Surfaces presenting broadband antireflective (AR) properties in the visible (Vis), near- and mid-infrared (IR) regions are of great importance for the development of devices that require ultra-high optical transmission for various applications (hyperspectral cameras, detectors operating in poor lightning conditions, etc.). Elimination of Fresnel reflection over wide wavelength range can be achieved with graded-index coatings in which the refractive index progressively decreases from that of the substrate to that of the ambient medium. Because of the unavailability of optical materials with very low refractive indexes that closely match the refractive index of the air, such films are however not realizable directly using standard deposition methods. As an alternative, this work aims at developing architectures made of a variety of materials (Ge, Si, MgF2, SiO2, TiO2, ITO) as single- or multi-layers by PVD processes (electron beam evaporation, ion beam sputtering) using oblique angle deposition (OAD). This geometry allows obtaining tilted columnar layers with tailored refractive indexes promoted by the introduction of porosity by shadowing effect . Knowing finely the nanostructure in these systems is crucial because it plays a critical role on their functionalities and strongly depends on the growth parameters (type of process, deposition angle, temperature, etc.) . In this contribution we will show that advanced (scanning) transmission electron microscopy (S)TEM is a unique tool to provide new key insights related to the nanostructure of such systems, and to overcome important barriers related to the optimization of the optical performances in already efficient AR surfaces made by OAD. Fundamental aspects, not achievable with other techniques, will be reported with a high level of details using imaging, spectroscopies and tomography: column morphologies, porosity distribution and gradient, composition heterogeneities (core-shell, intermixing interfaces). These data obtained at the nanoscale will be directly correlated with the growth processes. Implementation of this nanostructural information into models (effective medium approximation) and simulations (finite-difference time domain) will also be addressed to gain better description of the optical response (determined from Vis/IR ellipsometries and spectrophotometry) in such systems.
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