Absorption coefficient in type-II GaAs/AlAs short-period superlattices

Optical-absorption experiments have been performed on short-period GaAs/AlAs superlattices of type-II in a waveguiding configuration at low temperature. We measured the absolute absorption coefficients of optical transitions involving electrons confined in X states in AlAs layers and holes confined in {Γ} states in GaAs layers. Complementary photoluminescence and photoluminescence-excitation experiments show clearly the origin of the transitions and support our results. Theoretical calculations of the effective dielectric tensor and absorption coefficient in the exciton resonance region are presented. The {Γ}-X mixing of electron states is explicitly taken into account and the dependence on the parity of layers is discussed. From comparison between experimental and theoretical values of the absorption coefficient we deduce the value of {Γ}-X coupling coefficient. The study of the photoluminescence decay time yields radiative exciton lifetimes and allows us to calculate the excitonic localization area in our structures.