Determination of refractive index of submicron-thick films using resonance shift in a four-layer slab waveguide

The measurement of refractive index of very thin films at the order of ten to hundred nanometers is cumbersome and usually requires employing sophisticated techniques such as the spectral ellipsometry. In this paper we describe a simple contact method for measuring the refractive index of thin films. Here we have used the prism-coupling technique for characterizing samples prepared as four-layer slab waveguides. The waveguide resonance condition can be calculated by solving simple analytic transcendental equations for the slab waveguide. Then the captured mode position as a function of cladding thickness is used for probing the refractive index of cladding layer. We used indium-tin-oxide layer on glass as the substrate and polysulfone with known refractive index as the material for testing the method. In the paper we provide the theoretical background of the method, demonstrate the experimental results obtained during the implementation of the technique as well as discuss its main strengths and flaws.