Germanium (Ge) single crystal is a chemically inert material with high hardness, good thermal conductivity, and insoluble in water. It exhibits high transparency in a broad wavelength range from 2μm to infrared 12μm. It is a very common infrared window used in various fields, such as infrared imaging systems and infrared spectrometer systems. The absorption coefficient of Ge single crystal at 10.6 μm is very small, so it is an ideal material for CO₂ laser lenses, windows, and output coupling mirrors. Ge single crystal is also used as a base material for various infrared filters.