Silicon (Si) is commonly used as a subtrate material for inraredr reflectors and windows in the 1.5u m- 8um region.The strong absorption band at  9um makes it usuitable for CO2 laser tranmission applications but it is frquently used for laser mirrors because of its high thermal conductivity and low density. Silicon is also a useful transmitter in 20 um range.

Silicon (Si) is the most abundant electropositive element in the Earth's crust. lt's a metalloid with a marked metallic luster and very brittle. lt is usually tetravalent in its compounds, although sometimes they are bivalent and purely electropositive in its chemical behaviour. Moreover, pentacoordinated and hexacoordinated silicon

compounds are also known.

Silicon lenses are popular for many applications in the IR. They are typically used for imaging applications in the 3-5 um 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 um spectral range.