Material - Zinc Selenide ZnSe CVD

ZnSe is used for optical windows, lenses, mirrors, prisms and optical blanks, plates, disks, sheets, slabs particularly for infrared applications ZnSe (Zinc Selenide) crystalZinc Selenide (ZnSe) — transparent in wide spectral range from yellow (visible) to far IR. ZnSe material is a chemically inert, non-hygroscopic and highly pure product that is very effective in many optical applications due to its extremely low bulk losses, high resistance to thermal shock and stability in virtually all environments, easily machined. To obtain superior transmittance, ZnSe crystals are grown by Chemical Vapor Deposition process (CVD). Zinc Selenide CVD is polycrystalline material, demonstrates superior CO2 laser transmittance and is employed in the transmission optical components used in CO2 laser processing. ZnSe (Zinc Selenide) Chemical Vapor Deposition (CVD) material is produced by synthesis from zinc vapour and H2Se gas, forming as sheets on graphite susceptors (ZnSe-CVD processes). Chemical purity of CVD-ZnSe 99.999%. It has à polycrystalline structure; the grain size of Zinc Selenide CVD material is controlled to produce maximum strength. The transmission range of Zinc Selenide ZnSe-CVD is (0.5 – 20) microns. ZnSe CVD used for high power CO2 laser optics at 10.6 microns, for protective optics windows or optical elements in FLIR (forward looking infrared) thermal imaging equipment, optics for medical and industrial applications (optical plane parallel windows (plates, disks, sheets, slabs, blanks), wedged windows, Brewster windows, optical meniscus, spherical and cylindrical lenses, prisms, resonator mirrors of CO2 laser). Single crystal ZnSe Zinc Selenide is available, but is not common, it has lower bulk losses and thus more effective for CO2 optics.

ZnSe (ZINC SELENIDE) base properties

Parameter Value
Physical properties of Zinc Selenide (ZnSe)
Density 5.27 g/cm3
Melting Point 1525°C (dissociates about 700°C)
Thermal Conductivity 18 W/(m K) at 298K
Thermal Expansion 7.1 x 10-6/°C at 273K
Hardness Knoop 120 with 50g indenter
Specific Heat Capacity 339 J/(kg K)
Young's Modulus (E) 67.2 GPa
Bulk Modulus (K) 40 GPa
Apparent Elastic Limit 55.1 MPa (8,000psi)
Poisson Ratio 0.28
Resistivity ~1012 (Ohm x cm)
Chemical properties of Zinc Selenide ZnSe CVD
Solubility 0.001 g/100g water
Molecular Weight 144.33
Crystal Structure polycrystalline ZnSe (CVD) Laser grade, grain size (50-70) microns ZnSe (CVD) IR optical grade, grain size (20-100) microns
Optical properties of ZnSe Zinc Selenide CVD
Transmission Range 0.6 - 21.0 microns
Refractive Index 2.4028 at 10 microns
Reflection Loss 31.11% at 10.6 µm (2 surfaces)
dn/dT +61 x 10-6/°C at 10.6µm at 298K
dN/dµ = 0 5.5µm
Index of Absorption ZnSe-CVD Laser grade 0.0005 (1/cm) at 10.6 microns ZnSe-CVD IR optical grade 0.003 (1/cm) at 10.6 microns

ZnSe (ZINC SELENIDE) refractive index

Wavelength, µm 2.75 5.00 7.50 9.50 11.0 12.5 13.5
Refractive Index 2.44 2.43 2.42 2.41 2.40 2.39 2.38
Wavelength, µm 15.0 16.0 16.9 17.8 18.6 19.3 20.0
Refractive Index 2.37 2.36 2.35 2.34 2.33 2.32 2.31
Because of high refractive index (n=2.4 at 10.6 microns), Fresnel losses of Zinc Selenide polished surface are equal 17% (at one side), thus the transmission of ZnSe polished window equals 68.9% at 10.6 micron. We offer two sides antireflection (AR/AR) coating both for one wavelength (10.6 microns or another) and for the spectral range (3 – 12) microns. Transmission of AR/AR coated ZnSe Zinc Selenide CVD window — up to 99%. Spectral transmittance (T) of ZnSe laser grade uncoated polished window (thick. 2mm), range (2.5–20) microns. For visible range T greater than or equal to 62% at 633 nm. ZnSe laser grade spectral transmittance Spectral transmittance (T) of Zinc Selenide IR optical grade uncoated polished window (thick. 2mm), range (2.5–20) microns. ZnSe IR grade spectral transmittance

ZnSe Optics.

ZnSe Windows, ZnSe Lenses and ZnSe prisms.

CVD ZnSe (Zinc Selenide) optics is widely used in spectral range 0.6-16.0 microns as optical components in many infrared spectral devices, FLIR (Forward Looking Infrared) systems, thermal imaging equipment and medical systems. Due to low absorption of material the ZnSe optics is used for high-power CO2 lasers. The following infrared optical details are made of CVD ZnSe: different kinds of prisms, lenses, blanks and windows. All optical products are manufactured of the usual standard sizes and max sizes of CVD ZnSe details according to max available ingots - up to (250x500x20) mm.

ZnSe optical windows or plates have modification as plane parallel windows, wedges (wedged windows), protective windows and Brewster windows.

ZnSe plane parallel windows are widely used as transfer optics and substrates for a variety of applications, such as laser windows, mirrors, beamsplitters, plate polarizers, output couplers, dichroic filters and beam combiners.

ZnSe wedges (wedged windows) are used as transfer beam splitting or beam deviation optics.

ZnSe Brewster windows are uncoated windows used as output window of CO2 laser chamber for selection of s- and p-polarization of laser radiation. In this case the window beam incident angle equals Brewster angle. Also it's possible another application of Brewster window, namely pair of Brewster windows can be used as Polarizer-Analyzer-Attenuator of transmitted radiation.

ZnSe protective windows - uncoated or AR / AR coated windows designed to withstand the conditions in industrial operations.

ZnSe Spherical lenses. It’s possible different ZnSe lenses configuration. ZnSe spherical lenses are used in infrared beam delivery systems for collimating, focusing, imaging and matching.

ZnSe spherical plano-convex lenses are used as the most inexpensive focusing transmitting optics (positive focal length). And for better focusing the lens convex surface should be turned to the incident parallel beam.

ZnSe meniscus lenses (spherical convex- concave lenses) are used as the better focusing optical component, which have minimal spherical aberration and so minimal focal spot size for incident parallel beam, (in case R1>R2 - positive focal length, positive meniscus).

ZnSe spherical biconvex lenses are used as focusing and imaging transmitting optics (positive focal length).

ZnSe spherical plano-concave lenses are used as diverging transmitting optics (negative focal length), for example: for compact collimating system.

ZnSe spherical biconcave lenses are used as diverging transmitting optics (negative focal length).

ZnSe CVD cylindrical lenses are used for infrared applications including CO2 laser optics: usually - ZnSe cylindrical plano-convex lenses and ZnSe cylindrical plano-concave lenses.

ZnSe prisms have modification as right angle prisms, ATR prisms and Dove prisms.

ZnSe right angle prisms are used for beam turning, for minimizing of optical systems dimensions and image converting.

ZnSe ATR prisms (Attenuated Total Reflection prism) are used in spectral analysis of liquids and gases.

Dove prisms are used to rotate an image, and angle of image rotation depends on the prism rotate angle.

Application of ZnSe and ZnS in aircraft FLIR (Forward-Looking Infra-Red) external optics

Although optical CVD ZnSe (Zinc Selenide) is a pretty convenient material for 3-5 & 8-12 μm applications (good spectral transmittance up to 12 μm, possibility of adjustment at the He-Ne laser wavelength 633 nm) some areas require more properties like erosion resistance. Being quite soft material (Knoop hardness 120) optical ZnSe is easily worn while landing / take-off of aircraft.

Solution one is external high density antireflection coating (HD-AR) which is a dense fluoride-oxide thin film generated by vacuum electron beam deposition. Main advantage of this solution is obtaining good transmittance and less erosion affect. This ZnSe optics coating is performed in Crystaltechno company which produces ZnSe domes with 200 mm diameter and two side AR&HD-AR coating. Although this is quite cost-effective solution of ZnSe surface protection it does not provide a perfect erosion resistance comparing to ZnS (Zinc Sulfide) domes or windows.

Solution two is ZnS (Zinc Sulfide) which is harder material (Knoop 160) having less density at the same time. There are two types of ZnS for different applications: multispectral (vis+IR) and regular ZnS which is nontransparent in visible range. There are few manufacturers producing multispectral ZnS material like II-IV Infrared and Vitron GmbH. Being more expensive (1.2-1.5 times than ZnSe) due to additional heat isostatic pressing stage ZnS material is widely used in aircraft FLIR devices, missiles. Other disadvantage of ZnS optics is its rain erosion with required external AR-HD coating.

Table 1. ZnSe vs ZnS: comparison of properties

Parameter ZnSe
Zinc Selenide
Zinc Sulfide
Refractive index at 10.6 micron, n 2.4 2.2
Reflectance losses at 10.6 µ m 29,1% 24,6%
Density, g/cc 5,27 4,08
Thermal expansion index, 1/ °ree; С 7,1х10-6 6,6х10-6
Knoop hardness, 50 g indenter 120 160
Young modulus, GPa 67,2 74,5

Third solution is combining technologies of ZnS and ZnSe in one CVD process when thin layer of ZnS is deposited on ZnSe crystal thus leading to transmittance like ZnSe and erosion behavior like ZnS has. This way is offered by Rohm & Haas recently bought by DOW company nfrared_materials/tuftran/tds/tuftran.pdf.
Reliable future of 3-5 & 8-12 μm multichannel systems pushes ZnSe and ZnS domes to further development. Very promising technology being developed nowadays is CVD diamond deposition on ZnSe substrate which provides the same transmittance but extreme erosion resistance as diamond is known as hardest optical material.