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Germanium Crystal

Germanium is a high index material that is used to manufacture Attenuated Total Reflection (ATR) prisms for spectroscopy. Germanium Crystal refractive index is such high that Germanium makes an effective natural 50% beamsplitter without the need for coatings. Germanium is also used extensively as a substrate for production of optical filters. Germanium covers the whole of the 8-14 micron thermal band and is used in lens systems for thermal imaging. Germanium can be AR coated with Diamond producing an extremely tough front optic.

Property of Germanium Crystal

Transmission Range : 1.8 to 23 µm (1)
Refractive Index : 4.0026 at 11 µm (1)(2)
Reflection Loss : 53% at 11 µm (Two surfaces)
Absorption Coefficient : <0.027 cm-1 @ 10.6 µm
Reststrahlen Peak : n/a
dn/dT : 396 x 10E-6 /°C (2)(6)
dn/dµ = 0 : Almost constant
Density : 5.33 g/cm³
Melting Point : 936 °C (3)
Thermal Conductivity : 58.61 W m-1 K-1 at 293K (6)
Thermal Expansion : 6.1 x 10E-6/°C at 298K (3)(4)(6)
Hardness : Knoop 780
Specific Heat Capacity : 310 J Kg-1 K-1 (3)
Dielectric Constant : 16.6 at 9.37 GHz at 300K
Youngs Modulus (E) : 102.7 GPa (4) (5)
Shear Modulus (G) : 67 GPa (4) (5)
Bulk Modulus (K) : 77.2 GPa (4)
Elastic Coefficients : C11=129; C12=48.3; C44=67.1 (5)
Apparent Elastic Limit : 89.6 MPa (13000 psi)
Poisson Ratio : 0.28 (4) (5)
Solubility : Insoluble in water
Molecular Weight : 72.59
Class/Structure : Cubic Diamond, Fd3m

- Germanium crystal's high index is desirable for the design of lenses that might not otherwise be possible.
- Germanium crystal has nearly the highest density of the IR transmitting materials, a consideration when designing for weight restricted systems.
- Germanium crystal is subject to thermal runaway; the hotter it gets, the more the absorption increases. Pronounced transmission degradation starts at about 100°C and begins rapidly degrading between 200°C and 300°C, resulting in possible catastrophic failure of the optic.
- Germanium crystal is generally less expensive than ZnSe and Cleartran.
- Germanium crystal is diamond turnable.

Typical application: Thermal imaging, FLIR (Forward Looking Infrared)

Products manufactured: Lenses, Aspheric Lenses, Binary (Diffractive) Lenses, Windows, Optical Beamsplitters, Optical Filters, Wedges.

Surface Finish: Typical 40-20 or 60-40 scratch-dig. Diamond Turned 120 Angstroms rms or better.
Surface Flatness: Range from 1/2 wave to 2 waves @633nm depending on the system performance requirements.
AR coating: BBAR for 3 to 5µm, 8 to 12µm, and the 3 to 12µm spectral regions. Other specialized bands are possible from 2 to 12 µm.