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World Academy of Science, Engineering and Technology, International Journal of Mathematical, Computational, Physical, Electrical and Computer Engineering

The Emission Spectra Due To Exciton-Exciton Collisions In Gaas/Algaas Quantum Well System

Surendra Kumar Pandey

Published Nov 28, 2012• 0 citations
Publisher

Abstract

Optical emission based on excitonic scattering processes becomes important in dense exciton systems in which the average distance between excitons is of the order of a few Bohr radii but still below the exciton screening threshold. The phenomena due to interactions among excited states play significant role in the emission near band edge of the material. The theory of two-exciton collisions for GaAs/AlGaAs quantum well systems is a mild attempt to understand the physics associated with the optical spectra due to excitonic scattering processes in these novel systems. The four typical processes considered give different spectral shape, peak position and temperature dependence of the emission spectra. We have used the theory of scattering together with the second order perturbation theory to derive the radiative power spontaneously emitted at an energy ħω by these processes. The results arrived at are purely qualitative in nature. The intensity of emitted light in quantum well systems varies inversely to the square of temperature, whereas in case of bulk materials it simply decreases with the temperature.

DOI
10.5281/zenodo.1332277

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Optical emission based on excitonic scattering processes becomes important in dense exciton systems in which the average distance between excitons is of the order of a few Bohr radii but still below the exciton screening threshold. The phenomena due to interactions among excited states play significant role in the emission near band edge of the material. The theory of two-exciton collisions for GaAs/AlGaAs quantum well systems is a mild attempt to understand the physics associated with the optical spectra due to excitonic scattering processes in these novel systems. The four typical processes considered give different spectral shape, peak position and temperature dependence of the emission spectra. We have used the theory of scattering together with the second order perturbation theory to derive the radiative power spontaneously emitted at an energy ħω by these processes. The results arrived at are purely qualitative in nature. The intensity of emitted light in quantum well systems varies inversely to the square of temperature, whereas in case of bulk materials it simply decreases with the temperature.
This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.
An examination of the contribution of excitons, compared to the continuum states, to the optical properties of GaAs/AlGaAs multiple quantum wells illuminates the mechanisms underlying nonlinear absorption and nonlinear refraction and yields information which is useful for optimizing devices which incorporate these materials. We have measured the absorption as a function of intensity at three different wavelengths—above the band edge, on the excitonic resonance, and far below the band edge—to separate the effects. We find that different well thicknesses have different saturation intensities both for excitonic and continuum absorption.
Excitons are bound electron-hole pairs in semiconductors whose interactions influence optical properties and determine their collective behavior and emergent correlated phases. Despite the theoretical and experimental significance of these phenomena, a microscopic theory of exciton-exciton interactions that takes into account the full composite nature of these quasiparticles in systems with an arbitrary single-particle Hamiltonian is lacking. Here, we present a general theory of exciton-exciton interactions that builds upon our previous publication [Physical Review B 113, 155440 (2026)], which applied only to conventional semiconductors with weak band hybridization. By contrast, the theory presented here is valid in virtually any crystalline semiconducting system and can be applied to the study of biexciton formation and correlated excitonic phases in systems where band topology, quantum geometry, and moiré effects play a significant role. In addition to the analytical theory, we also present results in simple systems with flat bands with a nontrivial quantum metric and show that the biexcitonic spectrum differs strongly from that in the case without quantum geometry. Our theory paves the way for accurate quantitative calculations of biexciton spectra and excitonic many-body phases in modern condensed-matter systems.
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.
We examine the resonance fluorescence from localized excitons in GaAs/AlGaAs quantum wells with increasing excitation power and for different temperatures. Extending our experimental setup by a microscope objective with high numerical aperture in the cryostat, the detection of the emission from localized excitons is possible. We find a nonlinear behaviour of the emitted intensity with increasing laser power, which is explained as a transition of the emission from excitonic to electron-hole plasma states due to many-body effects between excited carriers. This is supported by our theoretical description based on the semiconductor Bloch equations.
Different aspects of the photoluminescence from semiconductor quantum-confined structures are studied in this dissertation, for a better understanding of fundamental physics of semiconductors.The precursor of any photoluminescence study is the characterization of the linear optical properties of the semiconductor structure. High resolution absorption measurements were performed in order to study the interplay of disorder and acoustic phonon scattering in a quantum well. Also, reflectivity measurements, together with a fitting procedure based on the transfer matrix formalism, are used to determine the thickness of samples.Excitons are atom-like quasi-particles, formed from a bound electron-hole pair. They follow a Bose-Einstein statistic, so in principle it is possible to achieve an excitonic Bose-Einstein condensate. Time resolved photoluminescence measurements were performed over an extensive range of lattice temperatures and carrier concentrations, in order to determine the fraction of excitons formed from the electron-hole plasma in a quantum well, after non-resonant excitation. The experimental spectra were compared to a pure plasma calculation first, then excitons were taken into account. The highest fraction of formed excitons is found for low temperatures and intermediate carrier densities. This fraction is found to be very small, and this has clear implications on the excitonic Bose-Einstein condensation studies.The photoluminescence emitted left and right from a quantum well is interfered in a modified Mach-Zender interferometer. It is shown that the light emitted on the two paths will interfere for a V-shape geometry and will not for any other paths.A structure formed by placing a quantum well in a field antinode of a resonant planar microcavity exhibits normal mode coupling: splitting of the resonance spectral line. The coherence properties of the photoluminescence from a normal-mode-coupling microcavity are studied using another version of the Mach-Zender interferometer. The degree of coherence measured in this way depends greatly on the pump wavelength and intensity, ranging from zero to 0.8. However, direct observation of the emission speckle shows significant coherence in all cases. The difference is explained by the different methods used to evaluate the coherence.The strong coupling between a quantum dot and a photonic crystal nanocavity is investigated by observation of photoluminescence. A new method of tunning the cavity wavelength by deposition of a thin film of solid Xenon on all the surfaces of the sample is presented. The method allows the scanning of the cavity wavelength with about 5 nm without a decrease in the quality factor and without changing the temperature.Finally, an extensive study of the quality factors of quantum dot photonic crystal nanocavities is presented. The role of the quantum dot ensemble absorption is investigated. At higher excitation levels, lasing is observed and discussed.