Experimental Demonstration of Higher-Order Topological States in Photonic Metasurfaces

We demonstrate and visualize the emergence of a topological transition and opening of a Dirac cone by directly exciting the bulk modes of the higher-order topological metasurface via solid-state immersion spectroscopy. The open nature of the metasurface is then utilized to directly image topological boundary states. We show that, while the topological domain walls host 1D edge states, their bending induces 0D higher-order topological modes confined to the corners.