Materials: Germanium

Germanium (Ge)

Germanium is a versatile infrared material commonly used in imaging systems and instruments in the 2 to 12 microns spectral region. It is used as a substrate for lenses, windows, and output couplers for low-power CW as well as pulsed TEA, CO2 lasers. The lowest absorbing Ge optics are limited to a throughput power range of 50 to 100 watts before thermal lensing or thermal runaway. These problems are minimized by properly heatsinking the optic and carefully cleaning contaminated optical surfaces.

Ge is non-hygroscopic and non-toxic, has good thermal conductivity, excellent surface hardness, and good strength. II-VI carefully monitors the 10.6µm Ge absorptivity for use in laser applications to assure that thermal runaway and fracture do not occur.

One ideal laser application for Ge is a > 99% reflecting back mirror for a CO2 laser with built-in power meters. A high-reflecting, low-absorption dielectric coating is on one side and an AR coating on the other. The laser operates on the feedback from the high-reflecting side, but enough power leaks through for monitoring by a power meter. II-VI supplies these optics for a number of commercial laser applications.

Material Properties

Optical Properties

Bulk Absorption Coefficient @ 10.6µm

< =0.03 /cm

Temperature Change of Refractive Index @ 10.6µm

408 x 10-6/°C

Thermal Properties

Thermal Conductivity @ 20° C

0.59 W/cm/°C

Specific Heat

0.31 J/g/°C

Linear Expansion Coefficient @ 20° C

5.7 x 10-6/°C

Mechanical Properties

Young’s Modulus

100 GPa (14.0 x 106 psi)

Rupture Modulus

93 MPa (13,500 psi)

Knoop Hardness

692 kg/mm2

Density

5.32 g/cm3

Poisson’s Ratio

0.27


Wavelength
(µm)

Index

Wavelength
(µm)

Index

Wavelength
(µm)

Index

Wavelength
(µm)

Index

2.2

4.0879

5.0

4.0160

7.8

4.0061

10.6

4.0028

2.4

4.0732

5.2

4.0149

8.0

4.0057

10.8

4.0026

2.6

4.0599

5.4

4.0139

8.2

4.0053

11.0

4.0025

2.8

4.0523

5.6

4.0128

8.4

4.0049

11.2

4.0024

3.0

4.0451

5.8

4.0118

8.6

4.0044

11.4

4.0023

3.2

4.0399

6.0

4.0107

8.8

4.0042

11.6

4.0023

3.4

4.0311

6.2

4.0097

9.0

4.0040

11.8

4.0022

3.6

4.0289

6.4

4.0092

9.2

4.0038

12.0

4.0021

3.8

4.0267

6.6

4.0087

9.4

4.0036

12.2

4.0020

4.0

4.0245

6.8

4.0083

9.6

4.0034

12.4

4.0020

4.2

4.0223

7.0

4.0079

9.8

4.0033

12.6

4.0019

4.4

4.0205

7.2

4.0074

10.0

4.0031

12.8

4.0019

4.6

4.0189

7.4

4.0070

10.2

4.0030

13.0

4.0018

4.8

4.0172

7.6

4.0066

10.4

4.0029