Production of 1s quadrupole-orthoexciton polaritons in Cu₂O by by two-photon pumping

We have studied the angular (at 1.8 K) and temperature dependence (from 1.8 to 70 K) of the time-integrated luminescence spectra of 1s orthoexcitons from the yellow series in Cu₂O arising from resonant two-photon pumping to both the 1s and 2s levels. For resonant pumping to the 1s level, the direct (denoted as X_o) emission is strongly enhanced at low temperature. It is forward directed, however, with an angular width substantially larger than the divergence of the excitation beam; excitation to the 2s level (which subsequently decays into a 1s level) results in a more isotropic angular distribution of X_o emission. The lifetime of the X_o emission resulting from resonant excitation to the 1s level at 1.8 K is ∼2 ns, shorter than the decay time of thermalized orthoexcitons. The results support the idea that resonant two-photon excitation to the 1s level results primarily in a quadrupole-orthoexciton-polariton formation, whereas 2s excitation does not. The behavior of the phonon-assisted luminescence is essentially independent of the mode of excitation.