Corner states of light in photonic waveguides

The recently established paradigm of higher-order topological states of matter has shown that not only edge and surface states1,2 but also states localized to corners, can have robust and exotic properties3–9. Here we report on the experimental realization of novel corner states made out of visible light in three-dimensional photonic structures inscribed in glass samples using femtosecond laser technology10,11. By creating and analysing waveguide arrays, which form two-dimensional breathing kagome lattices in various sample geometries, we establish this as a platform for corner states exhibiting a remarkable degree of flexibility and control. In each sample geometry we measure eigenmodes that are localized at the corners in a finite frequency range, in complete analogy with a theoretical model of the breathing kagome7–9,12–14. Here, measurements reveal that light can be ‘fractionalized,’ corresponding to simultaneous localization to each corner of a triangular sample, even in the presence of defects. Higher-order topologically protected states, localized to corners of photonic waveguide arrays, are experimentally demonstrated.