Maser

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First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at center is the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps. Charles Townes and first maser.jpg
First prototype ammonia maser and inventor Charles H. Townes. The ammonia nozzle is at left in the box, the four brass rods at center is the quadrupole state selector, and the resonant cavity is at right. The 24 GHz microwaves exit through the vertical waveguide Townes is adjusting. At bottom are the vacuum pumps.
A hydrogen radio frequency discharge, the first element inside a hydrogen maser (see description below) Hydrogen maser.gif
A hydrogen radio frequency discharge, the first element inside a hydrogen maser (see description below)

A maser ( /ˈmzər/ , an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953. Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical work leading to the maser. Masers are used as the timekeeping device in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes and deep space spacecraft communication ground stations.

Contents

Modern masers can be designed to generate electromagnetic waves at not only microwave frequencies but also radio and infrared frequencies. For this reason Charles Townes suggested replacing "microwave" with the word "molecular" as the first word in the acronym maser. [1]

The laser works by the same principle as the maser, but produces higher frequency coherent radiation at visible wavelengths. The maser was the forerunner of 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 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 1952 in Ottawa, [2] with a summary published in the June 1953 Transactions of the Institute of Radio Engineers Professional Group on Electron Devices, [3] and simultaneously by Nikolay Basov and Alexander Prokhorov from Lebedev Institute of Physics at an All-Union Conference on Radio-Spectroscopy held by the USSR Academy of Sciences in May 1952, subsequently published in October 1954.

Independently, 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 gigahertz. [4] Townes later worked with Arthur L. Schawlow to describe the principle of the optical maser, or laser, [5] of which 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. [1]

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 coherent radiation.

Some common types

Twenty-first century developments

In 2012, a research team from the National Physical Laboratory and Imperial College London developed a solid-state maser that operated at room temperature by using optically pumped, pentacene-doped p-Terphenyl as the amplifier medium. [7] [8] [9] 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 diamonds containing Nitrogen-Vacancy defects. [10] [11]

Uses

Masers serve as high precision frequency references. These "atomic frequency standards" are one of the many forms of atomic clocks. Masers were also used as low-noise microwave amplifiers in radio telescopes, though these have largely been replaced by amplifiers based on FETs. [12]

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. [13] This maser used deeply refrigerated helium to chill the amplifier down to a temperature of four 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 21 MPa (3,000 psi) through a 150 μm (0.006 in) micrometer-adjustable entry to the chamber. The whole system noise temperature looking at cold sky (2.7 kelvins in the microwave band) was 17 kelvins. 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 watts, and hence the total signal power received was only -169 decibels with respect to a milliwatt (dBm).

Hydrogen maser

A hydrogen maser. Hmaser.svg
A hydrogen maser.

The hydrogen maser is used as an 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 energy levels of atomic hydrogen. Here is a brief description of how they work:

Astrophysical masers

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 (H2O), hydroxyl radicals (•OH), methanol (CH3OH), formaldehyde (HCHO), and silicon monoxide (SiO). Water molecules in star-forming regions can undergo a population inversion and emit radiation at about 22.0  GHz, creating the brightest spectral line in the radio universe. Some water masers also emit radiation from a rotational transition at a frequency of 96 GHz. [15] [16]

Extremely powerful masers, associated with active galactic nuclei, are known as megamasers 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, [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. [17]

When the laser was developed, Townes and 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. [18] In modern usage, devices that emit in the X-ray through infrared portions of the spectrum are typically called lasers, 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 lasers), irasers (infrared lasers), masers (microwave masers), and rasers (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.

See also

Related Research Articles

Laser Device which emits light via optical amplification

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

Laser science branch of optics that describes the theory and practice of lasers

Laser science or laser physics is a branch of optics that describes the theory and practice of lasers.

In science, specifically statistical mechanics, a population inversion occurs while a system exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy states. It is called an "inversion" because in many familiar and commonly encountered physical systems, this is not possible. This concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.

Spectroscopy Study involving matter and electromagnetic radiation

Spectroscopy is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency, predominantly in the electromagnetic spectrum, though matter waves and acoustic waves can also be considered forms of radiative energy; recently, with tremendous difficulty, even gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO) and laser interferometry. Spectroscopic data are often represented by an emission spectrum, a plot of the response of interest, as a function of wavelength or frequency.

Stimulated emission process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron, causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field.

Laser diode semiconductor laser

A laser diode, (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction. Laser diodes can directly convert electrical energy into light. Driven by voltage, the doped p-n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.

Laser cooling variety of techniques where atomic samples are cooled via interacting with lasers

Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled down to near absolute zero. Laser cooling techniques rely on the fact that when an object absorbs and re-emits a photon its momentum changes. For an ensemble of particles, their thermodynamic temperature is proportional to the variance in their velocity. That is, more homogeneous velocities among particles corresponds to a lower temperature. Laser cooling techniques combine atomic spectroscopy with the aforementioned mechanical effect of light to compress the velocity distribution of an ensemble of particles, thereby cooling the particles.

Charles H. Townes American physicist

Charles Hard Townes was an American physicist. Townes worked on the theory and application of the maser, for which he obtained the fundamental patent, and other work in quantum electronics associated with both maser and laser devices. He shared the 1964 Nobel Prize in Physics with Nikolay Basov and Alexander Prokhorov. Townes was an adviser to the United States Government, meeting every US President from Harry Truman (1945) to Bill Clinton (1999).

Quantum optics (QO) is a field of research that uses semi-classical and quantum-mechanical physics to investigate phenomena involving light and its interactions with matter at submicroscopic levels. In other words, it is quantum mechanics applied to photons or light.

Gordon Gould American inventor

Gordon Gould was an American physicist who is often credited with the invention of the laser.. Gould is best known for his thirty-year fight with the United States Patent and Trademark Office to obtain patents for the laser and related technologies. He also fought with laser manufacturers in court battles to enforce the patents he subsequently did obtain.

Ali Javan Iranian-American Physicist

Ali Javan was an Iranian-American physicist and inventor. He was the first to propose the concept of the gas laser in 1959 at the Bell Telephone Laboratories. A successful prototype, constructed by him in collaboration with W. R. Bennett, Jr., and D. R. Herriott was demonstrated in 1960. His other contributions to science have been in the fields of quantum physics and spectroscopy.

Here, is a list of initialisms and acronyms used in laser physics, applications and technology.

Astrophysical maser

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.

Doppler cooling laser cooling technique

Doppler cooling is a mechanism that can be used to trap and slow the motion of atoms to cool a substance. The term is sometimes used synonymously with laser cooling, though laser cooling includes other techniques.

Ruby laser Solid-state laser with ruby as gain medium

A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960.

Sound amplification by stimulated emission of radiation

Sound amplification by stimulated emission of radiation (SASER) refers to a device that emits acoustic radiation. It focuses sound waves in a way that they can serve as accurate and high-speed carriers of information in many kinds of applications—similar to uses of laser light.

Photomixing is the generation of continuous wave terahertz radiation from two lasers. The beams are mixed together and focused onto a photomixer device which generates the terahertz radiation. It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources, quantum cascade laser and ultrashort pulsed lasers with photoconductive switches as used in terahertz time-domain spectroscopy. The advantages of this technique are that it is continuously tunable over the frequency range from 300 GHz to 3 THz, and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10−8 W.

An X-ray laser is a device that uses stimulated emission to generate or amplify electromagnetic radiation in the near X-ray or extreme ultraviolet region of the spectrum, that is, usually on the order of several of tens of nanometers (nm) wavelength.

Anthony E. Siegman was an electrical engineer and educator concerned with masers and lasers. He was president of the Optical Society of America in 1999 and was awarded the Esther Hoffman Beller Medal in 2009.

James P. Gordon American physicist

James Power Gordon was an American physicist known for his work in the fields of optics and quantum electronics. His contributions include the design, analysis and construction of the first maser in 1954 as a doctoral student at Columbia University under the supervision of C. H. Townes, development of the quantal equivalent of Shannon's information capacity formula in 1962, development of the theory for the diffusion of atoms in an optical trap in 1980, and the discovery of what is now known as the Gordon-Haus effect in soliton transmission, together with H. A. Haus in 1986. James P. Gordon was a member of the National Academy of Engineering and the National Academy of Science.

References

  1. 1 2 3 Charles H. Townes – Nobel Lecture, page 63
  2. American Institute of Physics Oral History Interview with Weber
  3. Mario Bertolotti (2004). The History of the Laser. CRC Press. p. 180. ISBN   978-1420033403.
  4. Gordon, J. P.; Zeiger, H. J.; Townes, C. H. (1955). "The Maser—New Type of Microwave Amplifier, Frequency Standard, and Spectrometer". Phys. Rev. 99 (4): 1264. Bibcode:1955PhRv...99.1264G. doi:10.1103/PhysRev.99.1264.
  5. Schawlow, A.L.; Townes, C.H. (15 December 1958). "Infrared and Optical Masers". Physical Review. 112 (6): 1940–1949. doi:10.1103/PhysRev.112.1940.
  6. The Dual Noble Gas Maser, Harvard University, Department of Physics
  7. Brumfiel, G. (2012). "Microwave laser fulfills 60 years of promise". Nature. doi:10.1038/nature.2012.11199.
  8. Palmer, Jason (16 August 2012). "'Maser' source of microwave beams comes out of the cold". BBC News. Archived from the original on 2012. Retrieved 23 August 2012.
  9. Microwave Laser Fulfills 60 Years of Promise
  10. Liu, Ren-Bao (March 2018). "A diamond age of masers". Nature. 555 (7697): 447–449. Bibcode:2018Natur.555..447L. doi:10.1038/d41586-018-03215-3. PMID   29565370.
  11. Scientists use diamond in world's first continuous room-temperature solid-state maser, phys.org
  12. "Low Noise Amplifiers - Pushing the limits of low noise". National Radio Astronomy Observatory (NRAO).
  13. Macgregor S. Reid, ed. (2008). "Low-Noise Systems in the Deep Space Network" (PDF). JPL.
  14. Time and Frequency From A to Z: H
  15. Neufeld, David A.; Melnick, Gary J. (1991). "Excitation of Millimeter and Submillimeter Water Masers in Warm Astrophysical Gas". Atoms, Ions and Molecules: New Results in Spectral Line Astrophysics, ASP Conference Series (ASP: San Francisco). 16: 163. Bibcode:1991ASPC...16..163N.
  16. Tennyson, Jonathan; et al. (March 2013). "IUPAC critical evaluation of the rotational–vibrational spectra of water vapor, Part III: Energy levels and transition wavenumbers for H216O". Journal of Quantitative Spectroscopy and Radiative Transfer. 117: 29–58. Bibcode:2013JQSRT.117...29T. doi:10.1016/j.jqsrt.2012.10.002.
  17. Taylor, Nick (2000). LASER: The inventor, the Nobel laureate, and the thirty-year patent war. New York: Simon & Schuster. ISBN   978-0-684-83515-0.
  18. Taylor, Nick (2000). LASER: The inventor, the Nobel laureate, and the thirty-year patent war. New York: Simon & Schuster. pp. 66–70. ISBN   978-0-684-83515-0.

Further reading