Topological transition and topological line defects in dielectric photonic crystals

We propose a topological photonic crystal (PC) with split hexagonal ring structure by removing six isosceles trapezoids on the ring. By changing the size of trapezoid and hexagonal ring, we realize Dirac point and multi topology transition of the quadruple accidental degenerate. We simulate the quantum spin Hall effect (QSHE) and find that the EM wave (from 149.9 THz to 155.96 THz) almost achieves lossless transmission in the system, and is immune to sharp bends, cavity and disorder. At the junction of trivial and nontrivial photonic crystals (PCs), the topological trivial is replaced with the ordinary honeycomb lattice to construct line defect. We prove that the line defect has a non-spin-mixing defect mode with topological protection. The PC with line defect mode has high transmission efficiency, robust unidirectionality, and high optical local performance, which can not only be used in lasers, all-optical integrated circuits, optical communication systems and semiconductor technologies, but also will enrich the exploration of topological physics.