Si (Silicon) is one of the hardest optical materials. Silicon optical components are widely used in various infrared applications operating in the wavelength range from 1.2 to 7 μm, including IR imaging or IR spectroscopy. Optical quality Silicon is typically lightly doped (5 – 40 ohms/cm) to prevent absorption bands and provide better transmission beyond 10 microns. Low silicon density (2.329 g/cm3) is ideal for weight-sensitive applications.

Silicon is used for windows and lenses primarily in the 2-6 µm range, which is an ideal choice for IR-illuminated lenses.

Tunable infrared silicon lenses are grown by the Czochralski extraction (CZ) process and contain some oxygen. To avoid this, single crystal Si material can be produced using the floating zone (FZ) process. Optical silicon is usually lightly doped (5 to 40 ohm*cm) for best transmission above 10 µm.

Due to its physical properties, light weight and hardness, silicon is widely used as a substrate material for laser mirrors (mostly for CO2 lasers).