Non-linear coupling of polariton and dark exciton states in semiconductor microcavities

We report a new mechanism of non-linear coupling of optically active and dark crystal states. We observe experimentally pronounced beats of the intensity of photoluminescence from a bottleneck region of the exciton-polariton band in a microcavity in the strong coupling regime and at strong pumping. These beats are extremely sensitive to the pumping intensity and vanish for weak pumping. We show theoretically that coherent polariton-polariton scattering which leads to the mixing between bright and dark exciton states can be responsible for this effect.