Fig. 2: Emergence of multi-dimensional boundary states at the interface between two topologically distinct photonic crystals.
From: Higher-order quantum spin Hall effect in a photonic crystal
a The combination of an ordinary insulator and a topological insulator with a line-shape boundary (blue dashed line). b The measured (yellow area) and the simulated (solid line) spinful edge states in the projected band structure. The unidirectional propagation of light to the c right and d left directions with an right circular polarized (RCP) and a left circular polarized (LCP) source, respectively. e The numerically calculated eigenvalues (left panel) of and measured transmission (right panel) of the hexagonal meta-structure (lower inset). The bulk, edge and corner states are represented by gray, blue and yellow dots, respectively. f The field distribution of the numerically calculated (lower panel) and experimentally excited (upper) corner states at 9.760 GHz.
