The thermalization of resonantly excited two-dimensional excitons in $\mathrm{GaAs}/{\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ superlattices is studied using time-resolved photoinduced intersubband absorption. Resonantly photogenerated excitons are sharply distributed in momentum space around the wave vector of their parent photons. We measured the time it takes for these excitons to redistribute evenly over the whole superlattice Brillouin zone and found it to be a few tens of picoseconds. This time depends on the initial density of excitons $N_X$ as $N_X^{-0.7}$ and on the superlattice period $L_z$ approximately as $L_z^{-8}.$ We discuss an excitonic momentum space self-diffusion model, which describes the strong dependence on the superlattice period. We conjecture that exciton-exciton scattering is the dominant cause for this diffusion.

• Received 12 August 2001

DOI:https://doi.org/10.1103/PhysRevB.65.153302