In this paper, the behavior of a plasmonic filter structured around a metal - air -metal waveguide incorporating a washer - like resonator was analyzed. The transmittance spectrum and magnetic field (Hz) distributions were analyzed using the finite element method (FEM) within the COMSOL Multiphysics software. The impact of various geometrical parameters - such as width of the washer (d), the size of the gap width (g), the external radius of the washer, and the material configuration on optical characteristics including transmission, reflection, and absorption was thoroughly examined. Findings indicate that enlarging the value of d leads to a redshift in the transmission spectrum and strengthens resonant interaction, whereas narrowing g weakens the optical coupling and shift the spectrum toward the blue region. Parameters such as the full width at half maximum (FWHM) and the quality factor are significantly influenced by both the radius of the washer and the total structural length, underlining the balance between resonance sharpness and transmission strength. The designed filter demonstrates a transmission rate surpassing 70%, a bandwidth around 10 nm, and a quality factor reaching 70. Additionally, a comparative analysis using the Drude model revealed that silver surpasses gold in terms of transmission efficiency and resonance quality, making it more suitable for high - efficiency optical filter applications. These insights contribute to the advancement of tunable plasmonic systems for use in areas like optical sensing, spectral filtering.
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