Editing Laser (section)

== Types and operating principles ==
{{Further|List of laser types}}

[[File:Commercial laser lines.svg|thumb|upright=1.5|Wavelengths of commercially available lasers. Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The color codifies the type of laser material (see the figure description for more details).]]

=== Gas lasers ===
{{Main|Gas laser}}

Following the invention of the HeNe gas laser, many other gas discharges have been found to amplify light coherently.
Gas lasers using many different gases have been built and used for many purposes. The [[helium–neon laser]] (HeNe) can operate at many different wavelengths, however, the vast majority are engineered to lase at 633&nbsp;nm; these relatively low-cost but highly coherent lasers are extremely common in optical research and educational laboratories. Commercial [[Carbon dioxide laser|carbon dioxide (CO<sub>2</sub>) lasers]] can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6&nbsp;μm; such lasers are regularly used in industry for cutting and welding. The efficiency of a CO<sub>2</sub> laser is unusually high: over 30%.<ref>{{cite web |url=http://www.phy.davidson.edu/stuhome/jimn/co2/pages/CO2Main.htm |last=Nolen |first=Jim |title=The Carbon Dioxide Laser |publisher=Davidson Physics |access-date=August 17, 2014 |author2=Derek Verno|archive-date=October 11, 2014 |archive-url=https://web.archive.org/web/20141011053320/http://www.phy.davidson.edu/stuhome/jimn/co2/pages/CO2Main.htm |url-status=live}}</ref> [[Ion laser|Argon-ion]] lasers can operate at several lasing transitions between 351 and 528.7&nbsp;nm. Depending on the optical design one or more of these transitions can be lasing simultaneously; the most commonly used lines are 458&nbsp;nm, 488&nbsp;nm and 514.5&nbsp;nm. A nitrogen [[TEA laser|transverse electrical discharge in gas at atmospheric pressure]] (TEA) laser is an inexpensive gas laser, often home-built by hobbyists, which produces rather incoherent UV light at 337.1&nbsp;nm.<ref>{{cite web
  |last = Csele
  |first = Mark
  |title = The TEA Nitrogen Gas Laser
  |work = Homebuilt Lasers Page
  |year=2004
  |url = http://www.technology.niagarac.on.ca/people/mcsele/lasers/LasersTEA.htm
  |access-date =September 15, 2007 |archive-url = https://web.archive.org/web/20070911190723/http://www.technology.niagarac.on.ca/people/mcsele/lasers/LasersTEA.htm <!-- Bot retrieved archive --> |archive-date = September 11, 2007}}</ref> Metal ion lasers are gas lasers that generate [[deep ultraviolet]] wavelengths. [[Helium]]-silver (HeAg) 224&nbsp;nm and [[neon]]-copper (NeCu) 248&nbsp;nm are two examples. Like all low-pressure gas lasers, the gain media of these lasers have quite narrow oscillation [[linewidth]]s, less than 3 [[GHz]] (0.5 [[picometers]]),<ref>{{cite web
  |title = Deep UV Lasers
  |publisher = Photon Systems, Covina, Calif
  |url = http://www.photonsystems.com/pdfs/duv-lasersource.pdf
  |archive-url = https://web.archive.org/web/20070701004933/http://www.photonsystems.com/pdfs/duv-lasersource.pdf
  |archive-date = July 1, 2007
  |access-date =May 27, 2007 }}</ref> making them candidates for use in [[fluorescence]] suppressed [[Raman spectroscopy]].

[[Lasing without inversion|Lasing without maintaining the medium excited into a population inversion]] was demonstrated in 1992 in [[sodium]] gas and again in 1995 in [[rubidium]] gas by various international teams.<ref>{{cite journal |title=Lasing without inversion |year=2000 |last1=Mompart |first1=J. |last2=Corbalán |first2=R. |journal=J. Opt. B |volume=2 |issue=3 |doi=10.1088/1464-4266/2/3/201 |bibcode=2000JOptB...2R...7M |pages=R7–R24 |s2cid=121209763 }}</ref><ref>{{cite book |last=Javan |first=A. |year=2000 |chapter=On knowing Marlan |title=Ode to a quantum physicist: A festschrift in honor of Marlan O. Scully |publisher=Elsevier}}</ref>{{Page missing|date=January 2024}} This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths so that the likelihood for the ground electrons to absorb any energy has been canceled.

==== Chemical lasers ====

[[Chemical laser]]s are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high-power lasers are especially of interest to the military; however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications. As examples, in the [[hydrogen fluoride laser]] (2700–2900&nbsp;nm) and the [[deuterium fluoride laser]] (3800&nbsp;nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of [[ethylene]] in [[nitrogen trifluoride]].

The first chemical laser was demonstrated in 1965 by Jerome V. V. Kasper and [[George C. Pimentel]] at the University of California, Berkeley. It was a [[hydrogen chloride]] laser operating at 3.7 micrometers.<ref>{{cite journal | last=Gupta | first=Devaryan | title=Laser Technology Applications: A gift to Humanity | journal=International Journal of Applied Research | publisher=AkiNik | volume=1 | issue=7 | date=2016-09-29 | issn=2394-5869 | pages=476–486 | url=https://www.allresearchjournal.com/archives/?year=2015&vol=1&issue=7&part=H&ArticleId=2626 | access-date=2025-03-13}}</ref>

==== Excimer lasers ====

[[Excimer laser]]s are a special sort of gas laser powered by an electric discharge in which the lasing medium is an [[excimer]], or more precisely an [[exciplex]] in existing designs. These are molecules that can only exist with one atom in an [[excited state|excited electronic state]]. Once the molecule transfers its excitation energy to a photon, its atoms are no longer bound to each other, and the molecule disintegrates. This drastically reduces the population of the lower energy state thus greatly facilitating a population inversion. Excimers currently used are all [[:Category:Noble gas compounds|noble gas compounds]]; noble gasses are chemically inert and can only form compounds while in an excited state. Excimer lasers typically operate at [[ultraviolet]] wavelengths, with major applications including semiconductor [[photolithography]] and [[LASIK]] eye surgery. Commonly used excimer molecules include ArF (emission at 193&nbsp;nm), KrCl (222&nbsp;nm), KrF (248&nbsp;nm), XeCl (308&nbsp;nm), and XeF (351&nbsp;nm).<ref>{{cite book
 |first=D. |last=Schuocker |year=1998
 |title=Handbook of the Eurolaser Academy
 |publisher=Springer |isbn=978-0-412-81910-0}}</ref>{{Page missing|date=January 2024}}
The molecular [[fluorine]] laser, emitting at 157&nbsp;nm in the vacuum ultraviolet, is sometimes referred to as an excimer laser; however, this appears to be a misnomer since F<sub>2</sub> is a stable compound.

=== Solid-state lasers ===

[[File:Starfield Optical Range - sodium laser.jpg|thumb|A 50 W [[frequency addition source of optical radiation|FASOR]], based on a Nd:YAG laser, used at the [[Starfire Optical Range]]]]

[[Solid-state laser]]s use a crystalline or glass rod that is "doped" with ions that provide the required energy states. For example, the first working laser was a [[ruby laser]], made from [[ruby]] ([[chromium]]-doped [[corundum]]). The [[population inversion]] is maintained in the dopant. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flash tube or another laser. The usage of the term "solid-state" in laser physics is narrower than in typical use. Semiconductor lasers (laser diodes) are typically ''not'' referred to as solid-state lasers.

[[Neodymium]] is a common dopant in various solid-state laser crystals, including [[yttrium orthovanadate]] ([[Neodymium-doped yttrium orthovanadate|Nd:YVO<sub>4</sub>]]), [[yttrium lithium fluoride]] ([[Nd:YLF]]) and [[yttrium aluminium garnet]] ([[Nd:YAG]]). All these lasers can produce high powers in the [[infrared]] spectrum at 1064&nbsp;nm. They are used for cutting, welding, and marking of metals and other materials, and also in [[spectroscopy]] and for pumping [[dye laser]]s. These lasers are also commonly [[second-harmonic generation|doubled]], [[third-harmonic generation|tripled]] or quadrupled in frequency to produce 532&nbsp;nm (green, visible), 355&nbsp;nm and 266&nbsp;nm ([[ultraviolet|UV]]) beams, respectively. Frequency-doubled [[diode-pumped solid-state]] (DPSS) lasers are used to make bright green laser pointers.

[[Ytterbium]], [[holmium]], [[thulium]], and [[erbium]] are other common "dopants" in solid-state lasers.<ref>{{Cite book |url=https://books.google.com/books?id=tygEGtu1b7MC&q=%C2%A0Ytterbium,+holmium,+thulium,+and+erbium+are+other+common+%22dopants%22+in+solid-state+lasers |title=Handbook of Optics, Third Edition Volume V: Atmospheric Optics, Modulators, Fiber Optics, X-Ray and Neutron Optics |last1=Bass |first1=Michael |last2=DeCusatis |first2=Casimer |last3=Enoch |first3=Jay |last4=Lakshminarayanan |first4=Vasudevan |last5=Li |first5=Guifang |last6=MacDonald |first6=Carolyn |last7=Mahajan |first7=Virendra |last8=Stryland |first8=Eric Van |date=2009-11-13 |publisher=McGraw Hill Professional |isbn=978-0-07-163314-7 |language=en |access-date=July 16, 2017 |archive-date=February 8, 2023 |archive-url=https://web.archive.org/web/20230208064634/https://books.google.com/books?id=tygEGtu1b7MC&q=%C2%A0Ytterbium,+holmium,+thulium,+and+erbium+are+other+common+%22dopants%22+in+solid-state+lasers |url-status=live}}</ref>{{Page missing|date=January 2024}} Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF<sub>2</sub>, typically operating around 1020–1050&nbsp;nm. They are potentially very efficient and high-powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. [[Holmium]]-doped YAG crystals emit at 2097&nbsp;nm and form an efficient laser operating at [[infrared]] wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.

[[Titanium]]-doped [[sapphire]] ([[Ti-sapphire laser|Ti:sapphire]]) produces a highly [[tunable laser|tunable]] [[infrared]] laser, commonly used for [[spectroscopy]]. It is also notable for use as a mode-locked laser producing [[ultrashort pulse]]s of extremely high peak power.

Thermal limitations in solid-state lasers arise from unconverted pump power that heats the medium. This heat, when coupled with a high thermo-optic coefficient (d''n''/d''T'') can cause thermal lensing and reduce the quantum efficiency. Diode-pumped thin [[disk laser]]s overcome these issues by having a gain medium that is much thinner than the diameter of the pump beam. This allows for a more uniform temperature in the material. Thin disk lasers have been shown to produce beams of up to one kilowatt.<ref>C. Stewen, M. Larionov, and A. Giesen, "Yb:YAG thin disk laser with 1 kW output power", in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, H. Injeyan, U. Keller, and C. Marshall, ed. (Optical Society of America, Washington, D.C., 2000) pp. 35–41.</ref>

=== Fiber lasers ===
{{Main|Fiber laser}}

Solid-state lasers or laser amplifiers where the light is guided due to the [[total internal reflection]] in a single mode [[optical fiber]] are instead called [[fiber laser]]s. Guiding of light allows extremely long gain regions, providing good cooling conditions; fibers have a high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. [[Erbium]] and [[ytterbium]] ions are common active species in such lasers.

Quite often, the fiber laser is designed as a [[double-clad fiber]]. This type of fiber consists of a fiber core, an inner cladding, and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a single-mode fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region while still having a high numerical aperture (NA) to have easy launching conditions.

Pump light can be used more efficiently by creating a [[fiber disk laser]], or a stack of such lasers.

Fiber lasers, like other optical media, can suffer from the effects of [[photodarkening]] when they are exposed to radiation of certain wavelengths. In particular, this can lead to degradation of the material and loss in laser functionality over time. The exact causes and effects of this phenomenon vary from material to material, although it often involves the formation of [[Color center (crystallography)|color centers]].<ref>{{Cite web |last=Paschotta |first=Rüdiger |title=Photodarkening |url=https://www.rp-photonics.com/photodarkening.html |url-status=live |archive-url=https://web.archive.org/web/20230625023025/http://www.rp-photonics.com/photodarkening.html |archive-date=June 25, 2023 |access-date=2023-07-22 |website=www.rp-photonics.com |date=February 2, 2007 |language=en}}</ref>

=== Photonic crystal lasers ===

[[Photonic crystal]] lasers are lasers based on nano-structures that provide the mode confinement and the [[Density of states|density of optical states]] (DOS) structure required for the feedback to take place.{{Clarify|date=February 2009}} They are typical micrometer-sized{{dubious|date=November 2010}} and tunable on the bands of the photonic crystals.<ref>{{cite journal |title=Ultraviolet photonic crystal laser |first=X. |last=Wu |volume=85 |issue=17 |date=October 25, 2004|journal=Applied Physics Letters |doi=10.1063/1.1808888|arxiv = physics/0406005 |bibcode = 2004ApPhL..85.3657W |page=3657 |s2cid=119460787 |display-authors=etal}}</ref>{{Clarify|date=February 2009}}

=== Semiconductor lasers ===
{{Main|Semiconductor lasers}}

[[File:Diode laser.jpg|thumb|A 5.6 mm 'closed can' commercial laser diode, such as those used in a [[CD player|CD]] or [[DVD player]]]]

Semiconductor lasers are [[diode]]s that are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal forms an optical resonator, although the resonator can be external to the semiconductor in some designs.

Commercial [[laser diode]]s emit at wavelengths from 375&nbsp;nm to 3500&nbsp;nm.<ref>{{cite web |url=http://www.hanel-photonics.com/laser_diode_market.html |title=Laser Diode Market |publisher=Hanel Photonics |access-date=Sep 26, 2014 |archive-date=December 7, 2015 |archive-url=https://web.archive.org/web/20151207211944/http://hanel-photonics.com/laser_diode_market.html |url-status=live }}</ref> Low to medium power laser diodes are used in [[laser pointer]]s, [[laser printer]]s and CD/DVD players. Laser diodes are also frequently used to optically [[laser pumping|pump]] other lasers with high efficiency. The highest-power industrial laser diodes, with power of up to 20&nbsp;kW, are used in industry for cutting and welding.<ref>{{Cite web |url=https://www.industrial-lasers.com/articles/print/volume-29/issue-3/features/high-power-direct-diode-lasers-for-cutting-and-welding.html |title=High-power direct-diode lasers for cutting and welding |website=industrial-lasers.com |access-date=August 11, 2018 |archive-date=August 11, 2018|archive-url=https://web.archive.org/web/20180811195507/https://www.industrial-lasers.com/articles/print/volume-29/issue-3/features/high-power-direct-diode-lasers-for-cutting-and-welding.html |url-status=live}}</ref> External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-[[linewidth]] radiation, or ultrashort laser pulses.

In 2012, [[Nichia]] and [[OSRAM]] developed and manufactured commercial high-power green laser diodes (515/520&nbsp;nm), which compete with traditional diode-pumped solid-state lasers.<ref>{{cite web |url=http://www.nichia.co.jp/en/product/laser.html |title=LASER Diode |work=nichia.co.jp|access-date=March 18, 2014 |archive-date=March 18, 2014 |archive-url=https://web.archive.org/web/20140318093016/http://www.nichia.co.jp/en/product/laser.html |url-status=live}}</ref><ref>{{cite web |url=http://www.osram-os.com/osram_os/en/products/product-catalog/laser-diodes/visible-laser/green-laser/index.jsp |title=Green Laser |date=August 19, 2015 |work=osram-os.com |access-date=March 18, 2014 |archive-date=March 18, 2014 |archive-url=https://web.archive.org/web/20140318104254/http://www.osram-os.com/osram_os/en/products/product-catalog/laser-diodes/visible-laser/green-laser/index.jsp |url-status=live}}</ref>

Vertical cavity surface-emitting lasers ([[VCSEL]]s) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes. As of 2005, only 850&nbsp;nm VCSELs are widely available, with 1300&nbsp;nm VCSELs beginning to be commercialized<ref>{{cite web |url=http://lfw.pennnet.com/Articles/Article_Display.cfm?ARTICLE_ID=243400&p=12 |title=Picolight ships first 4-Gbit/s 1310-nm VCSEL transceivers |work=Laser Focus World Online |date=December 9, 2005 |access-date=May 27, 2006 |archive-url=https://web.archive.org/web/20060313161940/http://lfw.pennnet.com/Articles/Article_Display.cfm?ARTICLE_ID=243400&p=12 |archive-date=March 13, 2006}}</ref> and 1550&nbsp;nm devices being an area of research. [[VECSEL]]s are external-cavity VCSELs. [[Quantum cascade laser]]s are semiconductor lasers that have an active transition between energy ''sub-bands'' of an electron in a structure containing several [[quantum well]]s.

The development of a [[silicon]] laser is important in the field of [[optical computing]]. Silicon is the material of choice for [[integrated circuits]], and so electronic and [[silicon photonic]] components (such as [[optical interconnect]]s) could be fabricated on the same chip. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However, recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as [[indium(III) phosphide]] or [[gallium(III) arsenide]], materials that allow coherent light to be produced from silicon. These are called [[hybrid silicon laser]]s. Recent developments have also shown the use of monolithically integrated [[nanowire lasers]] directly on silicon for optical interconnects, paving the way for chip-level applications.<ref name="nwls">{{cite journal|title=Monolithically Integrated High-β Nanowire Lasers on Silicon |first1=B. |last1=Mayer |first2=L. |last2=Janker |first3=B. |last3=Loitsch |first4=J. |last4=Treu |first5=T. |last5=Kostenbader |first6=S. |last6=Lichtmannecker |first7=T. |last7=Reichert |first8=S. |last8=Morkötter |first9=M. |last9=Kaniber |first10=G. |last10=Abstreiter |first11=C. |last11=Gies |first12=G. |last12=Koblmüller |first13=J.J. |last13=Finley |date=January 13, 2016 |journal=Nano Letters |volume=16 |issue=1 |pages=152–156 |doi=10.1021/acs.nanolett.5b03404 |pmid=26618638 |bibcode=2016NanoL..16..152M}}</ref> These heterostructure nanowire lasers capable of optical interconnects in silicon are also capable of emitting pairs of phase-locked picosecond pulses with a repetition frequency up to 200&nbsp;GHz, allowing for on-chip optical signal processing.<ref name="nwpl" /> Another type is a [[Raman laser]], which takes advantage of [[Raman scattering]] to produce a laser from materials such as silicon.

Semiconductor [[quantum dot laser]]s use [[quantum dot]]s as the active laser medium. These lasers exhibit device performance that is closer to gas lasers and avoid some of the disadvantages of traditional semiconductor laser media. Improvements in [[modulation bandwidth]], [[lasing threshold]], [[relative intensity noise]], linewidth enhancement factor and temperature insensitivity have all been observed. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot size and composition. This allows quantum dot lasers to be fabricated to operate at wavelengths previously not possible using semiconductor laser technology.<ref>{{Cite web|url=https://www.fujitsu.com/global/about/resources/news/press-releases/2004/0910-01.html|title=Fujitsu, University of Tokyo Develop World's First 10Gbps Quantum Dot Laser Featuring Breakthrough Temperature-Independent Output - Fujitsu Global}}</ref>

=== Dye lasers ===
[[File:Coherent 899 dye laser.jpg|thumb|Close-up of a table-top dye laser based on [[Rhodamine 6G]]]]

[[Dye laser]]s use an organic dye as the gain medium. The wide gain spectrum of available dyes, or mixtures of dyes, allows these lasers to be highly tunable, or to produce very short-duration pulses ([[on the order of]] a few [[femtosecond]]s). Although these [[tunable laser]]s are mainly known in their liquid form, researchers have also demonstrated narrow-linewidth tunable emission in dispersive oscillator configurations incorporating solid-state dye gain media. In their most prevalent form, these [[solid-state dye lasers]] use dye-doped polymers as laser media.

[[Bubble laser]]s are dye lasers that use a [[Bubble (physics)|bubble]] as the optical resonator. [[Whispering gallery mode]]s in the bubble produce an output spectrum composed of hundreds of evenly spaced peaks: a [[frequency comb]]. The spacing of the whispering gallery modes is directly related to the bubble circumference, allowing bubble lasers to be used as highly sensitive pressure sensors.<ref name="miller">{{cite journal |last1=Miller |first1=Johanna |title=Bubble lasers can be sturdy and sensitive |journal=Physics Today |date=2024 |volume=77 |issue=3 |pages=12–14 |publisher=American Institute of Physics |doi=10.1063/pt.xafv.lnix |doi-access=free |bibcode=2024PhT....77c..12M }}</ref>

=== Free-electron lasers ===
[[File:FELIX.jpg|thumb|The free-electron laser ''FELIX'' at the FOM Institute for Plasma Physics Rijnhuizen, [[Nieuwegein]]]]

[[Free-electron laser]]s (FEL) generate coherent, high-power radiation that is widely tunable, currently ranging in wavelength from microwaves through [[terahertz radiation]] and infrared to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term ''free-electron''.

=== Exotic media ===

The pursuit of a high-quantum-energy laser using transitions between [[Nuclear isomer|isomeric states]] of an [[atomic nucleus]] has been the subject of wide-ranging academic research since the early 1970s.  Much of this is summarized in three review articles.<ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C. |last3=Gol'danskii |first3=V. I.|year=1981 |title=Approaches to the development of gamma-ray lasers |journal=Reviews of Modern Physics |volume=53 |issue=4 |pages=687–744 |bibcode = 1981RvMP...53..687B |doi = 10.1103/RevModPhys.53.687}}</ref><ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C. |year=1995 |title=Recent proposals for gamma-ray lasers |journal=Laser Physics |volume=5 |issue=2 |pages=231–239}}</ref><ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C. |year=1997 |title=Recoilless gamma-ray lasers |journal=Reviews of Modern Physics |volume=69 |issue=4 |pages=1085–1117 |bibcode=1997RvMP...69.1085B|doi=10.1103/RevModPhys.69.1085 |url=https://zenodo.org/record/1233965 |access-date=June 13, 2019 |archive-date=July 28, 2019 |archive-url=https://web.archive.org/web/20190728114208/https://zenodo.org/record/1233965 |url-status=live}}</ref> This research has been international in scope but mainly based in the former Soviet Union and the United States.  While many scientists remain optimistic that a breakthrough is near, an operational [[gamma-ray laser]] is yet to be realized.<ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C. |year=1982 |title=Is the time ripe? Or must we wait so long for breakthroughs?|journal=Laser Focus |volume=18 |issue=6 |pages=6&8}}</ref>

{{Anchor|multiphoton2016-01-30}}Some of the early studies were directed toward short pulses of neutrons exciting the upper isomer state in a solid so the gamma-ray transition could benefit from the line-narrowing of [[Mössbauer effect]].<ref>{{cite journal |last=Solem |first=J.C. |year=1979 |title=On the feasibility of an impulsively driven gamma-ray laser |journal=Los Alamos Scientific Laboratory Report LA-7898 |doi=10.2172/6010532 |osti=6010532}}</ref>{{Page missing|date=January 2024}}<ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C.|year=1979 |title=Maximum density and capture rates of neutrons moderated from a pulsed source |journal=Nuclear Science & Engineering |volume=72 |issue=3 |pages=281–289 |url=http://www.ans.org/pubs/journals/nse/a_20384|doi=10.13182/NSE79-A20384 |bibcode=1979NSE....72..281B |access-date=January 13, 2016 |archive-date=February 7, 2016 |archive-url=https://web.archive.org/web/20160207090745/http://www.ans.org/pubs/journals/nse/a_20384 |url-status=live}}</ref> In conjunction, several advantages were expected from two-stage pumping of a three-level system.<ref>{{cite journal |last1=Baldwin |first1=G.C. |last2=Solem |first2=J.C. |year=1980|title=Two-stage pumping of three-level Mössbauer gamma-ray lasers |journal=Journal of Applied Physics |volume=51 |issue=5 |pages=2372–2380 |bibcode = 1980JAP....51.2372B |doi = 10.1063/1.328007}}</ref> It was conjectured that the nucleus of an atom embedded in the near field of a laser-driven coherently-oscillating electron cloud would experience a larger dipole field than that of the driving laser.<ref>{{cite conference |last=Solem |first=J.C. |title=AIP Conference Proceedings |year=1986 |chapter=Interlevel transfer mechanisms and their application to grasers |conference=Proceedings of Advances in Laser Science-I, First International Laser Science Conference, Dallas, TX 1985 (American Institute of Physics, Optical Science and Engineering, Series 6) |volume=146 |pages=22–25 |doi=10.1063/1.35861 |bibcode=1986AIPC..146...22S |chapter-url=https://digital.library.unt.edu/ark:/67531/metadc1105924/ |access-date=November 27, 2018 |archive-date=November 27, 2018 |archive-url=https://web.archive.org/web/20181127110611/https://digital.library.unt.edu/ark:/67531/metadc1105924/ |url-status=live}}</ref><ref>{{cite conference |last1=Biedenharn |first1=L.C. |last2=Boyer |first2=K. |last3=Solem |first3=J.C. |title=AIP Conference Proceedings |year=1986 |chapter=Possibility of grasing by laser-driven nuclear excitation |conference=Proceedings of AIP Advances in Laser Science-I, Dallas, TX, November 18–22, 1985 |volume=146 |pages=50–51|doi=10.1063/1.35928|bibcode=1986AIPC..146...50B}}</ref> Furthermore, the nonlinearity of the oscillating cloud would produce both spatial and temporal harmonics, so nuclear transitions of higher multipolarity could also be driven at multiples of the laser frequency.<ref>{{cite conference |last1=Rinker |first1=G.A. |last2=Solem |first2=J.C. |last3=Biedenharn |first3=L.C. |editor1-first=Randy C |editor1-last=Jones |title=Calculation of harmonic radiation and nuclear coupling arising from atoms in strong laser fields |book-title=Proc. SPIE 0875, Short and Ultrashort Wavelength Lasers |conference=1988 Los Angeles Symposium: O-E/LASE '88, 1988, Los Angeles, CA, United States |date=April 27, 1988 |publisher=International Society for Optics and Photonics |volume=146 |pages=92–101 |doi=10.1117/12.943887 |series=Short and Ultrashort Wavelength Lasers}}</ref><ref>{{cite journal |last1=Rinker |first1=G. A. |last2=Solem |first2=J.C. |last3=Biedenharn |first3=L.C. |year=1987 |title=Nuclear interlevel transfer driven by collective outer shell electron excitations |journal=Proceedings of the Second International Laser Science Conference, Seattle, WA (Advances in Laser Science-II) |editor=Lapp, M. |editor2=Stwalley, W.C. |editor3=Kenney-Wallace G.A. |publisher=American Institute of Physics |location=New York |volume=160 |pages=75–86 |oclc=16971600}}</ref><ref>{{cite journal |last=Solem |first=J.C. |year=1988 |title=Theorem relating spatial and temporal harmonics for nuclear interlevel transfer driven by collective electronic oscillation |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |volume=40 |issue=6 |pages=713–715 |url=https://zenodo.org/record/1253954 |bibcode=1988JQSRT..40..713S |doi=10.1016/0022-4073(88)90067-2 |access-date=September 8, 2019 |archive-date=March 18, 2020 |archive-url=https://web.archive.org/web/20200318062519/https://zenodo.org/record/1253954 |url-status=live}}</ref><ref>{{cite journal |last1=Solem |first1=J.C. |last2=Biedenharn |first2=L.C. |year=1987 |title=Primer on coupling collective electronic oscillations to nuclei |journal=Los Alamos National Laboratory Report LA-10878 |url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/19/009/19009581.pdf |bibcode=1987pcce.rept.....S |page=1 |access-date=January 13, 2016 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304060942/http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/19/009/19009581.pdf |url-status=live}}</ref><ref>{{cite journal |last1=Solem |first1=J.C. |last2=Biedenharn |first2=L.C.|year=1988|title=Laser coupling to nuclei via collective electronic oscillations: A simple heuristic model study |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |volume=40 |issue=6 |pages=707–712 |bibcode = 1988JQSRT..40..707S |doi = 10.1016/0022-4073(88)90066-0 }}</ref><ref>{{cite conference |last1=Boyer |first1=K. |last2=Java |first2=H. |last3=Luk |first3=T.S.|last4=McIntyre |first4=I.A.|last5=McPherson |first5=A.|last6=Rosman |first6=R.|last7=Solem |first7=J.C.|last8=Rhodes |first8=C.K. |last9=Szöke |first9=A. |year=1987 |title=Discussion of the role of many-electron motions in multiphoton ionization and excitation |book-title=Proceedings of International Conference on Multiphoton Processes (ICOMP) IV, July 13–17, 1987, Boulder, CA |editor=Smith, S. |editor2=Knight, P. |publisher=Cambridge University Press |location=Cambridge, England |pages=58 |osti=10147730}}</ref><ref>{{cite journal |last1=Biedenharn |first1=L.C. |last2=Rinker |first2=G.A. |last3=Solem |first3=J.C. |year=1989 |title=A solvable approximate model for the response of atoms subjected to strong oscillatory electric fields |journal=Journal of the Optical Society of America B|volume=6 |issue=2 |pages=221–227 |bibcode=1989JOSAB...6..221B|doi=10.1364/JOSAB.6.000221 |url=https://zenodo.org/record/1235650 |access-date=June 13, 2019 |archive-date=March 21, 2020 |archive-url=https://web.archive.org/web/20200321181221/https://zenodo.org/record/1235650 |url-status=live}}</ref>

In September 2007, the [[BBC News]] reported that there was speculation about the possibility of using [[positronium]] [[annihilation]] to drive a very powerful [[gamma ray]] laser.<ref name="Fildes">{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/6991030.stm |title=Mirror particles form new matter |first=Jonathan |last=Fildes |date=September 12, 2007 |work=BBC News |access-date=May 22, 2008 |archive-date=April 21, 2009 |archive-url=https://web.archive.org/web/20090421143709/http://news.bbc.co.uk/2/hi/science/nature/6991030.stm |url-status=live}}</ref> David Cassidy of the [[University of California, Riverside]] proposed that a single such laser could be used to ignite a [[nuclear fusion]] reaction, replacing the banks of hundreds of lasers currently employed in [[inertial confinement fusion]] experiments.<ref name="Fildes" />

Space-based [[X-ray laser]]s pumped by nuclear explosions have also been proposed as antimissile weapons.<ref>{{cite journal |first=Jeff |last=Hecht |title=The history of the x-ray laser |journal=Optics and Photonics News |volume=19 |issue=5 |date=May 2008 |pages=26–33 |doi=10.1364/opn.19.5.000026|bibcode = 2008OptPN..19R..26H}}</ref><ref>{{cite magazine |first=Clarence A. |last=Robinson |title=Advance made on high-energy laser |magazine=Aviation Week & Space Technology |date=February 23, 1981 |pages=25–27}}</ref> Such devices would be one-shot weapons.

Living cells have been used to produce laser light.<ref>{{cite news |url=https://www.bbc.co.uk/news/science-environment-13725719 |title=Laser is produced by a living cell |first=Jason |last=Palmer |date=June 13, 2011 |newspaper=BBC News |access-date=June 13, 2011 |archive-date=June 13, 2011 |archive-url=https://web.archive.org/web/20110613112054/http://www.bbc.co.uk/news/science-environment-13725719 |url-status=live}}</ref><ref>{{cite journal |title=Single-cell biological lasers |author1=Malte C. Gather |author2=Seok Hyun Yun |name-list-style=amp |date=June 12, 2011 |journal=Nature Photonics |doi=10.1038/nphoton.2011.99 |volume=5 |issue=7 |pages=406–410|bibcode=2011NaPho...5..406G}}</ref> The cells were genetically engineered to produce [[green fluorescent protein]], which served as the laser's gain medium. The cells were then placed between two 20-micrometer-wide mirrors, which acted as the laser cavity. When the cell was illuminated with blue light, it emitted intensely directed green laser light.

=== Natural lasers ===

Like [[astrophysical maser]]s, irradiated planetary or stellar gases may amplify light producing a natural laser.<ref>{{cite news |last1=Chen |first1=Sophia |title=Alien Light |url=https://spie.org/news/photonics-focus/janfeb-2020/astrophysical-lasers?SSO=1 |access-date=9 February 2021 |work=[[SPIE]] |date=1 January 2020 |archive-date=April 14, 2021 |archive-url=https://web.archive.org/web/20210414143405/https://spie.org/news/photonics-focus/janfeb-2020/astrophysical-lasers?SSO=1 |url-status=live}}</ref>  [[Mars]],<ref>{{cite journal |last1=Mumma |first1=Michael J |title=Discovery of Natural Gain Amplification in the 10-Micrometer Carbon Dioxide Laser Bands on Mars: A Natural Laser |journal=[[Science (journal)|Science]] |date=3 April 1981 |volume=212 |issue=4490 |pages=45–49 |doi=10.1126/science.212.4490.45 |pmid=17747630 |bibcode=1981Sci...212...45M |url=https://www.science.org/doi/10.1126/science.212.4490.45 |access-date=February 9, 2021 |archive-date=February 17, 2022 |archive-url=https://web.archive.org/web/20220217063028/https://www.science.org/doi/10.1126/science.212.4490.45 |url-status=live}}</ref> [[Venus]], and [[MWC 349]] exhibit this phenomenon.