Excitonic effects in miniband and Wannier-Stark absorption spectra

We compare experimental and theoretical electroabsorption spectra of strongly coupled GaAs/AlAs superlattices for the full field range from the Franz-Keldysh (low field) to the Wannier-Stark (high field) limit. It is shown that in spite of the additional localization mechanism induced by the electronhole Coulomb interaction the interference between Wannier-Stark and Franz-Keldysh type absorption remains visible over the whole miniband width. However, a strong asymmetry between the lower and upper miniband edge is found which can be explained by the Coulomb coupling of higher to lower levels. At the position of the lower miniband edge the coupling is strongly reduced leading to sharp resonance's of the quasi -2D Wannier-Stark excitons which merge into the 3D miniband exciton for low fields.