Localization dynamics of excitons in disordered semiconductor quantum wells

Exciton transport in nanomaterials is sensitive to fluctuations in the confinement potential that are intrinsic to heterogeneous solid-state systems. Redistribution of exciton population manifests as spectral diffusion in which the exciton energy shifts. It is generally assumed that increase or decrease in the exciton energy are equally probable. We show that this assumption is not necessarily valid using two-dimensional coherent spectroscopy on a disordered GaAs quantum well. High-energy excitons relax into lower-energy localized states over a time scale of tens of picoseconds at low sample temperatures $(\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. A transition to uniform spectral diffusion of excitons is observed as the temperature is increased to $\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Numerical simulations reveal the contribution of exciton-phonon interactions to spectral diffusion of excitons. These results provide a perspective on the process of dynamic localization and the effect of the correlation length of disorder on spectral diffusion of excitons.