Zinc Selenide (ZnSe) is the most popular material for Infrared application. Due to very wide transmission range covering 0.6 to 20 µm CVD grown ZnSe high optical quality material is used to manufacture optical components (windows, mirrors, lenses etc.) for high power IR lasers.
Zinc Selenide is produced by synthesis from zinc vapour and H2Se gas, forming as sheets on graphite susceptors. This material is soft and non-hygroscopic. It is microcrystalline in structure, the grain size being controlled to produce maximum strength. Single crystal ZnSe is available, but is not common but has been reported as having lower absorption and thus more effective for CO2 optics.
The internal transmittance of Zinc Selenide is very high (absorption <= 0.0005cm-1 at 10.6mm), the relatively high refractive index (2.4 at 10.6m m) cause reflection losses of nearly 30%. Unless such losses can be tolerated, Zinc Selenide optics should always be antireflection coated. Because of the high refractive index, single and double layer antireflection coatings can be very effective.
Although Zinc Selenide has this clear advantage over Silicon and Germanium, unfortunately it is not an easy material from which to produce optics. Firstly, it is not a naturally occurring material and has to be synthesized using a difficult process (CVD). Secondly, the dust, which is generated when Zinc Selenide is ground and polished constitutes a significant health hazard as a cumulative toxin.