AbstractThe effect of two CW laser beams resonant with the polariton dispersion of a GaAs planar microcavity is discussed. A new coherent polariton emission is observed when the pump and the probe beams are both resonant with the dispersion at finite k‐vectors. This feature is the idler of an optical parametric amplifier (OPA) and is generated by probe‐stimulated scattering of pairs of polaritons from the pump. OPA behaviour is also observed in the optical parametric oscillator (OPO) regime. A study is made of the effect of varying the probe power on the interaction between the pump OPO and the probe OPA states; the OPA idler is found to be a much stronger emission than the signal of the OPO on its own. A simple theoretical model is used to simulate the pump‐probe interactions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)