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.