Microstructure And Optical Properties Of Mg/In/Hf And Ho Codoped Lithium Niobate Crystals

In our research, three crystals materials series: the Mg: Ho: LiNbO₃ crystals, In: Ho: LiNbO₃ crystals and Hf: Ho: LiNbO₃ crystals were grown by the Czochraski method. The influence of growing technological parameters (temperature gradient, rotation speed and pulling rate et al) on the quality of crystals was studied, high quality crystals without macroscopic defect, but with good optical homogeneity were grown under optimum conditions.The concentration of doped ions in crystal was measured by inductively coupled plasma atomic-emission spectrometry. The way that segregation coefficient of Mg, In and Hf in LiNbO₃ crystal and Li/Nb ratio change as Mg, In and Hf doping concentration varies was fully studied. The result indicates: the segregation coefficient of Mg, In and Hf decrease as any of Mg²⁺, In³⁺ and Hf⁴⁺ doping concentration increase, wherever increase In³⁺ concentration only decrease In segregation coefficient , but the increase segregation coefficient of Ho.The defect structures of Mg: Ho: LiNbO₃, In: Ho: LiNbO₃ and Hf: Ho: LiNbO₃ crystals and the influence of the Mg, In and Hf doped concentration variation on doped ions'occupation sites in crystals were investigated thoroughly by the infrared OH- absorption spectra, UV-Vis absorption spectra, X-ray powder diffraction and the differential thermal analysis. The experiment results show that Mg, In and Hf are all first replace the N b4L+i, when their concentration below their own threshold value. When Mg, In and Hf doped concentration equal or above their own threshold value, all defect N bL⁴⁺i in crystal are replaced, Mg²⁺, In³⁺ and Hf⁴⁺ ions occupy the normal Li and Nb sites. However, the defects complexes formed by those three ions'occupation in the Li sites and Nb sites are different: Mg²⁺ mainly occupy the Li site, forming 3 Mg +L i -Mg3N-b; In³⁺ mainly occupy the Li and Nb site, forming the In 2L+i -In2N-b; Hf⁴⁺ mainly occupy the Nb, forming the Hf L3i+ -3HfN-b.By using the upconversion instrument, the upconversion luminescence spectra of Mg: Ho: LiNbO₃, In: Ho: LiNbO₃ and Hf: Ho: LiNbO₃ crystals excited by 808nm wavelength source were examined. We found how doping concentrations of Mg, In and Hf affect their optic strength of upconversion spectra, clarified that Mg: Ho: LiNbO₃, In: Ho: LiNbO₃ and Hf: Ho: LiNbO₃ upconversion luminescence mechanisms are all two-photon process.Phenomenological intensity parameters of Mg: Ho: LiNbO₃, In: Ho: LiNbO₃ and Hf: Ho: LiNbO₃ crystal were calculated with ultraviolet-visible absorption spectra test results applying the Judd-Ofelt theory. The spontaneous emission transition probability, radiative lifetime, fluorescence branching ratio and integrated emission cross section partial energy levels of Ho³⁺ were obtained at the same time. The result shows: doping of Mg, In and Hf alter the strength of 9 Ho absorption peaks as well as many other absorption spectra characteristics such as some absorption peak red shift; The absorption peak value of Ho: LiNbO₃ doping with Mg²⁺, In³⁺ is obviously enhanced when the doped concentration surpass threshold to 10mol% and 5mol%; Different from the absorption peak changing rule of Ho: LiNbO₃ doping with Mg²⁺, In³⁺, the peak values decline when Hf⁴⁺doped concentration below the threshold concentration, the peak value increase when Hf⁴⁺ doped concentration was above the threshold.The optical damage resistance ability of Mg: Ho: LiNbO₃, In: Ho: LiNbO₃ and Hf: Ho: LiNbO₃ crystals were studied by both the transmitted beam pattern distortion method and the light-induced scattering exposure energy flux threshold method. The experimental results show that the optical damage resistance ability enhanced as the concentration of Mg, In and Hf increase, when Mg, In and Hf doped concentration reaches or above their own threshold value, the optical intensity threshold improves at least two orders of magnitude, however, the optical damage resistance ability weakened as the concentration of Mg and Hf ions surpass the concentration to 10mol% and 8mol%.