Influence of roughness, porosity and grain morphology on the optical properties of ultrathin Ag films

The dielectric function ε(ω) determines the optical behavior of thin and ultrathin metal films, crucial for many research areas and devices. Surprisingly, even for the paradigmatic case of evaporated Ag films, a large scattering of ε(ω) exists, mainly because the film is modelled by neglecting grain structure and surface roughness. Here, we quantitatively describe the optical - morphological relationships for Ag films in the 5.4 to 63 nm range thermally evaporated on fused silica. The experimental reflectance and transmission spectra are reproduced by a multilayer model employing an effective medium approximation for each layer. For the 9 nm thick film we describe how the film grain size and shape, porosity and surface roughness determine the reflectance and transmission spectra, and the total film ε(ω). Moreover, we provide the thickness dependent trend of the morphology-optical response correlation thus obtaining a complete quantitative description of the Ag films behavior up to the transition to the bulk regime. These results make this work a milestone in the morphology-optical understanding for ultrathin metal films and open new perspectives in the interpretation of thin film properties and in the design of the optical response of a tailored system.