Effect of [Li]/[Nb] ratios on the absorption and up-conversion emission spectra in In:Yb:Ho:LiNbO3 crystal

In:Yb:Ho:LiNbO3 crystals with high optical quality were grown by the Czochralski method with various ratios of [Li]/[Nb], that is 0.94, 1.05, 1.20 and 1.38 in the melt. The UV-VIS-NIR absorption spectra of In:Yb:Ho:LiNbO3 crystals were measured. The transition intensity parameters Ωt (t = 2, 4 and 6), spectroscopic quality factor (X) and the lifetimes of Ho3+ in In:Yb:Ho:LiNbO3 crystals were all evaluated by the Judd-Ofelt theory. The spectroscopic quality factor Ω4/Ω6 of In:Yb:Ho:LiNbO3 crystals with the [Li]/[Nb] ratio of 1.38 was found to be 1.69, which is comparable with those found in widely used Ho3+ doped crystals. Furthermore, up-conversion emission spectra were determined and analyzed under 980 nm LD excitation in the In:Yb:Ho:LiNbO3 crystals. The results revealed that In:Yb:Ho:LiNbO3 crystal with the [Li]/[Nb] ratio of 1.38 is a promising material for 2 μm wavelength radiation.