Momentum redistribution times of 2D excitons measured by transient resonantly induced intersubband absorption

We apply a transient interband-pump-intersubband-probe technique, to directly measure the time it takes for resonantly photoexcited excitons in GaAs/AlGaAs superlattices to redistribute in momentum space. We determine the redistribution time and its excitation density and superlattice periodicity dependence from the temporal evolution of the conduction intersubband absorption spectrum. We find that resonantly excited heavy-hole excitons, at moderate densities, redistribute slowly acid reach thermal distribution within a few tens of ps after the pulsed excitation. This redistribution time is nearly inversely proportional to the square root of the initial density of the photoexcited excitons and it depends on the periodicity of the superlattice structure. The smaller the periodicity in direct space is, the longer is the redistribution time. This is due to the relatively inefficient exciton-exciton scattering, and the small momentum that each resonantly excited exciton carries. From measurements performed on three samples of different periodicity we find that the redistribution time increases faster than the superlattice Brillouin-zone length squared. (C) 1998 Elsevier Science B.V. All rights reserved.