Coexistence of Wannier-Stark transitions and miniband Franz-Keldysh oscillations in strongly coupled GaAs-AlAs superlattices

Using differential photocurrent spectroscopy we have studied the field dependent absorption in a GaAs/AlAs superlattices, which have only one monolayer AlAs barriers, with unpreceded dynamical resolution. We are able to resolve a symmetric Wannier-Stark (WS) fan up to an index of plus/minus 9 and for the first time Franz-Keldysh (FK) oscillations across the whole energy width of the lowest combined miniband. A modulation of the WS transitions by FK oscillations (and vice versa) is clearly visible. We also present results or theoretical calculations which describe in detail this coexistence of the linear WS fan and the nonlinear fans of the FK oscillations.