Publication # 132

The optical properties are investigated of undoped and n-modulation doped multiple quantum well structures (MQWS) under quasistationary high excitation conditions. The characteristic nonlinear optical behaviour is due to an electron-hole plasma. Its density, temperature, and the renormalized bandgap are determined via photoluminescence and pump and probe beam measurements. A model is developed for the line shape analysis of the plasma luminescence taking into account the broadening of the single particle density of states with results deviating from those in literature, but very good agreement is achieved with the experimental findings from pump and probe beam spectroscopy. The study of the renormalization of the higher, unpopulated subbands reveals that the pure intersubband screening is weak with respect to the shift of the fundamental bandgap. The exciton resonances of the higher subbands are found to be stable against high plasma densities in the fundamental subbands and are mainly bleached by direct population. The idea of a rigid shift of the whole subband structure must be clearly rejected