Ultra-high-Q Fano resonance based on coupled topological corner states in Kagome photonic crystals.

Higher-order topological photonic crystals offer a promising route for robust light confinement. This work presents a Fano resonance sensing system via near-field coupling between two topological corner states (TCSs) in a Kagome photonic crystal. The coupling generates an odd-symmetric supermode with suppressed radiative loss and an ultra-high eigenmode quality factor. Integrating the coupled TCSs with a topological edge waveguide forms a cavity-waveguide system supporting sharp Fano resonance. The proposed system exhibits excellent performance, with a sensitivity of 562.75 nm/RIU and a maximum figure of merit of 1.22×107RIU-1 achieved for the odd-symmetric supermode. This platform paves the way for advanced higher-order topology applications in integrated photonics and sensing.