Interband optical transitions in GaAs-Ga1-xAlxAs superlattices in an applied electric field

We have investigated the light absorption in a GaAs-Ga1-xAlxAs superlattice in the presence of an applied electric field. Using Houston functions to represent the valence and conduction states we have calculated the transition rates between the valence and conduction subbands for different values of the field. Both the Franz-Keldysh shift and Franz-Keldysh oscillations emerge from the formalism. The absorption edge as a function of photon energy varies exponentially and has small oscillations superimposed on it. It is followed by a flat region characteristic of a two-dimensional electron gas. The use of Houston functions is justified by computing the tunneling probability between adjacent subbands and showing that it is negligibly small.