Effect of Hf⁴⁺ doping on structure and enhancement of upconversion luminescence in Yb: Tm: LiNbO₃ crystals

A series of Yb:Tm:LiNbO₃ crystals doped with x mol% Hf⁴⁺ ions (x=2, 4, and 6) were grown by the Czochralski method. The dopant occupancy and defect structure of Hf:Yb:Tm:LiNbO₃ crystals were investigated by x-ray diffraction and infrared transmission spectra. The influence of Hf⁴⁺ ions concentration on UV-VIS-NIR absorption spectra of Hf:Yb:Tm:LiNbO₃ crystals was discussed. The upconversion luminescence of Hf:Yb:Tm:LiNbO₃ crystals was obtained under 980 nm excitation. Strong emissions were observed at 475 nm in the blue wavelength range and 651 nm in the red wavelength range. Remarkably, enhancement of the red and blue upconversion luminescence was achieved by tridoping Hf⁴⁺ ions.