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Ultraviolet Optics

Acronym: UV optics

Definition: optical elements for use with ultraviolet light

German: Ultraviolett-Optik

Category: general optics

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Ultraviolet light behaves in many ways just as visible light, and it can thus in principle be used with the same times of optical elements as visible or near-infrared light: focusing and imaging lenses, mirrors, optical windows, beam splitters, prisms, polarizers, waveplates, optical filters etc. UV mirrors and lenses are also available in aspheric versions, and anti-reflection coatings are available. However, there are certain special aspects to be observed for applications in the UV region:

Examples for UV-transparent Media

UV optics are often made from highly purified calcium fluoride (CaF2), which has a very low UV absorption, high homogeneity, low birefringence, relatively high hardness (compared with other fluoride materials), high physical stability, and high optical damage threshold. It can be used down to ≈ 160 nm and is thus suitable for use, e.g., with argon fluoride (ArF) excimer lasers. However, it is brittle, naturally anisotropic, and hygroscopic. Similar properties are obtained for other purified fluorides such as magnesium fluoride (MgF2) and lithium fluoride (LiF). In addition to good UV transparency, such fluorides also offer good infrared properties up to wavelengths of 5 μm and beyond.

As an alternative, UV-grade fused silica can be used even for wavelengths down to ≈ 200 nm, whereas the cheaper standard-grade fused silica has significant attenuation already below 260 nm.

Similar constraints apply to materials chosen for making dielectric coatings, for example in the form of anti-reflection coatings. A frequently used AR coating material is magnesium fluoride (MgF2). Often, manufacturers offer UV anti-reflection coatings without revealing what material(s) they are made of.

Another possible material choice is artificial diamond, which is transparent down to ≈ 230 nm and very robust, but expensive.

For nonlinear frequency conversion, one often uses borate crystals such as LBO and BBO, which exhibit relatively good UV transparency and high resistance.

Some optical fibers can be used in the near-ultraviolet spectral region, although with relatively high propagation losses. Fiber delivery of ultraviolet light is usually not feasible for shorter wavelengths and/or high optical powers.

Common UV Wavelengths

In laser technology, a few ultraviolet wavelengths are particularly common:

Therefore, certain optical elements like lenses and mirrors are specifically developed for such wavelengths and may then be sold as excimer optics, for example.

EUV Optics: Essentially Limited to Reflective Optics

In the EUV region, basically all solid materials are relatively strongly absorbing, and even air causes strong attenuation below ≈ 200 nm, so that e.g. lithography with vacuum UV or EUV light below 100 nm has to be performed in vacuum. Bragg mirrors can still be made for the EUV region, e.g. with molybdenum/silicon (Mo/Si) structures, which allow, e.g., ≈ 70% reflectivity at 12 nm wavelength to be reached. Due to this limited reflectivity, EUV optics have to be designed with the smallest possible number of reflecting surfaces.

Suppliers

The RP Photonics Buyer's Guide contains 36 suppliers for ultraviolet optics. Among them:

See also: ultraviolet lasers, optical materials, scattering
and other articles in the category general optics

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