GaAs/AlGaAs quantum wells (n-i-n structures) have been investigated (an interwell exciton in these systems is an electron-hole pair spatially separated by a narrow AlAs barrier). Under resonance excitation by circularly polarized light, the luminescence line of interwell excitons exhibits a significant narrowing and a drastic increase in the degree of circular polarization of photoluminescence with increasing exciton concentration. It is found that the radiative recombination rate significantly increases under these conditions. This phenomenon is observed at temperatures lower than the critical point and can be interpreted in terms of the collective behavior of interwell excitons.     

Interface-related effects on confined excitons in GaAs/AlxGa1−xAs single quantum wells

Confined excitons in non-abrupt GaAs/Al_xGa_1−xAs single quantum wells are studied. The graded interfaces are described taking into account fluctuations in their thickness a and positioning with respect to the abrupt interface picture. Numerical results for confined (0,0),(1,1) and (0,2) excitons in GaAs/Al_0.3Ga_0.7As quantum wells show that while the interfacial fluctuations produce small changes (<0.5 meV) in the exciton binding energies, the confined exciton energies can be red- or blue-shifted as much as 25 meV for wells with mean width of 50 Å and 2 ML wide interfaces.     

Inter-island energy transfer and in-plane exciton migration in AlGaAs/GaAs quantum wells detected by exciton dynamics

We present the results of optical, steady-state and time-resolved studies of photoluminescence and photoluminescence excitation in high-quality Al_0.3Ga_0.7As/GaAs quantum wells in which the presence of large (larger than the exciton radius) atomically flat islands can be inferred, identical to the case of interrupted MBE growth. Migration of excitons towards lower-lying energy states induced by local potential fluctuations and/or progressive localisation has been revealed and the transition rate between quantum well regions 24 to 25 monolayers thick has been derived to be 290 ps⁻¹.     

Linear and nonlinear optical properties of excitons in semiconductor quantum wells and microcavities

Linear and nonlinear light propagation in single and multiple quantum wells and in semiconductor microresonators are studied on the basis of Maxwell’s equations. The treatment includes radiative broadening of quantum-confined excitons, radiative coupling between quantum wells as well as coupling of quantum wells to the cavity field of a microresonator for steady state or ultrashort pulse excitation. The dynamical evolution of the coherent quantum-well polarization under the influence of many-body effects is studied within a microscopic model. The theory is used to investigate the influence of exciton saturation and dephasing on pulse propagation and excitonic normal-mode coupling.     

Spontaneous light emission from fractional-dimensional excitons

We apply the concept of fractional-dimensional excitons to study the process of light emission in quantum confined systems. We focus on a single parameter , known as the degree of dimensionality and which is related to the exciton coherence volume. We compute rates of light emission due to free excitons in GaAs/Al_xGa_1-xAs quantum wells as functions of and link them to experimental observations. The rates are compared with those of quantum well excitons embedded in a microcavity.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Creation of spin-oriented 2P excitons in CdS by two-photon magneto-absorption

We report experimental results on the creation of 2P excitons in CdS by the two-photon magneto-absorption process involving a simultaneous absorption of visible and infrared photons. For a magnetic field parallel to the c axis and particularly for a visible beam propagating along the c axis, spin-oriented 2P (A) excitons are created from the A (Γ₉) valence band. We explain these results on the basis of the main contribution of the 1SΓ₅ (A) exciton intermediate state.     

DC transformer: Dream or reality?

We propose to use the exciton coupling between electrons and holes in different quantum wells to reach a strong Coulomb drag effect. The drag has to be really strong below the Mott transition when the most of the carriers are bound in excitons. We suggest to use the exciton drag for fabrication of DC transformer. Preliminary estimates for Si/SiO₂/Si structure give the Mott transition temperature of the order of 100 K.     

Specific features in reflectance and absorbance spectra of one-dimensional resonant photonic crystals

The optical spectra of resonant Bragg and quasi-Bragg quantum-well structures are studied theoretically. The existence of special frequencies in the reflectance and absorbance spectra at which the reflectance or absorbance does not depend on the number of quantum wells in a structure is explained analytically. It is shown that the reflectance and absorbance spectra of structures in which the quantum-well width substantially exceeds the exciton Bohr radius also contain special frequencies and that the smooth spectral component is determined primarily by the interaction of the light wave with the quantum-well ground-state exciton.