Intersubband relaxation of hot excitons in GaAs quantum wells

We have experimentally studied the time evolution of the exciton population in a higher subband of GaAs quantum wells, below the free-carrier continuum. The lifetime of the exciton formed by an electron of the lowest subband and a heavy hole of the second subband in GaAs quantum wells is determined by time-resolved luminescence as 130+or-20 ps. This result is consistent with theoretical estimations of intersubband scattering by acoustic phonon emission. We discuss subband transitions of excitons in quantum wells as a new appealing concept for optically pumped coherent sources in the meV range.