Ultraviolet and visible upconversion emission in Tb3+/Yb3+ co-doped fluorophosphate glasses

Ultraviolet and visible upconversion emissions in Tb³⁺/Yb³⁺ co-doped YF₃–BaF₂–Ba(PO₃)₂ glasses were observed under 980-nm laser diode excitation. The dependence of the emission intensities of Tb³⁺ on the pump power reveals that two-photon processes account for blue cooperative emission of Yb³⁺ at 476 nm and green upconversion emission of Tb³⁺ at 543 nm, and three-photon processes for ultraviolet emission of Tb³⁺ in the wavelength range of 379–435 nm. The effects of Tb³⁺ concentration on the emission intensity and the lifetime of Tb³⁺ and Yb³⁺ are investigated in detail. It is found that the cooperative energy transfer from a pair of excited Yb³⁺ ions to a ground Tb³⁺ ion is responsible for the appearance of blue and green upconversion emissions due to the ⁵D₄→⁷F _J (J=6,5,4,3) transitions of Tb³⁺, and the resonance energy transfer from Yb³⁺ to Tb³⁺ accounts for the population on the ⁵D₃,⁵G₆ level and ultraviolet upconversion emission.