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physica status solidi (a)

Exciton–Light Coupling in Quantum Wells in the Presence of Inhomogeneous Broadening

Alexey Vitalievich Kavokin, Maria R. Vladimirova, Lucio Claudio Andreani, Giovanna Panzarini, Jeremy J. Baumberg

Published Nov 1, 1997• 2 citations
Publisher

Abstract

Maxwell equations for light incident on a quantum well at the exciton resonance frequency have been solved taking into account the inhomogeneous broadening of the exciton resonance. Inhomogeneous and homogeneous mechanisms of broadening can be easily distinguished in time-resolved reflection or transmission spectra where the inhomogeneous broadening causes Gaussian-like decay of the signal and oscillations. Experimentally predicted nonmonotonic time-resolved signal in transmission has been detected in a system of five GaAs/AlGaAs quantum wells.

DOI
10.1002/1521-396x(199711)164:1<189::aid-pssa189>3.0.co;2-5

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Maxwell equations for light incident on a quantum well at the exciton resonance frequency have been solved taking into account the inhomogeneous broadening of the exciton resonance. Inhomogeneous and homogeneous mechanisms of broadening can be easily distinguished in time-resolved reflection or transmission spectra where the inhomogeneous broadening causes Gaussian-like decay of the signal and oscillations. Experimentally predicted nonmonotonic time-resolved signal in transmission has been detected in a system of five GaAs/AlGaAs quantum wells.
Summary form only given.The coherent response of excitons in semiconductor nanostructures, as measured in four wave mixing (FWM) experiments, depends strongly on the inhomogeneous broadening of the exciton transition. We investigate GaAs-AlGaAs single quantum wells (SQW) of 4 nm to 25 nm well width. Two main mechanisms are found to be important. First, the excitation-induced dephasing FWM signal (EID), which leads to a strong dependence of the signal on the angle between the linear input polarizations. The presence of the EID in the mainly homogeneously broadened sample (25 nm) is shown by the beating between EID and two-photon coherence (TPC) at the exciton for negative delay and co-linear polarized excitation. It vanishes for cross-polarized excitation, thus excluding the importance of local-field effects.
We investigate the four-wave mixing response of semiconductor quantum wells having a predominantly inhomogeneous broadening of the exciton states. The influence of the biexcitons on the nonlinear emission, in proximity to the lowest exciton resonance, is studied. Using about 1 ps long pulses, we show that biexcitons are created for all excitation energies lying within the broadened exciton absorption line, and that their binding energy does not depend on the exciton energy. Reducing the temporal width of cross-linearly polarized incident laser fields to about 0.1 ps, we observe a drastic changeover from a prompt response to a photon echo, and quite interesting beating phenomena, visible in time-integrated as well as in time-resolved experiments, appear. Our observations compare well with the numerical solutions of the optical Bloch equations for a five-level scheme. The influence of the inhomogeneous broadening on the beating phenomena is discussed.
Coherent transient optical spectroscopy techniques provide a means of studying the relaxation of electronic excitations in solids. Using these techniques the authors examine the relaxation of excitons in GaAs-AlGaAs multiple quantum wells at low temperature. Localization of excitons due to interface disorder results in inhomogeneous broadening of the absorption spectrum and relaxation due to migration between localization sites. The temperature dependence and energy dependence of the relaxation indicate the presence of complex scattering mechanisms. The dependence of the optical response on the polarization of the incident excitation fields is shown to result in a dramatic change of the dephasing and broadening characteristics of the optical response. This response is interpreted in terms of the presence of both localized and delocalized excitons. Mechanisms for the polarization dependence are discussed.>
We investigate a GaAs/(Ga,Al)As Fabry-P\'erot microcavity, into which (In,Ga)As quantum wells have been inserted. The cavity is wedge shaped, i.e., the detuning between the bare-exciton resonance and the bare optical cavity mode depends on the spatial position on the sample. Linear transmission spectra reveal a well-resolved Rabi splitting of 8 meV at resonance, an inhomogeneously broadened exciton transition of 4--5-meV width, and an 0.7-meV-wide Fabry-P\'erot mode. The time-resolved transmission exhibits deep beatings and a subpicosecond exponential decay: a behavior similar to that foreseen in the strong-coupling regime and in the absence of electronic disorder. Conversely, the four-wave mixing response appears weakly influenced by the cavity and not much different from what is expected for bare excitons. A photon echo, dephasing times as long as 50 ps, and only weak Rabi oscillations are observed. The experimentally observed features can be explained by a model based on the numerical solutions of the Maxwell-Bloch equations. This model confirms the dramatic influence structural disorder in the quantum wells has on the coherent nonlinear exciton-photon dynamics.
The optical response of radiatively coupled semiconductor multiple-quantum-well structures is investigated theoretically. It is shown that the transverse optical field leads to a coupling of exciton states within each well which causes a radiative decay and a mixing of excitonic resonances. Simultaneously, the field-induced long-ranged coupling between different wells leads to collective effects that are very sensitive to the detailed geometry of the structure. For a quantum-well spacing equal to an integer multiple of half the optical wavelength inside the medium, it is predicted that the collective effects cause a stimulated decay of electronic excitations that should be observable in either transmission or reflection geometry. On the other hand, in a quarter wavelength structure, the light-induced coupling causes an interwell energy transport and a splitting of the excitonic resonances, that should be observable as quantum beats in the time-resolved transmitted or reflected signal.