Multiphonon decay in halide and oxide glasses at high excitation density: a comparison

The scientific and applied interests for so-called infra-red glasses, the very subject of this conference, are exclusively linked to such glass low phonon energy. It provides their infra red transparency and good quantum efficiency when doped, trough reduced multiphonon interactions. Recently we have shown that multiphonon decay in oxide glasses could be reduced at high excitation level and that this was linked with a saturation of the accepting vibration modes. The saturation coming from a common phonon diffusion volume for two excited ions. In this paper, after recalling the basics of multiphonon nonradiative decay, we shall present recent results on rare-earth doped halide glasses in comparison with oxide glasses having various phonon energies. Again the low phonon energy of halide glasses brings a different behaviour with respect to oxide glasses: sometime no saturation can be observed for certain levels because accepting modes seem not to be excited whatever the phonon diffusion volume in the halide glass. In fact we show that up-conversion do play a role in fluoride grasses partially masking the phonon bottleneck. Because the key parameter for such multiphonon process is the ratio between inter electronic levels energy gap and the phonon energy, halide glasses can be said to behave homothetically with oxide glasses. This confirms a likely generalisation for our theoretical approach and allows to predict likely similar behaviour for other multiphonon processes described by energy gap laws such as sidebands and infra-red absorption bleaching, phonon assisted energy transfer saturation.