The dynamics of resonantly excited 1s-HH excitons in GaAs single quantum wells is studied by picosecond transient reflection and photon echo. The energy relaxation and transport properties of the excitons are probed, making use of the surface electric field. At low exciton densities and 5 K, transport appears to be absent, and superradiant decay takes the place of free excitons with characteristic times ranging from 5 to 13 ps, depending on the well width. The polarization decay is governed by energy relaxation, and a coherence length is found for these free excitons of ϳ200 nm. Low in the exciton resonance, we measure a much slower radiative decay and smaller coherence lengths, due to localization. Here both radiative decay and pure dephasing contribute to the polarization dephasing rate. At higher exciton densities and temperature, transport is observed, induced by exciton-exciton and exciton-phonon collisions. For free excitons it is demonstrated that the increased polarization dephasing rate is exclusively due to exciton transport, and not to pure dephasing. For localized excitons, pure dephasing processes prevail in T 2 , and thermally activated transport is found. ͓S0163-1829͑97͒05847-5͔

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Accumulated photon echo1 (AccPE) is an ultra-sensitive tool for studying optical dephasing of excitations in solids. The essential prerequisite for the succesful application of this technique is the presence of metastable states in the optical pumping cycle, preventing the system from a quick return to the ground state. A frequency grating in the ground state may than be formed and serve to create a stimulated echo. The technique has been applied to study long-lived optical impurity centers and bound excitons in solids. Here we present the first exploratory measurements on short-lived excitons in semi-insulating GaAs, and we will discuss the accumulation mechanisms. These mechanisms hold promise for general application of AccPE to study excitons in semiconductors.

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