Radial order dependence of the gas sensing sensitivity of whispering gallery mode microspheres

Silica microspheres were coated with a porous silica layer. The wavelength shift of the whispering gallery modes of the coated microsphere was used to detect small traces of ammonia vapors. The sensing sensitivities of different modes of the same microsphere were found to differ greatly, which is attributed to the modes having different radial indices. This effect was found to be more pronounced in the microspheres coated with thicker coatings.