Enhanced saturation in structural colors via addition of a refractive-index-matching layer for bound states in the continuum metasurfaces

While quasi-bound states in the continuum (Q-BICs) enable vivid structural colors, their performance is often compromised by higher-order resonances at short wavelengths that limit spectral purity. This paper presents a dual-layer all-dielectric metasurface that overcomes this challenge through refractive index matching. The design not only suppresses short-wavelength resonances but also enhances the far-field coupling of the main resonance. It achieves continuous hue and brightness tuning across the visible spectrum, delivering colors with exceptional monochromaticity (<5 nm FWHM), high saturation (>90%), and a wide gamut covering 150.2% of sRGB. This study provides a robust solution for advanced display, data storage, and anti-counterfeiting applications.