Quasi-ballistic transport of excitons in quantum wells

The transport of quantum-well excitons is investigated in the model system ZnSe on the length scale of the light wavelength and on the timescale of the first inelastic scattering processes. The experiments are based on confocal microscopy including a solid immersion lens in the setup to achieve a spatial resolution of 250 nm.The excitons propagate in the quantum-well plane quasi-ballistically for several 100 nm only affected by elastic scattering due to interface disorder. The first inelastic scattering process, which destroys the coherence of the exciton wave function, is the interaction with acoustic phonons after about 30 ps. This process results in the reversal of the excitonic propagation direction which is directly reflected by a spatial oscillation in the temporal evolution of the photoluminescence spot. From the oscillation one can deduce simultaneously the coherence time and length of the excitonic transport. We describe the transition to diffusive exciton transport with increasing temperature and the influence of strong interface disorder.