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Photonics

Excitons in Shallow GaAs/Al0.03Ga0.97As Quantum Wells

Roman S. Nazarov, Matthew A. Maksimov, Yu. P. Efimov, Sergei A. Eliseev, Vyacheslav A. Lovcjus, Yury V. Kapitonov

Published Dec 26, 2025• 1 citation
Publisher

Abstract

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.

DOI
10.3390/photonics13010019

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•Jan 1, 2015•0 citations
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.
Three-beam time-integrated transient four-wave mixing (TFWM) is used to probe both light-hole (LH) and heavy-hole (HH) exciton coherences in weakly disordered GaAs quantum wells. The period of the quantum beats between the LH and the HH exciton coherences changes as a function of first-second pulse delay. A theoretical model for the TFWM signal, including exciton dephasing rates that vary with frequency across the inhomogeneous linewidth, reproduces the observations. The inclusion of inhomogeneous dephasing also improves the correspondence between theory and experiment when data as a function of both first-second and second-third pulse delays are simultaneously examined.
The theoretically predicted rapid superradiant decay of coherent 2D excitons on a time scale of approximately 10 ps [1] is expected to be strongly enhanced in multiple QW (MQW) Bragg structures consisting of N identical QWs separated by (Ga,Al)As barriers the thickness of which was adjusted so that the QWs had a distance d equal to one half of the heavy-hole exciton resonance wavelength λ in the medium [2]. This enhancement originates from efficient radiative coupling of excitons excited in different wells via the exchange of transverse photons. Recently we have published [3] degenerate-four-wave-mixing experiments with fs optical pulses on GaAs QW Bragg structures which demonstrate that the radiative lifetime in high-quality GaAs single QWs (SQW) is approximately 5 ps and decreases to less than 2 ps in Bragg samples with N=10. These studies revealed that radiative recombination provides the dominant contribution to the homogenous linewidth of the 2D exciton in high-quality samples and exceeds nonradiative contributions caused by scattering with sample imperfections in contrast to previous assumptions.
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.