The acronym maser stands for Microwave Amplification by Stimulated Emission of Radiation. This name can be slightly misleading however, as a maser resembles a laser but operates anywhere between the microwave and radio frequency regions of the electromagnetic spectrum.
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. For their research in this field Townes, Basov and Prokhorov were awarded the Nobel Prize in Physics in 1964.
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.
Masers serve as high precision frequency references, as, for example in an atomic clock. They are also used as elctronic 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 it along the strip of polished ruby. As the radio wave moves through the crystal, it knocks electrons into different orbits. As the electrons move to their new lower energy orbits, they add to the wave that knocked them down, leading to a population inversion as occurs in a laser. 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 don't add together, and are therefore filtered out, leading to a highly coherent induced emission.
Masers are also currently being studied by the United States Military as a non-lethal form of crowd control. A maser can be used to heat up the water molecules of the skin, causing a burning sensation without truly damaging a target. Masers used in such a fashion would be vehicle-mounted, and it is possible that they could be lethal if concentrated on a target for an extended period of time.
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 at temperatures of only about 4 kelvins. This reduces the noise from electrons, nuclei, and other charged particles that the molecular motion of heat can bounce around. However, helium is used in other masers for different reasons.
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.
J.R. Singer, Masers, John Whiley and Sons Inc., (1959)
Last updated: 05-06-2005 14:20:11