Editing Optical coating (section)

===Fano-resonant optical coatings===

Fano-resonant optical coatings (FROCs) represent a new category of optical coatings.<ref name="elkabbash2021fano">ElKabbash, Mohamed, et al. [https://www.nature.com/articles/s41565-020-00841-9 "Fano-resonant ultrathin film optical coatings"], Nature Nanotechnology, vol.&nbsp;16, no.&nbsp;4, pp.&nbsp;440–446, 2021, Nature Publishing Group UK London. {{doi|10.1038/s41565-020-00841-9}}.</ref> FROCs exhibit the photonic [[Fano resonance]] by coupling a broadband nanocavity, which serves as the continuum, with a narrowband Fabry–Perot nanocavity, representing the discrete state. The [[Interference (wave propagation)|interference]] between these two resonances manifests as an asymmetric Fano-resonance line-shape. FROCs are considered a separate category of optical coatings because they enjoy optical properties that cannot be reproduced using other optical coatings. Mainly, semi-transparent FROCs act as a beam splitting filter that reflects and transmits the same color, a property that cannot be achieved with [[Transmission (optics)|transmission filters]], [[dielectric mirror]]s, or semi-transparent metals.

FROCs enjoy remarkable [[Structural coloration|structural coloring]] properties, as they can produce colors across a wide color gamut with both high brightness and high purity.<ref name="elkabbash2023fano">ElKabbash, Mohamed, et al. [https://www.nature.com/articles/s41467-023-39602-2 "Fano resonant optical coatings platform for full gamut and high purity structural colors"], Nature Communications, vol.&nbsp;14, no.&nbsp;1, p.&nbsp;3960, 2023, Nature Publishing Group UK London. {{doi|10.1038/s41467-023-39602-2}}.</ref> Moreover, the dependence of color on the angle of incident light can be controlled through the dielectric cavity material, making FROCs adaptable for applications requiring either angle-independent or angle-dependent coloring. This includes decorative purposes and anti-counterfeit measures.

FROCs were used as both monolithic spectrum splitters and selective solar absorbers, which makes them suitable for hybrid solar-thermal energy generation.<ref name="elkabbash2021fano"/> They can be designed to reflect specific wavelength ranges, aligning with the energy band gap of photovoltaic cells, while absorbing the remaining solar spectrum. This enables higher photovoltaic efficiency at elevated optical concentrations by reducing the photovoltaic's cell temperature. The reduced temperature also increases the cell's lifetime. Additionally, their low infrared emissivity minimizes thermal losses, increasing the system's overall optothermal efficiency.<ref name="elkabbash2021fano"/>