We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-$Q$ resonances arising from a distortion of symmetry-protected bound states in the continuum. We develop a rigorous theory of such asymmetric periodic structures and demonstrate a link between the bound states in the continuum and Fano resonances. Our results suggest the way for smart engineering of resonances in metasurfaces for many applications in nanophotonics and metaoptics.

Enhancing optical nonlinearity: Intense pulses of light interacting with a dielectric material can induce optical nonlinear behavior, whereby the frequency of the output light can be doubled or tripled or excited to even higher harmonics of the input light. Usually this interaction is weak and occurs over many thousands of wavelengths, typically requiring the combination of bulk volumes of material with a confining cavity. Using a mechanism of light confinement called bound states in the continuum, Koshelev et al. show that enhanced second-harmonic generation can be obtained in nanoscale subwavelength cylinders of a dielectric material. The results on these optical nanoantennas offer a platform for developing integrated nonlinear nanophotonic devices. Science , this issue p. 288

Recent progress in nanoscale optical physics is associated with the development of a new branch of nanophotonics exploring strong Mie resonances in dielectric nanoparticles with a high refractive index. The high-index resonant dielectric nanostructures form building blocks for novel photonic metadevices with low losses and advanced functionalities. However, unlike extensively studied cavities in photonic crystals, such dielectric resonators demonstrate low quality factors ($Q$ factors). Here, we uncover a novel mechanism for achieving giant $Q$ factors of subwavelength nanoscale resonators by realizing the regime of bound states in the continuum. In contrast to the previously suggested multilayer structures with zero permittivity, we reveal strong mode coupling and Fano resonances in homogeneous high-index dielectric finite-length nanorods resulting in high-$Q$ factors at the nanoscale. Thus, high-index dielectric resonators represent the simplest example of nanophotonic supercavities, expanding substantially the range of applications of all-dielectric resonant nanophotonics and meta-optics.

Nonradiating sources of energy have traditionally been studied in quantum mechanics and astrophysics but have received very little attention in the photonics community. This situation has changed recently due to a number of pioneering theoretical studies and remarkable experimental demonstrations of the exotic states of light in dielectric resonant photonic structures and metasurfaces, with the possibility to localize efficiently the electromagnetic fields of high intensities within small volumes of matter. These recent advances underpin novel concepts in nanophotonics and provide a promising pathway to overcome the problem of losses usually associated with metals and plasmonic materials for the efficient control of light‐matter interaction at the nanoscale. This review paper provides a general background and several snapshots of the recent results in this young yet prominent research field, focusing on two types of nonradiating states of light that both have been recently at the center of many studies in all‐dielectric resonant meta‐optics and metasurfaces: optical anapoles and photonic bound states in the continuum . We discuss a brief history of these states in optics, as well as their underlying physics and manifestations, and also emphasize their differences and similarities. We also review some applications of such novel photonic states in both linear and nonlinear optics for the nanoscale field enhancement, a design of novel dielectric structures with high‐ Q resonances, nonlinear wave mixing, and enhanced harmonic generation, as well as advanced concepts for lasing and optical neural networks.