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Modern infrared antireflection coatings can effectively increase transmission of windows and lenses. Such coatings materials include commonly chalcogenides, fluorides, silicon, germanium and oxides. It is known that chalcogenides and fluorides are characterized by relatively low mechanical strength and stability against enhanced humidity. Oxide films typically have high absorption in the MWIR and LWIR. Silicon and germanium have high refractive index and are rarely used as an upper layer. Some authors offer using diamond-like carbon (DLC) film as a protective layer for multilayer coatings (MLC). DLC thin films are characterized by high infrared transparency, high mechanical hardness, low friction coefficient and chemical inertia. In this paper we show that high performance broadband and durable infrared antireflection coatings on Ge substrates can be designed and fabricated. The initial design of an antireflective coating is a gradient-index system. Then the layers are replaced with three-layer equivalent periods. The thickness of the upper DLC layer is limited by required mechanical properties of the coating and from our experiments should be not less than 100 nm. DLC film deposited using an End-Hall ion beam deposition technique. The remaining layers were deposited using an electron beam evaporator with ion bombardments. The average transmission of the samples with MLC + DLC surface is 93% in the wavelength range between 3 and 12 μm. On the opposite surface, a broad-band antireflection coating was deposited. The peak transmission is about 97%. The coatings are passed mechanical (adhesion, abrasion) and environmental (temperature cycle, humidity) durability tests. The result is improved significantly in comparison with conventional infrared windows.
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