NONEQUILIBRIUM DYNAMICS IN A QUASI-2-DIMENSIONAL ELECTRON-PLASMA AFTER ULTRAFAST INTERSUBBAND EXCITATION

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