Optical gas sensing by evaluating ATR leaky mode spectra

Abstract

The angular reflectivity spectrum of a glass-silver-polymer-air multilayer structure is investigated for gas-sensor applications. Minima of reflectance are caused by surface plasmon polariton (SPP) excitation at the silver-polymer interface and by attenuated total reflection (ATR) leaky modes coupled into the polymer film. Changes in refractive index of the polymer (here Teflon ®AF) due to absorption of vapour molecules in the surrounding atmosphere lead to line shifts of the resonances in the reflectivity spectrum. For several vapours (benzene, toluene, xylene, etc.) maximum index changes of about Δ n = 0.02 are observed with an accuracy better than 0.0001. A dynamic and more accurate measurement is obtained by recording the reflected intensity as a function of time at a fixed observation angle, the angle of the SPP resonance. The recorded intensity is then a measure for the refractive-index changes of the Teflon ®AF coating. In a modified set-up for application purposes, a divergent reflected beam is recorded by a CCD camera and the line shifts of two leaky modes are monitored simultaneously with a time resolution of a few microseconds. On-line evaluation of these shifts allows the time-resolved observation of the refractive-index changes. A first miniaturization step is achieved by laser-beam patterning of PMMA substrates.