Second-harmonic and sum-frequency generation from dielectric metasurfaces empowered by multiple quasibound states in the continuum

Abstract All-dielectric metasurfaces associated with the physics of bound states in the continuum (BICs) provide a novel platform to enhance nonlinear effects at the nanoscale. In this work, we demonstrate sharp enhancements of second-order nonlinear processes including second-harmonic (SHG) and sum-frequency generation (SFG) from a dielectric metasurface with the assistance of multiple quasi-BIC resonances at different input wavelengths. By resonantly pumping the AlGaAs metasurface under oblique incidence, nonlinear emissions are significantly enhanced across the visible wavelength regime from 500 nm to 600 nm. Thanks to the presence of multiple quasi-BIC resonances in the metasurface, enhanced SH signals at the respective nonlinear wavelengths can be achieved, and the SF emissions at 10 different wavelengths is further implemented and boosted by simultaneous excitations of any two quasi-BIC resonances. High-efficiency second-order nonlinear processes assisted by multiple quasi-BIC resonances present the potential to expand the functionalities of quasi-BIC metasurfaces in the nonlinear regime.