Monocrystal Germanium is a most widely used infrared material for windows and optical elements in thermal-imaging systems and IR spectrograph systems in the 2 to 12 microns waveband. It has excellent hardness and high thermal conductivity, and it is insoluble in water.

Due to its good mechanical property, good thermal conductivity, and the low absorption in 10.6 micron, monocrystal germanium is an ideal substrate material for CO2 laser lenses, windows and output couplers for low-power CW as well as IR filters.