Optical phase-shift dynamics in surface-modified transparent polymers: Application of wavefront distortion analysis to refractive index (RI)-based sensor development

Optical sensors which are based on slight changes in refractive index (RI) resulting from interaction of an optical substrate with some target species utilize a variety of techniques to quantify the RI modification. Detection schemes based on light propagation within optical fibers measure RI changes by transmission efficiency and/or interferometric techniques. Ellipsometry measures changes in surface RI through amplitude and phase changes for the p- and s- components which alter the angle of reflection for polarized light. An alternate method of relating RI variation to surface adsorption in transparent polymers has been explored using a wavefront sensor of the Shack-Hartmann type. The basic concept has been demonstrated by passing a collimated laser beam through a thin film of polystyrene on glass and analyzing the transmitted wavefront for optical phase-shift of the film relative to the glass in the presence of volatile organic compounds. Changes in magnitude of the overall distortion could be related both to the concentration of the vapor and the solvent/polymer solubility parameter match.