Nonlinear Optical Frequency Conversion In Lithium Niobate Subwavelength Thin-Film Waveguide

Nonlinear optical frequency conversion is an important means to extend the laser frequency window,and nonlinear crystal is an important functional material to realize these functionalities.Lithium niobate(LN)has been widely used in integrated optics due to its superior electro-optical,acousto-optical and nonlinear optical properties,as well as its wide transparency from 350 nm to 5200 nm.Recently,a single-crystal lithium niobate on insulator(LNOI)has aroused great interest among researchers.Subwavelength LN film has been developed as a promising nonlinear integrated optical material platform due to its strong beam confinement effect and rich optical properties.To date,a variety of highly integrated and high-performance LN thin film devices have been successfully manufactured,including electro-optical modulators,microdisc resonators,filters,etc.However,,reports about the physics of light transport in single-crystal LN thin film waveguide and the nonlinear optical frequency conversion process are relatively rare.In addition,the fabrication of LN chip-level devices is expensive and time-consuming.Therefore,the mode dispersion relation and basic physical characteristics of nonlinear optical interaction and frequency conversion in LN subwavelength film structures should be further discussed.At the same time,it is very important to develop more efficient and high-precision theoretical analysis methods to qualitatively and quantitatively analyze the nonlinear interaction efficiency of these structural devices.These requirements have not been fulfilled in current theoretical and experimental works.In this thesis,a nonlinear coupled mode theory system which fully takes into account the dielectric constant and refractive index birefringence characteristics of LN crystals as well as the anisotropic nonlinear susceptibility tensor is established for the second harmonic generation(SHG)in single crystal subwavelength LN thin films.It considers the SHG effect between different waveguide modes and involves complex nonlinear interaction of the vector light field.The main content of this paper is as follows:Firstly,the mode dispersion curves corresponding to TE and TM polarization modes in LN thin film waveguides with different thicknesses are analyzed and calculated based on the electromagnetic field theory of LN waveguide,in which the wavelength range covers the entire broad band from ultraviolet to infrared.Furthermore,we systematically study the phase matching conditions required for the conversion of fundamental-light waveguide mode to second-harmonic waveguide mode by SHG effect.By using the mode order,polarization state,thickness,symmetry and other physical parameters of the waveguide,a series of modal phase matching schemes can be found.It is noticed that mode phase matching often occurs only in very limited discrete pump base frequency wavelengths.Then,under the model of phase matching conditions,starting from nonlinear Maxwell's equations,we develop nonlinear coupled mode theory of LN thin film,calculate effective nonlinear susceptibility,derive the analytical solution to the equations in the condition of small signal and big signal,and obtain the explicit relation between the nonlinear frequency conversion efficiency and the pump light intensity,polarization,wavelength,and thin-film length.This allows one to determine the optimal phase matching scheme.Our analytical theory and formula can greatly promote the deep understanding of the physical mechanism of nonlinear optical interaction in micro/nanoscale single crystal LN thin films,and provide a convenient and accurate quantitative evaluation tool for SHG conversion efficiency of pumped laser.This analytical theory is also beneficial to the performance improvement and functional innovation of micro-nanoscale LN thin film nonlinear optical devices.