Nonlinear interaction of silica photonic crystals with ammonia vapor

Abstract

Two mechanisms of gas adsorption in silica photonic crystals are shown. It is found that ammonia adsorption, in contrast to water, is nonlinear depending on its concentration. At low concentration, ammonia molecules form hydrogen bonds with silanol groups of the silica surface. At a high concentration, a contribution from dissolution of ammonia in previously adsorbed water becomes noticeable. Nonlinear dependence allows to create an optical ammonia sensor with large dynamic range (from 1 to 105 ppm) which at the same time has high sensitivity at low concentrations. A novel approach is proposed for accurate measurements of gas adsorption in photonic crystal materials. Keywords: Photonic crystal, Adsorption, Water vapor, Ammonia, Fabry-Perot interferometer, Thin film devices