Evaluating Optical Techniques for Determining Film Structure: Optical Invariants for Anisotropic Dielectric Thin Films

A model-free method, based on optical invariants, of analyzing the reflectivity of thin adsorbed dielectric films is extended to dielectric waveguide techniques such as optical waveguide lightmode spectroscopy (OWLS). The reliability of film parameters determined from three general types of techniques (OWLS, scanning angle reflectometry, and scanning angle ellipsometry) is compared in the context of uniaxially anisotropic adsorbed films, with major axis perpendicular to the film. Expressions for the invariants for stratified anisotropic films in terms of simple moments of the layer optical distribution are presented. These general methods hold out the promise of more extended analysis of the optical response of thin films adsorbing on dielectric waveguides, while avoiding the pitfalls of specific optical models.