Nonlinearities in emission from the lower polariton branch of semiconductor microcavities

Stimulated scattering of spin-polarized lower branch polaritons (LP) arising from their bosonic character is reported. The observations are achieved in a microcavity close to zero detuning between the uncoupled exciton and photon modes, under conditions of circularly polarized resonant excitation into the upper polariton branch. The key role played by the bosonic nature of the LP states is demonstrated by measurements of the degree of circular polarization of the emitted light $P_{\mathrm{LP}}.$ At low-excitation densities, $P_{\mathrm{LP}}$ is of the order of 30% in the whole range of wave vectors $k<\mathrm{}{2.10}^4\hspace{0.5em}{\mathrm{cm}}^{-1}.$ However, at higher excitation intensities, $P_{\mathrm{LP}}$ increases drastically, up to 80%, for LP’s with $k\approx 0,$ and is accompanied by the appearance of strong nonlinearities in the LP emission intensity whilst P remains unchanged for weakly coupled polaritons with $k>\mathrm{}{10}^4\hspace{0.5em}{\mathrm{cm}}^{-1}.$