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