Exciton dynamics in GaAs/AlxGa1-xAs quantum wells

The changes induced in the optical absorption spectrum of a GaAs/AlxGa1-xAs multiple quantum well due to a photoexcited carrier distribution are reexa mined. We use a femtosecond pump-probe technique to excite excitons and fre e electron-hole pairs. We find that for densities up to 10(11) cm(-2) the s aturation of exciton absorption is caused both by a reduction of the oscill ator strength and by a broadening of the exciton resonance, which exhibit q uite different temporal evolutions. The restoring of the initial value of t he reduced oscillator strength is determined by the free-carrier lifetime, which we have measured to be 65 ps, whereas the temporal evolution of the b roadening is due to the exciton-exciton interaction and determined by the e xciton lifetime, which is found to be 410 ps. We have measured the oscillat or strength saturation densities and shown that at low temperature the infl uence of free electron-hole pairs on the exciton oscillator strength is twi ce as effective as the influence of other excitons. [S0163-1829(99)11011-7] .