ULTRAFAST DYNAMICS OF INTERSUBBAND RELAXATION IN GAAS QUANTUM-WELLS - HOT-CARRIER AND PHONON POPULATIONS EFFECTS

The dynamics of intersubband relaxation in GaAs quantum wells and the role of hot carriers and the phonon distributions have been investigated using two different optical techniques with femtosecond resolution: 1) time-resolved photoluminescence and 2) pump and probe experiments. The (2-->1) intersubband relaxation times have been measured as functions of well widths (100 Angstrom < L(well) < 220 Angstrom), under different experimental conditions (15K < T-lattice < 300K, and 1x10(10) cm(-2) < excitation densities < 1x10(12) cm(-2)). The electron intersubband relaxation time is deduced from the decay time of the n=2 well luminescence (or differential transmission) intensity. For thin wells (<150 Angstrom), a fast intersubband (2-->1) relaxation time less than or equal to 3 ps has been measured. For thicker wells, the measured decay times are found to be critically dependent on the excitation conditions (vary from 5 ps to 40 ps). The well width dependence of the intersubband relaxation time does not show the strong dependence (2 orders of magnitude) predicted theoretically for electron-LO phonon scattering. Our results show that the hot phonon populations and the slow carrier cooling rate limit the observation of subpicosecond relaxation time. For thick well widths, our results also suggest that hot carriers effects play an important role in the intersubband relaxation mechanisms.