Ge is a high refractive index material (4.0 @ 10 microns) with good long wavelength transmission in the infrared. At elevated temperatures the free carrier absorption becomes important and at 200 °C it is almost opaque. The high index permits very novel lens designs and ATR sampling of samples with high refractive index or high carbon black content.
Far-IR Transmission – 1 mm thick Ge window
Far-IR Transmission – 4 mm thick Ge window
Melting Point: 936 °C
Density: 5.33 g/cm3
Solubility in H2O: Insoluble
Hardness: 780 kg/mm2
Short Wavelength Limit: 5,500 cm-1 (1 mm)
Long Wavelength Limit: 432 cm-1 (1 mm), 574 cm-1 (4 mm)
The high refractive index of Ge permits design of BBAR coatings with greater than 98% transmission. It may also be coated to control depth of penetration in ATR applications.
Short and Long Wavelength Limits defined for which transmissivity is greater than 50% of stated crystal thickness.