Effect of [Li]/[Nb] ratios on UV-VIS-NIR spectroscopy and up-conversion luminescence in Hf:Dy:LiNbO₃ crystals

A series of Hf:Dy:LiNbO₃ crystals with various [Li]/[Nb] ratios ([Li]/[Nb]=0.946, 1.05, 1.20 and 1.38) in the melt were grown by the Czochralski method. The UV-VIS-NIR absorption spectra of Hf:Dy:LiNbO₃ crystals were measured, and Judd-Ofelt (J-O) theory is used to evaluate the transition intensity parameters Ω_{2, 4, 6} and spectroscopic quality factor (X). The X of Hf:Dy:LiNbO₃ crystals with the [Li]/[Nb] ratio of 1.38 was found to be 2.11. The up-conversion luminescence at around 475 nm, 582 nm, 653 nm and 749 nm of Dy³⁺ ions were observed under 800 nm excitation. The energy transfer mechanisms of up-conversion process were analyzed in detail, and luminescence lifetime was measured. The results show that Hf:Dy:LiNbO₃ crystal is an excellent candidate for laser medium.