Density dependence of carrier-carrier-induced intersubband scattering in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

Photoluminescence lifetimes of the $n=2\mathrm{}$ level in a large quantum well show a clear nonmonotonic dependence on the density of optically generated carriers. Varying the power density over five orders of magnitude we prove directly the high efficiency of carrier-carrier interaction for intersubband scattering when longitudinal-optical phonon emission is suppressed. For low densities, the observed $n=2\mathrm{}$ decay times get shorter (from 40 down to 5 ps) as the density is increased. At high densities Pauli blocking reduces significantly the intersubband scattering rates.