Phonon bottleneck effect in multiphonon non-radiative transitions of rare-earth ions

Abstract Fluorescence transients of resonantly excited RE multiplets were recorded as function of excitation density in different glasses. Multiphonon relaxation rates are reduced at high excited state concentration for large energy gaps. Such behavior is analysed in a statistical approach and explained on the basis of a theoretical model for the microscopic process. We suggest a phonon bottleneck effect on radiationless relaxation rate related to an accepting modes saturation. At high excited state density, the accepting modes are simultaneously filled by several ions lying in a phonon diffusion volume. The phonon diffusion length I c in the host deduced from our model is compared with the phonon mean-free path independently derived.