Rare-earth ion distribution in sol–gel glasses co-doped with Al3+

Terbium spectroscopy is used to probe clustering of rare earth (RE) ions in sol–gel glasses. We use pulsed laser experiments to study energy transfer among Tb3+ dopants, and analyze the fluorescence decay curves using the Inokuti–Hirayama model. The analysis yields new insights about the role of Al3+ co-doping. Assuming a multipolar interaction, we derive an effective Tb3+ concentration for a series of samples with varying amounts of RE. When actual doping concentration is varied over 2 orders of magnitude, the effective Tb3+ concentration of centers that emit light changes by only a factor of 10. The results indicate that Al3+ co-doping is only effective at dispersing RE ions when the ratio of Al:RE is 10:1 or greater. At higher RE concentrations, most ions reside in clusters; 5D3 fluorescence is observed from a minority of isolated ions. This interpretation contradicts earlier work that used fluorescence line narrowing to demonstrate that Al3+ co-doping caused RE dispersal at high doping levels.