Study of the influence of the sol-gel silica layer thickness on sensitivity of the coated silica microsphere resonator to ammonia in air

In this paper we investigate experimentally the influence of the sol gel layer thickness on the sensitivity of the proposed ammonia sensor based on a silica microsphere resonator coated with a layer of silica sol gel. The operating principle of the sensor relies on the excitation of whispering gallery modes (WGMs) in the coated silica microsphere using the evanescent field of the tapered fiber. A change in the ammonia concentration is detected by measuring the wavelength shift of the WGMs in the transmission spectrum of the tapered fiber. We demonstrate that sensitivity to ammonia initially increases with the increase of the coating thickness, followed by its saturation at higher values of the silica layer thickness. We demonstrate that when the coating thickness reaches a certain threshold value, the WGMs are no longer observed. The results of the study are useful for the design and optimization of sensors based on microsphere resonators, for applications in gas sensing.