Instrumentation and measurement
A brief guide to UV – Visible – Near IR optical materials
August 13, 2018
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 UV, visible and near IR optical materials with some useful insights and a few things to look out for:
Calcium Fluoride, CaF2:
Probably the most frequently used IR and UV material when balancing performance and practicality against cost and availability. Inexpensive grades are available for standard IR spectroscopy, but purer (more expensive) material is available for Raman and Far UV (122 to 200 nm) applications.
Having a cubic lattice structure, CaF2 is optically isotropic (non bi-refringent), highly-homogeneous and can be polished to the highest of standards (eg. surface flatness up to Lambda/20, peak to valley).
Barium Fluoride, BaF2:
More mechanically fragile than CaF2but provides slightly wider transmission into the LWIR. It is frustratingly short of the full 8-12 micron atmospheric bandpass window (by a micron), and is prone to cleaving under thermal shock.
Nevertheless BaF2 still has useful application as an IR window with a wide variety of sizes available. It works well within the limitations of thermal shock and can be polished to a good standard of cosmetic finish and flatness.
Magnesium Fluoride, MgF2:
One of the two available materials especially suited to upper-VUV applications (the other is LiF). MgF2is tough, amenable to working and can take a very high cosmetic polish needed for Far UV transmission.
Being uniaxial, MgF2 is intrinsically birefringent so is most commonly cut perpendicular to its optic-axis (c-axis) to remove unwanted effects. (Its sister material, LiF, is not birefringent but is more delicate to handle and sensitive to atmospheric moisture. We feel sometimes that nature has stacked the odds against us in material selection!).
Lithium Fluoride, LiF:
Lithium Fluoride has the furthest transmission into the VUV and is used for specialised applications with photon energies as high as 11.7 eV (105 nm). This material cleaves easily and must be worked with extreme care. Polishing, particularly steep radii, often causes surface “rip-out” which has to be very carefully controlled.
It is also used for X-ray monochromator plates where it’s lattice spacing behaves as a diffracting element providing the spectral discrimination needed for analysis purposes.
Sapphire is often used for its extreme toughness and strength and is one of the hardest natural substances on the planet (next to diamond)!
Because of this it is highly resistant to scratches and abrasion whilst having a wavelength range suitable for use in the UV, visible, and near infra-red.
Crystal Quartz, SiO2:
Quartz is positive birefringent and is mined naturally, but more commonly produced synthetically in large, long-faceted crystals. Be careful not to confuse terminology in this material, as “Fused Quartz” is often used to denote the glassy non-crystalline form better known as Silica.
At temperatures >490°C, Crystal Quartz starts to revert to glassy state, a process which is complete by 530°C. At this time, as annealling is unlikely, fracture of the component is probable.
Fused Silica, SiO2:
Fused Silica is the glassy form of Quartz and is thus isotropic. Fused Silica is tough and hard and has a very low expansion. Normal varieties of UV grade Fused Silica contain water which gives strong absorption in the IR. Water-free varieties of Fused Silica are available for near IR applications.
Keep an eye out for part 2 which will focus on near, mid, and far IR optical materials. 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.