In this article, we present a passive plasmonic metasurface sensor, the square annular cavity array (SACA), designed for ultrasensitive refractive index (RI) detection, structural color modulation, and multiwavelength spectral filtering. This sensor is built on a multilayer plasmonic architecture comprising silver (Ag), silicon nitride (Si \(_3\) N \(_4\) ), and silicon dioxide (SiO \(_2\) ), featuring tunable square annular nanocavities with gap sizes ranging from 10 to 110 nm. These cavities are engineered to support hybrid plasmon–Fabry–Pérot resonances over a wavelength range of 400 to 1800nm. Finite element method (FEM) simulations conducted in COMSOL and optimized using the Nelder–Mead algorithm reveal the highest sensitivity of 800 nm/RIU, a spectral figure of merit (FOM) of 85.23, a quality factor (Q-factor) of 167.61, and a dots per inch (DPI) value of 169,333. The SACA sensor displays distinct chromatic transitions influenced by changes in the refractive index and angle of incidence. These effects are quantitatively assessed using the CIE 1931 color space under standard D65 illumination, facilitating label-free visual detection. Angle-resolved analysis reveals polarization-dependent mode splitting of up to 30 \(^{\circ }\) , facilitating multiplexed spectral filtering and sensing. The normalized sRGB color gamut coverage is calculated to be 85.90% under RI modulation, indicating a design balance between visual expressiveness and functional spectral performance. By integrating high RI sensitivity, tunable spectral response, and real-time colorimetric feedback within a compact passive structure, the SACA sensor offers significant advantages. This design provides a versatile solution for point-of-care diagnostics, lab-on-chip optics, and integrated photonic applications.
