The term anisotropy refers to a non-uniform, direction-dependent material property. There are many materials of scientific and/or technological interest that show optical, magnetic, thermal, electrical, or acoustical anisotropy. For example, many of us have been camping and have had to split wood with an axe so that it can be more easily handled and burned. Anyone that has done this knows that wood is much easier to split in one direction than in others. In other words, wood is an anisotropic material. Optically anisotropic materials interact with light in a way that depends on the orientation of the material with respect to the direction and polarization of the incident light. For example, Figure 1a shows the propagation of two polarized beams of light in an anisotropic material. While both light beams are traveling in the same direction, their electric fields are polarized in orthogonal directions and experience different material properties (notice that nx and ny, which are indices of refraction, must be different because the wavelengths of light in the material are different for the different polarization states of the beam). Examples of anisotropic materials include crystals in which atomic distances and angles vary as a function of the direction in the crystal.

In this article we discuss the simplest type of anisotropy analyzed in spectroscopic ellipsometery: that of transparent uniaxial thin films with different refractive indices in the in-plane (referred to as ordinary) direction and the out-of-plane (referred to as extraordinary) directions. This type of anisotropy is common for organic materials, including many polymers and small molecules, as well as for tetragonal and hexagonal crystals. 1, 2 Figure 1b shows the unit cell of a primitive tetragonal lattice. While all the angles are the same in this structure (90), the distance between the atoms in either the top or bottom planes, a, is different from the bond length in the z-direction, b. Uniaxial an-