Tb~(3+)-Yb~(3+) cooperative down and up conversion processes in Tb_(0.81)Ca_(0.19)F_(2.81):Yb~(3+) single crystals

摘要

The energy transfer processes between Tb~(3+) and Yb~(3+) ions are studied in Tb-based fluoride single crystals. These are very transparent from the UV to the IR wavelength regions, except for the characteristic absorption lines of the rare-earth ions under study. In contrast with previously reported Tb~(3+)-doped glasses and oxide powders, these crystals containing a high Tb~(3+) concentration present two major advantages for the study of the energy transfer processes in the Tb~(3+)-Yb~(3+) ion pair. Firstly, the adverse influence of host defects is minimized with the use of high quality crystals. Secondly, the high Tb~(3+) concentration guarantees a much higher absorption cross-section of UV light, and consequently these crystals have real potential for practical applications. Photolumi-nescence spectra in the visible-IR wavelength region demonstrate the existence of efficient down and up conversion processes by crossed excitation and emission characteristics of Tb~(3+) and Yb~(3+) ions. In the down conversion process, Tb~(3+)(5D_4) → 2Yb~(3+)(~2F_(5/2)), two IR photons are emitted for each Tb~(3+) ion deexcited by second-order energy transfer to two Yb~(3+) ions. The total quantum efficiency of the down conversion process in the fluoride system is shown to increase linearly with the Yb~(3+) concentration, reaching its maximum at the Yb~(3+) solubility limit in the fluoride host. Further, efficient up conversion by two- and three-IR photon absorption is observed.