Silicon (Si) is commonly used as a substrate material for infrared reflectors and windows in the 1.5 μm-7 μm region. The strong absorption band at 9 μ m makes it unsuitable for CO2 laser transmission applications but it is frequently used for laser mirrors because of its high thermal conductivity and low density.
Silicon (Si) is the most abundant electropositive element in the Earth's crust. It's a metalloid with a marked metallic luster and very brittle. It is usually tetravalent in its compounds, although sometimes they are bivalent and purely electropositive in its chemical behavior.
Silicon lenses are popular for many applications in the IR. They are typically used for imaging applications in the 3-5 μm region of the spectrum, making them excellent for security and military applications. They can also be used to collimate infrared lasers. These lenses are coated with a BBAR coating that provides 98% transmission in the 3-5 μm spectral range.