Abstract

Using the thin layer equivalence principle, which determines the equivalent index of two very thin layers having the same total physical thickness, we argue that the index squared of co-evaporated films is given by the sum of the index squared of the constituent materials weighted by their volume fractions. This turns out to be in agreement with the Drude model. A more difficult question is: what is the wavelength dependence or dispersion of the co-evaporated thin film? To answer that, we started with the derived expression for the mixed index and applied a dispersion relation for each of the constituents. The dispersion was expressed in terms of a chromatic coordinate expansion, which assumes an ultraviolet band edge and a far-infrared cutoff. Each constituent material is characterized by a dispersion coefficient indicating how much of the chromatic coordinant is needed to express the proper index dispersion. The dispersion coefficient of the mixed layer is thus obtained in terms of the dispersion coefficients of the constituents and the volume fraction of the mixing. It is shown that it is possible to control the dispersion of thin-film coatings through co-evaporation.

© 1986 Optical Society of America