Merging bound states in the continuum for high-efficiency second-harmonic generation

Subwavelength resonant metasurfaces empowered by bound states in the continuum (BICs) have recently emerged as promising platforms for boosting optical nonlinearities as they support high-quality (Q) quasi-BIC resonances with strong electromagnetic-field confinement. To date, most such studies have focused on optical resonances, termed symmetry-protected quasi-BICs, because they can be readily accessible in broken-symmetry structures. However, for traditional designs of which meta-atoms are placed on low-refractive-index substrates, their smallest achievable geometric symmetries impose a limitation on the realization of ultrahigh-Q resonances and thereby on the harmonic generation efficiency. Here we demonstrate a strategy that allows us to improve the Q factors of quasi-BICs over a broad range of geometrical asymmetry parameter values. Specifically, using asymmetric dielectric metasurfaces placed above a perfectly reflecting mirror, we construct merging BICs in the geometrical parameter space, which pushes the square asymptotic behavior of the Q factors of traditional configurations into the power of 4 or even 6. We demonstrate that merging-BIC designs can remarkably enhance the second-harmonic generation efficiency. This work suggests a route to suppress out-of-plane radiation losses of metasurfaces with potential applications to nonlinear optics, lasing, and sensing. locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon Physics Subject Headings (PhySH)Fano resonanceNonlinear opticsPhotonics