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Physical review. B, Condensed matter

Influence of the dielectric environment on the radiative lifetime of quantum-well excitons

Dirk Ammerlahn, Bernd Grote, S. W. Koch, J. Kühl, Matthias Hübner, R. Hey, Klaus Ploog

Published Feb 15, 2000• 13 citations
Publisher

Abstract

Due to the large mismatch in the refractive index of semiconductor material and air, time-integrated and spectrally resolved four-wave mixing experiments reveal strong influence of the cladding layer thickness, its refractive index, and the reflectivity of the sample-air interface on the radiative dephasing time of excitons in single quantum wells and multiple quantum well structures. A cladding layer thickness of \ensuremath{\lambda}/2 and \ensuremath{\lambda}/4 enhances and decreases, respectively, the radiative contribution to the exciton linewidth and causes radiative shifts to higher and lower energies, respectively. All our experiments are well described by solutions of the semiconductor Maxwell-Bloch equations.

DOI
10.1103/physrevb.61.4801

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Due to the large mismatch in the refractive index of semiconductor material and air, time-integrated and spectrally resolved four-wave mixing experiments reveal strong influence of the cladding layer thickness, its refractive index, and the reflectivity of the sample-air interface on the radiative dephasing time of excitons in single quantum wells and multiple quantum well structures. A cladding layer thickness of \ensuremath{\lambda}/2 and \ensuremath{\lambda}/4 enhances and decreases, respectively, the radiative contribution to the exciton linewidth and causes radiative shifts to higher and lower energies, respectively. All our experiments are well described by solutions of the semiconductor Maxwell-Bloch equations.
Time-integrated four-wave mixing experiments reveal strong influence of the cladding layer thickness d1, its refractive index and the reflectivity of the sample–air interface on the radiative dephasing time Trad of excitons in single quantum wells and multiple quantum well structures. A cladding layer thickness of λ/2 and λ/4 decreases and increases, respectively, Trad. Furthermore, we show that the superradiance is not limited to the heavy-hole exciton but can be observed even on excited and light-hole–exciton states as well as on correlated electron–hole-pair states. All findings agree fairly well with solutions of the semiconductor Maxwell-Bloch equations.
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.
Time- and spectrally resolved reflectivity measurements on multiple-quantum-well structures excited by ultrashort laser pulses are presented. In Bragg structures, where the interwell distance amounts to half the optical wavelength at the exciton resonance, superradiant coupling of the polarization dynamics within different quantum wells leads to an enhancement of the reflectivity strength and linewidth. We show that the superradiance is not limited to the heavy-hole exciton but can be observed even on excited and light-hole exciton states as well as on correlated electron-hole pair states. These coupling effects grow with the number of quantum wells. For our samples containing 60 and 30 quantum wells, pronounced superradiant enhanced reflectivity is present even at high excitation intensities and high temperatures, showing the insensitivity of radiative coupling with respect to incoherent scattering processes. Our findings are in good agreement with solutions of the semiconductor Maxwell-Bloch equations.
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.
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.
The coherent emission from excitons corresponding to different subband transitions in a 500 Åbroad GaAs/AlGaAs quantum well is studied by spectrally resolved transient four-wave mixing (FWM). For the simultaneous excitation of several subband transitions, the dephasing times of the lowest exciton and higher- order exciton transitions are determined from the width of the resonances in the FWM spectrum. The decay of the spectrally integrated FWM signal as a function of the time delay between the excitation laser pulses does not reflect the dephasing times but is determined by the laser pulse width for multi-subband excitation. This is indicative of quantum interference between exciton and continuum transitions. Many-body Coulomb effects have to be included in a theoretical treatment of these experimental observations.