Measurements of exciton diffusion by degenerate four-wave mixing inCdS1−xSex

We performed transient-grating experiments to study the diffusion of excitons in ${\mathrm{CdS}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Se}}_{\mathit{x}}$ mixed crystals. The decay of the initially created exciton density grating is well described for t≤1 ns by a stretched-exponential function. For later times this decay changes over to a behavior that is well fitted by a simple exponential function. During resonant excitation of the localized states, we find the diffusion coefficient (D) to be considerably smaller than in the binary compounds CdSe and CdS. At 4.2 K, D is below our experimental resolution which is about 0.025 ${\mathrm{cm}}^2$/s. With increasing lattice temperature (${\mathit{T}}_{\mathrm{lattice}}$) the diffusion coefficient increases. It was therefore possible to prove, in a diffusion experiment, that at ${\mathit{T}}_{\mathrm{lattice}}$≤5 K the excitons are localized, while the exciton-phonon interaction leads to a delocalization and thus to the onset of diffusion. It was possible to deduce the diffusion coefficient of the extended excitons as well as the energetic position of the mobility edge.