Tailoring Plasmonic Waveguide-Coupled Cavities for CO₂ Sensing

Plasmonic devices can guide and control light on the subwavelength scale, but their optical behavior is sensitive to minute changes in geometrical/material parameters; however, it requires a rigorous theoretical framework to obtain a high level of structural control and optimize their performance. This study investigates the coupling of a metal-dielectric-metal (MDM) waveguide with a dielectric resonator, yielding a transmittance spectrum with three distinct resonance modes. The physical mechanisms, governing transmission behavior, and the effects of geometrical parameters on mode characteristics are thoroughly analyzed. The dielectric resonator is replaced with polyhexamethylene biguanide (PHMB) functional material, whose refractive index (RI) changes upon exposure to CO₂ gas. Comprehensive simulations and analysis reveal that multiple modes can be effectively utilized for CO₂ sensing application, demonstrating high sensitivities: mode 1 at 126.9 pm/ppm, mode 2 at 133.58 pm/ppm, and mode 3 at 202.2 pm/ppm. All modes maintain a higher sensitivity and outperform previously reported CO₂ sensors. Consequently, hybrid plasmonic integration emerges as a promising strategy for next-generation sensors, outperforming conventional single-mode designs through an improved accuracy, multifunctionality, and enhanced measurement reliability.