Time-resolved two-photon excitation of long-lifetime polaritons

Recent studies of two-photon excitation of exciton-polaritons in microcavities have considered the possibility of an allowed absorption process into the 2p-state of the excitons that participate in the polariton effect. Here we report time-resolved measurements of two-photon excitation directly into the lower polariton states, invoking the 1s state of the excitons. Although this process is forbidden by symmetry for light at normal incidence, it is allowed at a nonzero angle of incidence due to state mixing. We examine the polarization dependence of two-photon absorption at finite k both theoretically and experimentally. Previous results should be reevaluated in light of the mechanism observed here.