Response of excitonic absorption spectra to photoexcited carriers in GaAs quantum wells

The changes induced in the optical-absorption spectrum of a GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum well due to photoexcited carrier distributions are reexamined. We use a picosecond pump-probe technique to excite carriers in a sample having wells of thickness 210 Å and barriers of thickness 100 Å. We find that for densities up to 1×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$ the peak of the exciton line of the lowest-energy exciton becomes bleached; however the spectrally integrated area does not change, indicating that the carriers reduce the absorption peak by collision broadening rather than by phase-space filling or exchange. We also observe shifts of the exciton line to higher energy (‘‘blueshift’’) or to lower energy (‘‘redshift’’) depending on the energy of the pump photons. The shift can be ascribed to competing effects of band-gap renormalization and the change in the binding energy of the exciton, although a detailed theoretical description is still needed.