Research On Ultraviolet Light-induced Absorption And Third-order Nonlinear Optical Effects In Doped LiNbO₃

Lithium niobate,(LiNbO3,LN)is one of the most important synthetic crystals because of its good electro-optic,piezoelectric and nonlinear optical properties.Due to its excellent photorefractive properties,LiNbO3 crystal has been one of the most widely used photorefractive materials in holographic volume storage.Except for holographic-storage,its photorefractive effect(optical damage)is damage for a number of other nonlinear optical applications,such as second harmonic generation,optical parametric oscillator.Mg O,Zn O,In2O3 and Sc2O3 can be used as doping impurities to suppress the photorefractive effect.The resistance ability for optical damage of the doped LiNbO3 crystal has a threshold effect and the doped LiNbO3 crystal will be affected by the third nonlinear optical effects under strong light irradiation.The study of the third nonlinear optical effects of damage-resistance impurities doped LiNbO3 crystal is therefore of practical significance.In this dissertation,the optical damage and the third-order nonlinear optical effects of different concentrations Sc3+,Mg2+,Zn2+doped and pure near-stoichiometric LiNbO3 crystals are studied through experiments.In this dissertation,firstly,the pump-probe method is used to measure the ultraviolet-light-induced absorptions(UVLIA)of optical damage resistance impurities Sc3+?Mg2+?Zn2+ doped LiNbO3 crystals.The filtered mercury-xenon lamps with the center wavelength of 365 nm are used as the pump source and a continuous-wave diode laser operating at 785 nm is used as the probe beam.The steady-state light-induced absorptions(LIA)with respect to the different doping concentrations are obtained.The polarization state of the probe beam is changed in order to measure the effect of polarization on the light-induced absorption.The results show that in Sc:LiNbO3 crystals when the Sc doping concentrations are 0.1mol% and 0.2mol%,there is considerable LIA,which is larger than that of pure LiNbO3 crystal and when the Sc doping concentrations are 0.5mol%,0.6mol% and 0.8mol%,the light-induced absorption is extremely suppressed,which is much smaller than that of pure LiNbO3 crystal.For Mg:LiNbO3 crystals,when the Mg doping concentration is 1mol%,there is considerable LIA,which is larger than that of pure LiNbO3 crystal and when the Mg doping concentrations are 2mol%,3mol% and 4mol%,the light-induced absorption is substantially suppressed.Meanwhile,for Zn:LiNbO3 crystals,when the Zn doping concentration is 1mol%,there is considerable LIA,which is larger than that of pure LiNbO3 crystal and when the Zn doping concentration is 2mol%,the light-induced absorption is effectively suppressed.The doping threshold of Sc3+?Mg2+ and Zn2+ doped near-stoichiometric LiNbO3 crystals are deduced to be 0.2-0.5mol%,1-2mol% and 1-2mol%,respectively.Then we can see the LIA has a good corresponding relationship with the resistance ability for optical damage in these crystals.When the concentration of doped impurities is below its photorefractive threshold,there is considerable light-induced absorption and when the concentration of doped impurities is above its photorefractive threshold the light-induced absorption is suppressed.To verify the resistance ability for optical damage of crystals,the transmitted light spot distortion method and light scattering threshold of energy flow method are used,where a sapphire coherent SF laser with a wavelength of 532 nm is used.The results are in good agreement with the above LIA results,and it can be concluded that the resistance ability for optical damage of the crystal can be judged by measuring the UVLIA.Secondly,the third-order nonlinear optical effects of the doped crystals are studied through Z-scan method.The third-order nonlinear absorption coefficient ? is measured by the open aperture Z-scan method,and the third-order nonlinear refractive coefficient ? is measured by the close aperture Z-scan method.By simulating the experimental curves,the values of the third-order nonlinear refractive coefficient and the third-order nonlinear absorption coefficient are obtained for all crystals.The results show that the absolute values of ? and ? both decrease with the increasing doping concentration and the sign of ? changes around photorefractive threshold.For Sc:LiNbO3 crystals,when the Sc doping concentrations are 0.1mol% and 0.2mol%,the ? is negative and when the Sc doping concentrations are 0.5mol%,0.6mol% and 0.8mol%,the ? is positive.For Mg:LiNbO3 crystals,when the Mg doping concentration is 1mol%,the ? is negative and when the Mg doping concentrations are 2mol%,3mol% and 4mol%,the ? is positive.For Zn:LiNbO3 crystals,when the Zn doping concentration is 1mol%,the ? is negative and when the Zn doping concentration is 2mol%,the ? is positive.In conclusion,the UVLIA,the third-order nonlinear refractive coefficient and the third-order nonlinear absorption coefficient of Sc3+,Mg2+,Zn2+ doped near-stoichiometric LiNbO3 crystals are measured,and the UV-light-induced absorption method may be used to evaluate the resistance ability for optical damage and the impurity threshold concentration.The results are helpful for nonlinear optical applications of LiNbO3 crystals doped with photorefractive resistance impurity.