TIME-RESOLVED STUDIES OF INTERSUBBAND RELAXATION IN GAAS ALGAAS QUANTUM-WELLS BELOW THE OPTICAL PHONON ENERGY USING A FREE-ELECTRON LASER/

The problem of intersubband relaxation in GaAs/GaAs quantum wells, whe re the energy separation of the two lowest subbands is smaller than th e optical phonon energy, is considered. Time resolved pump-and-probe m easurements are performed with a far-infrared free-electron laser on t wo multiquantum well samples with similar thicknesses (approximate to 300 Angstrom), but different doping and mobilities. The measured lifet imes are shorter than could be explained by acoustic phonon emission a lone. Monte-Carlo calculations show the importance of electron-electro n scattering for thermalization of the hot electron distribution funct ion and subsequent optical phonon emission from the long thermal tail. (C) 1996 Academic Press Limited