The effects of indium segregation on exciton binding energy and oscillator strength in GaInAs/GaAs quantum wells

We solve analytically the Schrödinger equation taking into account the shape changes of GaInAs/GaAs quantum wells due to indium segregation during the MBE growth by using transfer matrix method. The indium compositional profiles of the quantum wells are provided using the phenomenological model. The fundamental transition energy, binding energy and oscillator strength of excitons as a function of indium segregation coefficient R$R$ and well width are studied. For narrow wells (less than 40 ML), the exciton binding energy and oscillator strength decrease, but for wide wells (larger than 40 ML), increase with increasing the segregation coefficient R$R$. It is shown that indium segregation degrades the optical properties and results in a blue-shift of exciton transition energy in GaInAs/GaAs quantum wells.     

Control of the oscillator strength of the exciton in a single InGaN-GaN quantum dot

We report direct evidence for the control of the oscillator strength of the exciton state in a single quantum dot by the application of a vertical electric field. This is achieved through the study of the radiative lifetime of a single InGaN-GaN quantum dot in a p-i-n diode structure. Our results are in good quantitative agreement with theoretical predictions from an atomistic tight-binding model. Furthermore, the increase of the overlap between the electron and hole wave functions due to the applied field is shown experimentally to increase the attractive Coulomb interaction leading to a change in the sign of the biexcitonic binding energy.     

High-resolution nonlinear laser spectroscopy of exciton relaxation in GaAs quantum wells

This paper describes measurements of exciton relaxation in GaAs/AlGaAs quantum well structures based on high resolution nonlinear laser spectroscopy. The nonlinear optical measurements show that low energy excitons can be localized by monolayer disorder of the quantum well interface. We show that these excitons migrate between localization sites by phonon assisted migration, leading to spectral diffusion of the excitons. The frequency domain measurements give a direct measure of the quasi-equilibrium exciton spectral redistribution due to exciton energy relaxation, and the temperature dependence of the measured migration rates confirms recent theoretical predictions. The observed line shapes are interpreted based on solutions we obtain to modified Bloch equations which include the effects of spectral diffusion.     

Phonon heating of two-dimensional exciton gases in GaAs/AlGaAs quantum wells

We report the first measurements of the interaction of non-equilibrium phonons with two-dimensional exciton gases (2DExGs). The rise in the effective temperature of the 2DExG produced by the phonons depends on the width of the quantum well and the exciton sheet density and hence on the ratio τ^?1 (ex-ph)/τ^?1 (ex-ex). The dependence of the effective temperature rise on this ratio is attributed to the non-equilibrium frequency distribution of the phonons incident on the 2DExG.     

Long-range spatial correlations in the exciton energy distribution in GaAs/AlGaAs quantum wells

Variations in the width of a quantum well (QW) are known to be a source of broadening of the exciton line. Using low temperature near-field optical microscopy, we have exploited the dependence of exciton energy on well width to show that in GaAs QWs, these seemingly random well-width fluctuations actually exhibit well-defined order-strong long-range correlations appearing laterally, in the plane of the QW, as well as vertically, between QWs grown one on top of the other. We show that these fluctuations are correlated with the commonly found mound structure on the surface. This is an intrinsic property of molecular beam epitaxial growth.     

Bose-einstein condensation of exciton polaritons in high-Q planar microcavities with GaAs quantum wells

The walls of the box which contains matter represent a membrane that allows the relativistic quantum vacuum to pass but not matter. That is why the pressure of matter in the box may be considered as the analog of the osmotic pressure. However, we demonstrate that the osmotic pressure of matter is modified due to interaction of matter with vacuum. This interaction induces the nonzero negative vacuum pressure inside the box, as a result the measured osmotic pressure becomes smaller than the matter pressure. As distinct from the Casimir effect, this induced vacuum pressure is the bulk effect and does not depend on the size of the box. This effect dominates in the thermodynamic limit of the infinite volume of the box. Analog of this effect has been observed in the dilute solution of ³He in liquid ⁴He, where the superfluid ⁴He plays the role of the non-relativistic quantum vacuum, and 3He atoms play the role of matter.     

Photoluminescence saturation in InGaAs/GaAs single quantum wells

Saturation of the photoluminescence associated with the 11H transition in the InGaAs single quantum wells is observed under high intensity optical excitation. At the onset of saturation, a spill-over of the photoluminescence occurs into the GaAs cladding layers as the excitation intensity is increased. The measurements are used to determine a limiting value of the quantum efficiency of the quantum-well associated photoluminescence.     

Minimum of oscillator strength of excitons in ultra-narrow GaAs/AlGaAs quantum wells:: Theory and experiment

A theoretical and experimental study of the exciton in ultra-narrow quantum wells (QWs) is performed. A crossover from strong (separate localization of electron and hole levels) to weak confinement (with localization of excitonic center of mass) is predicted to occur for decreasing thickness, and is characterized by a minimum of the oscillator strength per unit area. Cathodoluminescence measurements performed on a series of GaAs/Al_0.35Ga_0.65As QWs with thicknesses from one to eight monolayers show a minimum of the oscillator strength, in agreement with theory, and indicate that the crossover from strong to weak confinement occurs at a thickness of about three monolayers for this composition.     

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.     

Investigation of the mechanisms of exciton coherence relaxation in single GaAs/AlGaAs quantum wells by the methods of excitonic induction

The mechanisms of exciton coherence relaxation in GaAs quantum wells in the linear mode have been experimentally investigated. An experimental technique has been developed for measuring the total phase relaxation rate, rates of reversible and temperature-irreversible excitonic phase relaxation, and the radiative decay rate Γ _R of excitonic polarization. The experimental values of Γ _R for a quantum well of specified thickness, obtained for a series of samples, have a spread not larger than 15%. This accuracy made it possible to estimate the shape of the dependence of Γ _R on the well thickness L _Z . It is experimentally found that Γ _R is temperature-independent up to 80 K.