In the MWIR and LWIR wavelength ranges, germanium (Ge), a substance with a high refractive index, is frequently employed in thermal imaging camera lenses. Ge is frequently utilized as a substrate for the manufacturing of optical filters due to its superior mechanical hardness and chemical resistance (aside from acids) as well as the high refractive index that reduces aberrations. Alfa Chemistry can custom fabricate optical components of virtually any shape and size to customer requirements using Ge materials. Contact us if you require assistance.
Advantages and Uses of Germanium Materials
Ge has distinct mechanical and optical characteristics. Germanium of the optical grade may be monocrystalline or polycrystalline. It is a preferred material for thermal and night vision because to its high transmittance in the 8 to 14 μm range, which covers thermal infrared radiation. It also has a high refractive index, making it possible to create optics with large opening apertures. In the transmission spectrum, Ge exhibits extremely little dispersion. Additionally, Ge possesses great surface hardness, strong strength, non-hygroscopicity, non-toxicity, and good thermal conductivity.
Fig 1. Optical absorption spectra for bulk Ge and tensile strained Ge on Si. (Colace L, et al. 2009)
But because germanium is sensitive to high temperatures and loses optical quality over 100 ℃, heatless designs are necessary. Despite having excellent internal transmittance and thermal conductivity, germanium is not suited for applications requiring a wide temperature range due to its significant temperature-dependent refractive index.
Ge is also a semiconductor material and can be either n-type or p-type. Because of its substantially lower absorption coefficients, n-type is preferred for IR applications.
Fig 2. Calculated absorption efficiency versus active layer thickness for strained Ge at various wavelengths. (Colace L, et al. 2009)
Properties of Germanium Materials
| Density | 5.33 g/cc |
| Molecular Weight | 72.59 |
| Solubility | Insoluble in water |
| Class/Structure | Cubic Diamond, Fd3m |
| Melting Point | 936 ℃ |
| Hardness | Knoop 780 |
| Refractive Index | 4.0026 at 11 μm |
| Transmission Range | 1.8 ~ 23 μm |
| Reflection Loss | 53% at 11 μm |
| Absorption Coefficient | <0.027 cm-1 @ 10.6 μm |
| dn/dT | 396x10-6/°C |
| Youngs Modulus (E) | 102.7 GPa |
| Shear Modulus (G) | 67 GPa |
| Bulk Modulus (K) | 77.2 GPa |
| Thermal Expansion | 6.1 x 10-6/°C at 298K |
| Thermal Conductivity | 58.61 W/m/K at 293K |
| Dielectric Constant | 16.6 at 9.37 GHz at 300K |
| Specific Heat Capacity | 310 J Kg-1 K-1 |
| Poisson Ratio | 0.28 |
| Elastic Coefficients | C11=129; C12=48.3; C44=67.1 |
About refractive index parameters.
"No" means ordinary light.
| µm | No | µm | No | 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 |
Reference
- Colace L, et al. (2009). "Germanium on Silicon for Near-Infrared Light Sensing." IEEE Photonics Journal. 1(2): 69-79.