Spectroscopic studies of excitons in cuprous oxide: Natural crystals and synthetic thick films on magnesium oxide

We observed exciton photoluminescence from Cu₂O natural bulk crystals under two-photon excitation to the 1s, as well as to the 2s level, over a wide temperature range. The direct recombination emission, denoted as X_o, and a phonon-assisted, electric-dipole radiative transition involving $G-12$ longitudinal optical phonons, denoted as X_o-$G-12$, were observed for 1s excitation. We have studied the angular (at 1.8K) and temperature (from 1.8K to 297K) dependence of the time integrated photoluminescence intensity of both emission features. For resonant pumping to the 1s level, the direct 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 1.8K is ∼2ns, 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. A theory involving the Green's function and coupled photon-exciton wave equations was developed to simulate the coherent polariton propagation.; To study excitons in a spatially confined geometry, we developed an ex situ growth technique to obtain single-crystal like Cu₂O thick films on MgO substrates. The optical absorption spectrum exhibits the exciton absorption series up to n = 5p at low temperature. 1s orthoexciton direct emission and phonon-assisted emission features were observed to split into three (on MgO (110)) and two (on MgO (111)) peaks. The distortion of film crystal structure and the effective Hamiltonian including a deformation potential were used to explain the energy level splitting and shift of the 1s orthoexcitons in Cu₂O films. Cu₂O dots and waveguides were also prepared by the ex situ growth technique. Distinguishable exciton emission peaks were observed from the waveguides. Only broad exciton emission features were observed from Cu₂O dots.