Optical constants in the IR from thin film interference and reflectance: the reststrahlen region of muscovite mica

A new technique for determining the IR optical constants of materials that can be formed into thin films is presented. At a given wavelength the thickness of the film t, the index of refraction n, and the extinction coefficient k combine to produce interference effects in the film, which in turn control reflectance from the film. When reflectance is plotted vs thickness the resultant curve is a unique function of n and k. Values of n and k are determined by curve fitting. The technique is illustrated using thin films of muscovite mica, and values of n and k are reported for wave numbers from 1200 to 400 cm-1, which include the reststrahlen region of mica.