Concentration and excitation effects in multiphonon non-radiative transitions of rare-earth ions

Non-radiative transitions in rare-earth ion doped systems are usually considered to be concentration and excitation intensity independent. Here we consider theoretically and experimentally a saturation in the capacity for the modes of the lattice to dissipate energy. Some first experimental results of this new effect in the case of Yb3+ doped berate glasses are presented showing that at excited ion densities larger than 10(18) cm(-3), non-radiative multiphonon transitions can be completely quenched well before amplification by stimulated emission is reached except when concentration rises above 8 x 10(20) cm(-3). The experiment is well described at lower concentration by considering that the excited ions can share a common phonon density of states. At higher concentration, the common accepting modes diffusion length is reduced from 23 to 8 Angstrom and a non-saturable radiative relaxation rate background of about 575 s(-1) is observed which can be viewed as an increase in the effective accepting mode coupling strength by a factor 1.33.