ENERGY TRANSFER BETWEEN DEFECTS AND RARE-EARTH IONS IN GARNET CRYSTALS.

The current hypothesis in the literature concerning the cathodoluminescent spectrum of rare-earth-doped YAG assumes that energy is transferred between the defects and dopant ions through the initial thermal production of excited carriers, initially located at the defect, into the conduction band; their subsequent deexcitation leads to excited states of the rare-earth ion. It is proposed that energy can be transferred directly between the dopants and defects through a Foerster-Dexter nonradiative transfer mechanism. The degree of overlap of the emission spectrum and the dopant's absorption spectrum determines the magnitude of the nonradiative energy transfer. In particular, the temperature dependence of the cathodoluminescence of cerium-doped, terbium-doped, and europium-doped YAG and the temporal decay of excited states of these materials can be understood by this latter model. The model one assumes for energy transfer is important for understanding the nature of the defects in YAG. It is shown that electrons localized at oxygen vacancies in the crystals are responsible for the defect luminescence in YAG and the observed energy transfer properties.