Effects of carrier relaxation on excitonic nonlinear absorption in GaAs quantum wells

The authors use time-resolved differential transmission spectroscopy with time-resolution better than 100 fs to study exciton absorption saturation in by optically excited carriers in GaAs/AlGaAs multi-quantum wells (MQWs). They perform pump-probe experiments using near infrared pump pulses tunable from 1.44 eV to 1.56 eV and a white-light continuum probe. Pump-induced transmission changes are recorded as differential transmission spectra (DTS). These spectra show bleaching of the excitonic transitions at various subband edges due to carrier-induced loss of exciton oscillator strength and broadening of the excitonic absorption lines. The former contribution to the saturation of the exciton absorption peaks can be separated from pure broadening (and shift) of the exciton by integrating the DTS over the width of the broadened exciton line. The main mechanisms leading to exciton bleaching are phase space filling (PSF) and exchange effects, which are directly related to the occupation of carrier states, and long-range Coulomb screening