High sensitivity infrared gas sensing and filtering using metasurfaces plasmonic ring resonators

This work investigates a plasmonic metal–insulator–metal (MIM) ring structure periodically coupled to dual resonators and a defect cell, engineered for infrared filtering and gas sensing. The optical response is modeled numerically using the finite element method in COMSOL Multiphysics, enabling accurate analysis of strong field confinement and loss in the subwavelength MIM geometry. By tuning the geometrical parameters of the rectangular and lateral resonators and of the defect cavity, we show that the transmission spectrum, band-gap position, and defect resonance wavelength can be precisely controlled, and that introducing a defect resonator into a periodic dual-resonator array generates a sharp, highly localized mode inside the plasmonic band gap. Small variations of the refractive index in the gas-filled device lead to clear, nearly linear shifts of this defect resonance, yielding a high sensitivity of about 1920 nm/RIU together with a Q-factor above 100 and a low detection limit, so that sensitivity, spectral selectivity, and detection capability are simultaneously enhanced. These results highlight the novelty of combining periodic band-gap engineering, dual-resonator coupling, and defect optimization in a single MIM platform, and demonstrate that the proposed structure can function as both a narrowband photonic filter and a high-performance infrared gas sensor in a compact, integrated configuration.