Embedded topological edge states from reversed two-dimensional photonic crystals

Abstract

Abstract Topological edge states have attracted great attention for their exotic properties such as the asymmetrical light propagation, which can be exploited in many application areas especially in integrated photonics. In this work, we propose the design of a novel structure that supports topological edge states residing inside the bulk bands. The purely dielectric structure comprises two reversed two-dimensional photonic crystals (PCs) with the simplest triangular lattices. As the energy of the p and d orbits in the two PCs are reversed, the topologically trivial and non-trivial states can directly be formed without any intermediate transition required. The topological edge states occur at the interface between the two PCs, in analogy to the quantum spin Hall (QSH) effect in electronic systems. Due to the unique configuration, the edge states can extend themselves into the semi-bulk bands and become the embedded topological edge states, which breaks the constraint that the edge states can only exist inside the band gap. Our simulation results have demonstrated the existence of the pseudospin states and the spin-locked propagation with topological protection. A spin locked unidirectional transmission of topological edge states along the direction of Γ K - Γ M or Γ M - Γ K has also been demonstrated, respectively, in a compound structure.