Excitation effects in non-radiative multiphonon decays of rare earth doped laser materials

The non-radiative decay process of rare earth ions in solids, which are known to constitute a whole class of laser materials, has been considered up to now to be independent of excitation intensity. We shall show here that the multiphonon non-radiative decay probabilities of rare earth ions in glasses are reduced at high excitation state densities for the larger energy gaps. The classical exponential energy gap law is shown to 'rotate' at higher excitation around the 3.2-phonon point The observed effect is described in term of a spatial saturation of the accepting mode term with effective diffusion lengths ranging between 54 and 20 Angstrom for excited state density from 10(17) to 10(19) cm(-3). This effect which is predicted to be general for multiphonon processes at large gives hints for the obtention of laser effects from otherwise non-radiative transitions provided the excitation density is sufficient to allow the coupling between ions through a common phonon diffusion volume. (C) 1997 Elsevier Science B.V.