Giant Nonlinear Response of Subwavelength Dielectric Resonators Enhanced by Bound States in the Continuum

We reveal that nonlinear effects at the nanoscale can be enhanced by bound states in the continuum. We demonstrate SHG from subwavelength AlGaAs nanoantennas with record-high conversion efficiency that exceeds by two orders of magnitude the conversion efficiency observed at the conditions of magnetic dipole Mie resonances. This gain is achieved due to the engineering of the mode structure that allows exploiting a high-Q supercavity mode at the fundamental frequency. These results constitute a significant step towards the development of highly-efficient frequency conversion metadevices in the subwavelength circuitry.