Near-Field Characterization of Higher-Order Topological Photonic States at Optical Frequencies.

Higher-order topological insulators (HOTIs) represent a new type of topological system, supporting boundary states localized over boundaries, two or more dimensions lower than the dimensionality of the system itself. Interestingly, photonic HOTIs can possess a richer physics than their original condensed matter counterpart, supporting conventional HOTI states based on tight-binding coupling, and a new type of topological HOTI states enabled by long-range interactions. Here, a new mechanism to establish all-dielectric infrared HOTI metasurfaces exhibiting both types of HOTI states is proposed, supported by a topological transition accompanied by the emergence of topological Wannier-type polarization. Two kinds of near-field experimental studies are performed: i) the solid immersion spectroscopy and ii) near-field imaging using scattering scanning near-field optical microscopy to directly observe the topological transition and the emergence of HOTI states of two types. It is shown that the near-field profiles indicate the displacement of the Wannier center across the topological transition leading to the topological dipole polarization and emergence of the topological boundary states. The proposed all-dielectric HOTI metasurface offers a new approach to confine the optical field in micro- and nano-scale topological cavities and thus paves the way to achieve a novel nanophotonic technology.