Nonequilibrium Dynamics in a Quasi-Two-Dimensional Electron Plasma after Ultrafast Intersubband Excitation

The dynamics of electrons in GaInAs/AlInAs quantum wells is studied after excitation from the $n=1\mathrm{}$ to the $n=2\mathrm{}$ conduction subband. Femtosecond pump-probe experiments demonstrate for the first time athermal distributions of $n=1\mathrm{}$ 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.