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Anti-reflective (AR) coatings are indispensable for an excellent imaging of optical systems. Common AR coating systems consist of layer stacks of alternating low and high refractive index materials with the residual reflection depending mainly on the low refractive index (LRI) of the last layer relative to air. However, conventional LRI materials are limited to SiO2 (n = 1,46 at 532 nm) and MgF2 (n = 1,38 at 532 nm), where MgF2 is not environmentally stable. Nanostructures with an adjustable effective LRI in the range of 1.07 to 1.25 are an attractive alternative to these materials. Integrated as the last layer in the stack system, these structures significantly improve the optical performance compared to a conventional interference coating system, as a broadband AR coating can be realized, which is less sensitive to high angles of light incidence. Nanostructures can be produced using various methods - e.g. wet chemical or lithographic. However, these methods are expensive and time-consuming, as often more than one manufacturing step is necessary. At the Fraunhofer Institute IOF in Jena, self-assembling AR nanostructures have already been successfully fabricated for several years using a conventional plasma-ion-assisted-deposition (PIAD) technology. Thereby organic material is deposited on the substrate via thermal evaporation and subsequently self-assembling nanostructures are formed by an ion plasma source. Melamine and Uracil are already being used successfully as organic material. The advantage of this method is the generation of nanostructures in one process which is cost- and time-effective. In this talk, we want to introduce Xanthine, another organic material that is also highly promising for the generation of nanostructures exhibiting an LRI. We will demonstrate the formation of these nanostructures during plasma etching and investigate their optical performance for the use as an AR coating.
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