Ordered-mesoporous-silica-thin-film-based chemical gas sensors with integrated optical polarimetric interferometry

A technology to make integrated optical chemical gas sensors using sol-gel surfactant-templated mesoporous thin films was demonstrated. The sensor was prepared by dip coating of an ordered mesoporous silica thin film onto a tapered thin $\mathrm{Ti}{\mathrm{O}}_2$ layer sputtered on top of a tin-diffused glass waveguide. The sensor employs single-beam polarimetric interferometry to detect molecular adsorption in the mesoporous silica film because the adsorption can lead to a change of the phase difference $\left(\Delta \varphi \right)$ between the fundamental transverse electric $\left({\mathrm{TE}}_0\right)$ and magnetic $\left({\mathrm{TM}}_0\right)$ modes propagating in the waveguide. The sensor is sensitive to humidity and $\mathrm{N}{\mathrm{H}}_3$ gas at room temperature (RT). The sensitivity of $\Delta \varphi >180°$ was detected for $40\min$ exposure of the sensor to $2\mathrm{ppm}$ of $\mathrm{N}{\mathrm{H}}_3$ in dry air. The sensor was recoverable from the $\mathrm{H}{\mathrm{N}}_3$ exposure but its recovery was too slow to cause a detectable $\Delta \varphi$ per unit time. The slow response and recovery of the sensor were ascribed to slow diffusion of gaseous molecules in the laterally oriented tube-like pores of the silica film.