FEMTOSECOND INTERSUBBAND RELAXATION AND POPULATION-INVERSION IN STEPPED QUANTUM-WELL

have investigated intersubband relaxation rates in a stepped quantum w ell at room temperature using differential transmission spectroscopy w ith subpicosecond time resolution. The dynamics of the subband populat ions are derived from the experimentally observed reduction of oscilla tor strength of the corresponding exciton transitions. In the stepped quantum well the relaxation through longitudinal optical-phonon emissi on from n=3 to 1 (25 ps) is slower than that from 2 to 1 (220 fs), due to the reduced wave function overlap and larger wave vector required for intersubband scattering. When the n=3 state is pumped, a populatio n inversion between n3 and n2 (which are separated by 7 THz) is observ ed. (C) 1996 American Institute of Physics.