Compact multimode bandpass plasmonic filter based on a dual-spiral resonator coupled to metal–insulator–metal waveguides for refractive index sensing application

The development of compact, high-performance plasmonic filters capable of multimode operation remains a critical challenge in integrated photonic sensing. In this work, a multimode bandpass plasmonic filter based on dual-spiral resonators is proposed and numerically investigated. The resonator is comprised of left- and right-handed variants and coupled to metal–insulator–metal (MIM) waveguides. Leveraging the self-similar geometry of spiral resonators, the design supports four spectrally distinct transmission peaks within the 600–1200 nm near-infrared window, with high transmittance (up to 76.4%) and narrow full width at half-maximum (as low as 15.2 nm). Using two-dimensional finite-difference time-domain (FDTD) simulations with a Drude–Lorentz model for silver, it is demonstrated that the spiral architecture enables independent spectral tuning of resonant modes through geometric parameters such as the winding angle and coupling gap. When employed as a refractive index (RI) sensor, the device exhibits sensitivities ranging from 801 to 896 nm/RIU across its modes, with figure-of-merit (FoM) values ranging from 35.6 to 58.9RIU −1 across its modes. The compact footprint (520×540nm 2 ) and robust multimode response make this platform highly suitable for label-free, multi-analyte biosensing and on-chip spectral multiplexing, offering a significant advance over conventional ring- or stub-based plasmonic filters.