Maser
From Freepedia
A maser is a device that produces coherent electromagnetic waves through amplification due to stimulated emission. Historically the term came from the acronym "microwave amplification by stimulated emission of radiation", although modern masers emit over a broad portion of the electromagnetic spectrum. This has lead some to replace "microwave" with "molecular" in the acronym, as suggested by Townes [1]. When optical coherent oscillators were first developed, they were called optical masers, but it has become more common to refer to these as lasers. See the section on terminology below for more on this.
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History
Theoretically, the principle of the maser was described by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by USSR Academy of Sciences in May 1952. They subsequently published their results in October 1954. Independently, Charles H. Townes, J. P. Gordon, and H. J. Zeiger built the first maser at Columbia University in 1953. The device used stimulated emission in a stream of energised ammonia molecules to produce amplification of microwaves at a frequency of 24 gigahertz. Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, which Theodore H. Maiman first demonstrated in 1960. For their research in this field Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964.
Uses
Masers serve as high precision frequency references, as, for example in an atomic clock. They are also used as electronic amplifiers in radio telescopes. For more information about frequency reference masers, see atomic clock.
Another example is given by telescopic masers, which may use arrays of chromium atoms in an insulating aluminium oxide crystal as amplifiers, pumping the energy in at a different radio frequency. That is, they use polished strips of synthetic ruby. As the input signal comes in, a gold comb (made of gold because it is slow to corrode and change shape) distributes the signal along the strip of polished ruby. As the radio wave moves through the crystal, it knocks electrons into different atomic orbitals. As the electrons move to their new lower energy orbits, they add to the wave that knocked them down, leading to a population inversion. The comb-fingers are spaced so that the desired radio waves add together as they move down the crystal. This means that unwanted radio waves will not add together, and are therefore filtered out, leading to a highly coherent induced emission.
An example of a maser which operates in the radio frequency range of the spectrum, is the spin-exchange Zeeman maser. Here, coupled spins from different atoms can exchange (ie. sometimes from the electronic to the nuclear) in order to generate a coherency in the system that radiates in, for example, the VLF region of the electromagnetic spectrum. These types of masers are sometimes called rf-lasers or rasers for short (though most often simply called masers).
See [2] for an expansive list of articles on masers.
Masers are also currently being studied by the United States Military as a form of vehicle-mounted riot control device. A maser can be used to heat up water molecules in the skin, producing a burning sensation without causing permanent damage, unless concentrated on a target for an extended period of time.
Technology
There are a number of types of masers. Generally one can break these down into type as the gas masers, solid masers, and as yet only hypothesized liquid masers. In each of these categories there are many different subtypes; for example, solid state masers come in a number of varieties; such as two level solid state masers, and three level cavity masers.
In operation, some masers use liquid helium for cooling to temperatures around 4 kelvins. This reduces the noise from vibrating electrons, nuclei, and other charged particles.
Several different chemicals are capable of masing. These include water (H2O), hydroxyl ion (HO−), ammonia (NH3), methanol (CH3OH), formaldehyde (CH2O), silicon monoxide (SiO), and the hydrogen ion (H+). The dual noble gas maser [3] is an example of a masing medium which is nonpolar.
Masers in Nature
Masers also occur in nature in interstellar space. Water molecules in star-forming regions can undergo a population inversion and emit radiation at 22 GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a vibrational mode at 96 GHz.
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 of stimulated emission has since been extended to more devices and frequencies, and so the original acronym is sometimes modified, as suggested by Charles H. Townes [4], to "molecular amplification by stimulated emission of radiation." 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.
Initially, visible light oscillators based on stimulated emission were called optical masers, but this terminology has become uncommon. It is more conventional now to refer to devices that emit in the X-ray through infrared portions of the spectrum as lasers, and devices that emit in the microwave region and below as masers. There is some debate over whether maser or laser is the correct generic term for all devices that produce coherent electromagnetic waves through stimulated emission.
Distinct names have been proposed for devices that emit in each portion of the spectrum, including grasers (gamma ray lasers), xasers (x-ray lasers), uvasers (ultraviolet lasers), lasers (visible lasers), irasers (infrared lasers), masers (microwave masers), and rasers (rf masers). Most of these terms never caught on, however. All but maser, laser, and raser are now obsolete, and the latter is uncommon.
In the study of astrophysics, a maser usually refers specifically to a device that emits microwave radiation.
Masers in Science Fiction
Masers are the most recognizable weapon in the Godzilla movie monster series as well as Toho's other monster movies. Maser tanks are often deployed against monsters. The maser tank fires a bolt of electricity, presumably created by amplified microwaves. As in most science fiction, the science behind the maser tank is dubious and not supported by real world physics.
The maser tank is also present as a unit in Outpost 2. It is one of the weaker units.
In the film Batman Begins (2005), the villain Ra's Al Ghul tries to destroy Gotham City using a device he refers to as a "focused microwave emitter," which seems to be some sort of maser.
Arvin Sloane refers to a Maser in the show Alias, Season 3, Episode 7 during a private meeting with the CIA.
External links
- arXiv.org search for "maser"
- Noble gas Maser
- Marine Corps Times — 'The People Zapper'
- The Hydrogen Maser Clock Experiment
See also
References
- 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.
