Zinc Selenide

Material
ZnSe

Zinc Selenide  is the most popular material for infrared applications. Due to the very wide transmission range covering 0.6 to 20 µm, CVD grown ZnSe high optical quality material is used to manufacture optical components (windows, mirrors, lenses etc.) for high power IR lasers.

Zinc Selenide is produced by synthesis from zinc vapour and H2Se gas, forming as sheets on graphite susceptors. This material is soft and non-hygroscopic. It is microcrystalline in structure, the grain size being controlled to produce maximum strength. Single crystal ZnSe is available, but is not common but has been reported as having lower absorption and thus more effective for CO2 optics.

The internal transmittance of Zinc Selenide is very high (absorption <= 0.0005cm-1 at 10.6mm), the relatively high refractive index (2.4 at 10.6mm) cause reflection losses of nearly 30%. Unless such losses can be tolerated, Zinc Selenide optics should always be antireflection coated. Because of the high refractive index, single and double layer antireflection coatings can be very effective.

Although Zinc Selenide has this clear advantage over Silicon and Germanium, unfortunately it is not an easy material from which to produce optics. Firstly, it is not a naturally occurring material and has to be synthesized using a difficult process (CVD). Secondly, the dust, which is generated when Zinc Selenide is ground and polished constitutes a significant health hazard as a cumulative toxin.
Properties
Optical Properties
Transmission range in µm @10% min. 0.48 - 20
Transmission range in µm @50% min. 0.5 - 20
Refractive index @ 633nm 2.5906
(2.4028 @10.6µm)
Reflection losses in % at 1 surface 17.05 @ 10µm
Reflection losses in % at 2 surfaces 29.1 @ 10.6µm
dn/dT in 1/K 61 · 10-6
Physical Properties
Density in g/cm³ 5.27
Melting point in °C Subl. @ >500
Specific heat capacity in J/(kg · K) 339
Thermal Conductivity in W/(m · K) 18
Thermal Expansion in 1/K 7.1 · 10-6 @ 271K
Dielectric constant 9.12
Solubility in water in g/100g unsoluble
Mohs hardness 4
Knoop hardness in kg/mm² 110
Material type Polycrystalline, synthetic
Crystal type cubic
Crystal structure F43m
Lattice constant in Å a = 5.67
Elastic coeficients in GPa C11 = 86.4
C12 = 51.5
C44 = 40.2
Young's Modulus (E) in GPa 70.3
Shear Modulus (G) in GPa 28.9
Bulk Modulus (K) in GPa 63.1
Rupture Modulus in MPa 55
Apparent elastic limit in MPa 55.1
Poisson ratio 0.3
Spectral Properties