Excimer laser

An excimer laser (sometimes, and more correctly, called an exciplex 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', while exciplex is short for 'excited complex'. 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 energized state and can give rise to laser light in the ultraviolet range.[1]

The UV light from an excimer laser is well absorbed by biological matter and organic compounds. 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.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).

Excimer lasers

The first excimer laser was invented in 1970[2] by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the 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, developed by many groups in 1975[3] was the use of noble gas halides (originally XeBr). These groups include the United States Government's Naval Research Laboratory[4], the Northrop Research and Technology Center[5], the Avco Everett Research Laboratory[6], and Sandia Laboratories[7].

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	Relative Power
Ar₂*	126 nm
Kr₂*	146 nm
F₂	157 nm	10
Xe₂*	172 & 175 nm
ArF	193 nm	60
KrF	248 nm	100
XeBr	282 nm
XeCl	308 nm	50
XeF	351 nm	45
CaF₂	193 nm
KrCl	222 nm	25
Cl₂	259 nm
N₂	337 nm	5

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.

For electric discharge pump see: Nitrogen laser.

Uses

The high-power ultraviolet output of excimer lasers makes them useful for surgery (particularly eye surgery), for lithography for semiconductor manufacturing, and for dermatological treatment. Excimer laser light is typically absorbed within the first billionth of a meter (nanometer) of tissue. The website howstuffworks.com states:

"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."

This quote is a bit misleading. The beam output from an excimer is in general multimode and not of good quality when compared to other lasers. In laser drilling systems the excimer is employed similar to a conventional light source. The accuracy comes from the imaging system and the fact that UV light has a short wavelength.

In 1980 - 1983, Dr. Samuel Blum was working with Dr. Rangaswamy Srinivasan and Dr. James Wynne at IBM’s T. J. Watson Research Center when they observed the effect of the ultraviolet excimer laser on biological materials. Intrigued, they investigated further, finding that the laser made clean, precise cuts that would be ideal for delicate surgeries. This resulted in a fundamental patent[8] and Drs. Blum, Srinivasan, and Wynne were elected to the National Inventors Hall of Fame in 2002. Subsequent work introduced the excimer laser for use in angioplasty [9]. Kansas State University pioneered the study of the excimer laser which made LASIK surgery possible [1]

Excimer lasers are quite large and bulky devices, which is a disadvantage in their medical applications, although their size is rapidly decreasing with ongoing development.