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Journal of the Optical Society of America B

Dynamics of doubly resonant Raman scattering and resonant luminescence in ultrathin InAs/GaAs quantum wells

J. Brübach, Jos E. M. Haverkort, Joachim H. Wolter, P. D. Wang, Nikolay N. Ledentsov, Clivia Marfa Sotomayor Torres, A. E. Zhukov, Petr S. Kop'ev, V. M. Ustinov

Published Jun 1, 1996• 2 citations
Publisher

Abstract

We have studied the transient behavior of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6-monolayer-thick InAs layer embedded in a GaAs matrix. The sharp emission line appears under resonant excitation of the light-hole (lh) exciton transition, whose energy separation from the hh exciton transition amounts to one GaAs LO phonon. We have measured the decay time of the sharp emission line in addition to the lifetime of the hh and the lh excitons, using time-correlated single-photon counting and two-wavelength picosecond time-resolved pump–probe transmission techniques. We find that the decay time of the sharp emission line is equal to the lh exciton lifetime for excitation on resonance but that it strongly decreases for excitation off resonance. We conclude that, whereas the sharp line originates from doubly resonant Raman scattering for off-resonant excitation, it is predominantly due to resonant luminescence for resonant excitation. We derive a lh exciton dephasing time of 40 ps, which suggests that the length scale of potential fluctuations from disorder is larger than the lh exciton Bohr radius.

DOI
10.1364/josab.13.001224

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We have studied the transient behavior of a sharp emission line, which emerges close to the heavy-hole (hh) exciton photoluminescence line in a 1.6-monolayer-thick InAs layer embedded in a GaAs matrix. The sharp emission line appears under resonant excitation of the light-hole (lh) exciton transition, whose energy separation from the hh exciton transition amounts to one GaAs LO phonon. We have measured the decay time of the sharp emission line in addition to the lifetime of the hh and the lh excitons, using time-correlated single-photon counting and two-wavelength picosecond time-resolved pump–probe transmission techniques. We find that the decay time of the sharp emission line is equal to the lh exciton lifetime for excitation on resonance but that it strongly decreases for excitation off resonance. We conclude that, whereas the sharp line originates from doubly resonant Raman scattering for off-resonant excitation, it is predominantly due to resonant luminescence for resonant excitation. We derive a lh exciton dephasing time of 40 ps, which suggests that the length scale of potential fluctuations from disorder is larger than the lh exciton Bohr radius.
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 a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when dephasing dominates the spectral width (homogeneous broadening), we find that the rise time of the incoherent luminescence signal is given by TIii 2/2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result that coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for dephasing due to exciton–phonon interaction, as the rise time of the emission decreases dramatically when the sample temperature is increased.
Secondary emission from GaAs quantum wells following resonant pulsed excitation is studied in the time domain. Selective excitation of heavy- and light-hole excitons is accomplished in a two-pulse excitation experiment which preserves sub-ps time resolution. After the initial rise the emission decays exponentially at a fast rate and then with another much slower exponential decay at longer times. The variations in the initial decay times with excitation density and sample temperature are similar to the variation in dephasing rates measured in four-wave mixing experiments. The decay in the visibility of heavy- and light-hole exciton beating is consistent with the decay rates measured for both species separately.
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.
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.
Resonant photoexcitation of a predominantly homogeneously broadened exciton line in a GaAs quantum well results in a nonexponential time-resolved photoluminescence decay transient due to the decrease in the exciton-exciton scattering contribution to the homogeneous linewidth, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{h}}$, as the exciton population decays. This result confirms the theoretically predicted relationship between the exciton radiative recombination lifetime and the homogenous linewidth (and coherence area) of the exciton state.