Ultraviolet light is light with a wavelength shorter than ≈ 400 nm, the lower limit of the visible wavelength range.
Different definitions are used for distinguishing different spectral regions:
- The near-UV spectral region ranges from 400 nm down to 300 nm. The middle-UV region ranges from 300 to 200 nm, and shorter wavelengths from 200 nm down to 10 nm belong to the far-UV region. Still shorter wavelengths belong to the extreme UV (EUV).
- The term vacuum UV (below ≈ 200 nm) refers to the wavelength range where a vacuum apparatus is often used, because the light is strongly absorbed in air. The vacuum UV includes the far and extreme UV.
- UVA stands for the range from 320 to 400 nm, UVB for 280–320 nm, and UVC for 200–280 nm.
However, the precise definitions of these spectral regions vary in the literature.
UV light finds a wide range of applications, including UV disinfection of water and tools, UV curing of adhesives, quality control for many materials and exciting fluorescence for analytical purposes.
Essential Properties of Ultraviolet Light
Compared with visible light, ultraviolet light is different in essentially two different respects:
- The short wavelength allows precise focusing and the generation of very fine structures (provided that a light source with high spatial coherence is used). This is utilized in UV photolithography, as used e.g. for the fabrication of microelectronic devices such as microprocessors and memory chips. Future generations of microprocessors will have even finer structures and will require photolithography in the EUV region. Powerful EUV sources and the corresponding photoresists are currently being developed.
- The photon energy is higher than the bandgap energy of many substances. As a consequence, ultraviolet light is strongly absorbed by many substances, and the induced excitation can lead to changes in the chemical structure (e.g. breaking of bonds). This is important for laser material processing (e.g. for laser ablation, pulsed laser deposition, and for the fabrication of fiber Bragg gratings), and for sterilization of water or medical instruments. UV light can also damage the human skin (see below), and particularly UVC light has germicidal effects. When ultraviolet light interacts with trace hydrocarbons in air, it can lead to the deposition of organic films on nearby surfaces; such kind of photocontamination can e.g. degrade the quality of nonlinear crystals in UV laser sources.
Generation of Ultraviolet Light
The technology of lasers for the generation of ultraviolet light faces various challenges; nevertheless, there are a few kinds of ultraviolet lasers which can directly generate UV light: some bulk lasers (e.g. based on cerium-doped crystals such as Ce:LiCAF), fiber lasers, laser diodes (mostly GaN-based), dye lasers, excimer lasers, and free electron lasers. Another way of generating ultraviolet light is by nonlinear frequency conversion of the outputs of near-infrared lasers. The article on ultraviolet lasers gives more details.
Particularly for the EUV region, gas discharges (e.g. with xenon or with tin vapor) or laser-induced plasmas are used for generating UV radiation with high powers of multiple watts or even dozens of watts. However, such sources do not emit coherent radiation.
In many cases, UV radiation is generated with devices other than lasers. Of particular importance are gas discharge lamps (e.g. mercury tubes), but light-emitting diodes (UV LEDs) are also attracting interest for a range of applications.
For handling UV light, special UV optics are required. Important material parameters for UV applications are a low bubble and inclusion content, good homogeneity of the refractive index, a small birefringence, and the potential for polishing surfaces with very small roughness. Particularly for applications with intense UV lasers, the long-term resistance against UV light is also important. 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. As an alternative, UV-grade fused silica can be used for wavelengths down to ≈ 200 nm, whereas the cheaper standard-grade fused silica has significant attenuation below 260 nm. Another possible material choice is diamond, which is transparent down to ≈ 230 nm and very robust, but expensive.
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.
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 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.
Ultraviolet light is dangerous for the eyes (particularly for wavelengths in the range 250–300 nm) and for the skin (particularly for 280–315 nm), because it can cause cataracts or photokeratitis of the eye's lens and skin cancer, apart from hyperpigmentation and erythema. Lower doses, not yet causing acute effects, can accelerate aging of the skin. Therefore, work with UV light sources, in particular with UV lasers, demands special precautions for laser safety. For example, UV beams in open optical setups usually have to be enclosed with metal tubes.
The RP Photonics Buyer's Guide contains 15 suppliers for ultraviolet light sources. Among them:
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