RENARD

Optical metasurfaces, thin nanostructured layers, have the ability to manipulate light on a local scale, with independent functions in several spectral bands. For example, the infrared thermal emission of a surface can be modified and controlled using optical nanocavities. Although they are widely used to control phase, polarisation and diffraction, very little work has been done yet on controlling light scattering, especially because the structures are often periodic. However, in a defence context, a surface will have an optical response that will enable it to be detected, identified and possibly acted upon. This optical detection is highly dependent on the scenarios considered, but is fundamentally based on the radiation emitted directly by the object or on the radiation reflected back, and can cover several spectral bands. In a civilian context, obtaining emission and scattering behaviour in the visible and infrared range can be used to fight counterfeiting, but it is also of interest for photovoltaics and the windows of buildings or vehicles that need to combine several visual and thermal functions. The aim of the RENARD project is to develop metasurfaces using disordered arrays of antennas to control both infrared emissivity and scattering behaviour, angularly and spectrally. These devices combine the properties of state-of-the-art resonant structures with knowledge recently developed in the field of scattering. The optical response of a textured surface will depend on 2 scales. The microscopic scale is usually described using Maxwell's equations with resonant elements to control the level of absorption or transmission. The second scale is at a more macroscopic level and corresponds to the organisation of the nanostructures, which can also be described as a structure factor and will rigorously determine the permitted directions in which the light is reflected. The project aims to combine these two elements to demonstrate responses that would be compatible with stealth-type applications. The RENARD project is based on theoretical developments on disordered structures and the development of models to enable the design, optimisation and manufacture of three families of components. The first family of components are coatings with an abnormal scattering radiation pattern (angular energy control). The second family of components are coatings combining angular energy control and resonant behaviour, particularly for spectral emission and scattering bands (spectral energy control). The third family of components are coatings with the properties of the two previous families, but on non-planar surfaces. The project aims to achieve experimental characterisation of these different devices manufactured by laser lithography, and to develop a large-area laboratory mock-up on a flexible substrate to illustrate the camouflage capability with respect to infrared cameras. This project has several objectives: 1. To develop a model and implement a code for simulating disordered sets of resonant structures. 2. Demonstrate the simultaneous spectral control of scattering and thermal emission by a metasurface. 3. Demonstrate non-Lambertian angular control of scattering by a disordered metasurface. 4. Design metasurfaces that combine spectral and angular control (identical or different) over several spectral bands independently. 5. Extend these results to non-planar surfaces.