BK7

BK7 is an excellent general purpose material widely used in visible and NIR optical components. BK7 is a borosilicate crown optical glass. 

                 Transmission Range: 330 nm~2100 nm

                 Thermal Expansion Coefficient: 7.5X10^-6 /K

                 Density:2.52 g/cm³ 

Fused Silica

Fused silica is a widely used material with high chemical purity, good thermal characteristics and excellent homogeneity. Very good thermal expansion characteristic is an outstanding feature of fused silica. Fused Silica is primarily used in wider thermal application. Fused silica includes UV grade and IR grade. 

                 Typical index Homogeneity: < 8 x 10^-6

                 Thermal Expansion Coefficient: 0.58 x 10^-6 /K (0℃to200℃)

                 Density:2.201 g/cm³

Quartz
Although crystal quartz is made of SiO2, it is different from fused silica, it is a positive uniaxial birefringent crystal. It features its very small birefringent refractive index difference, so it is always used in waveplates and other polarization control application.
 

                 Transmission Range: 200 nm~2300 nm

                 Thermal Expansion Coefficient: ||a=13.2 x 10^-6/K, ||c=7.1 x 10^-6/K

                 Density:2.65 g/cm³

CaF2

CaF2 is a cubic single crystal. It has good transmission from UV to middle infrared wavelength. It is always used in excimer laser components. While, it is a little hygroscopic and susceptible to thermal shock.
 

                 Transmission Range: 170nm~7800nm

                 Thermal Expansion Coefficient: 18.85 x 10^-6/K (20℃to60℃)

                 Density:3.18 g/cm³

MgF2

MgF2 is a tetragonal positive birefringent crystal. MgF2 has good chemical, mechanical and thermal characteristics compared with CaF2. Due to its high vacuum UV transmission and high damage threshold, MgF2 is usually used in VUV and excimer laser windows, polarizers, lenses and etc.
 

                 Lattice Constant: a=4.621A; c=3.053A

                 Mohs Hardness: 6

                 Density:3.17g/cm³

                 Transparence Range: 120nm ~ 7000nm

                 Thermal Conductivity: 0.0075W/m/K

                 Thermal Expansion Coefficient: ||a=13.7x10^-6/K
||c=8.48x10^-6/K

                 Index of Refraction: @405nm no=1.3836, ne=1.3957

Sapphire

Sapphire is a single crystal aluminum oxide (Al2O3). Its Mohs hardness is 9, it is one of the hardest materials. Sapphire has good mechanical strength, and it is also transparent from visible to IR spectrum. So, it is always used as window material where scratch resistance is required.
 

                 Lattice Constant: a =4.75 A; c =12.97A

                 Transparent Range: 180nm ~ 4500nm

                 Density:3.98g/cm³

                 Mohs Hardness: 9

                 Melting Point:2030℃

                 Thermal Conductivity: 0.04W/m/K

                 Expansion Coefficient: 8.4x10^-6/K

                 Refractive Index: 1.755@1um

Silicon

Silicon is a good IR material , it can be used from 1.2 to 7 microns, its peak transmission range is in 3-5 micron. Silicon can be used in some circumstances where weight is a concern as that the density of Silicon is similar to optical glass. Silicon material is widely used in security and military applications.
 

                 Transmission Range: 1200 nm~7000 nm

                 Thermal Expansion Coefficient: 2.55x10^-6/K

                 Density:2.33g/cm³

Germanium

Germanium is widely used in far infrared wavelength, it is transparent from 2 micron to 17 micron. Germanium also has higher refractive index about 4 at 10.6 micron. Germanium is very popular in FLIR and imaging applications.
 

                 Transmission Range: 2000 nm~17000 nm

                 Thermal Expansion Coefficient: 5.5x10^-6/K

                 Density:5.33g/cm³

Zinc Selenide

ZnSe is a material for production of optical window and lens for IR range. It is transparent from 0.5 micron to 19 micron. It is mainly used in CO2-laser application for its excellent imaging characteristics and high resistance to thermal shock. 
                 Transmission Range: 500 nm~19000 nm

                 Thermal Expansion Coefficient: 7.6x10^-6/K

                 Density:5.27g/cm³