Femtosecond intersubband relaxation and population inversion in stepped quantum well

Abstract

We have investigated intersubband relaxation rates in a stepped quantum well at room temperature using differential transmission spectroscopy with subpicosecond time resolution. The dynamics of the subband populations are derived from the experimentally observed reduction of oscillator strength of the corresponding exciton transitions. In the stepped quantum well the relaxation through longitudinal optical-phonon emission 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 population inversion between n3 and n2 (which are separated by 7 THz) is observed.