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5 June 2018 Enhanced far-UV reflectance of Al mirrors protected with hot-deposited MgF2
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Mirrors based on Al protected with a MgF2 film provide high reflectance over a broad spectral range down to the wavelength of 120 nm in the Far UV (FUV). After more than 50 years since the development of this technology, a significant FUV reflectance enhancement has been obtained in the last years. Such enhancement originates mostly in the higher transparency of the MgF2 protective layer deposited on a hot Al-coated substrate. Research has been conducted at GOLD to measure the dependence of the FUV reflectance enhancement with MgF2 deposition temperature. A reflectance enhancement was found for freshly-prepared samples; moreover, the reflectance degradation over time of Al films protected with hot-deposited MgF2 was also smaller than for the coatings deposited at room temperature. A reflectance as high as 90% was measured at 121.6 nm (hydrogen Lyman α line) for aged samples. A FUV reflectance enhancement was also obtained on samples fully deposited at room temperature and later annealed in vacuum. The reflectance of Al mirrors as a function of MgF2 deposition temperature, as well as of post-deposition annealed mirrors, and their stability over time is presented. Structural data on film roughness, density, and main crystal orientations for mirrors with a MgF2 film deposited both at room temperature and at 250°C are also presented.
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Luis Rodríguez-de Marcos, Nuria Gutiérrez-Luna, Lucía Espinosa-Yáñez, Carlos Honrado-Benítez, José Chavero-Royán, Belén Perea-Abarca, and Juan I. Larruquert "Enhanced far-UV reflectance of Al mirrors protected with hot-deposited MgF2", Proc. SPIE 10691, Advances in Optical Thin Films VI, 106910T (5 June 2018);

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