Many of the more advanced AR techniques exhibit excellent performances in terms of bandwidth (spectral and spatial), particularly those employing gradient index profiles or 3D nanostructures. Such approaches can indeed cope with the challenges associated with beams exhibiting complex spatiotemporal shapes. However, the practical realization of such AR structures is quite complex, requiring extreme control capabilities over fabrication processes, and is often not very repetitive. Moth-eye-like AR structures, for example, rely on continuously varying surface profiles with very fine features that are difficult to optimize and to fabricate accurately or in a repetitive manner. To date, the commonly used AR approach in optics is based on thin films. The basic AR coating employs a single layer placed between one medium (e.g., air) and another (e.g., glass). By choosing the layer thickness to be a quarter wavelength and its refractive index to be the geometric mean of the indices of the two media, it is possible to cancel completely the reflectivity at a specific wavelength. Employing multilayer coatings can yield broadband AR. However, achieving this in practice is challenging due to the difficulty in obtaining transparent materials with specific refractive indices that do not necessarily exist23<\/a>. Nevertheless, optimal performances can be traded for realizability by employing intensive numerical optimization procedures based on existing materials.<\/p>\n Despite the practical success of thin-films AR coating, there are applications for which a monolithic approach (i.e. the AR coating consists of a single material) is advantageous24<\/a>. For example, practical AR for high-power applications25<\/a>,26<\/a> is difficult to attain, mainly because of the different thermal expansion coefficients associated with different materials. Consequently, thin-film AR coatings are susceptible to thermal damage under high-power illumination conditions27<\/a>.<\/p>\n In this paper, we present a hybrid broadband (spectral and spatial) AR scheme, utilizing nanostructure arrays in a multi-layer metasurface configuration with periodicity in the order of a wavelength (see Fig.\u00a01<\/a>). This scheme offers a rigorous design and fabrication approach, allowing for a quick realization flow with good control over the spectral reflectivity. It also allows for optimizing the geometry and obtain more repetitive realization (in terms of obtaining as specific design) than that of moth-eye-like approaches. In parallel, the monolithic, single-material, nature of structure supports high-power applications as well as operation at very high temperature conditions. Furthermore, such AR structures can also be realized using additive manufacturing techniques such as 3D printing and Nano-Imprint Lithography.<\/p>\n
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