Field distribution of the <i>Z</i><sub>2</sub> topological edge state revealed by cathodoluminescence nanoscopy

Photonic topological insulators with robust boundary states can enable great applications for optical communication and quantum emission, such as unidirectional waveguide and single-mode laser. However, because of the diffraction limit of light, the physical insight of topological resonance remains unexplored in detail, like the dark line that exists with the crystalline symmetry-protected topological edge state. Here, we experimentally observe the dark line of the Z2 photonic topological insulator in the visible range by photoluminescence and specify its location by cathodoluminescence characterization, and elucidate its mechanism with the p-d orbital electromagnetic field distribution which calculated by numerical simulation. Our investigation provides a deeper understanding of Z2 topological edge states and may have great significance to the design of future on-chip topological devices.