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Zinc Selenide

Zinc Selenide (ZnSe) is a polycrystalline, semiconductor that readily transmits from 600nm to 21µm, with around 70% transmission between 1-15µm. Produced by chemical vapour deposition (CVD) zinc selenide boasts a lot of impressive properties including:

  • High purity
  • High refractive index homogeneity
  • Low bulk loss (from absorption and scattering)
  • Chemically inert
  • Highly resistant to thermal shock
  • Insoluble in water
  • Non-hygroscopic
  • High damage threshold

Laser grade ZnSe IR lenses are often used as CO2 laser focal lenses as they transmit at 10.6µm, with minimal absorption. It is also the only infrared material which also transmits at 632.8nm which means it works for the CO2/HeNe laser pairing.

Dependent on the lens type, zinc selenide can be used in a number of ways for laser applications:

  • Meniscus lenses are used to focus a laser beam to a smaller spot size with minimal spherical aberration. In IR applications they perform better than a planoconvex lens at shorter focal lengths.
  • Planoconvex lenses are a more cost effective lens for focussing the laser to a point, however they do produce more spherical aberration to their meniscus counterpart and therefore the resulting beam diameter is larger.
  • Cylindrical lenses are often used in beam circularisation to circularise an elliptical laser diode beam. Usually two cylindrical lenses are used as individually they collimate the beam in one axis only, so two would be needed to collimate both axes.
  • Aspheric lenses eliminate spherical aberration in optical systems and therefore produce a reduced spot size in laser diode collimation, they can also remove the need for multiple spherical optics in one system, and can therefore be more cost effective. Zinc selenide can be diamond turned to give precision, high quality aspheric lenses.

Zinc selenide beam combiners are also used when a CO2 and HeNe laser are used in conjunction with each other. The beam combiner, positioned at 45° to the CO2 laser, will transmit the CO2 beam (10.6µm) and reflect the HeNe beam (632.8nm) at 45° angle into the path of the CO2 laser beam, combining the two.

CO2 lasers are based on using a gas mixture as the gain medium. In a CO2 laser, this gas mixture is usually carbon dioxide (CO2), helium (He) and nitrogen (N2). Fundamentally, through the excitation of the nitrogen and carbon dioxide molecules within a tube, a beam of photons is emitted from the laser medium. A CO2 laser emits a beam at 10.6µm, which will not be visible to the human eye.
CO2 laser systems are often paired with a HeNe laser, which emits light at 632.8nm (visible red-light) which is then used align the CO2 laser beam.
CO2 lasers have many applications including welding, cutting, engraving and medical purposes. The wattage of the laser will vary depending on the application, but the power outputs are usually between 20-200 watts.

Zinc selenide is the chosen infrared substrate for a large number of optical components:

  • Zinc selenide optical windows are often used as protective windows in both FLIR/thermal imaging applications. They are also used in CO2 laser systems where they are used as resonator windows to separate the gas medium from the resonator optics. Often these protective windows would have a BBAR coating to reduce the reflection back into the system and maximise the transmission.
  • ZnSe can be used as a substrate for infrared filters, especially longpass filters due to its sharp cut-on and broadband transmission over longer IR wavelengths.
  • Infrared beamsplitters are crucial in metrology applications such as FTIR spectroscopy and interferometry, and zinc selenide beamsplitters are often used due to their broad transmission band. Zinc selenide beamsplitters can be designed to work over most wavebands between 1-20µm. A beamsplitter and BBAR coating would be applied to the front and back surface, respectively, of a zinc selenide blank. The antireflective coating is needed to improve the transmission and lower the back surface reflectance, which is considerable due to zinc selenide’s high refractive index (2.4028 at 10.6µm).
  • Zinc selenide is often the material of choice for Attenuated Total Reflection (ATR) crystals/prisms in FTIR spectroscopy, due to its high refractive index and chemical resistance.

Please note, zinc selenide is a hazardous material and so precautions should be taken when handling it.

Knight Optical supplies high quality zinc selenide optics including windows, ranging from 7-70mm in diameter, and lenses ranging from 5mm to 25.4mm. Knight Optical can also provide custom zinc selenide optical components to meet your exact needs, from small micro optics to large diameters. We can also ensure your zinc selenide is coated to your specification, as in laser applications it is essential that your coating has a high damage threshold to withstand the laser beam without damaging the coating or optic.

Contact our multilingual technical sales team and discover how Knight Optical’s high quality Zinc Selenide optical components and service can improve your instrumentation and supply chain experience.

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