Antireflective coatings (ARC) are widely used on lenses to higher their transmittance properties by suppressing the light reflection occurring at the optical interface when a refractive index mismatch is present. Usually, ARC are made by the deposition of thin films of different materials, i.e refractive index, to create destructive interferences in-between the multiple reflections happening in these layers. However this method is not suited to obtain AR properties over wide wavelength ranges. The emergence of new nanostructuration methods like Oblique Angle Deposition (OAD) or nanoimprint lithography has offered the possibility to fabricate coatings with significantly higher performances overcoming this important limitation. Indeed, by creating structures that are small compared to the wavelength it is possible to obtain new effective refractive indices that can be used to design graded-index layers of ultra-high efficiency with broadband AR properties. In these approaches (OAD or nanoimprint lithography), essential problems that limit the transmittance performances are not frequently addressed: (1) the ideal profile of this gradient is usually designed by an empirically or an iterative process with no guaranty to obtain the best solution; (2) the effective medium approximation is often used to design gradient ARC without analyzing the limit of validity of this method regarding the nanostructure sizes; (3) the potentially strong influence of the scattering phenomena that can occur when nanostructures are not small in regard of the wavelength.
Our work is then devoted to bring a clear theoretical description compared to experimental results of these different limiting parameters for Moth-eye structure and multiple discrete nanostructured layers deposited by OAD. We have developed a method to design analytically the profile of refractive index, i.e the nanostructures shape profile, which will perfectly suppress the reflection for any given substrate and range of wavelength. We applied this method to optimize both a bilayer of SiO2 that we elaborated by OAD and a Moth-eye structure profile. Using Finite-Difference Time Domain (FDTD) methods, we calculated in particular the underestimated aspect of light scattering in these systems. To do so we applied FDTD simulations on the 3D morphology of the bilayer extracted from an electron tomography experiment. Finally we compared this result to those obtain for various Moth-eye structures.