Direct measurement of exciton diffusion in quantum wells

We have measured the diffusion of excitons in GaAs quantum wells by using spatial and time-resolved photoluminescence (PL) spectroscopy at liquid helium temperature. A displacement of the detected region (empty set 1.5 mu m) with respect to the laser spot allows us to monitor the lateral change of the PL time-dependence. With increasing displacement the maximum of the PL-intensity shifts systematically to later times. For a theoretical description an in-plane diffusion model is applied, with the diffusion constant D as the only fit parameter. We obtain a continuous increase of D from 15 cm(2) s(-1) to 80 cm(2) s(-1) for displacements from 0 to 4.2 mu m. A measurement with only spatial resolution leads to a diffusion constant of 20 cm(2) s(-1).