Crystalline MgF2 Whispering Gallery Mode Resonators as Optical Refractometric Sensors With Ultra-High Improved Sensitivity

In this work, we theoretically show a high sensitivity refractometric sensor based on whispering gallery mode (WGM) cylindrical resonators made of magnesium fluoride (MgF₂). This crystalline material has a refractive index (RI) which is only slightly more than that of the water (Δn ~0.05 refractive index unit (RIU)). This causes more evanescent field depth of a WGM resonance in an aqueous environment, which makes it a promising candidate for sensing applications. For analysis of propagated WGM resonances, we have used a verified approach based on Airy approximations, which describes an analytical form of WGM resonance wavelengths in the cylindrical resonators. The ultra-high bulk refractive index sensitivity (S) of 780.61 nm/RIU, is obtained with employing the fifth radial order of a WGM with transverse magnetic (TM) polarization and λ = 795 nm. This parameter shows an improvement factor of ≥ 2 times, compared with a state-of-the-art SOI ring resonator-based biosensor with a similar radius, reported lately. These results demonstrate the potential of the MgF₂ cylindrical resonator-based refractometric sensor as a feasible alternative platform for manufacturing high-performance photonics sensors with vital biological applications.