Evanescent wave spectroscopy for the determination of the optical constants of thin silver films deposited on a silver-halide fiber

An unclad silver-halide fiber was coated with a 0.1-mm silver layer and immersed in ethyl alcohol, and the fiber transmission in the range 2-20 μm was measured. The loss, induced by the metallic layer, was used to calculate the extinction coefficient, k’, and the index of refraction, n’, of the layer by fitting the experimental data with the time-independent perturbation theory in the complex region. The dependence of the extinction coefficient on the wavelength was found to be logarithmic, k’(λ) = k₀’ exp(aλ), and not linear, as predicted by others. This simple technique may be used to study the optical constants of thin metallic films.