DIRECT OBSERVATION OF THE LO PHONON BOTTLENECK IN WIDE GAAS ALXGA1-XAS QUANTUM-WELLS/

We report the direct observation of a bottleneck in electron cooling i n wide GaAs quantum wells. Intersubband lifetimes tau and their depend ence on intensity, lattice temperature T-L, and well width have been m easured using a ps excite-probe technique in wells with subband separa tion less than the longitudinal optical (LO) phonon energy. Above an e lectron temperature of about T-e=35 K the lifetime depends on T-e and is determined by LO-phonon emission. Below this bottleneck temperature acoustic phonons dominate the plasma cooling. An energy balance model of these interactions, with no adjustable parameters, gives good agre ement with our results. At electron temperatures below 35 K we determi ne tau=500 and 200 ps for samples of subband energy 19.5 and 26.6 meV, respectively.