Highly Efficient Second Harmonic Generation Based On Lithium Niobate Assisted By Dispersion Design Of Metamaterial

Since the laser has been invented,the field of nonlinear optics has been rapidly developed and applied.A typical example is second harmonic generation(SHG),which is an important nonlinear optical process,and have important applications in microscopic imaging,lasers and other fields.With the development of optical field detection at the sub-wavelength scale and the development of micro-nano optics manufacturing technology,nonlinear nanostructures that do not need to meet phase matching conditions have been extensively studied.The optical anapole mode produced by the interaction between the electric dipole(ED)and the toroidal dipole(TD)has the localization of the optical field and the non-radiation optical characteristics of the far-field and is often used to enhance the optical local field of nanostructures.Lithium niobate crystal(Li Nb O₃,LN)is a kind of optoelectronic material with many excellent characteristics.It has strong second-order nonlinear optical characteristics and is widely used in nonlinear optical frequency conversion.Hyperbolic metamaterials(HMMs),as a new type of artificial composite electromagnetic materials,have the advantage of flexible light field control.Therefore,this paper uses the optically tunable properties of HMMs and LN to construct micro-and nanostructures that can excite anapole modes,so that light can be more effectively localized in the micro-and nanostructures to enhance the interaction with the light,to obtain more efficient nonlinear optical conversion.This paper proposes and studies LN micro-and nanostructure based on HMM,which have higher refractive index contrast,and include a Si O₂ spacer layer to assist in enhancing the interaction between light and matter.To effectively improve the nonlinear optical response,the dimensional parameters of the micro-and nanostructure are simulated and optimized by the method of finite-difference time-domain(Lumerical FDTD Solutions),and the linear responses such as the light scattering characteristics of the micro-and nanostructure and the light field distribution are obtained by the numerical simulations.Under the excitation wavelength of the anapole mode,the normalized electric field intensity enhancement value can reach 35,which indicates that the electric field local effect of the micro-and nanostructure has been significantly enhanced.At the same time,the finite element method(COMSOL Multiphysics)has been used in combination with the multipole expansion analysis theory of electromagnetic field scattering to verify that the enhancement of the nonlinear local field of the structure is derived from the excited anapole mode.The local field enhancement effect can greatly improve the nonlinear conversion efficiency of the micro-and nanostructure,that is,the excitation wavelength of?=565.4 nm of the anapole mode is used as the fundamental frequency pump light to incident on the LN micro-and nanostructure to generate high-efficiency second harmonic signals.The influences of pump light conditions such as pulse duration and peak intensity on the second harmonic conversion efficiency are further explored,and the optical responses of incident vector beams to LN micro-and nanostructure are compared and analyzed.Theoretical study shows that the LN micro-and nanostructure with HMM as substrate can increase the second harmonic conversion efficiency to10^-4 when the incident source intensity of linearly polarized light is 11 GW/cm².By designing and changing the structures or size parameters of HMMs,the nonlinear optical wavelength of the micro-and nanostructure can be precisely controlled to achieve an efficient and tunable nonlinear conversion effect.In summary,the proposed design of LN micro-and nanostructure to excite the anapole mode provides an effective method for improving nonlinear conversion efficiency and efficient nonlinear optical control,which effectively enhances the mechanism of light-matter interaction with flexible controllability and universal applicability,it has large application potential in the fields of biological imaging and integrated nonlinear optical devices.The innovations of this thesis are:1.Combined with the dispersion design characteristics of HMMs,a higher refractive index contrast is formed with LN,and then the optical anapole mode is excited to achieve local enhancement of the optical field,to increase the nonlinear conversion efficiency to 10^-4.2.Based on the dispersion design of HMMs,the nonlinear effect of different spectral bands can be enhanced,that is,the tunable second harmonic spectral response can be realized.