Meta-optics and bound states in the continuum (Conference Presentation)

We review the physics of bound states in the continuum and their applications in meta-optics and metasurfaces. First, we discuss strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator’s aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference. Our theoretical findings are confirmed by microwave and optical experiments for the scattering of high-index subwavelength resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high quality factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices. Next, we discuss how bound states in the continuum can appear in metasurfaces. 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 that originate from the physics of bound states in the continuum. We demonstrate a direct link between the bound states in the continuum and the Fano resonances, and discuss a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.