Studies of exciton localization in quantum-well structures by nonlinear-optical techniques

Abstract

An exciton moving in a random potential is a promising model system for the study of localization effects, since its energy spectrum can be measured directly, and there are no complications resulting from Coulomb interaction. This paper reviews our work on the use of nonlinear techniques, such as hole burning and four-wave mixing, to detect the motion of two-dimensional excitons in thin GaAs–Al_xGa_1−x As heterostructures, in which the random potential comes from fluctuations in layer width. A clear distinction is found between the behavior of excitons below and above the absorption line center. Below the line center, hole burning is easy, and both spectral and spatial diffusion are slow, i.e., the excitons behave as if they are localized; above it the reverse is true. This is strong evidence for a mobility edge at the line center, which is the position predicted classically.