Femtosecond intersubband relaxation in GaAs quantum wells

We investigate the intersubband relaxation in GaAs quantum wells at room temperature using differential transmission spectroscopy with subpicosecond time resolution. The dynamics of the subband populations are derived from the experimentally observed reduction of oscillator strength of the corresponding exciton transitions. The obtained electron intersubband scattering times are 160 fs for relaxation from the second to the first subband in a 15-nm quantum well and 170 fs for relaxation from the third to the first subband in a 20-nm structure. These times are significantly shorter than those deduced from previous experiments and from some theoretical studies.