Nonequilibrium dynamics in a quasi-two-dimensional electron plasma after ultrafast intersubband excitation

The dynamics of electrons in GaTnAs/ALlnAs quantum wells is studied after excitation from the n = 1 to the n = 2 conduction subband. Femtosecond pump-probe experiments demonstrate for the first time athermal distributions of n = 1 electrons on a surprisingly long time scale of 2 ps. Thermalization involves intersubband scattering of excited electrons via optical phonon emission with a time constant of 1 ps and intrasubband Coulomb and phonon scattering. Ensemble Monte Carlo simulations show that the slow electron equilibration results from Pauli blocking and screening of carrier-carrier scattering.