Amaser is a device that producescoherentelectromagnetic waves (microwaves), through amplification bystimulated emission. The term is an acronym formicrowave amplification by stimulated emission of radiation.Nikolay Basov,Alexander Prokhorov andJoseph Weber introduced the concept of the maser in 1952, andCharles H. Townes,James P. Gordon, andHerbert J. Zeiger built the first maser atColumbia University in 1953. Townes, Basov and Prokhorov won the 1964Nobel Prize in Physics for theoretical work leading to the maser. Masers are used as timekeeping devices inatomic clocks, and as extremely low-noisemicrowave amplifiers inradio telescopes and deep-spacespacecraft communication ground-stations.
Modern masers can be designed to generate electromagnetic waves at microwavefrequencies andradio andinfrared frequencies. For this reason, Townes suggested replacing "microwave" with "molecular" as the first word in the acronym "maser".[1]
Thelaser works by the same principle as the maser, but produces higher-frequency coherent radiation atvisible wavelengths. The maser was the precursor to the laser, inspiring theoretical work by Townes andArthur Leonard Schawlow that led to the invention of the laser in 1960 byTheodore Maiman. When the coherent optical oscillator was first imagined in 1957, it was originally called the "optical maser". This was ultimately changed tolaser, for "light amplification by stimulated emission of radiation".Gordon Gould is credited with creating this acronym in 1957.
The theoretical principles governing the operation of a maser were first described byJoseph Weber of theUniversity of Maryland, College Park at the Electron Tube Research Conference in June 1952 inOttawa,[2] with a summary published in the June 1953 Transactions of the Institute of Radio Engineers Professional Group on Electron Devices,[3] and simultaneously byNikolay Basov andAlexander Prokhorov fromLebedev Institute of Physics, at anAll-Union Conference on Radio-Spectroscopy held by theUSSR Academy of Sciences in May 1952, published in October 1954.
Independently,Charles Hard Townes,James P. Gordon, and H. J. Zeiger built the first ammonia maser atColumbia University in 1953. This device used stimulated emission in a stream of energizedammonia molecules to produce amplification of microwaves at a frequency of about 24.0gigahertz.[4] Townes later worked withArthur L. Schawlow to describe the principle of theoptical maser, orlaser,[5] of whichTheodore H. Maiman created the first working model in 1960.
For their research in the field of stimulated emission, Townes, Basov and Prokhorov were awarded theNobel Prize in Physics in 1964.[6]
The maser is based on the principle of stimulated emission proposed byAlbert 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 aresonant cavity, feedback is created that can producecoherent radiation.
In 2012, a research team from theNational Physical Laboratory andImperial College London developed asolid-state maser that operated at room temperature by using optically pumped,pentacene-dopedp-Terphenyl as the amplifier medium.[8][9][10] It produced pulses of maser emission lasting for a few hundred microseconds.
In 2018, a research team fromImperial College London andUniversity College London demonstrated continuous-wave maser oscillation usingsynthetic diamonds containingnitrogen-vacancy defects.[11][12]
In 2025 a team fromNorthumbria University created a low-cost energy-efficient unit that works at room temperature and uses an LED as the source of emission.[13]
Masers serve as high precisionfrequency references. These "atomic frequency standards" are one of the many forms ofatomic clocks. Masers were also used aslow-noise microwave amplifiers inradio telescopes, though these have largely been replaced by amplifiers based onFETs.[14]
During the early 1960s, theJet Propulsion Laboratory developed a maser to provide ultra-low-noise amplification ofS-band microwave signals received from deep space probes.[15] This maser used deeply refrigerated helium to chill the amplifier down to a temperature of 4 kelvin. Amplification was achieved by exciting a ruby comb with a 12.0 gigahertzklystron. 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 21 MPa (3,000 psi) through a 150 μm (0.006 in) micrometer-adjustable entry to the chamber. The whole systemnoise temperature looking at cold sky (2.7 kelvin in the microwave band) was 17 kelvin. This gave such a low noise figure that theMariner IVspace probe could send still pictures fromMars back to theEarth, even though the output power of itsradio transmitter was only 15 watts, and hence the total signal power received was only −169 decibels with respect to amilliwatt (dBm).
The hydrogen maser is used as anatomic frequency standard. Together with other kinds of atomic clocks, these help make up theInternational Atomic Time standard ("Temps Atomique International" or "TAI" in French). This is the international time scale coordinated by theInternational 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 twohyperfine energy levels of atomichydrogen.
Here is a brief description of how they work:
Maser-like stimulated emission has also been observed in nature frominterstellar space, and it is frequently called "superradiant emission" to distinguish it from laboratory masers. These emissions are observed from molecules such as water (H2O),hydroxyl radicals (•OH),methanol (CH3OH),formaldehyde (HCHO),silicon monoxide (SiO), andcarbodiimide (HNCNH).[17] Water molecules instar-forming regions can undergo apopulation inversion and emit radiation at about 22.0 GHz, creating the brightestspectral line in the radio universe. Some water masers also emit radiation from arotational transition at afrequency of 96 GHz.[18][19]
Extremely powerful masers, associated withactive galactic nuclei, are known asmegamasers and are up to a million times more powerful than stellar masers.
The meaning of the termmaser 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 theelectromagnetic 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,[1] 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.[20]
When the laser was developed, Townes andSchawlow and their colleagues at Bell Labs pushed the use of the termoptical maser, but this was largely abandoned in favor oflaser, coined by their rival Gordon Gould.[21] In modern usage, devices that emit in theX-ray throughinfrared portions of the spectrum are typically calledlasers, and devices that emit in the microwave region and below are commonly calledmasers, regardless of whether they emit microwaves or other frequencies.
Gould originally proposed distinct names for devices that emit in each portion of the spectrum, includinggrasers (gamma ray lasers),xasers (x-ray lasers),uvasers (ultraviolet lasers),lasers (visible lasers),irasers (infrared lasers),masers (microwave masers), andrasers (RF masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except formaser andlaser.[citation needed]
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.
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