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 ${\mathrm{Cu}}_2\mathrm{O}$ arising from resonant two-photon pumping to both the $1s$ and $2s$ levels. For resonant pumping to the $1s$ level, the direct (denoted as $X_{\mathrm{o}})$ 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 $X_{\mathrm{o}}$ emission. The lifetime of the $X_{\mathrm{o}}$ 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.

• Received 22 March 2000

DOI:https://doi.org/10.1103/PhysRevB.63.125323