Collisional broadening of semiconductor microcavity polaritons

We measure the effect of intensity on the homogeneous linewidth of two high finesse semiconductor microcavities in the strong coupling regime using degenerate four-wave mixing. We find that the collisional broadening due to polariton-polariton scattering shows a threshold behaviour with intensity. At low intensities, a strong suppression is seen due to the small density of states available. Above threshold, the number of accessible states dramatically increases, giving a large increase in the collisional broadening. We also investigate the effect of modifying the dispersion curve, by detuning, on the threshold behaviour.