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Germanium

Germanium (Ge) is the preferred lens and window material for high performance infrared imaging systems in the 8~12 m wavelength band. Its high refractive index makes Ge ideal for low power imaging systems because of minimum surface curvature. Chromatic aberration is small, often eliminating the need for correction.

Germanium is most widely used for lenses and windows in IR systems operating in the 2~12 m range. Its transmission is very temperature sensitive, becoming opaque near 100C. Environment does not cause problems because Germanium is inert, mechanically rugged, and fairly hard.

Germanium is a high index material that is used to manufacture Attenuated Total reflection (ATR) prisms for spectroscopy. Its refractive index is such that germanium makes an effective natural 50% beamsplitter without the need for coatings. It 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. It can be AR coated with diamond producing an extremely tough front optic.

Application as window, lens, beamsplitter, ATR prism or filter in spectrometers and in thermal imaging.


Main Properties

 

Crystal Properties
Crystal Growth Method Czochralski (CZ) pulling techniques
Maximum Size <250mm
Optical Properties
Transmission Range 1.9~17µm
Reflection Loss, for two surfaces at 11µm 53%
dn/dt (@633nm), /C 396x10-6
dn/d=0 R.I. almost constant
Refractive Index See below
Wavelength (µm)
Refractive Index (n)
Wavelength (µm)
Refractive Index (n)
2.0
4.1079
9.0
4.0047
3.0
4.0446
10.0
4.0040
4.0
4.0242
11.0
4.0031
5.0
4.0153
12.0
4.0029
6.0
4.0106
12.5
4.0024
7.0
4.0076
13.02
4.0021
8.0
4.0053
15.0
4.0017
Transmission Curve See below
Crystallographic Properties
Syngony Cubic
Crystal Form Polycrystalline or Monocrystalline
Lattice Constant, Angstrom (Å) 5.66
Cleavability <111>, non-perfect
Physical Properties
Density, at 20 C 5.33
Mohs Hardness 6.3
Dielectric Constant for 9.37x109 Hz at 300 K 16.6
Melting Point, C 937
Thermal Conductivity, W/mK at at 293 K 59
Thermal Expansion, 1/K at 298 K 6.1x10-6
Specific Heat Capacity, J/(KgK) at 273-373 K 0.074
Bandgap, eV 0.67
Knoop Hardness, Kg/mm2 800
Youngs Modulus, Gpa 102.66
Shear Modulus, GPa 67.04
Bulk Modulus, GPa 77.86
Debye Temperature, K 370
Poissons Ratio 0.278
Elastic Coefficient C11=129, C12=48.3, C44=67.1
Apparent Elastic Limit 89.6 MPa (13000psi)
Chemical Properties
Chemical Formula Ge
Molecular Weight 72.59
Solubility in water
None
Solubility in acids Soluble

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