Energy transfer in clustered sites of Er^3+ ions in LiNbO_3 crystals

We investigated energy-transfer processes associated with clustered sites of Er3+ ions in LiNbO3 crystals leading to upconverted blue-green fluorescence and cross relaxation with the analyses of fluorescence transients measured by site-selective excitations. We found two clustered sites with distinct lifetimes of 3 and 16 μs at room temperature for the green fluorescing levels (4S3/2), which were much shorter than the lifetimes of isolated sites (27 μs) that are due to nonradiative energy-transfer processes. The green upconverted fluorescence originates from the energy transfer of one clustered site in the 4I7/2 levels, and/or of the other clustered site in the 4I11/2 levels. Our analyses verified that the fluorescence efficiencies of Er3+ ions in the clustered sites are significantly decreased because of their interionic nonradiative energy transfers. Therefore, the formation of clustered sites in Er3+-doped LiNbO3 crystals gives rise to deleterious effects on laser and amplifier applications.