Effect Of Hf⁴⁺ Concentration On Defect Structure And Optical Properties Of Yb:Tm:LiNbO₃ Crystals

Hf:Yb:Tm:LiNbO₃ crystals were successfully prepared by Czochralski method.The doping concentration of Hf⁴⁺was 2 mol%,4 mol%and 6 mol%,respectively.The defect structure and dopant occupancy of crystals with different Hf⁴⁺concentration were investigated.The influence of Hf⁴⁺concentration on the upconversion luminescence and optical damage resistance ability of Hf:Yb:Tm:LiNbO₃ crystals were discussed.The concentration of Hf⁴⁺,Yb³⁺and Tm³⁺ions were analyzed via inductive couple plasma atomic emission spectrometer.In addition,the relationship between the separation coefficient of Hf⁴⁺,Yb³⁺and Tm³⁺ions and Hf⁴⁺concentration were analyzed.It is found that the increase of Hf⁴⁺ion doping concentration leads to the decrease of Hf⁴⁺ion segregation coefficient,while that of Yb³⁺ion and Tm³⁺ion decreases gradually.The influence of Hf⁴⁺doping concentration on defect structure and doping occupancy in Hf:Yb:Tm:LiNbO₃ crystal was measured and discussed by IR absorption spectra,UV-VIS-NIR absorption spectra and X-ray powders diffraction.The result showed that Hf⁴⁺ion enters into LiNbO₃ crystal by priority replacing intrinsicNb_Li^4?10?defect in form ofHf_Li^3?10?when the doping concentration of Hf⁴⁺ion was below the threshold concentration.However,when the doping concentration of Hf⁴⁺ion reached or exceed the threshold concentration,Nb_Li^4?10?defects completely replaced,and Hf⁴⁺will occupy the Li site and Nb site in the form ofHf_Li^3?10?and Hf_N^-_b defects.The upconversion fluorescence of Hf:Yb:Tm:LiNbO₃ crystals was obtained under 980nm excitation in the range of 400-700nm.And the energy transfer mechanism of upconversion process was analyzed in detail.The strong emissions were observed at 475nm in the blue wavelength range and 651nm in the red wavelength range.Remarkably,enhancement of upconversion emissions was achieved by tridoping Hf⁴⁺ions.The light damage resistance of Hf:Yb:Tm:LiNbO₃crystal was analyzed by light scattering threshold energy flow method.It is found that the light damage resistance of Hf:Yb:Tm:LiNbO₃ crystal can be effectively improved by increasing the doping concentration of Hf⁴⁺ions.When Hf⁴⁺concentration increased to 6mol%,the optical damage resistance of the crystal was48 times of that of the 2mol%Hf⁴⁺doped crystal.Through systematic research and analysis,Hf?6mol%?:Yb?1mol%?:Tm?1mol%?:LiNbO₃ crystal has the best luminescent properties and the strongest ability of light damage resistance among this series.