Polariton-polariton scattering in semiconductor microcavities:: Experimental observation of thresholdlike density dependence

Polariton-polariton scattering in semiconductor microcavities has been studied by time-integrated, degenerate four-wave mixing using spectrally compressed pulses. We find a thresholdlike behavior for the density dependence of the scattering rate gamma in the lower polariton branch. At low densities scattering is suppressed due to the small density of final states for the scattering process. At higher density gamma increases as excitonlike states become accessible for the scattering and the scattering strength only depends on the exciton fraction of the initial states. The threshold density depends on the detuning between the polariton branches. The experimental findings are in qualitative agreement with calculations using the self-consistent Born approximation.