Tailoring the bound states in the multi-channel nonlinear plasmonic metasurfaces

Metasurfaces offer a paradigm shift in the nonlinear optics, controlling and enhancing of nonlinear effects at the nanoscale, and offering new opportunities for applications in nonlinear optics, plasmonics and electromagnetics. Particularly, nonlinear optical conversion efficiency is limited due to the inherent losses in metallic materials. Nonlinear metasurfaces based on bound states in the continuum(BIC) can effectively increase the Q value and are expected to further enhance the nonlinear efficiency. In this paper, a tri-capacitance-like metasurface supporting both Friedrich-Wintgen quasi-BIC and symmetry-protected quasi-BIC is proposed to generates multiple high-Q resonances in the spectrum. The coupling between perfect BIC and quasi-BIC mode in the multi-channel optical system corresponds to the storage and release of electromagnetic waves, respectively. At the same time, the electromagnetic field is further enhanced at the resonant frequency, which can significantly improve the second order nonlinear efficiency. In addition, due to the different generation mechanism, the frequency and amplitude of the two resonances are easy to tune independently, which facilitates the sum and difference frequency generation (SFG and DFG). Therefore, the proposed nonlinear metasurfaces in this paper are of wide research value in the fields of nonlinear optics, multi-channel communication and memory.