INFRARED ANISOTROPY AS A SURFACE PROBE

From an optical point of view, a crystal with cubic symmetry has only a small anisotropy of the order (a/lambda)2 due to spatial dispersion, where a is an atomic distance and lambda is the wavelength. Therefore any other anisotropy in its optical properties, such as its reflectan ce, must originate from its surface. We explore the optical anisotropy of polar crystals in the far-infrared where the optical response of t he system is determined by the ionic motions.