Abstract

Accurate measurements of coupling angles (m-lines method) corresponding to different propagation modes in a layer can reveal an optical anisotropy of the material constituting that layer. In the case of a layer obtained by evaporation condensation, this anisotropy can be directly correlated with the shape and the direction of the columns which are the main characteristics of the microstructure. Starting with the hypothesis of a birefringent structure with one of the optical axes oriented in the columnar growth direction, we have a calculation method for simultaneously determining the layer thickness and the indices corresponding to the principal axes. We show that the anisotropy depends in a critical way on the evaporation conditions: it is much marked when the layers have a more lacunar structure. And, for a coating made under oblique incidence, anisotropy increases with the angle of incidence. If the substrate is moved during deposition, the columnar growth is perturbed and anisotropy is reduced. Guided wave methods can be applied to layer stacks. With simple structures (up to two or three layers), optical anisotropy can be detected. For example, if the substrate is maintained in a fixed position during deposition of one of the layers, we can study the consequences on the propagation of TE or TM modes in the stack, assuming that the other layers do not exhibit any marked anisotropy. The results can then be compared with those obtained on single layers.

© 1986 Optical Society of America