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

Abstract 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 borate glasses are presented showing that at excited ion densities larger than 1018 cm−3, non-radiative multiphonon transitions can be completely quenched well before amplification by stimulated emission is reached except when concentration rises above 8 × 1020 cm−3. The experiment is well described at lower concentration by considering that the excited ions can share a common phnon density of states. At higher concentration, the common accepting modes diffusion length is reduced from 23 to 8 A 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.

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