Optical germanium : a complete guide
Germanium famously known as predicted by Mendeleev before its actual discovery in the end 19th century, is widely used in optics as a component material for IR applications. It has also interesting semi-conductor properties that we will not discuss into details in this guide.
Germanium window
Why is germanium a good optical material ?
While rather expensive (around 1300 USD/kg) Germanium offers interesting optical properties.
It has good transmission in the 8 to 14 µm range covering the thermal Infrared light, therefore it is a material of choice for thermal and night vision. It also shows a high refractive index which enable high open aperture angle optical component’s design.
Ge is also interesting for its low optical dispersion and therefore very low chromatic dispersion in it’s transmission spectrum.
Interestingly, due to its similar crystal structure to Diamond can be coated with a DLC layer (Diamond Like Carbon) which on top of reducing the reflection on it’s surface, adds a diamond strong protective layer. Finally Germanium is not transparent in the visible which can be useful to focus on IR light.
Unfortunately Germanium is sensible to high temperature with degradation of its optical properties over 100°C. For high temperature Mid-IR (8-13µm) alternative we recommend using BaF2 (! much lower transmission after 11µm) or ZnSe.
Germanium properties
Properties chart of optical germanium :
| Property | Values |
|---|---|
| Material name | Germanium |
| Chemical formula / Chemical element | Ge (atomic number 32) |
| Type of chemical element | Metalloid |
| Color | Grayish |
| Index of Refraction | 4 |
| Transmission range | 1.8µm to 23µm |
| Density | 5.35 g/cm3 |
| Hardness | 800 Knoop (MOHS 6) |
| Young’s modulus | 75.79 GPa |
| Thermal expansion | 6.1*10-6 °K-1 |
| Thermal conductivity | 59 W/(m*°K) |
| Melting point | 1210 °K |
Transmission curve of germanium
Transmission curve of 3mm thick Germanium window with AR + DLC coating.
Raw germanium have a relatively average transmission in the IR (less than 50%) but Anti-reflect or DLC treatments do improve a lot it’s transmittance.
What type of optical component can be made with germanium ?
Raw germanium material is rather hard but can still effectively be machined and polished into optical components:
- Optical windows : main usage of germanium with very good transmission in the IR and no transmission in the visible. Often, germanium windows are strengthened with DLC coating on one face.
- Lenses : germanium lenses are common for usage in thermal and night vision objectives.
- Aspherical germanium lenses : designed and used for enhanced optical properties and for applications that need weight saving. Germanium can be diamond turned into high surface quality aspheres.
- Wafers : as a semi conductor material, like Silicon, it is widely used as a thin substrate for electronic component.
- Germanium is also frequently used as a dopant in optical fiber’s silica optical cores. Due to the high quantities of fibers used for data transfer in the world, this application amounts for 30% of the world’s total germanium consumption.
It is also worth noting that germanium is relatively more brittle that silicon and therefore should be handle with care during machining.
General specifications for windows and lenses
| Properties | Values |
|---|---|
| Available dimensions | Diam 3 to D400mm |
| Dimensional tolerances | +/-0.01mm |
| Thickness | 1-50mm |
| Surface quality | S&D : 60/40 normal quality; 40/20 high quality |
| Flatness | 1/2 lambda |
| Parallelism | better than 10″ |
Above value are generic specifications for fabrication, specific better value may be available and need to be reviewed case by case.
Where to find a good germanium optical component supplier ?
Germanium optical components along with Silicon ones are made by precision optical suppliers usually focusing on IR applications. Due to high value of the material, specialization enables these companies less pressure on the stocks and lower risks of non-quality rejection.
Most of the raw material comes from China, Russia and USA being also important suppliers, quality material is available everywhere.
Don’t hesitate to ask SINOPTIX for more details or quote request for your component:
