Optical and nanostructural insights of oblique angle deposited layers applied for photononic coatings

Oblique angle deposition (OAD) is a nanostructuration method widely used to tune the optical properties of thin films. The introduction of porosity controlled by the deposition angle is used to develop the architecture of each layer and stack that enable modifying and optimizing the optical properties of the constituent layers for optimal design. However, optical properties of these nanostructured layers may differ greatly from those of dense layers due to the presence of anisotropy, refractive index gradient and scattering. This work focuses on OAD layers based on a reference photonic material such as SiO2 and it aims at taking into account all these effects in the description of the optical response. For that, the nanostructure has been analyzed with a complete SEM study and key parameters like the porosity gradient profile and aspect ratio of the nanocolumns were extracted. The samples were then characterized by generalized ellipsometry to evaluate the influence of morphological anisotropy and porosity gradient on the optical response of the films. Based on this microstructural study, an original optical model is presented to fit the features of new optical properties. A reliable correspondence is observed between the optical model parameters and the microstructure characteristics like the column angle and the porosity gradient. This demonstrates that such complex microstructural parameters can be easily accessed solely from optical measurements. All the work has enabled us to develop a two-layer anti-reflective coating that already demonstrate high level of transmission.