Polarization beats in ballistic propagation of exciton-polaritons in microcavities

The emission from a microcavity in the strong coupling regime excited resonantly by linearly polarized light has been measured with spatial, directional, and temporal resolution. We observe ballistic propagation of the excited polaritons as a ring spreading in real space. The linear polarization degree P of the emission at the ring is found to oscillate as a function of the azimuthal angle θ and of time t according to P=sin(2θ)sin(ΩLTt∕2), where ΩLT is the longitudinal-transverse splitting of the exciton-polariton modes. This dependence holds for all investigated excitation energies corresponding to different values of ΩLT. The theoretical model assuming ballistic propagation of exciton polaritons in the specific cavity structure yields a detailed agreement with the experimental data. The observed polarization beats have the same nature as the recently predicted optical spin Hall effect