Comparative analysis of elliptical cavities for refractive index sensing for biomedical and industrial gas detection applications

This work compares two ultra-high sensitive refractive index (RI) sensors based on metal–insulator-metal (MIM) waveguides coupled with an elliptical cavity for label-free, low-cost, and fast gas sensing for biomedical and industrial applications. The first sensor can be called a Side-Coupled Elliptical Cavity (SCEC), and the second sensor can be called a novel Ring Encapsulated Elliptical Cavity (REEC), which has an elliptical cavity inside an elliptical ring with a small gap. The transmission spectra of both sensors are investigated by finite element method (FEM) simulations. By optimizing the structural parameters and enhancing the light-matter interaction, the REEC sensor exhibits a maximum sensitivity (S) of 7078.12 nm/RIU and a figure of merit (FOM) of 16.3 RIU⁻¹, which are 91.06 % and 46.94 % higher than the SCEC sensor, respectively. The sensors are tested with dielectric materials of different RIs from 1 to 1.02. Notably, both sensors exhibit nearly identical resonant wavelengths. To evaluate efficiency, we introduce a new parameter: sensitivity per resonant wavelength (S/RW). A higher S/RW indicates superior sensitivity at a lower resonant wavelength, desirable for compact and cost-effective devices. The REEC sensor outperforms existing plasmonic MIM waveguide-based sensors in terms of S/RW. Furthermore, owing to its exceptional accuracy, the REEC sensor can detect gases such as helium, carbon dioxide, gaseous methanol, and gaseous ethanol. This makes it a promising candidate for diverse biomedical and industrial applications, raising exciting possibilities for real-world implementation.