JUDD-OFELT ANALYSIS OF Tb3+ AND UPCONVERSION STUDY IN Yb3+–Tb3+ CO-DOPED CALIBO GLASSES

We report on the study of the spectroscopic properties in Ytterbium–Terbium (Yb³⁺-Tb³⁺) co-doped calcium lithium borate (CaLiBO) glasses, with the study’s focus being on the upconversion process. Intensity parameters Ω_λ for CaLiBO:Tb³⁺ are determined by the Judd–Ofelt method to be Ω₂ = 15.5 × 10^-20 cm², Ω₄ = 1.90 × 10^-20 cm² and Ω₆ = 3.69 × 10^-20 cm². We have also obtained electric-dipole (and magnetic-dipole) radiative transition probabilities, branching ratios, and lifetime for the Tb³⁺:⁵D₄ and ⁵D₃ levels. In addition, an evaluation of the upconversion processes by luminescence and time-resolved spectroscopy were carried out. The upconversion rise and decay times, energy transfer probability from Yb³⁺ to Tb³⁺ ions, and the efficiency of the processes that depopulate the Tb³⁺:⁵D₄ level after resonantly pumping the Yb³⁺:²F_5/2 level were estimated. Our results showed that a cooperative energy transfer (CET) from two Yb³⁺ ions to one Tb³⁺ ion is the origin of the Tb³⁺ upconversion luminescence in the visible region. While, CET followed of the cross relaxation or/and excited state absorption is responsible for the upconversion luminescence in the ultraviolet region.