Silicon is commonly used as an optical window primarily in the 3 to 5 micron band and as a substrate for production of optical filters. It is frequently used for laser mirrors because of its high thermal conductivity and low density. Silicon is also useful as a transmitter in the 20 micron range. Large blocks with polished faces are also employed as targets in neutron physics experiments.

Optical Silicon is grown using the Czochralski method to economically produce silicon that is guaranteed to be 99.99% pure. Silicon windows are available as optical grade mono-crystalline and polycrystalline for use in infrared systems operating in the 3 – 5 micron range. Optical Silicon is generally lightly doped (5 to 40 Ω·cm) for best transmission above 10 microns. Silicon has a further pass band 30 to 100 microns which is effective only in very high resistivity uncompensated material.

Silicon windows are used as spectroscopic cell windows, Silicon substrates for multi-layer infrared filter coatings, IR spectroscopy and windows in infrared systems for infrared transmission. It is also useful for some ATR applications (Attenuated Total reflection).
Optical Properties
Transmission range in µm @10% min. 1.1 - 16
Transmission range in µm @50% min. 1.2 - 6.7
Refractive index @633nm 3.35268
(3.4223 @5µm)
Reflection losses in % at 1 surface 30.11 @3µm
Reflection losses in % at 2 surfaces 46.2 @5µm
dn/dT in 1/K 160 · 10-6
Physical Properties
Density in g/cm³ 2.3291
Melting point in °C 1412
Specific heat capacity in J/(kg · K) 731
Thermal Conductivity in W/(m · K) 163
Thermal Expansion in 1/K 2.62 · 10-6 @300K
Dielectric constant 13
Solubility in water in g/100g unsoluble
Mohs hardness 6.5
Knoop hardness in kg/mm² 1150
Material type Single crystal, synthetic
Crystal type cubic face-centered, diamond structure
Crystal structure Fd3m
Lattice constant in Å a = 5.43
Elastic coeficients in GPa C11 = 11
C12 = 65
C44 = 80
Young's Modulus (E) in GPa 131
Shear Modulus (G) in GPa 79.9
Bulk Modulus (K) in GPa 102
Rupture Modulus in MPa 130
Apparent elastic limit in MPa 124.1
Poisson ratio 0.221
Spectral Properties