Unidirectional propagation of coupled edge states in sandwich topological photonic crystals

Topological photonic crystals (PCs) with exotic optical properties such as helical edge states have tremendous potential applications in the fields of photonic integrated circuits. Here, we propose a sandwich PC structure based on trivial-nontrivial-trivial topology with a robust unidirectional light propagation through coupling of two helical edge states in the nontrivial topological region. We calculate dispersion relations of the sandwich structures and observe a robust unidirectional light propagation excited by an external point source with orbital angular momentum. To evaluate the stability of light flow, a Z-shaped corner is established and positions or radii of several cylinders are changed randomly. Results show that the unidirectional propagation remains stable in the imperfect structures. The configuration may find potential applications for the construction of nanophotonic circuits.