Germanium
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Germanium is a material commonly used for basic elements of acousto-optic (AO) instruments operating in the mid-IR region.
Germanium-based instruments are most prevalent in the wavelength range between 9 and 11 μm. Second most common region of their application is between 5 and 6 μm. There is also a less frequently occurring but nevertheless important region between 2 and 3 μm, where the most common material, paratellurite (TeO2) exhibits absorption bands.
We use monocrystalline germanium for optical elements including AO modulators and deflectors.
To determine the requirements for germanium components used in AO devices, refer to the following physical properties and key characteristics of AO-grade germanium.
Table 1. Key physical properties of germanium
| Density (298 K), g/cm3 | 5.323 |
| Tensile modulus, MPa | 72.4 |
| Mohs hardness | 6 |
| Fracture toughness, MPa1/2 | 1.004 (fracture plane 110) |
| Thermal shock resistance, °C | 125 |
| Poisson ratio, 125-375 K | 0.278 |
| Elastic constants, cm2/N (298 K) | S11 = 9.685 × 10-8 S12 = -2.70 × 10-8 S44 = 14.94 × 10-8 |
| Elastic coefficients, N/cm2 (298 K) | C11 = 13.16 × 106 C12 = 5.09 × 106 C44 = 6.69 × 106 |
| Young’s modulus, N/cm2 (298 K) | Y100 = 10.33 × 106 Y110 = 13.80 × 106 Y111 = 15.55 × 106 |
| Shear modulus, N/cm2 (298 K) | M100 = 6.69 × 106 M100 = 4.1 × 106 M111 = 4.9 × 106 |
For more details and other characteristics, visit our primary page on this material.
Table 2. Acousto-optical properties of germanium
| NSW | USW | M1, 10-7 cm2•s/g | M2, 10-18 s2/g | M3, 10-12 cm2•s/g |
| [111] | [111] | 10200 | 840 | 1850 |
| [100] | [010] | 1430 | 290 | 400 |
In table 2, NSW is the direction of normal vector of the sonic wave, USW is the direction of the displacement vector of the sonic wave. The coefficients Mi describe the acousto-optical quality factor of the crystal. The parameters depend on basic limitations on the crystal dimensions and the field of application.
The table below summarizes the quality parameters of our optical components made of germanium:
Table 3. Germanium component data sheet
| Material | optical-grade monocrystal |
| Free aperture, % | >90 |
| Diameter tolerance (width and length), mm | +0.0 / -0.1 |
| Thickness tolerance, mm | +/- 0.1 |
| Parallelism, arcmin | <= 5 |
| Surface quality, scr/dig: - for parts up to 3" - for parts up to 8" - for larger parts |
40/20 60/40 80/50 |
| Surface accuracy (overall – local error) at 633 nm, Newton’s rings | 1 – 1/2 |
Please pay attention that this article is only for your information. We do not supply raw germanium as well as semi-finished products. Our standard products are finished (polished, coated) parts.
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