Germanium is a group IV semiconductor with a diamond cubic crystal structure, which provides excellent optical homogeneity for high-performance infrared applications. For IR optics, only high-purity, single-crystal germanium is used, as impurities or crystal defects can cause absorption, scattering, or reduced transmission. Purity levels are typically above 99.999%, ensuring minimal interference with infrared signals. Physically, germanium has a density of approximately 5.33 g/cm³, a Vickers hardness of around 780 MPa, thermal conductivity of 60 W/m·K, and a coefficient of thermal expansion of 6 × 10⁻⁶/K.

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In terms of optical performance, germanium is opaque in the visible spectrum but highly transparent in the mid-wave infrared (2–5 μm) and long-wave infrared (8–14 μm) ranges. This property is due to its electronic band structure, with a bandgap of roughly 0.66 eV, which blocks visible light while allowing infrared photons in the thermal range to pass through. Its high refractive index of approximately 4.0 at 10 μm enables strong focusing and collimation of infrared radiation, though anti-reflective coatings are typically applied to reduce reflection, which is around 36% per uncoated surface. At room temperature, intrinsic absorption is minimal, but above 200°C, free-carrier absorption increases slightly, reducing transmission. Mechanically, germanium is brittle under tensile stress but strong under compression, making it ideal for robust IR windows while requiring careful handling.