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Accurate measurement of thin-polymeric-films index variations: application to elasto-optic effect and to photochromism

Attenuated total reflection (ATR) is commonly used for measuring small variations of the index of refraction of thin polymeric films. We show that a careful analysis of reflectivity dips, due to the excitation of guided modes in the film, allows a precise measurement of very small variations of the anisotropic index of refraction and of the thickness of the film. This is very useful for studying a wide variety of physical phenomena such as electro-optic effects, piezoelectricity, electrostriction, mechanical and thermal strains inducing elasto-optic effects (i.e., photoelasticity), and photochromism. In this paper we particularly emphasize the measurement of some components of the photoelastic tensor and of the index variations associated with cis-trans photoisomerization of azo dyes which have been clearly demonstrated in doped polymeric films. Kinetics of cis-trans optical pumping and of relaxation in darkness have been studied. It is noticeable that the layered configuration of samples and the guided waves excited by ATR are quite similar to the structure and to the propagation conditions of integrated optics devices.