Germanium (Ge)

Germanium (Ge) is a chemical element used extensively in optics, especially in the infrared (IR) part of the spectrum. Ge has a high refractive index (around 4.0 in the infrared region), making it highly effective in manipulating light in optical systems. It is transparent to infrared light, particularly in the 2-14 micrometer range, making it ideal for IR applications. Ge maintains its optical properties over a wide temperature range, which is important for applications requiring consistent performance under varying environmental conditions.

Optical Properties

Refractive Index: 4.0034

Thermal Co-efficient of Refractive Index: 35 – 40 x 10^-5

Transmission Range: 1.8 – 12.0 µm

Thermal Properties

Thermal Linear Expansion: 6.1 x 10^-6 /°C at 20°C

Thermal Conductivity: 59.8 W m⁻¹ K⁻¹  at 273 K

Specific Heat Capacity 319 J Kg⁻¹ K⁻¹

Melting Point: 938.2 °C

Mechanical Properties

Density: 5.33 g/cc

Knoop Hardness: 780

Young Modulus: 102.7 GPa

Shear Modulus: 67 GPa

Poisson Ratio: 0.278

Wavelength (µm)	Refractive Index (n)
2.058	4.1020
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	4.0170
6.238	4.0094
8.660	4.0043
9.720	4.0034
11.04	4.0026
12.00	4.0023
13.02	4.0021