Reflection spectroscopy for anisotropic materials: Direct determination of phase differences; validity of Fresnel formulas

The reflectance of an anisotropic crystal is generally a tensor quantity even at normal incidence. Phase differences among the tensor components can be obtained from polarized reflectance measurements. A simple method for doing so is described and tested here. Possible applications of phase-difference information range from checking Kramers-Kronig evaluations of absolute phases to substituting for the Kramers-Kronig procedure. In addition, the following is proved, subject to some simple assumptions: If reflection extinction directions exist, the standard Fresnel equations may be used to derive optical constants (n's and k's), regardless of the anisotropy of the crystal. Moreover, extinction directions come in perpendicular pairs.