Excimer laser
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An excimer laser is a form of ultraviolet chemical laser which is commonly used in eye surgery and semiconductor manufacturing. The term excimer is short for excited dimer, and refers to the chemical gain medium of the laser.
The first excimer laser was invented in 1971 by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the P. N. Lebedev Physical Institute in Moscow, using a xenon dimer (Xe2) excited by an electron beam to give stimulated emission at 172 nm wavelength. A later improvement was the use of noble gas halides (originally XeBr), invented (and patented) in 1975 by George Hart and Stuart Searles of the United States Government's Naval Research Laboratory.
Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (disassociative) ground state. This is because noble gases such as xenon and krypton are highly inert and do not usually form chemical compounds. However, when in an excited state (induced by an electrical discharge or high-energy electron beams, which produce high energy pulses), they can form temporarily-bound molecules with themselves (dimers) or with halides (complexes) such as fluorine and chlorine. The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) disassociates back into two unbound atoms. This forms a population inversion between the two states.
Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly speaking a misnomer (since a dimer refers to a molecule of two identical or similar parts): The correct but less commonly used name for such is exciplex laser.
The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet:
| Excimer | Wavelength |
|---|---|
| Ar2* | 126 nm |
| Kr2* | 146 nm |
| F2 | 157 nm |
| Xe2* | 172 & 175 nm |
| ArF | 193 nm |
| KrF | 248 nm |
| XeBr | 282 nm |
| XeCl | 308 nm |
| XeF | 351 nm |
| CaF2 | 193 nm |
| KrCl | 222 nm |
| Cl2 | 259 nm |
Excimer lasers are usually operated with a pulse rate of around 100 Hz and a pulse duration of ~10 ns, although some operate as high as 8 kHz and 30 ns. Their high-power ultraviolet output makes them useful for surgery (particularly eye surgery), for lithography for semiconductor manufacturing, and for dermatological treatment. They are quite large and bulky devices, which is a disadvantage in their medical applications, although their size is rapidly decreasing with ongoing development.
For electric discharge pump see: Nitrogen laser.de:Excimerlaser it:Laser a eccimeri hu:Excimerlézer ru:Эксимерный лазер