Silicon (Si)

A semiconductor material that is commonly used in infrared optical systems operating in the 3 to 5µm spectral band. The refractive index is near 3.4 throughout the range. Silicon is also useful as a transmitter in the 20µm to 300µm range.

Silicon is used as a mirror substrate for lasers because of its thermal conductivity, light weight, and hardness. It is also used for windows and lenses in the 1.2µm to 6.7µm range. Due to the strong absorption at 9µm, Silicon is not suitable for use with CO2 lasers as a transmitting optic but is widely used for CO2 mirrors.

Silicon has one of the lowest densities of the common infrared materials making it ideal for systems with weight constraints. The density of Silicon is only half that of Germanium, Gallium Arsenide and Zinc Selenide. Silicon is harder than Germanium and not as brittle. Silicon is the lowest material cost option of all the infrared materials.


Property Specification
Transmission Range 1.2µm to 7.0µm & from 25µm out to beyond 300µm
Density 2.329
Thermal Expansion 2.55 x 10-6 /°C @ 25°C
Surface Finish Typical specifications for surface quality in the infrared are a 40-20 scratch-dig in the 1.2 to 3µm spectral region & 60-40 scratch-dig for the 3-7µm area. Diamond Turned surface finishes of 120 Å rms or better are typical.
Surface Figure In the infrared, typical required surface figure ranges from 1/2 wave to 2 waves @ 0.6328µm & are usually specified depending on the system performance requirements.
AR Coating Options The most common anti-reflectance coating for Silicon is BBAR for 3 to 5µm. Many other specialized wavelength bands are possible within the 1.2 to 7.0µm range.
Typical Applications Thermal imaging, FLIR.
Products Manufactured Lenses, Aspheric Lenses, Binary (Diffractive) Lenses, Windows, Optical Beamsplitters, Optical Filters, Wedges.


Wavelength (µm)
Index of Refraction (n)