Photonic Higher‐Order Topological Insulator with Enlarged Non‐Trivial Bandgaps

Abstract The emergence of higher‐order topological insulators (HOTIs) has greatly expanded the family of topological materials. While the co‐dimensional bulk‐boundary correspondence is observed in platforms, such as acoustics and photonics, realizing three‐dimensional (3D) photonic HOTIs is relatively challenging due to the complex properties of electromagnetic waves such as polarizations, scattering, and refractive index. In this paper, a photonic HOTI with a simple multilayer structure that supports higher‐order hinge states is proposed. By inserting a central metallic pillar in the unit cell, the 3D bandgap can be well extended, enabling pure and distinguishable surface and hinge modes. The lattice is reconfigurable and flexible, allowing for hinge and surface waves to be generated by controlling the geometrical length of sub‐lattices. The idea of distinguished higher‐order hinge modes is also extended to enlarged higher‐orbital bandgaps. Furthermore, by introducing a central disclination in this photonic model, the one‐dimensional (1D) vertical disclination mode is obtained which is not seen in existing photonic HOTIs. The findings open the door for a high‐performance topological optical apparatus that features efficient one‐way light propagation and energy concentration.