Bound states in the continuum in symmetry broken resonator rings

The efficient control of light propagation and localization at subwavelength scales enabled by nanophotonics has led to tremendous advances in tailoring light/matter interaction. Traditionally, this nanophotonic enhancement has been associated with the plasmonic resonances in metallic nanostructures. All-dielectric nanophotonic approaches have been introduced to overcome the innate losses of plasmonic geometries by leveraging high refractive index nanostructures that can be engineered to provide both electric and magnetic Mie resonances [1] . Nevertheless, such structures are still affected by radiative loss, decreasing the achievable resonance line sharpness and therefore putting an upper limit on the resonance quality factor. Recently, metasurfaces incorporating the physics of photonic bound states in the continuum (BICs) have emerged as a breakthrough toolkit for reducing these losses [2] , enabling all-dielectric nanophotonics with ultrasharp resonances and enabling a wide range of applications from optical phase control [3] to sensing [4] , [5] . The two headline geometries for realizing BIC resonances have been two-dimensional (2D) linear arrays of resonators with broken in-plane symmetry, which allows for precise control over the Q-factor [6] , as well as single-element resonators, where BICs emerge in a carefully tailored regime of interfering Mie modes [7] .