An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micromachining.

Since the 1960s, excimer lasers have been widely used in high-resolution photolithography machines, one of the critical technologies required for microelectronic chip manufacturing.

The term excimer is short for 'excited dimer', while 'exciplex' is short for 'excited complex'. Most excimer lasers are of the noble gas halide type, for which the term excimer is, strictly speaking, a misnomer.

Excimer laser was proposed in 1960 by Fritz Houtermans. The excimer laser development started with the observation of a nascent spectral line narrowing at 176 nm  reported in 1971 by Nikolai Basov, V. A. Danilychev and Yu. M. Popov, at the Lebedev Physical Institute in Moscow, using liquid xenon dimer (Xe₂) excited by an electron beam. Spurred by this report, H.A. Koehler et al. presented a better substantiation of stimulated emission in 1972, using high pressure xenon gas. Definitive evidence of a xenon excimer laser action at 173 nm using a high pressure gas at 12 atmospheres, also pumped by an electron beam, was first presented in March 1973, by Mani Lal Bhaumik of Northrop Corporation, Los Angeles. Strong stimulated emission was observed as the laser's spectral line narrowed from a continuum of 15 nm to just 0.25 nm, and the intensity increased a thousand-fold. The laser's estimated output of 1 joule was high enough to evaporate part of the mirror coatings, which imprinted its mode pattern. This presentation established the credible potential of developing high power lasers at short wavelengths.

A later improvement was the use of noble gas halides (originally Xe Br) developed by many groups in 1975. These groups include the Avco Everett Research Laboratory, Sandia Laboratories, the Northrop Research and Technology Center, the United States Government's Naval Research Laboratory, which also developed a XeCl Laser that was excited using a microwave discharge, and Los Alamos National Laboratory.

An excimer laser typically uses a combination of a noble gas (argon, krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer (or in the case of noble gas halides, exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.

Laser action in an excimer molecule occurs because it has a bound (associative) excited state, but a repulsive (dissociative) ground state. 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 electrical discharge or high-energy electron beams), they can form temporarily bound molecules with themselves (excimer) or with halogens (exciplex) such as fluorine and chlorine. The excited compound can release 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) dissociates back into two unbound atoms. This forms a population inversion.^{[citation needed]}

The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet range of electromagnetic radiation: