Relation between interface symmetry and propagation robustness along domain walls based on valley topological photonic crystals

Abstract: Valley photonic crystals provide efficient designs for the routing of light through channels in extremely compact geometries. The topological origin of the robust transport and the specific geometries under which it can take place have been questioned in recent works. In this article, we introduce a design for valley photonic crystals with richer arrangement possibilities than the standard valley photonic crystals based on two holes of different sizes in the unit cell. Our approach is based on the permutation of three sets of rhombi in an hexagonal lattice to investigate the interplay between Berry curvature, valley Chern number and chirality of interfaces to achieve robust edge-modes propagation along domain walls. We study three types of interfaces with different symmetries: the non-chiral interface with glide-mirror symmetry commonly used in honeycomb-type valley crystals, and two chiral interfaces with or without inversion symmetry of the adjacent bulk lattices. In the latter case, no valley topology is expected. We show that for the three families, edges preserving the shape of the interface through 120{\deg} sharp corners can sustain edge-modes with comparable robustness. Moreover, interfaces with glide-mirror symmetry offer promising performances in circuits with more exotic configurations, like 60{\deg} and 90{\deg} corners or arbitrary curves in which valley preservation is not guaranteed. Our work raises questions about the topological origin of the robustness of transport in valley photonic crystals, discusses the role of the chirality of the interfaces in the propagation around sharp corners, and provides a lattice scheme with broad design possibilities.