Topological Photonics on a Small Scale.

The study of topological phases of light suggests novel opportunities for creating robust optical structures and on-chip photonic devices which are immune against scattering losses and structural disorder. However, many recent demonstrations of topological effects in optics use structures with relatively large scales. Here, the physics and realization of topological photonics on small scales, with the dimensions often smaller or comparable with the wavelength of light, are discussed. The recent experimental demonstrations of small-scale topological states based on arrays of resonant nanoparticles are highlighted and a novel photonic platform using higher-order topological effects for creating subwavelength highly efficient topologically protected optical cavities is discussed. Special attention is paid to the recent progress on topological polaritonic structures and the paper concludes with the vision on the future directions of nanoscale topological photonics and its impact on other fields.