Accelerating polariton relaxation in a two beam experiment

We have performed a two-beam experiment on a II-VI microcavity in the strong coupling regime. A pulsed non-resonant excitation creates a transient population of excitons of large in-plane wave-vector and a very weak polariton population at k,=0, center of the Brillouin zone. A cw laser beam resonantly injects polaritons at the inflexion point of the lower polariton branch and efficiently populates the k,=0 via polariton-polariton scattering. In the presence of the two laser beams, we observe a strong increase of the transient emission at k,=0 associated to a drastic modification of the emission dynamics. The signal gain can reach 200. We show that the key parameter controlling the gain is the cw polariton population at k,=0, which stimulates the polariton relaxation from large k states toward k,=0.