Theory of the angle-resonant polariton amplifier

A microscopic theory is presented for the giant amplification from microcav ities in the strong exciton-photon coupling regime, providing excellent agr eement with recent angle-resolved pump-probe experiments. The analytical an d numerical solutions give insight into the physics of the polariton parame tric amplifier. The coherent gain, due to polariton four-wave-mixing, has a threshold dependence on the pump power and is spectrally blueshifted from the lower polariton energy. The gain shift does not depend on the power, bu t on the unperturbed polariton linewidth.