Comparison of different dispersion models for single layer optical thin film index determination

We here determine the optical properties of different single-layer thin films containing Ta₂O₅, Si, Indium Tin Oxide and Au in the ultraviolet-visible and near infrared ranges. More specifically, we deduce the complex refractive index and thickness from the reflectance and transmittance measured using a spectrophotometer at normal incidence. One major difficulty is to find an appropriate selection of dispersion laws for various types of material (dielectric, semiconductors, and metals). For this purpose, a number of models have been investigated from a theoretical point of view in consideration of the Kramers-Kronig relation. These include the Forouhi-Bloomer model, combined with the modified Drude, Tauc-Lorentz and multiple-oscillator Tauc-Lorentz models. A global optimization procedure had to be employed because of the large number of parameters (from 3 to 15) required to describe the optical dispersion laws. The calculated reflectance and transmittance are in good agreement with experimental data and the complex refractive index is consistent with our knowledge and that already reported.