Fig. 1: Higher-order quantum spin Hall effect in a photonic crystal.

a The dimensional hierarchy of the higher-order topological insulator in a dielectric photonic crystal. Corner, edge and bulk states are represented by blue, green and red colors, respectively, and separated from each other in the frequency domain. b Two pseudospins defined by the in-plane magnetic field (represented by the purple arrows) configuration in the unit cell. c The scheme of achieving directional localization of psedospin-polarized corner states excited by a pseudospin-dependent source. The blue (red) star represents the position of a left circular (right circular) polarized light as a pseudospin-dependent source. The blue (red) sphere represents the position of the corner states with a pseudospin up (down) polarization (represented by arrows).