Impact of impurities on orthoexciton-polariton propagation in Cu2 O

Near the orthoexciton resonance in Cu2 O, light propagating through the medium is accompanied by a quadrupolar polarization, a quadrupole polariton. However, these orthoexciton-polaritons can lose their initial coherence because the excitonic component of the mode, a tightly bound electron-hole pair, is subject to wide-angle scattering by atomic-scale imperfections within the crystal. In order to examine this scattering by ambient impurities, we measured the angular dependence of the orthoexciton-polariton luminescence under resonant two-photon excitation at 2 K. A rather broad angular distribution is obtained at higher impurity concentrations. We provide a simplified polariton scattering model in order to describe the observed angular distribution. Various experimental data obtained from two natural-growth samples indicate that the orthoexciton-polariton propagation can strongly depend on the local impurity concentration.