Summary form only given. We report the use of semiconductor microcavities in which both electronic and photonic wavefunctions and their interaction can be specifically manipulated, to produce enormous optical amplification of ultrafast pulses. This relies on the strong coupling of excitons and photons to produce new polaritonic quasiparticles whose nonlinear interactions can be resonantly enhanced. The devices act as vertical cavity parametric amplifiers with an optical pump being efficiently downconverted into the incident signal beam. Unexpectedly, the optimum gain condition is found when the pump pulse is 25% elliptically polarised. This state produces at least double the output compared to either a circular or a linear pump pulse. By scanning through the entire range of pump polarisations, and resolving the polarisation of the emission, we are able to map out the characteristics of this phenomenon, which has also recently been seen for CW excitation. We propose a simple spin-dependent model for the parametric amplification, which infers the presence of a second-order stimulated scattering process that can be as strong as the first order scattering.