Effect Of Zn²⁺ Ion Doping And [Li]/[Nb] Ratio On Defect Structure And Optical Properties Of Zn:Dy:LiNbO₃ Crystals

Two sets of Zn:Dy:LiNbO₃ series crystal samples were grown by traditional pulling method with Zn²⁺ion doping concentration and[Li]/[Nb]as variables and suitable axial temperature gradient,rotating speed and pulling speed as well as annealing and polarization treatment to prepare suitable size labeled as Zn Dy-1,Zn Dy-3,Zn Dy-5,Zn Dy-7.Zn Dy_0.946,Zn Dy_1.05,Zn Dy_1.20 and Zn Dy_1.38.The effective condensation coefficients of Zn²⁺,Dy³⁺ions and[Li]/[Nb]inside the real crystal were measured and calculated by ICP-AES.It was found that the effective condensation coefficients of Zn²⁺and Dy³⁺decreased with the increase of Zn²⁺doping concentration,while for different[Li]/[Nb]samples,the effective condensation coefficients of Zn²⁺ions decreased with the increase of[Li]/[Nb],and that of Dy³⁺ions increased with the increase of[Li]/[Nb].Moreover,[Li]/[Nb]=1.20is closest to the stoichiometric ratio of Zn:Dy:LiNbO₃ crystal.X-ray diffraction analysis showed that neither of the two variables changed the crystal type,and the doped ions entered the crystal as substitutes.The unit cell volumes were all found computationally to increase first and then decrease.The change of OH^-absorption peak inside the crystals was tested by IR absorption spectroscopy.It was found that the substitution mechanism of Zn²⁺doping concentration exceeding the threshold would change,resulting in a new absorption peak of4Zn_Li⁺-OH_Li^--Zn_Nb^3-near 3527cm^-1.When[Li]/[Nb]approaches the stoichiometric ratio,the absorption peak at 3466cm^-1 appears,and when it is far away from the stoichiometric ratio,the intensity of the absorption peak at that location decreases or even disappears.The total exposure energy and birefringence gradient values of the two groups of crystal samples were measured and calculated by using the light scattering energy flow threshold method and the birefringence gradient method,respectively,so as to obtain the variation rule of the resistance to light damage and optical uniformity,and to analyze the mechanism of the change in combination with the defect structure.It is concluded that both increasing the concentration of anti-photo-damage ion Zn²⁺and[Li]/[Nb]can greatly improve the photo-damage resistance and optical uniformity of Zn:Dy:LiNbO₃ series crystals.Among them,the improvement of photo-damage resistance is not linear but the optical uniformity increases linearly,and the improvement of anti-light damage performance by increasing the concentration of Zn²⁺is better than that by increasing[Li]/[Nb].