Optical waveguide as a dual band high sensitivity biosensor based on grating cavity structure

In this study, a plasmonic waveguide structure based on a cavity design is developed and analyzed as a mid-infrared refractive index sensor. The designed structure has two circular cavities, each containing a disk at its center. This configuration exhibits dual-band optical responses, showing distinct performances across two separate frequency bands with high sensitivity. A slow wave-based spoof (grating) structure is implemented along the edges of the disks, effectively increasing the operational wavelength and, as a result, reducing the overall size of the structure as a miniaturization technique. Analytical results indicate that the combination of the disks and gratings achieves around a 60% miniaturization of electrical length in comparison with an empty cavity design based on the wavelength shift. Furthermore, the proposed structure is utilized as a refractive index sensor for the analysis of liquids with a refractive index in the range of 1 to 1.5. Simulation results reveal that the sensor achieves the maximum sensitivity of 1461(nm/RIU) and 4421(nm/RIU) for the first (λ₁), and second (λ₂) wavelength, respectively and the maximum figure of merit (FOM) of 106 RIU⁻¹ and 4650 RIU⁻¹ for these wavelengths are obtained. Simulations demonstrate high sensitivity and figure of merit (FOM), making the design suitable for compact, high-resolution refractive index sensing in the mid-infrared regime.