Indirect generation of quadrupole polaritons from dark excitons in Cu2 O

The momentum-dependent electron-hole exchange interaction splits the triply degenerate orthoexcitons into the bright and dark states in Cu2 O, depending on the direction of k inside the crystal. Employing resonant two-photon excitation along a (100) direction at 2 K, we selectively create the dark orthoexcitons that do not couple to the radiation field. However, these excitons can radiatively recombine due to subsequent conversion into the bright states. By measuring the polarization dependence, we find that the two bright states are equally populated due to cross relaxation. We also find that this bright orthoexciton luminescence is highly directional and well aligned with the initial laser propagating direction. This strongly indicates that propagating quadrupole orthoexciton-polaritons, which are the quantum superposition of bright orthoexcitons and photons, are indirectly generated. We find that the dark-to-bright relaxation mechanism can be significantly affected in the presence of hot excitons due to density-dependent scattering.