Silica Gel Coated Spherical Micro resonator for Ultra-High Sensitivity Detection of Ammonia Gas Concentration in Air

Abstract

A silica gel coated microsphere resonator is proposed and experimentally demonstrated for measurements of ammonia (NH3) concentration in air with ultra-high sensitivity. The optical properties of the porous silica gel layer change when it is exposed to low (parts per million (ppm)) and even ultra-low (parts per billion (ppb)) concentrations of ammonia vapor, leading to a spectral shift of the WGM resonances in the transmission spectrum of the fiber taper. The experimentally demonstrated sensitivity of the proposed sensor to ammonia is estimated as 34.46 pm/ppm in the low ammonia concentrations range from 4 ppm to 30 ppm using an optical spectrum analyser (OSA), and as 800 pm/ppm in the ultra-low range of ammonia concentrations from 2.5 ppb to 12 ppb using the frequency detuning method, resulting in the lowest detection limit (by two orders of magnitude) reported to date equal to 0.16 ppb of ammonia in air. In addition, the sensor exhibits excellent selectivity to ammonia and very fast response and recovery times measured at 1.5 and 3.6 seconds, respectively. Other attractive features of the proposed sensor are its compact nature, simplicity of fabrication.

Terahertz Gas-Phase Spectroscopy Using a Sub-Wavelength Thick Ultrahigh-Q Microresonator

“…Nevertheless, the experimental results clearly show that the THz disc microresonator with sub-wavelength thickness can detect water vapour concentrations as low as 3 ppmv by interrogation of the absorption line at 0.5569 THz. Noteworthy, the sensitivity of the THz disc microresonator solely originates from the large evanescent field, and has not been enhanced using humidity absorbing materials like silica gel or surface activation [49,50]. Consequently, the reduction in Q-factor is immediate and shows no hysteresis as is common for many commercial hygrometers.…”

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

“…The value of the quality factor was estimated from the experimental spectra as Q=λR ∕ λFWHM, where λR is the resonance wavelength and λFWHM is the full width at half-maximum of the resonant lobe, calculated by fitting the resonant dip with the Lorentz function. The WGM resonant wavelengths experience a red shift when the sensor is exposed to ammonia vapors [8]. Since direct measurement of the silica coating thickness on the microsphere surface is difficult to realize, in our experiments the coating thickness was evaluated by performing a Scanning Electron Microscopic (SEM) imaging of the cross-section of a 125 µm diameter optical fiber subjected to different number of coating cycles, cured and cleaved.…”

“…Later, Lin et al used this approximate model to calculate the thickness-dependent sensitivity of a zeolite-coated micro-sphere to ammonia gas [5]. Recently we demonstrated an ultra-high sensitivity ammonia sensor using a silica coated microsphere [8]. The selective to a particular analyte coating enhances the typically small microsphere surface-to-mass ratio, and most importantly, acts as an effective concentrator of the analyte molecules, absorbing them within the layer close to the surface.…”

Whispering Gallery Mode Optical Microresonators: Structures and Sensing Applications

“…Measured by the frequency offset method, the sensitivity was as high as 800 μm ppm −1 in the ultra‐low concentration range (2.5–12 ppb). The minimum detection limit for the experiment was 0.16 ppb …”