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

Abstract

High quality crystals with composition of Ca_0.95Tb_0.05F_2.05 and Ca_0.85Tb_0.05Yb_0.1F_2.15 were grown by Bridgman-Stockbarger 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 ⁵D₄→⁷F₅ were calculated to be 4.61 × 10⁻²² cm² and 4.60 × 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.