Spectroscopic study and visible luminescence of Tb3+ions in CaF2 crystal co-activated with Yb3+

High quality crystals with composition of Ca 0.95 Tb 0.05 F 2.05 and Ca 0.85 Tb 0.05 Yb 0.1 F 2.15 were grown by BridgmanStockbarger technique. Yb3+ ions were used as a sensitizer for Tb3+ ions. Optical properties were measured, including the absorption spectrum, emission spectrum, and fluorescence decay curves. Judd-Ofelt theory was used to analyze the spectroscopic parameters. Fluorescence branch ratio, transition probability, radiative and fluorescence lifetimes, and quantum efficiency were determined. The emission cross section of Tb3+ ions in Tb: CaF2 and Tb/Yb:CaF2 crystals for the green emission at 544 nm, corresponding to the transitions of 5 D 4 -> 7 F 5 were calculated to be 4.61 x 10-22 cm2 and 4.60 x 10-22 cm2, respectively. Our experiments have shown that doping with Yb3+ ions does not significantly affect the spectral properties of Tb3+ ions in CaF2 crystal. Quadratic dependence of the emission intensity of Tb3+ ion in Tb/Yb:CaF2 crystal on the intensity of a diode laser at 955 nm proved that the excitation of Tb3+ ion occurs due to the cooperative process of transferring the excitation from two Yb3+ ions to one neighboring Tb3+ ion. The Tb/Yb:CaF2 crystal can be considered as a promising up- converting medium for creating diode-pumped visible phosphorus and active laser media.