Nonlinearities in emission from the lower polariton branch of semiconductor microcavities

Stimulated scattering of spin-polarized lower branch polaritons (LP) arisin g from their bosonic character is reported. The observations are achieved i n a microcavity close to zero detuning between the uncoupled exciton and ph oton modes, under conditions of circularly polarized resonant excitation in to 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 po larization of the emitted light P-LP. At low-excitation densities, P-LP is Of the order of 30% in the whole range of wave vectors k < 2.10(4) cm(-1). However, at higher excitation intensities, P-LP increases drastically, up t o 80%, for LP's with k approximate to 0, and is accompanied by the appearan ce of strong nonlinearities in the LP emission intensity whilst P remains u nchanged for weakly coupled polaritons with k > 10(4) cm(-1) [S0163-1829(99 )51040-0].