Research On Terahertz Photonic Topological Insulator

With the continuous breakthrough of scientific research,the 6G era is coming,and THz is the key to the next generation of high-speed wireless communication predicted internationally.When preparing THz waveguide chip,a large number of optical devices need to be highly integrated into the waveguide chip,and the robustness of the device determines the overall reliability and manufacturing cost of the waveguide chip.In recent years,researchers have found that robustness is closely linked to the emerging field of topology in condensed matter,so the study of photonic topological insulators is receiving widespread attention.The excellent characteristics of photon topological insulators are due to their topological phase transitions similar to those of electron topological systems in classical condensed matter,and the key to topological phase transition lies in the inversion of the time reversal symmetry or invert symmetry,and then the band inversion occurs at the degeneracy point,which can be achieved in the optical system to imitate the quantum valley-Hall effect.In addition,photons are non-conserved particles with a finite lifetime,in which photons are affected by a variety of non-equilibrium processes,from the gain and absorption of dielectric materials to radiation losses caused by defects in the energy interaction of open systems and protection from destructive topologies.It is of great significance to study the practical application of valley-Hall-like and non-Hermitian photonic systems to THz waveguide devices.In this thesis,the topology of valley-Hall-like and total-loss non-Hermitian THz photonic topological insulators are studied by combining theoretical analysis and numerical simulation.The main research contents are as follows:1.A valley-Hall-like THz photonic topological insulator based on the rotation of the split-rings to break the C_3v symmetry is designed.Its unit cell is composed of three rotatable split-rings arranged similar to Kagome lattice.Rotating the split-ring in two ways can realize the transformation of the model from C_{3 v} symmetry to ₃C symmetry.After breaking C_3v symmetry,the Dirac degenerate point opens and there exist two band inversion and three band inversion.In addition,valley hall like structures with different topological phases can construct topological interface states with different valley projections.Positive refraction and negative refraction appear when incident into the background material.The negative refraction angle at0.102THz are-75.8°calculated by phase matching conditions,and have strong robustness to suppress backscattering transmission and overcome defect transmission.2.The concepts of non-Hermitian photonic topological insulator and pseudo-PT symmetric domain-wall system are proposed.In this thesis,non-Hermitian factors are introduced by considering the imaginary part of non-zero potential energy in condensed matter model.Firstly,the concept of non-Hermitian is introduced from the simplest one-dimensional SSH model,then the non-Hermitian topologies of honeycomb lattice and BBH model are discussed in the two-dimensional case,and a pseudo-PT symmetric domain-wall system based on non-Hermitian system is proposed,in which there exist"real value"edge states that can be predicted accurately.