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 which 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 non-zero angle of incidence due to state mixing. 
Chitra Gautham, Mark Steger, David Snoke, Ken West, Loren Pfeiffer
physics.optics cond-mat.mes-hall