The Study On The Third-order Nonlinear Optical Properties Of Lithium Niobate Thin Films

Lithium niobate(LiNbO₃,LN)is regarded as one of the important candidate materials for photonic integrated chips due to its wide transmission band,low intrinsic absorption,excellent nonlinear optical coefficient and electro-optical coefficient.The third-order nonlinear properties of LiNbO₃,nonlinear refractive index n2 and nonlinear absorption coefficient ? can significantly affect the performance of optoelectronic devices under strong light irradiation,which are very important optical parameters of LiNbO₃.The design and preparation of integrable optoelectronic devices usually require LiNbO₃ films,however,the growth process of LiNbO₃ films is often accompanied by deviations in atomic composition and differences in crystalline structure,which will significantly affect the third-order nonlinear properties of LiNbO₃ films,and then affect optoelectronic devices which are based on LiNbO₃ thin films.Therefore,it is of great significance and value to study the third-order nonlinear optical properties of LiNbO₃ thin films.At present,there is a lack of research on the influence of the atomic composition of LiNbO₃ film on its third-order optical nonlinearity.This thesis uses magnetron sputtering method to prepare LiNbO₃ thin film,and annealing treatment in 700?different atmosphere,and finally obtain different composition and crystalline LiNbO₃ thin film samples.The samples are characterized by material characterization methods and the samples are analyzed by Z-scan technology.The third-order nonlinear optical properties are studied and the nonlinear refractive index and nonlinear absorption coefficient are obtained.The main specific tasks are as follows:(1)The amorphous LiNbO₃ film was prepared by magnetron sputtering technology at room temperature,then the film was annealed at 700? in an argon atmosphere to obtain a polycrystalline LiNbO₃ film.The crystalline state and element composition of the LiNbO₃ film were determined by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and Raman spectrometer test(Raman).The linear optical properties and third-order nonlinear optical properties were determined by optical linear transmittance test(Transmission),ellipsometer test(n&?)and Z-scan.The XPS test data shows that the film is a niobium-rich LiNbO₃ film.The linear transmittance test result is calculated and the argon annealing film band gap(Eg)is 4.05 eV,which is relative to the room temperature film band gap(Eg)3.84 eV,indicating annealing treatment the crystallization of the process film leads to an increase in the band gap.The Z-scan results show that both the room temperature LiNbO₃ film and the LiNbO₃ film annealed in an argon atmosphere exhibit reverse saturation absorption characteristics and self-focusing effects at near-infrared wavelength of 1064 nm and pulse duration of 25 ps.The nonlinear refractive index n2 and non-linear absorption coefficient ? of the amorphous LiNbO₃ film at room temperature are obtained by calculation,which are determined to be 1.44×10-11 cm2/W and 1.94×10-7 cm/W,respectively.The n2 and ? value of the argon annealed crystalline LiNbO₃ film are 1.52×10-11 cm2/W and 3.34×10-7 cm/W,respectively.Indicating that the nonlinearity of the LiNbO₃ film treated by argon gas annealing is enhanced compared with the room temperature LiNbO₃ film.The nonlinear polarizability of the film is about 10-10 esu,which is 3 orders of magnitude larger than the reported nonlinear polarizability of LiNbO₃ single crystal of about 10-13 esu.(2)The amorphous LiNbO₃ films which prepared at room temperature are annealed at 700?in oxygen atmosphere to obtain polycrystalline oxygen-annealed LiNbO₃ films.The linear transmission test results calculated that the band gap(Eg)of the annealed film is 4.08 eV,which is relative to the room temperature film band gap(Eg)3.83 eV and it also shows that the crystallization of the film during the annealing process causes the band gap to increase.The Z-scan results show that both the room temperature LiNbO₃ film and the LiNbO₃ film annealed in oxygen atmosphere exhibit reverse saturation absorption characteristics and self-focusing effects.According to calculations,the n2 and ? value of the oxygen annealed crystalline LiNbO₃ film are 1.57×10-11 cm2/W and 3.68×10-7 cm/W,respectively.Indicating that the nonlinearity of the LiNbO₃ film treated by oxygen gas annealing is enhanced compared with the room temperature LiNbO₃ film.In theory,Nb-rich atoms can enhance the nonlinear refractive index of LiNbO₃ films.At the same time,there may be more Nb-O bonds in the oxygen-annealed film,and the electronic state of the transition metal element Nb that does not occupy the d orbital may also have a certain effect on the nonlinear refraction of LiNbO₃.In summary,the crystalline state of the LiNbO₃ film,incident light wavelength pulse duration,light intensity,doping of different elements or ions,and interatomic bonding will all have significant impacts on the third-order nonlinear properties of the film,then affect the application of on-chip photonic devices has an impact.(3)We designed an electro-optic switch with a resonant waveguide grating structure based on the electro-optic effect of LiNbO₃ and using graphene as a transparent electrode.We use Comsol software to perform numerical simulation calculations,and realize the calculation and regulation of electro-optical switches at a wavelength of 1550 nm.The applied voltage from 0 to 10 V can realize the adjustable transmittance of the device from 0 to 60%.