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physica status solidi (b)

Nonlinear Propagation of Laser Pulses in GaAs‐GaAlAs MQWS

Günter Manzke, K. Henneberger

Published Jun 1, 1988• 8 citations
Publisher

Abstract

Abstract A theoretical approach is presented for the description of nonlinear optical properties of GaAs‐GaAlAs MQWS connected with the blue‐shift of the 1s‐exciton resonance at high excitation. An experimental situation is considered where the MQWS is resonantly excited by a pump pulse of narrow spectral width creating a high density of excitons, and where the optical spectra are recorded by an ultrafast broad‐band probe pulse at different time delays. The densities of excitons in the single GaAs wells are determined by the simultaneous solution of the exciton diffusion and the optical problem of wave propagation. With increasing pump intensity jumps of the exciton densities in certain layers appear when the maximum of absorption crosses the pump frequency due to the blue‐shift of the resonance. The mechanism of the corresponding bistabilities in the reflection and transmission spectra is similar to that observed in connection with the quantum‐confined Stark effect.

DOI
10.1002/pssb.2221470234

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Quantum Electronics and Laser Science Conference•Nov 13, 2002•0 citations
Abstract A theoretical approach is presented for the description of nonlinear optical properties of GaAs‐GaAlAs MQWS connected with the blue‐shift of the 1s‐exciton resonance at high excitation. An experimental situation is considered where the MQWS is resonantly excited by a pump pulse of narrow spectral width creating a high density of excitons, and where the optical spectra are recorded by an ultrafast broad‐band probe pulse at different time delays. The densities of excitons in the single GaAs wells are determined by the simultaneous solution of the exciton diffusion and the optical problem of wave propagation. With increasing pump intensity jumps of the exciton densities in certain layers appear when the maximum of absorption crosses the pump frequency due to the blue‐shift of the resonance. The mechanism of the corresponding bistabilities in the reflection and transmission spectra is similar to that observed in connection with the quantum‐confined Stark effect.
Abstract The dynamics of optical nonlinearities connected with the blue‐shift of the exciton line in resonantly excited GaAs–GaAlAs MQWS's is investigated theoretically simulating a two‐beam experiment. Jumps in the optical spectra of a weak test pulse (optical switching), which follows the intense pump pulse at different time delays, are observed, basing on a strong increase of the absorption. A similar mechanism of the bistability is observed in connection with the electro‐absorptive effect.
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.
The lowest energy two-dimensional exciton in GaAs/GaAlAs multiquantum wells (MQW) is inhomogeneously broadened at low temperature by fluctuations in the layer width. The broadening permits probing of the excitons as a function of position within the inhomogeneous line[1]. At high densities interaction between the excitons becomes important and leads to saturation and to an increase of the homogeneous linewidth[2]. Using the picosecond pump/probe technique we have measured the saturation behavior as a function of exciton energy within the inhomogeneously broadened line. Figure 1 shows the transmission spectrum T of a weak narrowband probe (dotted) and the change in transmission ΔT of the probe induced by the presence of a pump for two different pump wavelength for 102 Å thick layers of GaAs separated by 200 Å layers of AlGaAs. An increase in transmission (holeburning) at the pump wavelength is observed showing that the interaction between resonant excitons is much stronger than between nonresonant excitons. The strength of the hole decreases rapidly on passing through the center of the line towards the high energy side. This is consistent with an increase in the homogeneous linewidth of the higher energy excitons[1,3]. By delaying the probe we measured the rate of decay of the hole which was found to depend strongly on pump intensity. At low intensity the hole lifetime approaches the luminescent lifetime but becomes shorter at exciton densities greater than ~109 excitons/cm2/layer. We attribute this to exciton-exciton scattering leading to spectral diffusion. The behavior was similar in 51 Å and 102 Å layers. In addition to holeburning, Figure 1 shows an increased absorption in the wings of the line whose shape is almost independent of pump wavelength and which is similar to that observed in bulk GaAs[2].
We present here the first observation of quasisteady state optical nonlinearities in the absorption and photoluminescence (PL) spectra of GaAs/AlAs type II heterostructures. Using pump-probe spectroscopy we observed deep modulation, ΔT ~ 0.5 (Δα =−50,000 cm−1), due to a >40-meV blue shift and bleaching of the heavy hole exciton absorption peak. In addition, a blue shift of up to ~10 meV of the indirect PL line is observed with increasing PL excitation intensity. These nonlinearities are understood in terms of spatially separated layers of electrons and holes that are produced by the optical excitation process.
We present measurements of differential transmission and four-wave mixing in GaAs quantum well structures at 1.8 K near the inhomogeneously broadened lowest heavy-hole (hh1) exciton resonance using narrow band cw excitation. The data show an increase in absorption and an excitation lifetime of order 1–10 μs outside the spectral hole produced by the pump. The long lifetime and the experimentally determined absence of excitation spatial diffusion in this region suggests that optical absorption produces electron-hole pairs that are correlated but separately localized due to disorder. A phenomenological model is proposed to explain the nonlinear response based on two-photon absorption.
Summary form only given. Since the original discovery of the excitonic optical Stark effect, substantial progress has been made in the understanding of this ultrafast nonlinear optical effect. Recently, a red shift of the excitonic absorption line has been reported using low intensity excitation detuned 4.5 meV below the 1s-heavy-hole (hh) resonance and cross-circularly-polarized pulses. This was attributed to higher-order Coulomb correlations, interpretable as bound and unbound two-exciton states and memory effects. For co-circularly polarized pulses the well-known blue shift was found. Here we present a related effect occurring in the higher-intensity regime. Standard pump-probe experiments were performed on high-quality 8.5 nm-thick In/sub 0.04/Ga/sub 0.96/As multiple quantum well (MQW) samples.