Refractometric gas sensing using a whispering gallery mode microresonator coated with a supra-micron sol-gel layer

Spherical microresonators have been used to measure small concentrations of ammonia vapors. By tracking the shift of a whispering gallery mode (WGM) frequency, we can detect small changes of the refractive index of the medium in which the WGM electric field is confined. These changes are correlated to the chemical content of the medium. When a porous layer of silica (SiO2) is coated onto a silica sphere surface, molecules of the gas analyte can diffuse into the layer and interact with the WGM electric field thus increasing the sensitivity of analyte detection. In this paper, we present a theoretical analysis of the phenomenon and demonstrate both theoretically and experimentally that the sensitivity of the WGM sensors for ammonia gas detection can be significantly increased by varying the thickness of the porous coating.