Unidirectional propagation control of helical edge states in topological spin photonic crystals

The unidirectional propagation of the pseudospin-locked optical waves can be achieved by using the helical edge states in two-dimensional topological spin photonic crystals. Although there have been lots of researches on topological spin photonic crystal, the unidirectional propagation regulation of helical edge states has been rarely studied up to now. Herein, by tuning the diameter D of the dielectric rod and the distance R between the center of the unit cell and the center of the rod in two-dimensional topological spin photonic crystal with C-6v symmetry, the effects of geometric parameters on the bandgap structure, topological properties and unidirectional propagation of helical edge states in topological spin photonics crystals are studied in detail. The results show that the parameter R has significant effects on the bandgap structure and topological properties of topological spin photonic crystal, and the diameter D of rod only affects the bandgap positions. The structures of helical edge states and their unidirectional propagations are closely related to the topological properties of the bandgaps and the frequency structures of photonic crystals on both sides of boundary. Without changing the topological properties of bandgaps, only changing the structures of the photonics crystals on either side of the boundary can also regulate the structures of the helical edge states and their unidirectional propagation. Thus, by adjusting the geometric parameters R and D of the topological spin photonic crystals, the helical edge states and their unidirectional propagations can be controlled. This study provides useful references for the selection and application of helical edge states in topological spin photonic crytals.