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

The changes induced in the optical-absorption spectrum of a GaAs/Alₓ{Ga}_{1{{-}}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 {} and barriers of thickness 100 {}. We find that for densities up to 1×{}10¹¹ cm^-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.