Characterization of ultrathin dielectric films with the prism-coupler method

We demonstrate a technique of applying the prism-coupler method to the characterization of dielectric films that are too thin to support enough guided modes in air for the normal application of the method. The technique is based on applying suitable index-matching liquids on the surface of the thin film to increase the number of effective indexes available for the determination of the refractive index and the thickness of the film. With this technique, even thin films that do not support any guided modes in air can be characterized. We apply the technique to the characterization of polymer thin films as thin as 100-200 nm and discuss its performance and limitation.