Nonlinear emission of microcavity polaritons in the low density regime

We present polariton emission measurements as a function of the excitation power in a III-V semiconductor microcavity. In a large range of power the polariton emission exhibits a clear nonlinear behavior. Since its energy, in-plane dispersion, and radiation pattern remain unchanged, no polariton bleaching occurs. A model, including phonon assisted relaxation stimulated by the polariton final state population, accurately describes the experimental variations of the nonlinearity amplitude and threshold with the detuning. [S0031-9007(98)08222-2].