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Le Journal de Physique Colloques

PROPAGATION EFFECTS AND OPTICAL NONLINEARITIES OF EXCITONS IN MULTIPLE QUANTUM-WELLS

Juan Ariel Levenson, Rama K. Raj, Izo I. Abram, G. Dolique, F. ALEXANDRE

Published Nov 1, 1987• 0 citations
Publisher

Abstract

We have investigated the mechanisms that give rise to the optical nonlinear response associated with excitons in 2D GaAs/GaAlAs Multiple Quantum-Well (MQW) structures. The analysis of the spectra has revealed in addition to the expected sources of excitonic optical non-linearity, temperature dependent propagation effects. These experiments bring to the fore the role of light propagation through a nonlinear medium.

DOI
10.1051/jphyscol:19875116

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We have investigated the mechanisms that give rise to the optical nonlinear response associated with excitons in 2D GaAs/GaAlAs Multiple Quantum-Well (MQW) structures. The analysis of the spectra has revealed in addition to the expected sources of excitonic optical non-linearity, temperature dependent propagation effects. These experiments bring to the fore the role of light propagation through a nonlinear medium.
The propagation of a laser pulse through a GaAs-GaAlAs MQW is investigated in the frame of classical optics. Optical spectra are calculated in the vicinity of the 1 s -exciton resonance of GaAs taking into account interference effects and nonlinear absorption caused by many-exciton effects at high excitation. The density of excited e-h pairs and intensities of transmitted and reflected beam are determined in dependence on the intensity of stationary incident laser beam by simultaneous solution of the problem of exciton diffusion and propagation of the electromagnetic wave. The observed strong nonlinearities refer to an optical bistable behaviour.
Recent studies of the transient behavior of absorption saturation in low temperature Multiple Quantum Wells (MQW) samples have revealed a surprisingly fast recovery of absorption (within 10 ps at 15 K) after excitation by intense ultrashort light pulses1. This has been attributed to the effect of the large stimulated recombination rate caused by amplified spontaneous luminescence guided along the MQW structure, as confirmed by streak camera observations of the edge-emitted luminescence1, and by measurements of time-resolved luminescence spectra with subpicosecond resolution2. This effect could be used to increase the speed of optical gates based on absorption saturation or carrier-induced nonlinear refraction, but a more complete understanding of the mechanisms involved is clearly needed in order to evaluate the interest of this approach.
Correlated electron-hole pairs, or excitons, in semiconductor nanostructures have been studied extensively over the past few decades. The optical response of excitons is complicated due to inhomogeneous broadening, presence of multiple states, and exciton-exciton interactions. In this work we bring new perspectives to exciton physics in semiconductor quantum wells (QWs) through two-dimensional coherent spectroscopy (2DCS).\nThe effect of QW growth direction on the optical properties of excitons is explored by studying (110)-oriented GaAs QWs. The homogeneous and inhomogeneous linewidths of the heavy-hole exciton resonance are measured. By probing the optical nonlinear response for polarization along the in-plane crystal axes [1-10] and [001], we measure different homogeneous linewidths for the two orthogonal directions. This difference is found to be due to anisotropic excitation-induced dephasing, caused by a crystal-axis-dependent absorption coefficient. The extrapolated zero-excitation density homogeneous linewidth exhibits an activation-like temperature dependence.\nSpectral diffusion of excitons in (001)-oriented QWs has been studied. We show that the spectral diffusion characteristics depend strongly on the sample temperature. Spectral diffusion is generally assumed to follow the strong-redistribution approximation, partly because of lack of any evidence to the contrary. We find that this assumption is violated at low sample temperatures for excitons in QWs; high-energy excitons preferentially relax due to a negligible phonon population at low temperatures. The frequency-frequency correlation function is measured through a numerical fitting procedure to quantify spectral diffusion for sample temperatures >20>20>20 K.\nExciton-exciton interactions affect the light-matter interactions in QWs significantly. We present an intuitive and simple model for these interactions by treating excitons as interacting bosons. We show that the polarization-dependent exciton dephasing rate in GaAs quantum wells is due to the bosonic character of excitons. We fit slices from simulated spectra to those from the experimentally measured spectra and show that interference between two different quantum mechanical pathways results in a slower dephasing rate for co-circular and co-linear polarization of optical excitation pulses. This interference does not exist for cross-linearly polarized excitation pulses resulting in a faster dephasing rate. Additionally, we were able to separately quantify inter- and intra-mode interactions between excitons through exciton-density-dependent measurements.
Semiconductor multiple quantum well structures exhibit very large, room temperature, nonlinear refractive effects associated with resolvable excitonic features [1]. These nonlinearities may find use in all-optical devices if sufficient refractive index change can be achieved before the exciton completely saturates [2]. A unique aspect of band gap resonant optical, nonlinearities in quantum wells is the spatial confinement of the generated carriers within the layers. We have exploited the excitonic refractive nonlinearity in GaAs/AlGaAs multiple quantum wells in two separate degenerate four wave mixing configurations. These compare the intra-well and cross-well contributions to highly anisotropic carrier diffusion and also allow measurement of carrier relaxation rates and the saturation of the excitonic nonlinear refraction with increasing intensity.