Multiple-beam interferometry with thin metal films and unsymmetrical systems

Abstract

Numerical calculations using the multilayer matrix method show that the interference fringes in the visible spectrum of light transmitted through two back-silvered transparent sheets separated by a dielectric film is greatly modified when a thin metal layer is present on each sheet surface. The layers cause a modulation of fringe intensity and a shift in fringe wavelength which depend on the refractive index and thickness of the layer. Fringes will be well resolved for <40 nm layers of metals with high reflectivity and low absorption. In regard to the use of this interferometer to determine surface separations in the Israelachvili surface forces apparatus, Israelachvili's equation is found to be no longer valid, and a multilayer matrix method is presented. Some unsymmetrical interferometers are also considered.