Enhanced resonance of quasi-bound states in the continuum in an asymmetric double-gap silicon split-ring metasurface with a gold ground plate

Symmetry-protected quasi-bound states in the continuum (quasi-BICs) in all-dielectric metasurfaces are known to exhibit quality ( Q ) factors inversely proportional to the square of the asymmetry parameter. While reducing the asymmetry parameter enhances the Q factor, it simultaneously imposes stricter nanofabrication tolerances, limiting experimental realization of high- Q resonance modes. To address this challenge, we propose an alternative approach to further increase the Q factor of the symmetry-protected quasi-BIC mode in the double-gap silicon split-ring metasurface by incorporating a gold ground plate separated by a SiO 2 dielectric spacing layer. By optimizing the dielectric layer thickness, we demonstrate a fourfold increase in the Q factor of the quasi-BIC mode relative to the double-gap split-ring metasurface structure without the gold ground plate with the same large asymmetry parameter. Furthermore, we show that this new structure significantly enhances third harmonic generations.