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Physical Review Letters

High-resolution nonlinear laser spectroscopy of heavy-hole excitons in GaAs/AlGaAs quantum-well structures: A direct measure of the exciton line shape

J. T. Remillard, H. Wang, Duncan G. Steel, Je Hoon Oh, Jagadeesh Pamulapati, P. Bhattacharya

Published Jun 12, 1989• 24 citations
Publisher

Abstract

We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.

DOI
10.1103/physrevlett.62.2861

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We report high-resolution nonlinear laser-spectroscopy studies of the n=1 heavy-hole exciton in GaAs multiple quantum wells. Using a new method of cw frequency-domain four-wave mixing, we first show that the excitation relaxation consists of both fast and slow components. We then show that the contribution of well-width fluctuations to inhomogeneous broadening can be elimiated enabling us to obtain a direct measure of the intrinsic exciton line shape. Under certain conditions, this line shape is non-Lorentzian and cannot be explained on the basis of simple excited-state relaxation dynamics.
We report high resolution nonlinear laser spectroscopy studies of excitation relaxation associated with the excitonic optical nonlinearity at room temperature and low temperature in GaAs/AlGaAs multiple quantum wells. Using a new method of cw frequency domain four wave mixing, we show that relaxation of the room temperature nonlinear optical response is characterized by free carrier recombination and ambipolar diffusion. At low temperature, the excitation relaxation for localized excitons is dominated by phonon assisted tunneling. In addition, we use four wave mixing methods to eliminate contributions to the excitation line shape from inhomogeneous broadening. The observed line shape is highly asymmetric and shows the presence of spectral diffusion due to the phonon assisted tunneling associated with the excitation relaxation.
We investigate the spectral properties of the excitonic nonlinear response in a GaAs quantum-well sample containing wells of different thicknesses. We observe both the heavy-hole biexciton and the light-hole biexciton in the transient four-wave mixing spectrum and estimate their binding energy dependence on well width. For moderate excitation densities the signal for negative delay at the heavy-hole exciton and biexciton resonances is due to two-photon transitions to the biexciton states through the exciton states. The temporal behavior of the transient four-wave mixing signal from the heavy-hole exciton resonance is strongly dependent on the spectral position of the laser. We show that this effect is due to interference phenomena and to excitation-induced dephasing.
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.
Current issues in the study of light-matter interactions are coherence and quantum interference. With ultrafast nonlinear spectroscopy such as transient four-wave mixing (TFWM) the imposed coherence on the optical excitations in e.g. GaAs multiple quantum wells has been studied. In this contribution, we study the influence of inhomogeneous broadening on quantum beats observed by spectrally resolved TFWM. In artificially grown semiconductor structures as e.g. quantum wells, inhomogeneities due to interface-disorder has, up till now, been inevitable. In the experiments, the light-hole exciton (lhx) and the heavy-hole exciton (hhx) in a 100 Å GaAs multiple quantum well are excited simultaneously with 150 fs pulses from a self mode-locked Ti:sapphire laser. The spectral profile is shown in the inset of Fig. 1.
We report a comprehensive study of heavy-hole (HH) and light-hole (LH) excitons in a shallow GaAs/Al0.03Ga0.97As single quantum well (QW) using two-dimensional photoluminescence excitation (PLE) spectroscopy, reflectivity in Brewster geometry, and time-resolved four-wave mixing (FWM) with polarization-resolved photon echo (PE) detection. The PLE measurements reveal well-resolved HH and LH exciton states with minimal inhomogeneous broadening, while reflectivity spectra indicate strong light–matter coupling and narrow exciton linewidths, reflecting the high structural quality of the QW. FWM experiments demonstrate two-pulse photon echoes with coherence times of T2≈39.5 ps for HH and T2≈16.2 ps for LH excitons. Polarization-resolved PE confirms that the observed signals originate from pure three-level excitonic systems without contributions from trions or donor-bound excitons. Compared to conventional GaAs/Al0.3Ga0.7As QWs, the shallow QW exhibits reduced HH-LH splitting, enhanced optical homogeneity, and robustness against above-barrier illumination, making it a promising platform for coherent optical control and information photonics applications.
We report time-resolved photoluminescence (TRPL) measurements of the radiative lifetime (τr) at 1.8K of resonantly excited heavy-hole excitons in a 27nm GaAs single quantum well as a function of exciton density. By the use of AlAs interface smoothing layers [1], the sample exhibits very little inhomogeneous broadening (≈0.15meV). The exciton centre of mass motion is therefore not significantly restricted by localisation in well width fluctuation islands. However, measurements of the exciton homogeneous linewidth (Γ h ) using time-resolved degenerate four wave mixing in the self-diffraction configuration (Fig.1) [2], additionally show that the sample imperfection scattering contribution to the Γ h of these delocalised excitons is also small (0.2meV). Thus, as indicated by the dependence of Γ h on exciton density (N x ), even at moderate exciton densities, exciton-exciton scattering is the dominant dephasing interaction within the photocreated exciton ensemble.