Anapole States and Toroidal Multipole Excitations in Photonic Metastructures

A growing research field has recently emerged on the investigation of toroidal multipole resonances and the closely related nonradiating anapole state in both dielectric and metallic metastructures working in the visible, near-infrared, and terahertz spectrum. Here we provide an overview on the anapole state and toroidal multipole excitations in photonic metastructures. We cover the key points of the underlying electromagnetic theory and discuss the practical realization of both dielectric and plasmonic metastructures, which exhibit a strong toroidal response. Then, we present examples of toroidal-enabled applications in various photonic platforms, which span from nanolasing, higher-harmonic generation, and cloaking to photodetection, filtering, and sensing.