Exciton dynamics in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

The changes induced in the optical absorption spectrum of a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}\mathrm{}$ multiple quantum well due to a photoexcited carrier distribution are reexamined. We use a femtosecond pump-probe technique to excite excitons and free electron-hole pairs. We find that for densities up to ${10}^{11}\hspace{0.5em}{\mathrm{cm}}^{-2}$ the saturation of exciton absorption is caused both by a reduction of the oscillator strength and by a broadening of the exciton resonance, which exhibit quite different temporal evolutions. The restoring of the initial value of the reduced oscillator strength is determined by the free-carrier lifetime, which we have measured to be 65 ps, whereas the temporal evolution of the broadening is due to the exciton-exciton interaction and determined by the exciton lifetime, which is found to be 410 ps. We have measured the oscillator strength saturation densities and shown that at low temperature the influence of free electron-hole pairs on the exciton oscillator strength is twice as effective as the influence of other excitons.