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Topological photonics allows for the deterministic creation of electromagnetic modes of any dimensionality lesser than that of the system. In the context of two-dimensional systems such as metasurfaces, topological photonics enables trapping of light in 0D cavities defined by boundaries of higher-order topological insulators and topological defects, as well as guiding of optical fields along 1D boundaries between topologically distinct domains. More importantly, it allows engineering interactions of topological modes with radiative continuum, which opens new opportunities to control light-matter interactions, scattering, generation, and emission of light. This review article aims at highlighting recent work in the field focusing on the control of radiation and generation of light in topological metasurfaces. 

In this paper, we explore the operation of a nonreciprocal non-Hermitian system consisting of a lossy magneto-optical ring resonator coupled to another ring resonator with gain and loss, and we demonstrate that such a system can exhibit non-reciprocity-based broken parity-time (PT) symmetry and supports one-way exceptional points. The nonreciprocal PT-phase transition is analyzed with the use of both analytical tools based on coupled-mode theory and two-dimensional finite element method simulations. Our calculations show that the response of the system strongly depends on the regime of operation – broken or preserved PT-symmetry. This response is leveraged to show that the system can operate as an optical isolator or a one-way laser with functionality tuned by adjusting loss/gain in the second ring resonator. The proposed system can thus be promising for device applications such as magnetically or even optically switchable non-reciprocal devices and one-way micro-ring lasers. 

Abstract 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.