EUV or XUV: Extreme Ultraviolet, the short-wavelength or high-energy end of the ultraviolet spectrum, from 120 (or 200) nanometers to about 10 nanometers (nm).
Atmospheric transmission in the ultraviolet decreases sharply with wavelength, and air absorption is so strong that wavelengths shorter than 150 to 200 nm must be studied in a vacuum. The first explorers of the EUV spectrum were astronomers using satellite instruments. The quest to squeeze more and smaller transistors onto semiconductor chips has changed that by shrinking chip features to dimensions on the scale of EUV waves. The semiconductor industry is now testing the first wave of EUV photolithography systems operating at 13.5 nm.
Such a sudden technological interest in a long-neglected part of the spectrum is a great recipe for muddied definitions of spectral bands. Astronomers consider the 121 nm Lyman alpha line of hydrogen to be the major landmark in the EUV spectrum, so they picked 120 nm as the long-wave end of the EUV band. However, the major technological landmark for the semiconductor industry is the 13.5 nm lithography wavelength, so they often define EUV as having a wavelength of 13.5 nm. The laser community uses a broader definition of 10 to 120 or 200 nm as it explores a broader range of applications, enabled by new techniques such as high-harmonic generation.
The EUV largely overlaps the older vacuum-ultraviolet (VUV) band, usually defined as 10 to 200 nm. A draft document from the International Standardization Organization (developed for space observations) also defines two other overlapping bands, the far-ultraviolet (FUV) at 122 to 200 nm and the germicidal Ultraviolet C (UVC) band at 100 to 280 nm.
XUV often is an alternative abbreviation for extreme ultraviolet, substituting the fashionable X for the relatively drab E, but the ISO lists it as an abbreviation for soft X rays at 0.1 to 10 nm. A quick Google search gives the impression XUV is the more popular form, with 5.6 million hits compared to 3.2 million for EUV and 1.8 million for VUV. But that's misleading because XUV also is shorthand for "crossover utility vehicle," a sport-utility vehicle based on a car rather than a truck. That usage may be popular on the Internet, but it was new to me--and for years I've been driving a Toyota RAV4, which is classed as an XUV.