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Optical bound states in the continuum (BICs) underpin the existence of strongly localized waves embedded into the radiation spectrum. Here we bring the concept of BICs to the field of high-harmonic generation and employ resonant dielectric metasurfaces to generate efficiently optical harmonics up to the 11th order. We design BIC-resonant metasurfaces with a broken in-plane symmetry for the lower harmonics and then observe a transition to the nonlinear regime for higher harmonics. Our approach bridges the fields of perturbative and nonperturbative nonlinear optics on the subwavelength scale.
Zograf et al. (Thu,) studied this question.