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Reports on Progress in Physics

Microwave and optical masers

J. E. Geusic, H. E. D. Scovil

Published Jan 1, 1964• 11 citations
Publisher

Abstract

The subject of masers is introduced by giving an historical account of events which led to the amplification of microwave radiation by an inverted population in a system of weakly interacting ammonia molecules. The introduction also outlines the development of masers up to the present. Sections 2 and 3 are concerned with maser fundamentals. Methods for obtaining inverted populations are discussed in § 2. Section 3 is devoted to the theory of maser gain and noise. This is followed by a discussion of microwave solid state masers. Microwave maser materials and the development of unidirectional travelling wave maser amplifiers are emphasized. Optical masers are treated in § 5. In this section the theory of optical amplifiers and oscillators is outlined. Also included in the theoretical discussion is a rather lengthy consideration of pump power requirements in optically pumped masers. This is followed by a survey of solid state, semiconductor and gaseous optical masers. The review ends with some comments on the extension of masers to new wavelength regions.

DOI
10.1088/0034-4885/27/1/306

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Since the demonstration of the possibility of achieving very low noise amplification of signals in the centimetric wavebands, a great deal of work has been carried out in various laboratories with a view to producing and testing devices. The major part of the present article focuses attention on one particular device, the three-level solid-state maser. The physical principles underlying the process of stimulated emission of radiation are in many cases unfamiliar to electronic engineers, and study of the growing literature on the subject can be confusing without a clear understanding of the physics involved. The present article outlines the general theory of emission and radiation processes, particularly as they apply to the microwave spectrum. A quantum mechanical description is employed throughout, and a brief and not very rigorous account is given of the relevant portions of this theory as it applies to the investigation of transistors between energy levels. Well-known expressions are derived for the operating characteristics of a three-level maser, particular attention being given to the significance of the various factors involved and their relationship to easily measured quantities. Little information is included concerning practical maser amplifiers; at the time of writing, few practical devices had been constructed. A survey of recent literature indicates the rapid progress which has since been made, and shows the interest in the development of masers.

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Summary This article is intended to give the reader some understanding of why masers have suddenly become of such great interest. It begins with an account of the background to the problem of microwave amplification and then passes to a description of the properties of the populations of energy levels in thermal and other kinds of equilibrium. Particular attention is paid to the way in which a population reversal can be used for amplification, the basis of the maser. The next considerations are related to the performance characteristics to be anticipated and the article ends with descriptions of the ammonia maser, the ruby 3-level maser and a brief mention of the 2-level maser work. In having to choose between describing the technological and physical problems which are involved the basic physics has seemed of more importance and has been stressed.

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By A. E. Siegman London: McGraw-Hill Publishing Co. Ltd. 1964. Pp. 582. Price 143s. 6d. This is a postgraduate book, partly for electronics engineers, who would find a good deal of the material difficult, because it is a comprehensive account of the subject which demands a good deal of familiarity with quantum mechanics. The contents include chapters on internal structure and energy levels of atoms, the magnetic properties of atoms, energy levels of paramagnetic maser materials, paramagnetic resonance and the dynamics of spin systems, the three level solid state maser and travelling wave maser.

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Also see the archival list of Science 's Compass: Enhanced Perspectives APPLIED PHYSICS:Enhanced: The Maser at 50

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In 1954, the first device demonstrating microwave amplification by stimulated emission of radiation was demonstrated. Fifty years on, some masers are widely used as amplifiers and high-stability oscillators. Other, more novel maser designs are enabling fundamental studies of quantum mechanics and are providing sensitive tests for quantum gravity theories. In his Perspective, Walsworth reviews these recent developments. In a related Perspective, Claussen reviews the discovery and study of naturally occurring masers in astronomical objects such as star-forming regions.

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General Analysis of Optical, Infrared, and Microwave Maser Oscillator Emission

J. R. Singer, S. Wang

The equations are generalized governing coherent emission from quantum mechanical amplifiers (microwave, infrared, and optical masers) using either electric or magnetic dipole transitions. Earlier analyses by Gordon, Zeiger and Townes, Feynman, Vernon and Hellworth, and Wang and Singer, are incorporated in this generalization. The principal new result of this analysis is that an amplitude modulation of the output is to be expected from all maser oscillators excepting those in which excited atoms are supplied at a notably greater rate than the depopulation rate due to coherent induced emission. This analysis also suggests that an energy population inversion of electric dipole moments can be performed using coherent light sources in the same manner as with magnetic dipole moments. It appears therefore that modulation of an optical or infrared maser (e.g., for communication purposes) may be easily accomplished by varying the population of excited states through control of the pumping energy. This modulation may be both by absorption and amplification. (Author)

Physical Review Letters•Apr 1, 1961•21 citations
The subject of masers is introduced by giving an historical account of events which led to the amplification of microwave radiation by an inverted population in a system of weakly interacting ammonia molecules. The introduction also outlines the development of masers up to the present. Sections 2 and 3 are concerned with maser fundamentals. Methods for obtaining inverted populations are discussed in § 2. Section 3 is devoted to the theory of maser gain and noise. This is followed by a discussion of microwave solid state masers. Microwave maser materials and the development of unidirectional travelling wave maser amplifiers are emphasized. Optical masers are treated in § 5. In this section the theory of optical amplifiers and oscillators is outlined. Also included in the theoretical discussion is a rather lengthy consideration of pump power requirements in optically pumped masers. This is followed by a survey of solid state, semiconductor and gaseous optical masers. The review ends with some comments on the extension of masers to new wavelength regions.
Since the demonstration of the possibility of achieving very low noise amplification of signals in the centimetric wavebands, a great deal of work has been carried out in various laboratories with a view to producing and testing devices. The major part of the present article focuses attention on one particular device, the three-level solid-state maser. The physical principles underlying the process of stimulated emission of radiation are in many cases unfamiliar to electronic engineers, and study of the growing literature on the subject can be confusing without a clear understanding of the physics involved. The present article outlines the general theory of emission and radiation processes, particularly as they apply to the microwave spectrum. A quantum mechanical description is employed throughout, and a brief and not very rigorous account is given of the relevant portions of this theory as it applies to the investigation of transistors between energy levels. Well-known expressions are derived for the operating characteristics of a three-level maser, particular attention being given to the significance of the various factors involved and their relationship to easily measured quantities. Little information is included concerning practical maser amplifiers; at the time of writing, few practical devices had been constructed. A survey of recent literature indicates the rapid progress which has since been made, and shows the interest in the development of masers.
The first solid-state masers to operate at room-temperature and ambient air-pressure were recently demonstrated using optically pumped spin-triplet states as the gain medium. In this Perspective, we briefly review the previous state-of-the-art in cryogenic solid-state masers and then discuss the development of the room-temperature solid-state maser: from the organic pentacene pulsed maser to the diamond nitrogen-vacancy continuous-wave maser. We characterize the operation of these masers as coherent microwave sources and ultra-low noise amplifiers before outlining how they can be adapted to act as model systems in which to explore room-temperature cavity quantum electrodynamics. After discussing challenges facing current embodiments of the room-temperature solid-state maser, we explore how they might be addressed or by-passed altogether through the development of alternative materials and masing mechanisms. Finally, we speculate on how the advent of masers that can operate in ambient conditions might lead to novel applications in metrology and quantum technologies.
Summary This article is intended to give the reader some understanding of why masers have suddenly become of such great interest. It begins with an account of the background to the problem of microwave amplification and then passes to a description of the properties of the populations of energy levels in thermal and other kinds of equilibrium. Particular attention is paid to the way in which a population reversal can be used for amplification, the basis of the maser. The next considerations are related to the performance characteristics to be anticipated and the article ends with descriptions of the ammonia maser, the ruby 3-level maser and a brief mention of the 2-level maser work. In having to choose between describing the technological and physical problems which are involved the basic physics has seemed of more importance and has been stressed.
By A. E. Siegman London: McGraw-Hill Publishing Co. Ltd. 1964. Pp. 582. Price 143s. 6d. This is a postgraduate book, partly for electronics engineers, who would find a good deal of the material difficult, because it is a comprehensive account of the subject which demands a good deal of familiarity with quantum mechanics. The contents include chapters on internal structure and energy levels of atoms, the magnetic properties of atoms, energy levels of paramagnetic maser materials, paramagnetic resonance and the dynamics of spin systems, the three level solid state maser and travelling wave maser.
In 1954, the first device demonstrating microwave amplification by stimulated emission of radiation was demonstrated. Fifty years on, some masers are widely used as amplifiers and high-stability oscillators. Other, more novel maser designs are enabling fundamental studies of quantum mechanics and are providing sensitive tests for quantum gravity theories. In his Perspective, Walsworth reviews these recent developments. In a related Perspective, Claussen reviews the discovery and study of naturally occurring masers in astronomical objects such as star-forming regions.
The equations are generalized governing coherent emission from quantum mechanical amplifiers (microwave, infrared, and optical masers) using either electric or magnetic dipole transitions. Earlier analyses by Gordon, Zeiger and Townes, Feynman, Vernon and Hellworth, and Wang and Singer, are incorporated in this generalization. The principal new result of this analysis is that an amplitude modulation of the output is to be expected from all maser oscillators excepting those in which excited atoms are supplied at a notably greater rate than the depopulation rate due to coherent induced emission. This analysis also suggests that an energy population inversion of electric dipole moments can be performed using coherent light sources in the same manner as with magnetic dipole moments. It appears therefore that modulation of an optical or infrared maser (e.g., for communication purposes) may be easily accomplished by varying the population of excited states through control of the pumping energy. This modulation may be both by absorption and amplification. (Author)