MOMENTUM REDISTRIBUTION TIMES OF RESONANTLY PHOTOGENERATED 2D EXCITONS

We apply a novel transient interband-pump intersubband-probe technique to directly measure the time it takes for resonantly photoexcited exc itons in GaAs/AlGaAs quantum well structures to redistribute in moment um space. The redistribution time and its excitation density dependenc e are determined from the temporal evolution of the conduction intersu bband absorption spectrum, which is an unambiguous measure of the elec trons phase space distribution. We find that resonantly excited heavy- hole excitons redistribute and reach thermal distribution at a slow ra te nearly proportional to their density. This is due to the inefficien t exciton-exciton scattering, and the small momentum that resonantly e xcited excitons carry.