Observation of Square-Root Higher-Order Topological States in Photonic Waveguide Arrays

Abstract

Recently, higher-order topological insulators (HOTIs), accompanied by topologically nontrivial boundary states with a codimension larger than one, have been extensively explored because of unconventional bulk-boundary correspondences. As a novel type of HOTIs, very recent works have explored the square-root HOTIs, where the topologically nontrivial nature of bulk bands stems from the square of the Hamiltonian. In this paper, 2D square-root HOTIs are experimentally demonstrated in photonic waveguide arrays written in glass using femtosecond laser direct-write techniques. Edge and corner states are clearly observed at visible light spectra. The dynamical evolutions of topological boundary states are experimentally demonstrated, which verify the existence of photonic corner states in two band gaps. The symmetry-protected corner states in the photonic square-root HOTI may have potential applications in information processing and lasing.2D photonic square-root HOTIs are realized in photonic waveguide arrays written in glass using femtosecond laser direct-write techniques. In addition to the spectral measurement, a fractional corner anomaly is analyzed for revealing the higher-order topology, and the confinement and robustness of the corner states are explored. These findings pave the way toward potential applications in photonic devices.