Masers

Type of physical science: Atomic physics

Field of study: Nonrelativistic quantum mechanics

A maser is an atomic or molecular system that amplifies microwave radiation through the principle of stimulated emission. The name is an acronym for microwave amplification by stimulated emission of radiation. Used principally in the detection of weak microwave signals, for example in radio astronomy, masers operate on the same principle as lasers, and their discovery was a crucial step in the development of lasers.

Overview

Masers--that is, microwave amplifiers operating on the principle of stimulated emission--operate on the same principles as their more familiar cousins, lasers, although the details of design and the practical uses of actual maser-based devices operating in the microwave region of the electromagnetic spectrum differ considerably from laser-based devices operating in the visible and infrared regions of the spectrum. While textbooks on quantum electronics published since the flowering of laser technology usually treat masers as a specialized type of laser, masers were the first type of quantum amplifier to be discovered and constructed; in fact, lasers were originally called "optical masers."

The operation of a maser or laser ultimately takes place at the molecular and atomic level and is dependent on the interaction between molecules and electromagnetic radiation. This interaction takes three forms: absorption, in which a quantum (photon) of light or other electromagnetic radiation is captured by a molecule in a lower-energy state and the molecule enters a higher-energy state; spontaneous emission, in which a molecule in a higher-energy state spontaneously emits a photon and reverts to the lower-energy state; and stimulated emission, in which a photon interacts with a molecule in the higher-energy state, causing the molecule to release a second photon and revert to the lower-energy state. This third type of interaction is the basis of maser and laser action.

Energy transitions in atoms and molecules are quantum processes; that is, for any given substance there are very specific wavelengths of electromagnetic radiation that will be absorbed or emitted. A few specific examples will serve to illustrate this principle. Sodium vapor has a pair of closely spaced emission/absorption lines in the yellow portion of the visible spectrum corresponding to a change in energy levels in the outer electron shell of the sodium atom.

Spontaneous emission from sodium atoms raised to the higher-energy state by electrical discharge is responsible for the characteristic yellow light of sodium vapor streetlights. When visible light passes through sodium vapor, light of these specific wavelengths is selectively absorbed. A ruby appears red because the crystal absorbs green light.

Pure water appears transparent to the human eye because it lacks emission/absorption lines in the visible spectrum. Water does, however, absorb certain microwave frequencies, a fact that has been exploited in the design of microwave ovens. Food heats because the water it contains absorbs microwaves; glass and plastic containers, with a different molecular structure, are transparent to these specific frequencies of microwaves, and the interior of the oven is constructed of reflective materials. A microwave oven is not a maser, but it contains elements used in the design of a maser.

Historically, the microwave absorption spectrum of water has been significant to the discovery of masers. In the United States, maser research developed from radar research, and the existence of specific frequencies of microwaves that were useless for radar and communications purposes because of absorption by water served to focus attention on the quantum processes involved. More recently, systematic mapping by radio astronomers of cosmic microwave emission and absorption attributable of the hydoxyl ion led to the unexpected discovery of cosmic masers.

The processes of absorption, spontaneous emission, and stimulated emission all occur in any mass interaction between matter and electromagnetic radiation; however, under normal conditions, absorption and spontaneous emission predominate. At thermal equilibrium, the ratio of molecules N₂ in the upper-energy state to the number of molecules N₁ in the lower-energy state is specified by Boltzmann's equation: N₂/N₁ = e - (E₂-E₁)/kT, where E₂ - E₁ is the difference in energy levels, kT is the absolute temperature, and e is a constant. The relationship between the value E₂ - E₁ and the wavelength v is specified by the equation hv = E₂ - E₁ where h is Planck's constant. Thus, the ratio N₂/N₁ is larger when the energy difference is small and is at high temperatures. At thermal equilibrium, the proportion of molecules in excited states corresponding to the emission of visible light is negligible, whereas molecules in excited states corresponding to microwave frequencies are numerous enough to affect the sensitivity of equipment used to measure microwave absorption spectra. In both cases, however, N₁ is greater than N₂, and absorption predominates. An inverted population in which N₂ is greater than N₁ is possible both in principle and in practice. Such a system is not in thermal equilibrium and is said to have a negative temperature because the Boltzmann equation is satisfied only for negative values of T. When a system with negative temperature is subjected to incident radiation of appropriate frequency, stimulated emission predominates over absorption, and the signal is amplified. Since the process occurs at the molecular level, and each photon of the incoming signal produces (by the process of stimulated emission) an additional photon coinciding exactly in wavelength and phase to the original, the amplified signal is extremely faithful to the original. A maser amplification device has an exceptionally low noise level and is capable of multiplying a weak signal manyfold without distorting it.

The largest practical hurdle to be overcome in the design of quantum amplification devices is the creation of an inverted population of molecules. An inverted population rapidly reverts to thermal equilibrium through relaxation processes--emission of a photon of electromagnetic radiation in a gas or spin-spin interactions with the crystal matrix in a solid-state system.

The principle and operation of a maser may perhaps best be explained in the context of an ammonia gas maser, which was the first type to be demonstrated. In practical applications, solid-state masers involving crystalline oscillators are more commonly used, but quantum processes in crystals are more difficult to visualize. The ammonia gas maser is based on the 23,870-megacycle energy transition between two configuration states in the ammonia molecule, which consists of three hydrogen atoms and a nitrogen atom arranged in a pyramid, with the nitrogen atom at its apex. The molecule exists in two symmetrically equivalent states, with the nitrogen atom either above or below the plane of the hydrogens. The two energy states of the ammonia molecule have a different distribution of electrical charge and can be separated in an asymmetrical electric field. In the maser, a beam of ammonia molecules passes through a cage of rods to which are applied large d-c voltages ,000 volts) of alternating sign, which deflect molecules in the lower-energy state but allow molecules in the higher-energy state to pass into a resonant cavity (essentially a box with reflective walls whose dimensions are even multiples of the wavelength in question, subject to externally applied electrical and magnetic fields). The cavity and the inverted population of molecules within it serve both as an extremely sensitive detector and amplifier of radiation at 23,870 megacycles and, if the flow of ammonia molecules is sufficiently great, as a self-sustained oscillator at this specific frequency. The device requires high voltages, low temperatures, and a vacuum to operate, and is quite impractical as a generator of microwaves or as a receiver for ordinary broadcast communications. This and other gas masers are effective only at the specific frequencies and were quickly supplanted by tunable solid-state masers.

The ammonia maser has been proposed as a frequency standard and atomic clock. The frequency and wavelength of electromagnetic radiation produced by quantum transitions in atoms and molecules are constant quantities, replicable in any laboratory, and capable of dividing both time and distance into identical, extremely small segments.

The operation of a solid-state maser depends on the paramagnetic properties of certain transitional elements in crystal lattices. To cite a representative example, Cr³⁺ chromium atoms in a ruby crystal lattice have a magnetic dipole moment associated with the spin of the electrons in the atom. The energy of an electron is quantized into two levels according to whether its magnetic moment, associated with the electronic spin, is parallel or antiparallel to the magnetic field. There are four possible orientations for these chromium atoms relative to a magnetic field, and four associated energy levels--known as paramagnetic or Zeeman levels--for electrons in the ground state. Transitions between these energy levels correspond to emission or absorption of microwave radiation. Since energy difference between the levels varies with the strength of the magnetic field, a maser based on paramagnetic resonance in a crystal can be tuned by varying the strength of the magnetic field. (One type of laser is also based on energy transitions in chromium atoms in a ruby crystal but employs the higher-energy orbital-level transition of electrons.)

The ruby solid-state maser employs three paramagnetic levels instead of two and relies on pumping with incident radiation rather than on physical separation of two populations of molecules to create an inverted population. A ruby crystal in a magnetic field is subjected to incident microwave radiation at the 1-3 transition frequency, until level 1 and level 3 are equally populated and further pumping is impossible. At this point, the probability of absorption and stimulated emission at the 1-3 frequency are equal; however, since populations at level 2 are unaffected by the pumping frequency, the system is inverted with respect to the 2-1 transition probability and will amplify a signal at this frequency.

Microwave three-level, solid-state masers operate most effectively at low temperatures because the difference in population between the energy levels at thermal equilibrium, and therefore the extent to which the equilibrium level of N² exceeds the nonequilibrium value of N¹, is greater the lower the temperature. For greatest sensitivity, masers used in radio astronomy are cooled with liquid helium to 4.2 Kelvins and the entire apparatus is enclosed in a Dewar flask.

Some solid-state masers are designed to operate on the cavity principle, but many are traveling-wave masers, in which the maser material is distributed along a slow-wave circuit and the signal is amplified continuously as it travels the length of the circuit. A traveling-wave maser can be designed so that the signal is amplified only in one direction.

Applications

The principal application of masers is in the detection of weak microwave signals in radio astronomy, radiometry, and space communications. Maser-generated microwave signals have been shown experimentally to be extremely accurate time and frequency standards and are used in this context in the ground-based navigation instrumentation of deep-space probes.

Masers are invaluable in radio astronomy because they are capable of amplifying extremely weak signals with low distortion and because the microwave spectra of heavenly bodies are of exceptional interest to astronomers. Most of what scientists know about the universe is gained from interpreting electromagnetic radiation; each segment of the electromagnetic spectrum yields different types of information. Fortunately, the earth's atmosphere is transparent to most wavelengths in the microwave range. Dark bodies that emit no visible light may be strong emitters of radio waves. The microwave spectra of interstellar dust clouds yield information about their temperature, molecular composition, distribution in space, and internal dynamics. The microwave energy that reaches the earth is the fingerprint of low-energy processes occurring throughout the universe and is particularly helpful in interpreting events that occur at low temperatures, such as early stages in the formation of stars. Radio telescopes coupled with masers have detected the spectral signatures of a wide variety of organic and inorganic molecules, including water, ammonia, sulfur dioxide, and ethyl alcohol, in interstellar matter, and documented the 3-Kelvin thermal microwave background radiation that permeates all space and that has been deduced to be residual radiation from the big bang.

Although initial development of masers was an offshoot of radar research, masers are of little practical importance to amplifiers of radar signals reflected from terrestrial sources, because weak radar signals propagated parallel to the earth's surface have a high noise level resulting from background radiation. In contrast, signals arriving at an amplifier in a direction perpendicular to the earth's surfaces have a low noise level, whether they come from an interstellar source or are reflected from an object in the solar system. Masers have been used to detect radar signals bounced off the Moon, planets, and artificial satellites.

Masers are an essential part of the ground instrumentation of deep-space probes. The National Aeronautics and Space Administration's (NASA's) Deep Space Network (DSN) was responsible for the first field-operational amplifying masers for low-noise, weak-signal operations. Microwave signals are used in communications with deep-space probes because they offer greater directivity and more frequency space than the longer-wave radio band. Television pictures of outer planets were relayed by the Voyager via the 3.5-centimeter microwave band to an array of radio telescope stations. This array functions not only as a receiver for weak microwave signals but also, because of its inherent prevision, as a sensitive device for measuring small changes in the distance between the stations, thus providing information on shifting of continental plates. This data may prove useful in earthquake prediction.

Radiometry is the measurement of thermal noise emission; it is used in remote sensing to create a thermal image of a landscape. In order to resolve detail in the weak thermal microwave emission of bodies at or near room temperature, sensitive amplifiers are necessary.

The use of masers in radio astronomy led to the surprising discovery of cosmic masers--regions of the sky that are sources of intense microwave radiation--whose characteristics suggest that they are produced by maser action on a cosmic scale. They were encountered by astronomers surveying the universe for microwave emissions characteristic of the hydroxyl (OH-) ion. Some of these intense microwave sources are associated with old giant stars; others are not associated with stellar objects but are small-diameter sources within interstellar dust clouds. Calculations based solely on the intensity of microwave emission suggest an improbable temperature for these sources of 10¹³ Kelvins. Their observed spectrum is also a discrete spectrum characteristic of quantum processes rather than the continuous electromagnetic spectrum produced by synchrotron radiation of a moving electron in a magnetic field.

Researchers concluded that the source of microwave radiation was not a thermal process, but rather maser action by molecular species being acted on by an intense infrared, optical, or ultraviolet radiation. The ultimate source of energy for cosmic masers is believed to come from rapid gravitational collapse within a cosmic dust cloud in the region of star formation. Cosmic masers are the earliest observable sign of star formation. Although man-made masers are typically low-energy-output devices, the energy output of cosmic masers is enormous.

Context

The discovery of the principle of stimulated emission of radiation, which with its subsequent ramifications ranks as one of the most important practical physical discoveries of the second half of the twentieth century, has its roots in Albert Einstein's classic 1916 paper on the theory of general relativity. Einstein predicted the process of stimulated emission on theoretical grounds, but it was not until 1954 before a device employing the principle was demonstrated.

The discovery and practical application of the maser principle was accomplished by two groups of scientists working independently of each other. One group consisted of Charles Hard Townes, J. P. Gordon, and H. G. Zeiger, working at Columbia University in the United States, the other of Nikolay Gennadiyevich Basov and Aleksandr Mikhailovich Prokhorov of the Lebedev Physics Institute in the Soviet Union. Townes, Basov, and Prokhorov shared the 1964 Nobel Prize in Physics for their pioneering work on masers and lasers. Gordon, Zeiger, and Townes described a working ammonia gas maser in their 1955 paper in PHYSICAL REVIEW entitled "The Maser--A New Type of Microwave Amplifier, Frequency Standard, and Spectrometer," in which they coined the acronym maser; Basov and Prokhorov published descriptions of a similar device in 1955. The use of electron paramagnetic resonance in solids was proposed by Nicolaas Bloembergen of Harvard University in 1956. While refinements in maser technology were being investigated, a race developed to produce a quantum amplifier that operated in the visible range of the electromagnetic spectrum--an optical maser, or laser.

Although no one anticipated the full implications of quantum amplifier technology, it was clear from the outset that the commercial and military possibilities were impressive. The first working laser was demonstrated by Theodore Maiman in 1960. In 1964, when the Nobel committee reviewed their rationale for awarding the prize, lasers were still experimental laboratory devices, while masers had become established in radio astronomy.

Shortly thereafter, laser technology entered a period of exponential growth, and lasers came to be used in an incredible array of commercial, industrial, military, and medical applications, as well as being indispensable tools in the laboratory. Today, there is scarcely any high-technology field that is not dependent in some way on lasers. The range of applications continues to grow rapidly, and laser technology is increasingly being incorporated into devices for personal use--for example, laser printers for personal computers.

In principle, a quantum amplifier could be designed to amplify in any range of the electromagetic spectrum, although actual design of the amplifying device itself must necessarily vary greatly depending on the wavelength and energy of the radiation employed. Practical quantum amplifiers exist that operate in the microwave, infrared, visible, and near-ultraviolet regions of the spectrum. A device quantum amplifier operating in the X-ray region of the spectrum would, because of the much higher energy involved, open up a whole new horizon of properties and applications, but despite active research in this area, a working X-ray "maser" has yet to be produced.

Principal terms

AMPLITUDE: the magnitude of displacement of an oscillating system from its equilibrium point; the amplitude of an electromagnetic signal is related to its intensity

COHERENT RADIATION: radiation in which components of the signal are in phase with one another

FREQUENCY: the number of oscillations of a system per unit time

MICROWAVE RADIATION: electromagnetic radiation of wavelengths ranging from 0.03 to 30 centimeters (frequency of 1-100 gigahertz)

QUANTUM PROCESS: refers to the fundamental particle-like properties of a wave or field

Bibliography

Bertolotti, Mario. MASERS AND LASERS: AN HISTORICAL APPROACH. Bristol, England: Adam Hilger, 1983. In this technical account intended for a reader with considerable background in the physical sciences, Bertolotti traces the many threads of research leading to the development of masers and lasers. Several chapters discuss the discoveries and lines of research that led to the discovery of masers, a detailed account of the principles and design of the first ammonia gas maser, and biographical data on the people responsible for the discovery. The book underscores connections between masers and lasers and the importance of the maser to the laser.

Cook, Alan H. CELESTIAL MASERS. New York: Cambridge University Press, 1977. A relatively technical astronomy text reviewing literature and research on cosmic sources of intense microwave radiation. Topics covered include the distribution of such sources in the sky, their relevance to theories of stellar evolution, and the rationale for considering them to be masers.

Siegman, A. E. MICROWAVE SOLID-STATE MASERS. New York: McGraw-Hill, 1964. A textbook written for graduate students in physics and engineering. Chapter 1, "Some Introductory Matters," contains clear definitions of terminology and descriptions (including illustrations) of types of masers, which are unusually helpful in visualizing the connection between theory and actual operation. A more general text than the title implies, since solid-state masers are the predominant type in use, and much of the theoretical discussion applies to liquid and gas phase masers, as well as to lasers.

Singer, J. R. MASERS. New York: John Wiley & Sons, 1959. Written before lasers came to dominate the literature on quantum amplifiers, this book gives detailed descriptions of the design and operation of gas and solid-state masers and the mathematical theory behind stimulated emission. Includes detailed bibliographies of the earlier literature.

Weber, Joseph. MASERS: SELECTED REPRINTS WITH EDITORIAL COMMENT. New York: Gordon & Breach, 1967. A useful compilation of original papers on the maser principle and various specific types of masers, with a general introduction to the topic. Includes Gordon, Zeiger, and Townes' pioneering 1955 paper "The Maser--A New Type of Microwave Amplifier, Frequency Standard, and Spectrometer."

Weber, Marvin J., ed. CRC HANDBOOK OF LASER SCIENCE AND TECHNOLOGY. Vol. 1 in LASERS AND MASERS. Boca Raton, Fla.: CRC Press, 1982. A reference book, intended for professionals in the physical sciences and engineering, which provides reviews and references for many aspects of laser and maser science, particularly applications and devices currently used by laboratories and industries. Masers are treated on pages 465-483 as a special class of miscellaneous lasers. Includes diagrams and descriptions of masers used in deep-space probe instrumentation and a discussion of cosmic masers.

Photon Interactions with Molecules

The Effect of Electric and Magnetic Fields on Quantum Systems