Intensity dependence of superradiant emission from radiatively coupled excitons in multiple-quantum-well Bragg structures S. Haas and T. Stroucken Department of Physics and Material Sciences Center, Philipps University, Renthof 5, D-35032 Marburg, Germany M. Hu ¨ bner and J. Kuhl Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany B. Grote, A. Knorr, F. Jahnke, and S. W. Koch Department of Physics and Material Sciences Center, Philipps University, Renthof 5, D-35032 Marburg, Germany R. Hey and K. Ploog Paul-Drude-Institut fu ¨r Festko ¨rperelektronik, Hausvogteiplatz 5-7, D-10177 Berlin, Germany Received 11 July 1997 Linear time-resolved reflection on the heavy-hole exciton transition of high-quality multiple GaAs quantum- well Bragg samples reveals enhanced radiative emission and accelerated decay of the coherent optical polar- ization due to radiative interwell coupling. It is shown that this superradiant mode gradually vanishes with increasing excitation intensity. Microscopic calculations attribute this decoupling to the carrier-carrier Cou- lomb interaction in the individual quantum wells leading to excitation-induced dephasing. The intricate density dependence is discussed comparing computed results for the excitation-dependent decay in single quantum wells and multiple-quantum-well Bragg structures. S0163-18299802923-3 I. INTRODUCTION Recently, it could be shown that the dynamics of optical excitations in semiconductor multiple-quantum-well MQW structures differs considerably from the dynamics in a nomi- nally identical single quantum well QW. 1–3 The origin for this difference lies in the nature of the radiative decay chan- nel that exists in a composite structure of reduced effective dimensionality. Whereas direct radiative decay of an exciton polariton in a bulk semiconductor is inhibited by the require- ment of momentum conservation, a QW exciton with an in- plane momentum  k    / c , where   is the excitation energy, can decay into a photonic state due to the lack of translational invariance in the growth direction of the struc- ture. Values of the radiative lifetime in the range of a few ps have been postulated theoretically 1,2,4,5 and could be ob- served experimentally only recently. 3,6,7 Since the field that is emitted by a QW can subsequently interact with other QW’s in the sample, the short radiative lifetime of the exci- ton causes an efficient radiative coupling of the QW’s that leads to collective effects. The dynamics of the collective excitations depends strongly on the number of QW’s N and the interwell separation d . A special situation arises in a MQW Bragg structure, where the interwell spacing d equals an integer multiple of half the exciton wavelength in the medium  hh . At low excitation intensities, the dominant coupling mechanism is a stimulated polarization decay due to reemitted photons. This type of coupling creates a so- called superradiant mode, which ideally is characterized by an N times enhanced radiative decay rate. The remaining N -1 ‘‘dark’’ modes then have a vanishing radiative coupling strength. According to theoretical predictions, 2 the formation of a superradiant state in a MQW Bragg structure depends criti- cally on the phase matching between optical excitations in different QW’s. Therefore, the dynamics of the optical po- larization is extremely sensitive to various dephasing mecha- nisms like scattering at impurities or phonons, interface roughness or excitation-induced dephasing. 8 Moreover, since the concept of superradiance is based on the existence of quasistationary coupled exciton-photon modes, the superra- diant mode cannot be an eigenstate of the nonequilibrium coupled semiconductor-photon system at higher excitation intensities. In this paper, we present theoretical and experimental in- vestigations on the influence of elevated excitation intensities on the dynamics and the formation of the superradiant mode in a MQW Bragg structure. Since an efficient radiative cou- pling requires phase coherence, only experiments able to identify those photons that are reemitted from the coherent polarization are suited to the study of superradiance. In the coherent regime, the reemission of photons is restricted to the direction of the transmitted and reflected excitation beam by the in-plane momentum conservation. Therefore, we present in this paper experimental and theoretical investiga- tions of the time-resolved reflected signal that directly reveal the enhanced photon emission and the accelerated radiative decay of the coherent polarization. Comparing results of a high-quality GaAs MQW Bragg sample to those of a single QW, the significant reduction of the radiative lifetime can be shown to result from the strong coupling of excitons with phase-coherent photons leading to a stimulated superradiant decay of the excitonic polarization. Similar changes of the exciton lifetime have been observed for QW’s located inside microcavities 9,10 or excitons irradiated by phase-controlled coherent optical pulse trains. 11 PHYSICAL REVIEW B 15 JUNE 1998-I VOLUME 57, NUMBER 23 57 0163-1829/98/5723/148609/$15.00 14 860 © 1998 The American Physical Society