Saturation effect on multiphonon relaxation rates

Abstract

Fluorescence decay time of resonantly excited RE multiplets were recorded as a function of excitation power density in different glass hosts. The multiphonon nonradiative relaxation rates are reduced at high excited state concentration for large energy gaps. The results are analysed in a statistical approach and the observed phonon bottleneck effect related to an accepting mode saturation. At high excited state density, several ions lying in the phonon diffusion volume simultaneously fill the common accepting mode set of the host. The critical distance below which excited ions share a common phonon bath is related to the phonon diffusion length lc in the host. Theoretical fit of experimental data allows the deduction of lc. The results are compared with the phonon mean free path in the host independently derived from sound velocity measurements and estimation of thermal conductivity and heat capacity of the investigated glasses.