Beam Splitting And Phase Advance-free Propagation In Artificial Structures Based On Topological Edge States

In recent years,topological physical phenomena have received wide attention in the field of optics,and various topological phases and topological edge states analogous to electronic systems have been successively confirmed.The study of topological characteristics based on artificial structures has many controllable freedoms and is also convenient for experimental observation,greatly promoting the development of topological band theory and topological functional devices.Currently,the realization of directional transport of topological edge state is mainly dependent on the two spin freedoms of the interface,valley spin and pseudospin.The theoretical design of artificial structures that simultaneously support both two spin freedoms has yet to be developed,and the experimental research is also lacked.Moreover,frequency-dependent beam splitting transmission of topological edge state is limited to the frequency control of the edge state dispersion,and the coupling effects between topological edge states and other optical propagation modes are not yet fully understood.In view of these problems,this thesis proposes a method to control the propagation direction of valley edge states and chiral edge states,a zero effective refractive index waveguide based on the coupling of chiral edge states and waveguide modes,and a Kekulé topological photonic crystal with hybirdized valley-spin and pseudospin,and studied important issues related to toological edge state transmission.The main research contents of this thesis are as follows:(1)A study on the beam splitting transmission of topological edge states has been carried out.In order to solve the probrem of single controlled method of topological edge state,a multi-port topological structure has been proposed,each port of which supports different frequency and dispersion of topological edge states,thus achieving frequencycontrolled multidirectional beam splitting transmission of topological edge states.In addition,a four-port topological structure based on a non-Hermitian system has been proposed,and the gain and loss of the material can be controlled to allocate the topological edge states in a specific ratio.In this thesis,the finite element based electromagnetic simulation software COMSOL Multiphysics(COMSOL)is used to simulate and analyze the structure,and the feasibility of regulating the directional transmission of topological boundary states has been theoretically proven.(2)To study the coupling effect of topological edge states and other optical propagation modes,a sandwich structure waveguide composed of "topological photonic crystal-air-topological photonic crystal" has been proposed,which can simultaneously support chiral edge states and waveguide modes.By adjusting the thickness of the air channel,it is possible to couple the chiral edge state and the waveguide mode around k =0,thus generating a linear band crossing and enabling ideal phase advance-free propagation.In experiments,dielectric cylinders were prepared using ceramic processing techniques and positioned in the desired positions.The electric field intensity and phase distribution in the air channel were excited and measured using a vector network analyzer and a near-field scanner.The experimental results show good consistency with the simulations,and phase-advance free propagation of electromagnetic waves was directly observed in the air channel at the degeneracy frequency of the modes.(3)A study has been conducted on topological structures that simultaneously support valley and pseudospin edge states.A dual-band gap Kekulé topological photonic crystal has been proposed,in which the valley edge state is originated from the exchange of different bands at the Brillouin zone corner,and pseudospin edge state is caused by the linear combination of two degenerate modes at the center of the Brillouin zone.Simulation and experimental results have shown the coexistence of these two types of topological edge states.Due to the different topological origins,valley and pseudospin edge states exhibit different refraction phenomena when coupled to free space through an interface.In addition,a gradient interface composed of different structure parameters of Kekulé topological photonic crystals has been proposed,which realizes rainbow capture of pseudospin topological edge states,that is,different frequencies of waves can be transmitted to different locations along the interface.In summary,this thesis investgates the control of topological edge state propagation behavior and the hybridization coupling effect between topological edge states and waveguide modes based on microwave topological structures.Frequency-controllable topological edge state beam splitting transmission and phase-advance free propagation of electromagnetic waves have been achieved in air channels.In addition,a dual-band gap topological structure has been proposed,which can support valley and pseudospin topological edge states in the low-and high-frequency bands,respectively.By optimizing the size of the topological structure,this study can be extended to the terahertz and optical frequencies.This study provides a flexible way for understanding and designing multifunctional topological optical devices,with potential applications in on-chip information propagation,optical sensing,nanolaser,and quantum communication.