Is ellipsometry suitable for sensor applications?

Ellipsometry is a powerful tool for optical characterization of surfaces and thin-films. Very favorable features for sensor applications are the in situ advantage, the possibility to work with non-labeled molecules and the high thickness resolution. Sub-nanometers resolution can be achieved in bioaffinity-based sensing and ppm-sensitivity in gas sensing. Ellipsometric sensor systems are based on monitoring changes in the thickness, the refractive index or the microstructure of a sensing layer. These changes are induced by the substance or process measured. A classification of sensing layers is proposed and discussed. One specific application, gas sensing based on sensor arrays, is discussed in some detail. However, the main objective is to critically discuss the possibilities for sensor applications based on ellipsometric read-out.