Zinc Selenide (ZnSe) Components
Zinc Selenide (ZnSe) is the most popular material for infrared application. Due to very wide transmission range covering 0.6 to 20 m m CVD grown ZnSe high optical quality material is used to manufacture optical components (windows, mirrors, lenses etc.) for high power IR lasers.
Physical Properties
| Crystal type | cubic |
| Lattice constant | a=5.657Å |
| Density | 5.27 g/cm3 at 25° C |
| Melting point | 1525 ° C |
| Refractive index | 2.417 - 2.385 @ 8 - 13 m m 2.40272 at 10.6 m m |
| Transmission band | 0.6 to 20 m m |
| Bulk absorption coefficient | 5 ´ 10-4 cm-1 @ 10.6 m m |
| Young's Modulus | 6.72 ´ 109 dynes/mm2 |
| Specific Heat at 25 ° C | 0.085 ca/g° C |
| Linear thermal expansion | 7.57 ´ 10-6 /° C at 20° C |
General Specifications of Zinc Selenide Optical Components
| Material | ZnSe |
| Surface Quality | 40-20 scratch & dig |
| Clear aperture | 90% of diameter |
| Diameter tolerance | +0.0, -0.1 mm |
| Thickness tolerance | +0.1, -0.25 mm |
| Surface Irregularity | l /2 per inch @ 633 nm over clear aperture |
| Coatings | Typical available coatings for ZnSe include BBAR for 0.8 to 2.5µm, 3 to 5µm, 1 to 5µm, 8 to 12µm, and the 3 to 12µm spectral regions and single wavelength AR coating at 10.6µm R£ 0.5% per surface. Many other specialized wavelength bands are possible within the 0.6 to 16µm range. |
Zinc Selenide custom made windows, lenses, Brewster windows, mirrors, beamsplitters and other components are available on request.
Zinc selenide most obvious and important advantage over other materials is its low absorption in the red end of the visible spectrum. This allows the ubiquitous helium neon laser to be used as a convenient and inexpensive alignment or sighting tool for infrared laser beams. Prealignment of optics may be necessary merely because the infrared beam is invisible to the eye, or in addition because the infrared laser beam is of such a high energy misdirected laser beam can be extremely dangerous to equipment or personnel.
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. Also, it is not a particularly hard substance and scratches easily.
Refractive Index
|
Wavelength, µm |
2.75 |
5.00 |
7.50 |
9.50 |
11.0 |
12.5 |
13.5 |
15.0 |
16.0 |
16.9 |
17.8 |
18.6 |
19.3 |
20.0 |
|
Refractive Index |
2.44 |
2.43 |
2.42 |
2.41 |
2.40 |
2.39 |
2.38 |
2.37 |
2.36 |
2.35 |
2.34 |
2.33 |
2.32 |
2.31 |
Transmission
Although 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.
Zinc selenide is not hygroscopic unlike certain salts which are used for windows in the infrared.
