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Germanium Ge
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. 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.
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Prisms & ATR
| Transmission Range | 1.8 to 23 μm (1) |
| Refractive Index | 4.0026 at 11 μm (1) (2) |
| Reflection Loss | 53% at 11 μm (2 surfaces) |
| Absorption Coefficient | <0.027 cm-1 @ 10.6 μm (typical) (7); 0.006 cm-1 @ 5.6 μm (typical) (7); <0.005 cm-1 @ 2.7 μm (7) |
| Reststrahlen Peak | n/a |
| dn/dT | +396 x 10-6 K-1 (2)(6) |
| dn/dμ = 0 | Almost constant |
| Density | 5.33 g/cc |
| Melting Point | 936°C (3) |
| Thermal Conductivity | 58.62 W m-1 K-1 @ 293 K (6) |
| Thermal Expansion | 6.1 x 10-6/°C @ 298 K (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 (13000psi) |
| Poisson Ratio | 0.28 (4)(5) |
| Solubility | Insoluble in water |
| Molecular Weight | 72.59 |
| Class/Structure | Cubic Diamond, Fd3m |
| μm | No |
|---|---|
| 2.058 | 4.102 |
| 2.153 | 4.0919 |
| 2.313 | 4.0786 |
| 2.437 | 4.0708 |
| 2.577 | 4.0609 |
| 2.714 | 4.0562 |
| 2.998 | 4.0452 |
| 3.303 | 4.0369 |
| 4.258 | 4.0216 |
| 4.866 | 4.017 |
| 6.238 | 4.0094 |
| 8.660 | 4.0043 |
| 9.720 | 4.0034 |
| 11.04 | 4.0026 |
| 12.00 | 4.0023 |
| 13.02 | 4.0021 |
Germanium is grown using the Czochralski technique by a small number of manufacturers in Belgium, USA, China and Russia. The refractive index of Germanium changes rapidly with temperature and the material becomes opaque at all wavelengths a little above 350K as the band gap floods with thermal electrons.
REFERENCES:
(1) Handbook Optical Constants, ed Palik, V1, ISBN 0-12-544420-6
(2) Li, Refractive Index of Germanium etc, J.Phys Chem, V9, p561, 1980
(3) Pearson & Brattain, Proc. Inst. Radio Eng. V43, p1794, 1955
(4) Fine, J.App.Phys, V24, p338, 1953
(5) Wortman & Evans, V36, (1), P153 (1965)
(6) Hawkins, Sherwood, Djotni: Mid IR Filters for astronomical and remote sensing instrumentation. Invited Paper SPIE Conference Glasgow 2008
(7) Manufactures Published Data