INTERSUBBAND RELAXATION OF PHOTOEXCITED HOT CARRIERS IN QUANTUM WELLS.

We have probed the dynamics of hot electrons in undoped GaAs/(GaAl)As quantum wells using time-resolved Raman techniques. By tuning the photon energy of a time-delayed weak probe, we study the n equals 1 to n equals 2 or n equals 2 to n equals 3 intersubband transitions by electronic Raman scattering. The lifetime and the intersubband scattering time of the electrons photoexcited on the lowest electronic subbands are determined separately. Calculation of the rate of intersubband scattering by longitudinal acoustical phonons accounts for the observation of relatively long lived electrons on the second subband of a 215 Angstrom MQWS. When the well is narrower, so that the n equals 2 to n equals 1 transition can occur with emission of a longitudinal optic phonon, we find that the lifetime of carriers in the n equals 2 band is too short to measure with our technique.