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Guide to optical materials - Part 2

15th Jun 2017
June 2017


A brief guide to IR optical materials 
 

Welcome to part 2 of Crystran's guide to optical materials, where we will be focusing on longer wave IR crystals. When selecting materials for optical applications outside the visible spectrum, there are many different factors to consider. The obvious variables include the transmitted wavelength range, refractive index, achievable surface specifications, and of course, price. However, sometimes there are other parameters which can be overlooked such as: durability in harsh environments, sensitivity to moisture, raw material availability, and in some cases even toxicity.
 
We have compiled a helpful list of commonly used IR optical materials with some useful insights and a few things to look out for:
 
Zinc Selenide, ZnSe:
Developed in the late 1970s, ZnSe has a transmission range that extends all the way up to 21 microns. Produced by CVD, ZnSe is micro-crystalline in structure, very homogenous and has a wide variety of coatings available.
Yellow/orange in colour, ZnSe is partially transmissive in the visible, allowing it to be used with a 633 nm (red) laser for alignment purposes. It oxidizes significantly at 300°C, exhibits plastic deformation at about 500°C and dissociates at roughly 700°C. For safety, ZnSe windows should not be used above 250°C in normal atmospheric conditions.
 
Zinc Sulphide, ZnS:
There are two grades of this material commercially available: FLIR grade is milky yellow and is how it results from initial production. Multispectral grade is created through an extra process of ­Hot Isostatic Press (HIP), and a clear water-white form is produced.
FLIR grade is tougher and less expensive than ZnSe, and is used particularly for front optics in exposed systems. The water-white multispectral grade is used for IR applications that require visible transparency as well as IR. Similar temperature restrictions to ZnSe apply. 
 
Germanium, Ge:
This material is largely used for 3 - 5 and 8 - 12 micron IR applications. It is harder than ZnSe but more brittle and can chip easily. Germanium has been known to exhibit internal strain which can be difficult to control as many companies do not have IR interferometers to perform wavefront transmission measurements.
A good range of efficient coatings are available and particularly diamond-like coatings (DLC) which make a very tough front optic in harsh environments. Ge has a high refractive index of 4.0, reducing the effect of spherical aberration in lenses, and making it a popular medium for ATR (Attenuated Total Reflectance) spectroscopy.
 
Silicon, Si:
Silicon is used as an optical window primarily in the 3 - 5 micron band and as a substrate for the production of optical filters. Large blocks of silicon with polished faces are also employed as neutron targets in physics experiments.
It is grown by Czochralski pulling techniques (CZ) and contains some oxygen which causes an absorption band at 9 microns. To avoid this, silicon can be prepared by a Float-Zone (FZ) process. Optical silicon is generally lightly doped (5 - 40 ohm cm) for best transmission above 10 microns. It has a further pass band between 30 - 100 microns which is effective only in very high-resistivity (very pure) material. Doping is usually boron (p-type) and phosphorus (n-type). As with germanium, hard coatings can make this a very versatile optic.
 
KRS5:
The name KRS5 comes from German research on IR materials from the early 20th century and literally means "Crystal from the Melting Pot #5". Of all the allocated numbers, only KRS5 and KRS6 had any promise and there is only a small amount of KRS6 left!
KRS5 still has major use in IR spectroscopy due to its wide bandwidth (0.6 - 40 microns) and relatively high refractive index. It is very soft but will take a polish which is good enough for Far IR use and because of its particular formulation it is virtually insoluble. This means that although it contains thallium, in practical use it is not especially hazardous.
 
Caesium Iodide, CsI:
Caesium Iodide has the deepest known IR transmission (up to 55 microns), and is sometimes used for components in the widest range spectrophotometers. An extremely soft material, CsI is notoriously difficult to polish, and so performance is compromised for transmission range.
Doped with thallium, CsI(Tl) is a useful scintillator which emits at a wavelength that is a good match for silicon photodiodes.
 
Salts, NaCl, KBr:
These materials are useful for Long-wave / Far IR applications where sensitivity to moisture is not a problem. These soft crystals deform under heat and pressure and can be forged in polished dies to create IR windows and lenses. 
A major use for salt crystals is in the manufacture of small disposable cell windows for spectroscopy, known as mini-cells. These windows have a depression of controlled thickness pressed into the surface.
 
 
 
Please do get in touch if you would like a copy of our handbook of optical materials which contains more detailed information on these materials, or head over to our website at www.crystran.co.uk 
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