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.

Photonic spin Hall effect mediated by bianisotropy.

Coupling of electric and magnetic responses of a scatterer, known as bianisotropy, enables rich physics and unique optical phenomena, including asymmetric absorption or reflection, one-way transparency, and photonic topological phases. Here we demonstrate yet another feature stemming from bianisotropic response, namely, polarization-dependent scattering of light by bianisotropic dielectric meta-atom with broken mirror symmetry, which yields a photonic analogue of spin Hall effect. Based on a simple dipole model, we explain the origin of the effect confirming our conclusions by experimental observation of photonic spin Hall effect both for a single meta-atom and for an array of them.

Far-field probing of leaky topological states in all-dielectric metasurfaces.

Topological phase transitions in condensed matter systems give rise to exotic states of matter such as topological insulators, superconductors, and superfluids. Photonic topological systems open a whole new realm of research and technological opportunities, exhibiting a number of important distinctions from their condensed matter counterparts. Photonic modes can leak into free space, which makes it possible to probe topological photonic phases by spectroscopic means via Fano resonances. Based on this idea, we develop a technique to retrieve the topological properties of all-dielectric metasurfaces from the measured far-field scattering characteristics. Collected angle-resolved spectra provide the momentum-dependent frequencies and lifetimes of the photonic modes that enable the retrieval of the effective Hamiltonian and extraction of the topological invariant. Our results demonstrate how the topological states of open non-Hermitian systems can be explored via far-field measurements, thus paving a way to the design of metasurfaces with unique scattering characteristics controlled via topological effects.