Gain measurements and band-gap renormalization inGaAsAlxGa1−xAsmultiple-quantum-well structures

Using a pump and probe technique with nanosecond excitation pulses we measure the nonlinear optical properties of GaAs/${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$As multiple-quantum-well (MQW) structures under quasistationary excitation conditions. The main aspects of our work are the phenomena of optical gain, the renormalization of the fundamental band gap and of the higher subbands $n_z=2,n_z=3\mathrm{}$), due to the electron-hole population of the lowest ($n_z=1\mathrm{}$) subbands. We present the first systematic analysis of the gain spectra to determine the renormalization of the $n_z=1\mathrm{}$ subband and we interpret the data in terms of effective exciton parameters in combination with a pure two-dimensional theory.