Enhanced optical transmission tuning in a subwavelength metallic slit with internal ridges

We present a rigorous frequency-domain finite-element formulation for modeling the diffraction of transverse magnetic polarized electromagnetic waves through metallic subwavelength slits. The approach incorporates realistic boundary conditions, open-domain truncation via perfectly matched layers, and material dispersion. Numerical simulations reproduce Fabry-Perot- like resonances for a reference slit and demonstrate that embedding symmetric internal ridges produces measurable spectral shifts, redistributes near-field hot spots, and modulates transmitted power by factors exceeding two compared to the plain slit. The proposed model provides compact, quantitative design rules for tuning resonance wavelengths and quality factors, enabling the engineering of subwavelength photonic components for filtering, sensing, and light manipulation.