Excimer laser

An excimer laser is a form of ultraviolet chemical laser which is commonly used in eye surgery and semiconductor manufacturing.

An excimer laser typically uses a combination of an inert gas (Argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation, a pseudo-molecule called a dimer is created, which can only exist in an energised state and can give rise to laser light in the ultraviolet range. This light is exceptionally well focussed and capable of very delicate control, and is well absorbed by biological matter and organic compounds. These properties make excimer lasers well suited to precision micromachining organic material (including certain polymers and plastics), or delicate surgeries such as eye surgery (LASIK).

Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact.

The term excimer is short for 'excited dimer'.

Excimer lasers

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.

Uses

Excimer laser light is typically absorbed within the first billionth of a meter (nanometer) of tissue. howstuffworks.com states that::"The Excimer laser is incredibly precise. It has the ability to focus a beam as small as 0.25 micrometres [and] capable of removing 0.5% of a human hair's width at a time."