Interband optical transitions in GaAs-Ga_1-xAl_xAs and InAs-GaSb superlattices

Optical properties of GaAs-Ga_1-xAl_xAs and InAs-GaSb superlattices are studied within the framework of tight-binding approximation. The momentum matrix elements between tight-binding orbitals are related to those between Brillouin-zone-center Bloch states computed by a full-zone k→⋅p→ theory. The optical matrix elements of transitions from several valence subbands to several conduction subbands are calculated as functions of the well width and of the wave vector. It is found that the mixing of the heavy- and light-hole components in the superlattice states gives rise to a large variation in the optical matrix elements as k→ moves away from the zone center. The band mixing in conjunction with the exciton effect leads to weak structures in the absorption spectrum which can account for the forbidden transitions observed in several recent experiments.