Coherent and incoherent polaritonic gain in a planar semiconductor microcavity

The gain processes in a semiconductor microcavity in the strong coupling regime have been studied by pump-probe experiments in transmission geometry. It is demonstrated that the nonlinear signal consists of two contributions, a coherent and an incoherent one. In agreement with recent reports, the coherent gain is identified as a parametric amplification process that is driven by the probe field and stimulates the scattering of polaritons into the ${\mathbf{k}}_{\Vert }=0\mathrm{}$ states. We attribute the incoherent gain to scattering of randomly distributed polaritons in the predominantly excitonic part of the lower polariton branch into states with zero wave number in the lower branch. Both processes are characterized by their polarization dependence and their sensitivity to the spectral position of the pump laser beams. They also show a pronounced threshold behavior versus the pump power.