Higher-Order Topological Corner States in a Specially Distorted Photonic Kagome Lattice

Kagome lattices have garnered significant interest over the decades due to their rich physics and implications for exotic superconductors and topological materials. In particular, the so-called "breathing Kagome lattice" (BKL) is often used as a prototypical model to understand higher-order topological insulators. Here it is demonstrated that higher-order corner states exist in a specially distorted Kagome lattice, resembling the "Inverse Hexagram" lattice distortion originally introduced in the study of charge density waves in Kagome metals. Importantly, it is found that such an Inverse Hexagram-like Kagome lattice with C6 rotational symmetry hosts two types of in-gap corner states, in contrast to a flat-cutting BKL of C3 symmetry that supports only one type of corner state under the tight-binding condition. It is shown that the nontrivial corner states exhibit a fractional corner anomaly, revealing the feature of higher-order topology. Using laser-written waveguide lattices as a photonic platform, these two types of corner states are experimentally observed, along with a direct comparison with the corner excitation at their trivial counterparts. Experimental observations are further corroborated by numerical simulations and stability analysis under random perturbation. These results may prove relevant and applicable to similar phenomena in other Kagome platforms beyond photonics. Inverse hexagram-distorted Kagome lattices emerged recently in the exploration of exotic materials. Here in photonics it is demonstrated that two types of higher-order corner states exist in such a specially distorted Kagome lattice. This finding unveils a new class of higher-order photonic topological insulators, expanding the repertoire of Kagome topological models. image