Design And Study Of The Topological Photonic Structures Based On Lithium Niobate

In recent decades,the field of topological photonics has developed rapidly.In the field oftopological optics,different topological states are described by introducing new parameters,namely topological invariants or topological orders.These topological invariants reflect the invariant properties of structures under local deformation.In recent years,a series of novel and special phenomena have emerged in the field of topological photonics,including optical Floquet topological insulator,optical quantum spin Hall effect,etc.These topological photonic phenomena have been widely used in quantum optics computation,optical field regulation,local enhancement of light.Photonic crystals,as common objects in optical systems,play an important role in the study of topological photonics.As a common semiconductor material,lithium niobate has excellent optical and electrical properties,including electro-optic effect,nonlinear optical effect,which provides an excellent optical material system for the study of topological photonics.Especially in recent years,the development of lithium niobate single crystal films has made it possible to fabricate lithium niobate micro/nano optical structures and devices.In this thesis,some typical topological photonic structures are designed based on Li Nb O₃ monocrystal thin films and their optical properties are systematically studied.The specific contents are summarized as follows:(1)A zigzag chain structure composed of lithium niobate nanocolumn was designed and its topological boundary states were studied.In this thesis,the winding and energy band models of zigzag structures are analyzed by the method of tight bound approximation,and the topological properties of zigzag structures are briefly analyzed.The boundary states and topological phase transition points of the structure are predicted by theoretical analysis results.Then,the simulation software was used to simulate the topological characteristics of the lithium niobate zigzag chain,and the corresponding situation of the structure to the incident light with different polarization under the odd or even number of particles was verified.When the structure is an odd number of particles,the magnetic field of boundary state excited by x-direction polarized incident light is localized at the upper right end,and the magnetic field of y-direction polarized incident light is localized at the lower left end.When the structure is an even number of particles,the boundary magnetic fields excited by x or y polarization are all localized at both ends.At last,we study the robustness of the lithium niobate zigzag chain,and the results show that the zigzag structure corresponding to the boundary states has a strong robustness in face of disordered disturbances or defects.(2)A one-dimensional non-Hermitian SSH structure composed of a one-dimensional resonant ring array of lithium niobate was designed and the non-Hermitian skin effect was studied.The non-Hermitian skin effect refers to the phenomenon that all the eigenstates of a structure are localized at the boundary.In this thesis,the anisotropic coupling between the modes of different rotation directions is introduced in the one-dimensional resonant ring array of lithium niobate.The energy band and eigenstate distribution of the non-Hermitian SSH model are analyzed by theoretical calculation.Furthermore,we use COMSOL software to simulate and calculate the eigenmodes of the one-dimensional non-Hermitian SSH resonant ring structure designed for lithium niobate.In the calculated results,we observe that all the eigenmodes are localized at the boundary.This work realizes the skin effect in the one-dimensional resonant ring array with simple structure.At last,we investigate the robustness of skin effect in non-Hermitian SSH resonant rings.The results show that the skin effect can always exist stably in the presence of disturbance.(3)In this thesis,the Bound states in the continuum(BIC)photonic crystal structures were designed based on Li Nb O₃ single-crystal films.The mode and radiation properties of Li Nb O₃quasi-BIC photonic crystal were studied.BIC is a kind of mode which can coexist with continuous domain mode and maintain the special existence of local characteristics.When the quality factor(Q)and resonance width become limited due to disturbance and other factors under BIC condition,BIC can realize quasi-BIC.We designed a lithium niobate elliptical cylinder array.By changing the angle between the lithium niobate elliptical cylinders,the transmission spectra of the structures under different symmetries are studied,together with the distribution and the Q factor of the field in the corresponding structure,obtained the corresponding position of BIC in the transmission spectrum,and verified the infinite Q value characteristic of BIC.In addition,we studied the relationship between the reflection spectrum of the structure and the radiation characteristics of the light field through the incident circularly polarized light,and obtained the reflection spectrum that changes with the polarization direction of the circularly polarized light.We can predict the radiation polarization of the mode light field by observing the position where the reflection disappears in the reflection spectrum.