Editing Maser

{{Short description|Device for producing coherent EM waves in the sub-visible spectrum}}
{{Other uses}}
[[File:Charles Townes and first maser.jpg|300px|thumb|upright=1.7 |The first prototype [[ammonia]] maser in front of its inventor [[Charles H. Townes]]. The ammonia nozzle is at left in the box, the four brass rods at center are the [[quadrupole ion trap|quadrupole]] state selector, and the resonant cavity is at right. The 24&nbsp;GHz microwaves exit through the vertical [[waveguide]] Townes is adjusting. At bottom are the [[vacuum pump]]s.]]
[[File:Hydrogen maser.gif|thumb|right|260px|A hydrogen radio frequency discharge, the first element inside a [[#Hydrogen maser|hydrogen maser]] (see description below)]]

A '''maser''' is a device that produces [[coherence (physics)|coherent]] [[electromagnetic wave]]s ([[microwaves]]), through amplification by [[stimulated emission]]. The term is an acronym for '''microwave amplification by stimulated emission of radiation'''. [[Nikolay Basov]], [[Alexander Prokhorov]] and [[Joseph Weber]] introduced the concept of the maser in 1952, and [[Charles H. Townes]], [[James P. Gordon]], and [[Herbert J. Zeiger]] built the first maser at [[Columbia University]] in 1953. Townes, Basov and Prokhorov won the 1964 [[Nobel Prize in Physics]] for theoretical work leading to the maser. Masers are used as timekeeping devices in [[atomic clock]]s, and as extremely low-noise [[microwave]] amplifiers in [[radio telescope]]s and deep-space [[spacecraft communication]] ground-stations.

Modern masers can be designed to generate electromagnetic waves at microwave [[frequencies]] and [[Radio wave|radio]] and [[infrared]] frequencies. For this reason, Townes suggested replacing "microwave" with "molecular" as the first word in the acronym "maser".<ref name="nobel_lecture">{{cite web |last =Townes |first =Charles H. |author-link =Charles H. Townes |date =1964-12-11 |title =Production of coherent radiation by atoms and molecules - Nobel Lecture |url =https://www.nobelprize.org/uploads/2018/06/townes-lecture.pdf |url-status =live |archive-url =https://web.archive.org/web/20200827171449/https://www.nobelprize.org/uploads/2018/06/townes-lecture.pdf |archive-date =2020-08-27 |access-date =2020-08-27 |archive-format =pdf |website =The Nobel Prize |page =63 |quote =We called this general type of system the maser, an acronym for microwave amplification by stimulated emission of radiation. The idea has been successfully extended to such a variety of devices and frequencies that it is probably well to generalize the name - perhaps to mean molecular amplification by stimulated emission of radiation.}}</ref>

The ''[[laser]]'' works by the same principle as the maser, but produces higher-frequency coherent radiation at [[Visible spectrum|visible]] wavelengths. The maser was the precursor to the laser, inspiring theoretical work by Townes and [[Arthur Leonard Schawlow]] that led to the invention of the laser in 1960 by [[Theodore Maiman]]. When the coherent optical oscillator was first imagined in 1957, it was originally called the "optical maser". This was ultimately changed to {{Em|laser}}, for "light amplification by stimulated emission of radiation". [[Gordon Gould]] is credited with creating this acronym in 1957.

==History==
The theoretical principles governing the operation of a maser were first described by [[Joseph Weber]] of the [[University of Maryland, College Park]] at the Electron Tube Research Conference in June 1952 in [[Ottawa]],<ref>[https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4941 American Institute of Physics Oral History Interview with Weber]</ref> with a summary published in the June 1953 Transactions of the Institute of Radio Engineers Professional Group on Electron Devices,<ref>{{cite book |date=2004 |title=The History of the Laser |author=Mario Bertolotti |publisher=CRC Press |page=180 |isbn=978-1420033403}}</ref> and simultaneously by [[Nikolay Basov]] and [[Alexander Prokhorov]] from [[Lebedev Physical Institute|Lebedev Institute of Physics]], at an ''All-Union Conference on Radio-Spectroscopy'' held by the [[USSR Academy of Sciences]] in May 1952, published in October 1954.

Independently, [[Charles H. Townes|Charles Hard Townes]], [[James P. Gordon]], and H. J. Zeiger built the first ammonia maser at [[Columbia University]] in 1953. This device used stimulated emission in a stream of energized [[ammonia]] molecules to produce amplification of microwaves at a frequency of about 24.0 [[hertz|gigahertz]].<ref>{{cite journal |last1=Gordon |first1=J. P. |last2=Zeiger |first2=H. J. |last3=Townes |first3=C. H. |title=The Maser—New Type of Microwave Amplifier, Frequency Standard, and Spectrometer |journal=Phys. Rev. |date=1955 |volume=99 |issue=4 |page=1264 |bibcode=1955PhRv...99.1264G |doi=10.1103/PhysRev.99.1264 |doi-access=free}}</ref> Townes later worked with [[Arthur Leonard Schawlow|Arthur L. Schawlow]] to describe the principle of the ''optical maser'', or ''laser'',<ref>{{cite journal |last1=Schawlow |first1=A.L. |last2=Townes |first2=C.H. |title=Infrared and Optical Masers |journal=Physical Review |date=15 December 1958 |volume=112 |issue=6 |pages=1940–1949 |doi=10.1103/PhysRev.112.1940 |doi-access=free |bibcode=1958PhRv..112.1940S}}</ref> of which [[Theodore Harold Maiman|Theodore H. Maiman]] created the first working model in 1960.

For their research in the field of stimulated emission, Townes, Basov and Prokhorov were awarded the [[Nobel Prize in Physics]] in 1964.<ref>{{cite web |title=The Nobel Prize in Physics 1964 |url=https://www.nobelprize.org/prizes/physics/1964/summary/ |access-date=2020-08-27 |website=NobelPrize.org |language=en-US}}</ref>

==Technology==
The maser is based on the principle of stimulated emission proposed by [[Albert Einstein]] in 1917. When atoms have been induced into an excited energy state, they can amplify radiation at a frequency particular to the element or molecule used as the masing medium (similar to what occurs in the lasing medium in a laser).

By putting such an amplifying medium in a [[resonant cavity]], feedback is created that can produce [[coherence (physics)|coherent radiation]].

===Some common types===
*Atomic beam masers
**Ammonia maser
**Free electron maser
**[[Hydrogen maser]]
*Gas masers
**Rubidium maser
*Liquid-dye and chemical laser
*Solid state masers
**Ruby maser
**[[Whispering-gallery wave|Whispering-gallery modes]] iron-sapphire maser
*Dual noble gas maser (The dual [[noble gas]] of a masing medium which is [[nonpolar]].<ref>[https://web.archive.org/web/20110228200716/http://www.cfa.harvard.edu/Walsworth/Activities/DNGM/old-DNGM.html The Dual Noble Gas Maser], Harvard University, Department of Physics</ref>)

===21st-century developments===
In 2012, a research team from the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] and [[Imperial College London]] developed a [[solid-state physics|solid-state]] maser that operated at room temperature by using optically pumped, [[pentacene]]-doped [[p-Terphenyl]] as the amplifier medium.<ref>{{cite journal |last1=Brumfiel |first1=G. |doi=10.1038/nature.2012.11199 |title=Microwave laser fulfills 60 years of promise |journal=Nature |year=2012 |s2cid=124247048}}</ref><ref>{{cite news |last=Palmer |first=Jason |title='Maser' source of microwave beams comes out of the cold |url=https://www.bbc.co.uk/news/science-environment-19281566 |access-date=23 August 2012 |date=16 August 2012 |agency=BBC News |archive-url=https://web.archive.org/web/20160729110004/http://www.bbc.co.uk/news/science-environment-19281566 |archive-date=July 29, 2016}}</ref><ref>[http://www.nature.com/news/microwave-laser-fulfills-60-years-of-promise-1.11199 Microwave Laser Fulfills 60 Years of Promise]</ref> It produced pulses of maser emission lasting for a few hundred microseconds.

In 2018, a research team from [[Imperial College London]] and [[University College London]] demonstrated continuous-wave maser oscillation using [[synthetic diamond]]s containing [[nitrogen-vacancy center|nitrogen-vacancy]] defects.<ref>{{cite journal |last=Liu |first=Ren-Bao |date=March 2018 |title=A diamond age of masers |journal=Nature |language=EN |volume=555 |issue=7697 |pages=447–449 |doi=10.1038/d41586-018-03215-3 |pmid=29565370 |bibcode=2018Natur.555..447L |doi-access=free}}</ref><ref>[https://phys.org/news/2018-03-scientists-diamond-world-room-temperature-solid-state.html Scientists use diamond in world's first continuous room-temperature solid-state maser], phys.org</ref>

==Uses==
Masers serve as high precision [[frequency reference]]s. These "atomic frequency standards" are one of the many forms of [[atomic clock]]s. Masers were also used as [[electronic amplifier|low-noise microwave amplifier]]s in [[radio telescope]]s, though these have largely been replaced by amplifiers based on [[field effect transistor|FET]]s.<ref>{{cite web |title=Low Noise Amplifiers – Pushing the limits of low noise |url=https://science.nrao.edu/facilities/cdl/low-noise-amplifiers |publisher=National Radio Astronomy Observatory (NRAO)}}</ref>

During the early 1960s, the [[Jet Propulsion Laboratory]] developed a maser to provide ultra-low-noise amplification of [[S-band]] microwave signals received from deep space probes.<ref>{{cite web |url=https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCANSO_sml-110804.pdf |title=Low-Noise Systems in the Deep Space Network |publisher=JPL |editor=Macgregor S. Reid |year=2008}}</ref> This maser used deeply refrigerated helium to chill the amplifier down to a temperature of 4&nbsp;[[kelvin]]. Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz [[klystron]]. In the early years, it took days to chill and remove the impurities from the hydrogen lines. 

Refrigeration was a two-stage process, with a large Linde unit on the ground, and a crosshead compressor within the antenna. The final injection was at {{convert|3000|psi|MPa|order=flip|abbr=on}} through a {{convert|0.006|in|um|abbr=on|order=flip}} micrometer-adjustable entry to the chamber. The whole system [[noise temperature]] looking at cold sky (2.7&nbsp;[[kelvin]] in the microwave band) was 17&nbsp;kelvin. This gave such a low noise figure that the [[Mariner IV]] [[space probe]] could send still pictures from [[Mars]] back to the [[Earth]], even though the output power of its [[radio transmitter]] was only 15&nbsp;[[watt]]s, and hence the total signal power received was only −169&nbsp;[[decibel]]s with respect to a [[milliwatt]]&nbsp;(dBm).

===Hydrogen maser===
{{Main|Hydrogen maser}}
[[File:Hmaser.svg|thumb|right|200px|A hydrogen maser.]]
The hydrogen maser is used as an [[atomic clock|atomic frequency standard]]. Together with other kinds of atomic clocks, these help make up the [[International Atomic Time]] standard ("Temps Atomique International" or "TAI" in French). This is the international time scale coordinated by the [[International Bureau of Weights and Measures]]. [[Norman Ramsey]] and his colleagues first conceived of the maser as a timing standard. More recent masers are practically identical to their original design. Maser oscillations rely on the stimulated emission between two [[hyperfine structure|hyperfine energy level]]s of atomic [[hydrogen]].

Here is a brief description of how they work:

*First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to a high-frequency [[radio wave]] discharge (see the picture on this page).
*The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a [[population inversion]] of the atoms. This is done in a way that is very similar to the [[Stern–Gerlach experiment]]. After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation.
*A high [[Q factor]] (quality factor) [[microwave cavity]] confines the microwaves and reinjects them repeatedly into the atom beam. The stimulated emission amplifies the microwaves on each pass through the beam. This combination of [[amplifier|amplification]] and [[feedback]] is what defines all [[oscillator]]s. The [[resonant frequency]] of the microwave cavity is tuned to the frequency of the hyperfine [[energy level#Energy level transitions|energy transition]] of hydrogen: 1,420,405,752 [[hertz]].<ref>{{cite journal |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-h |title=Time and Frequency From A to Z: H|journal=NIST |date=12 May 2010 }}</ref>
* A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent [[radio receiver]].
* The microwave signal coming out of the maser is very weak, a few [[picowatt]]s. The frequency of the signal is fixed and ''extremely'' stable. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of [[phase-locked loop]]s and a high performance [[quartz oscillator]].

==Astrophysical masers==
{{Main|Astrophysical maser}}
Maser-like stimulated emission has also been observed in nature from [[interstellar space]], and it is frequently called "superradiant emission" to distinguish it from laboratory masers. Such emission is observed from molecules such as water (H<sub>2</sub>O), [[hydroxyl radical]]s ([[OH masers|•OH]]), [[methanol]] (CH<sub>3</sub>OH), [[formaldehyde]] (HCHO), [[silicon monoxide]] (SiO), and [[carbodiimide]] (HNCNH).<ref>{{cite journal |last1=McGuire |first1=Brett A. |last2=Loomis |first2=Ryan A. |last3=Charness |first3=Cameron M. |last4=Corby |first4=Joanna F. |last5=Blake |first5=Geoffrey A. |last6=Hollis |first6=Jan M. |last7=Lovas |first7=Frank J. |last8=Jewell |first8=Philip R. |last9=Remijan |first9=Anthony J. |title=Interstellar Carbodiimide (HNCNH): A New Astronomical Detection from the GBT Primos Survey Via Maser Emission Features |date=2012-10-20 |url=https://iopscience.iop.org/article/10.1088/2041-8205/758/2/L33 |journal=The Astrophysical Journal |volume=758 |issue=2 |pages=L33 |doi=10.1088/2041-8205/758/2/L33 |arxiv=1209.1590 |bibcode=2012ApJ...758L..33M |s2cid=26146516 |issn=2041-8205}}</ref> Water molecules in [[star]]-forming regions can undergo a [[population inversion]] and emit radiation at about 22.0&nbsp;[[GHz]], creating the brightest [[electromagnetic spectrum|spectral line]] in the radio universe. Some water masers also emit radiation from a [[Quantum rotor model|rotational transition]] at a [[frequency]] of 96&nbsp;GHz.<ref>{{cite journal |last1=Neufeld |first1=David A. |last2=Melnick |first2=Gary J. |title=Excitation of Millimeter and Submillimeter Water Masers in Warm Astrophysical Gas |journal=Atoms, Ions and Molecules: New Results in Spectral Line Astrophysics, ASP Conference Series (ASP: San Francisco) |date=1991 |volume=16 |page=163 |bibcode=1991ASPC...16..163N}}</ref><ref>{{cite journal |last1=Tennyson |first1=Jonathan |display-authors=etal |title=IUPAC critical evaluation of the rotational–vibrational spectra of water vapor, Part III: Energy levels and transition wavenumbers for H<sub>2</sub><sup>16</sup>O |journal=Journal of Quantitative Spectroscopy and Radiative Transfer |date=March 2013 |volume=117 |pages=29–58 |doi=10.1016/j.jqsrt.2012.10.002 |bibcode=2013JQSRT.117...29T |doi-access=free|hdl=10831/91303 |hdl-access=free }}</ref>

Extremely powerful masers, associated with [[active galactic nuclei]], are known as [[megamaser]]s and are up to a million times more powerful than stellar masers.

==Terminology==
The meaning of the term ''maser'' has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation", which described devices which emitted in the microwave region of the [[electromagnetic spectrum]].

The principle and concept of stimulated emission has since been extended to more devices and frequencies. Thus, the original acronym is sometimes modified, as suggested by Charles H. Townes,<ref name="nobel_lecture" /> to "''molecular'' amplification by stimulated emission of radiation." Some have asserted that Townes's efforts to extend the acronym in this way were primarily motivated by the desire to increase the importance of his invention, and his reputation in the scientific community.<ref>{{cite book |last=Taylor |first=Nick |title=LASER: The inventor, the Nobel laureate, and the thirty-year patent war |year=2000 |publisher=Simon & Schuster |location=New York |isbn=978-0-684-83515-0}}</ref> <!--''Molecular'' is used here in the sense of [[kinetic theory]], where the base element of a kinetic system is a [[molecule]], even if it happens to be [[monatomic]]. This should not be confused with the usage of the term in the molecular sciences, where it refers to a [[bound state]] comprising two or more atoms.-->

When the laser was developed, Townes and [[Arthur Schawlow|Schawlow]] and their colleagues at Bell Labs pushed the use of the term ''optical maser'', but this was largely abandoned in favor of ''laser'', coined by their rival Gordon Gould.<ref>{{cite book |last=Taylor |first=Nick |title=LASER: The inventor, the Nobel laureate, and the thirty-year patent war |year=2000 |publisher=Simon & Schuster |location=New York |isbn=978-0-684-83515-0 |pages=66–70}}</ref> In modern usage, devices that emit in the [[X-ray]] through [[infrared]] portions of the spectrum are typically called [[laser]]s, and devices that emit in the microwave region and below are commonly called ''masers'', regardless of whether they emit microwaves or other frequencies.

Gould originally proposed distinct names for devices that emit in each portion of the spectrum, including ''grasers'' ([[gamma ray]] lasers), ''xasers'' (x-ray lasers), ''uvasers'' ([[ultraviolet]] lasers), ''lasers'' ([[visible light|visible]] lasers), ''irasers'' ([[infrared]] lasers), ''masers'' (microwave masers), and ''rasers'' ([[radio frequency|RF]] masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except for ''maser'' and ''laser''.{{citation needed|date=April 2022}}

==See also==
*[[List of laser types]]
*[[X-ray laser]]
*[[Gamma-ray laser]] ([[graser]])
*[[Sonic laser]] ([[Sonic laser|saser]])
*[[Spaser]]

==References==
{{Reflist}}

==Further reading==
* J.R. Singer, ''Masers'', John Whiley and Sons Inc., 1959.
* J. Vanier, C. Audoin, ''The Quantum Physics of Atomic Frequency Standards'', Adam Hilger, Bristol, 1989.

==External links==
{{Commons category|Masers}}
* [https://feynmanlectures.caltech.edu/III_09.html The Feynman Lectures on Physics Vol. III Ch. 9: The Ammonia Maser]
* [https://arxiv.org/find/grp_physics/1/ti:+maser/0/1/0/all/0/1?per_page=100; arXiv.org search for "maser"]
* {{cite web
|url=http://cfa-www.harvard.edu/hmc/
|title=The Hydrogen Maser Clock Project
|publisher=Harvard-Smithsonian Center for Astrophysics
|archive-url=https://web.archive.org/web/20061010151459/http://www.cfa.harvard.edu/hmc/
|archive-date=2006-10-10
}}
* [http://www.aip.org/history/exhibits/laser Bright Idea: The First Lasers] {{Webarchive|url=https://web.archive.org/web/20140424130338/http://www.aip.org/history/exhibits/laser/ |date=2014-04-24}}
* [https://physics.aps.org/story/v15/st4 Invention of the Maser and Laser], [[American Physical Society]]
* [https://web.archive.org/web/20061017080654/http://www.bell-labs.com/about/history/laser/ Shawlow and Townes Invent the Laser], [[Bell Labs]]

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[[Category:Laser types]]
[[Category:Microwave technology]]
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[[Category:20th-century inventions]]