Broad Band Antireflection Coating on Zinc Selenide from 2 to 12 μm Wavelength Region

摘要

Zinc selenide is one of the most useful materials in the infrared region transmitting in 0.6 to 15 μm spectral region. Its very low absorption, chemical stability and non-hygroscopic nature makes it useful for achromatizing optical components intended for multi spectral application, providing good imaging characteristics. In this paper, a broad band antireflection coating on zinc selenide for 2 to 12 μm spectral region is reported. The coating design approach involves gradient index concept where refractive index varies from that of incident medium to the substrate. To realize this gradient index profile refractive index is assumed to increase from 1.42 (thorium fluoride) for the lowest index available material to 2.4 of zinc selenide in six discrete optimized step gradient index layers. Further, the last five layers are replaced by equivalent layer system of two layers of real materials available, one thorium fluoride and other zinc selenide. This eleven layer coating deposited by e-beam evaporation system conforms to environmental stability standards and shows average transmission of 94% in 2 to 12 μm region with peak around 98% at 9 μm on a 5 mm thick zinc selenide substrate.