Chiral Plasmonic Filtering in Silver Nanowires via Point-Defect-Induced Mode Conversion and Interference

Silver nanowire plasmonic waveguides offer subwavelength light confinement for integrated photonic circuits. Typically, chiral surface plasmon polariton control depends on excitation polarization. Here, we introduce a nanogroove point defect as a new regulatory dimension to realize on-chip chiral filtering. Using finite-difference time-domain simulations together with scattering matrix method, we show that the nanogroove selectively couples the TM₀ and HE_-1 modes, and the filtering stems from constructive or destructive interference between direct transmission and mode-converted pathways. With optimized geometry (depth: 45 nm, width: 55 nm), a chiral extinction ratio of 2.77 is achieved. Experimentally, a nanogroove fabricated via focused ion beam milling exhibits a measured chiral extinction ratio of 1.96, in good agreement with theoretical predictions. Furthermore, the scattering matrix method allows systematic investigation of the influence of defect number and spacing on the filtering performance, predicting an enhanced ratio of ∼3.7 for two optimally spaced defects. This work demonstrates point-defect engineering as a compact, design-ready strategy for on-chip plasmonic devices.