Anisotropic behavior of thin film off-axes optical properties

Abstract

Often, materials such as TiO2 in thin-film form produced by classical evaporation exhibit columnar microstructure. An anisotropy of the refractive index is associated with this structural anisotropy. To describe the behavior of an optical wave propagating in the layer, one can choose a biaxal model, taking one of the three principal refractive indices (n1, n2, n3) in the direction of the main axis of the columns (n3). With such a model it is easy to write down the equations of propagation of an optical wave in a principal plane of the material [plane (n1, n3)]. Then we can study the propagation of the guided modes. From this study in the plane (n1, n3) of a single layer of TiO2, we obtain the refractive indices n1, n2, and n3 and the mechanical thickness. We have developed the formalism given by Teitler to calculate the eight reflexion and transmission coefficients in polarized light (RSS,RPP, RSP, RPS, TSS, TPP, TSP, TPS) in a plane of incidence that is rotated at an angle Φ from the principal plane (n1, n3) of the layer (the plane of the columns). We find that the coefficients RSP, RPS, TSP, and TPS, corresponding to a rotation of the polarization, are modulated with Φ. We have performed a comparison between the measurements and the calculated values of TSP and TPS. The agreement is very good when we use in our calculations the refractive indices n1, n2, and n3 that we obtained from the measurements of the guided modes in the plane (n1, n3).