Abstract

We use short-period optical transient grating techniques with a spatial resolution of 120 nm and a temporal resolution of 100 fs to investigate various fundamental prependicular (or cross well) transport mechanisms in Al_1−xGa_xAs/GaAs multiple quantum wells (MQW’s) and superlattices (SL’s). As indicated in Fig. 1, the short period grating was written in our samples by two copolarized nearly-counterpropagating pump pulses tuned to the excitonic resonance and read by an orthogonally polarized probe at the same wavelength. In contrast to other techniques, which use external fields, inhomogeneity in the structure, or both to drive the transport, this one allows the investigation of unbiased stuctures in which the well and barrier thicknesses do not vary throughout the sample. Specifically, by varying the sample geometry (well and barrier width and barrier height) and by varying the mean free path between collisions (by controlling the temperature and carrier density), we observe fundamentally different transport regimes in which its character changes from classical to quantum mechanical (as the nearest- neighbor well coupling is indreased) and from diffusive to ballistic (as the mean free path becomes comparable to the grating period).

© 1992 IQEC