Narrow-band filter of Fabry-Pérot cavity over a wide spectrum range based on second-order resonant peak

Currently, it remains a challenge for micro-transmission filters based on diffractive nanostructures to achieve a balance among the spectral range, transmission efficiency, and color purity. While plasmonic metal metasurfaces of Fabry-Pérot (FP) cavities offer a wide spectral range, they are hindered by large full width at half maximum (FWHM) and low transmission efficiency; on the other hand, all-dielectric FP cavities exhibit small FWHM and high transmission efficiency but narrow spectral range. This study presents an innovative second-order FP cavity structure, wherein the introduction of a metal layer modifies the electromagnetic field distribution inside the cavity, leading to a shift in resonance modes. By suppressing odd-order resonant peaks and retaining even-order resonant peaks, this approach breaks the inherent link between FWHM and free spectral range (FSR), significantly expanding the spectral range while simultaneously reducing FWHM. Leveraging the optical absorption of silicon and the selective suppression effect of a platinum layer, a filter with FWHM ranging from 6 nm to 13 nm and an average transmission efficiency exceeding 70% is fabricated over a wide spectral range from 550 nm to 850 nm. This achieves a narrow linewidth and high color purity across a broad transmission spectrum. A detailed comparison between measurement data of samples and simulation results strongly validates the effectiveness and reliability of the proposed filter design.