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quantum electronics[′kwän·təm ‚i‚lek′trän·iks]
the field of physics and technology that deals with methods for the amplification and generation of electromagnetic oscillations based on the use of the effect of stimulated emission, and also with the properties of quantum mechanical amplifiers and generators and with their use. The practical interest in quantum light generators (lasers) is due primarily to the fact that, unlike other light sources, they radiate light waves having very high directivity and monochromaticity. Quantum generators of radio waves differ from other radio apparatus in that the frequency of the oscillations generated is very stable; quantum magnetic amplifiers of radio-frequency waves are distinguished by their extremely low noise level.
Physical principles. Light and radio waves are electromagnetic radiation that can be emitted in batches called quanta (or photons) by atoms, molecules, and other quantum systems that have a certain amount of excess internal energy (excited particles). The internal energy of an atom or molecule can assume only certain strictly defined discrete values, called energy levels. A reduction in internal energy indicates a transition of an atom from a higher energy level to a lower one. If the excess energy is given off as a quantum of radiation, the radiation frequency v is determined by Bohr’s condition:
where h is Planck’s constant. Similarly, an increase in an atom’s internal energy indicates its transition from a lower energy level ε1 to a higher level ε2. If this increase results from the absorption of a quantum of radiation, then the frequency of the absorbed radiation is determined by the same condition (1). Thus, condition (1) determines the frequency of a spectral line of absorption or radiation that is characteristic for the particular particles. The interaction of the particles with surrounding particles and fields, as well as the shortness of their lifetime at a level, causes a washing out of the energy levels. As a result, condition (1) is met not for a single fixed value of the frequency v but rather for a range of frequencies, and the spectral lines broaden.
Excited particles are able to give up their energy in the form of quanta of radiation in two ways. They are unstable, and for each of them there is a certain probability of the spontaneous emission of a quantum of radiation (Figure l, a). The spontaneous emissions occur randomly; therefore, the spontaneous radiation has a chaotic character. Photons are emitted by different particles at different moments, and they have different frequencies, polarizations, and directions of propagation. The intensity of spontaneous radiation is proportional to the cube of the frequency and therefore drops sharply from light waves to radio-frequency waves. All nonlaser light sources (incandescent lamps, gas-discharge lamps, and so on) radiate light as a result of events of spontaneous emission. In the radio-frequency range the noise from electronic devices and the thermal radio-frequency radiation of hot bodies have the same characteristics.
Excited particles can emit photons by passing from a higher energy level ε2 to a lower level ε1 not only spontaneously but also when exposed to external radiation (stimulated), if the frequency of the external radiation satisfies condition (1) (Figure l, b). The probability of stimulated emission, which was predicted by A. Einstein in 1917, is proportional to the intensity of the stimulating radiation and can exceed the probability of the spontaneous process. Hence, the process of stimulated emission involves two quanta of radiation: the primary, or stimulating, quantum, and the secondary quantum, which is emitted by an excited atom. It is significant that the secondary quanta are indistinguishable from the primary quanta. They have exactly the same frequency, phase, and direction of propagation. This feature of stimulated emission, which is of fundamental significance for quantum electronics, was first pointed out by P. Dirac in 1927. The identical quanta form an electromagnetic wave that is an exact amplified copy of the initial radiation. As the number of events of stimulated emission per second increases, the intensity of the wave becomes greater, but its frequency, phase, polarization, and direction of propagation remain unchanged: coherent amplification of the electromagnetic radiation takes place.
For one particle the induced transitions from a higher energy level ε2 to a lower level ε1 (emission of a photon; Figure l, b) and from a lower to a higher level (absorption of a photon; Figure l, c) are equally probable. Consequently, the coherent amplification of a wave is possible only when the number of excited particles exceeds the number of unexcited particles. Under conditions of thermodynamic equilibrium there are fewer excited than unexcited particles—that is, the higher energy levels are populated by fewer particles than the lower levels, in accordance with the Boltzmann distribution for particles with respect to energy levels (Figure 2). During interaction of radiation with such a substance the radiation will be absorbed.
To achieve an amplifying effect, special measures must be taken so that the number of excited particles will exceed the number of unexcited particles. The state of a substance, even for only two energy levels of the particles, in which the higher level has a greater population that the lower is known as a state of population inversion. Such a substance is called active (an active medium) in quantum electronics. Stimulated emission in an active medium is used in quantum electronics for the amplification of electromagnetic waves (quantum mechanical amplifier) and for their generation (quantum generator). The feedback required for generation is accomplished by installing an active medium in a cavity resonator, in which standing electromagnetic waves can be excited. At some point in the resonator spontaneous transition of the particles in the active medium from a higher to a lower level inevitably takes place—that is, a photon is emitted spontaneously. If the resonator is tuned to the frequency of such a photon, it will not emerge from the resonator but, upon being repeatedly reflected from its walls, will generate a multitude of photons like itself, which in turn affect the active substance and produce more new events of stimulated emission of similar photons (feedback). As a result of such “multiplication” of the photons the electromagnetic energy in the resonator is built up, and a portion of the energy is passed on to the load by means of special devices (for example, a semitransparent mirror for light waves). If at a given moment the power of the stimulated emission exceeds the power caused by energy lost to heating the resonator’s walls, scattering of the radiation, and so on and to the useful radiation to external space (that is, if the conditions for self-excitation are fulfilled), then undamped oscillations are developed in the resonator—generation is stimulated.
By virtue of the properties of stimulated emission, the oscillations are monochromatic. All particles of the active substance operate cophasally because of feedback. The frequency of such a generator coincides to a high degree of accuracy with the emission frequency of the excited particles, although it is also considerably affected by mistuning of the resonator frequency with respect to the emission frequency of the particles. The intensity of the oscillations is controlled by the number of excited particles per second in each cubic centimeter of active medium. If the number of such particles is ∧, then the maximum possible power P of continuous emission per cubic centimeter of the medium is
P = ∧hv
Historical survey. Although Einstein’s and Dirac’s theorems on induced emission were formulated in connection with optics, the development of quantum electronics began in radiophysics. Under conditions of thermodynamic equilibrium the optical (higher) energy levels are virtually unpopulated, there are very few excited particles in the substance, and they transfer spontaneously to the lower energy levels because at the low densities of light energy spontaneous transitions are more probable than induced transitions. Therefore, although the concept of mono-chromaticity arose in optics, in this field there were no strictly harmonic oscillations and waves (oscillations having constant amplitude, frequency, and phase). In radio physics, on the other hand, the technology for producing harmonic oscillations by means of generators with oscillatory circuits and controlled positive feedback was developed immediately after the construction of the first spark transmitters. The polychromaticity of radiation in the optical range and the absence of techniques and concepts in optics that were well developed in radio physics, particularly the concept of feedback, were the reasons why masers appeared before lasers.
Until 1950, radio physics and optics developed along different paths. Quantum theories developed in optics, and wave theories developed in radio physics. The common character of radio physics and optics that results from the generality of the quantum nature of electromagnetic wave processes was not apparent until the appearance of microwave spectroscopy—the study of the spectra of molecules, atoms, and ions lying in the super high-frequency (SHF) range (1010−1011 Hz). An important feature of research in microwave spectroscopy (unlike research in optics) was the use of monochromatic sources of radiation, which provided enormously greater sensitivity, resolving power, and accuracy in microwave spectroscopes as compared to optical spectroscopes. No less important was the circumstance that in the radio-frequency range, unlike the optical range, the excitation levels under conditions of thermodynamic equilibrium were heavily populated and spontaneous emission was much weaker. As a result, stimulated emission directly influenced the observed value of the resonance absorption of radio-frequency waves by the substance being studied.
The reason for the population of excitation levels is the thermal motion of the particles, which at room temperature corresponds to an energy of about 4 × 10−14 erg. For visible light of wavelength λ = 0.5 microns (μ), the oscillation frequency v is 6 × 10−14 Hz and the energy of a quantum hv is 1 × 10−12 erg. For radio-frequency radiation of wavelength λ = 0.5 cm the oscillation frequency v is 6 × 1010 Hz and the energy of a quantum hv is 4 × 10−16 erg. Consequently, thermal motion can cause the excitation levels to be heavily populated at radio frequencies but not at optical frequencies.
As a result of the factors listed above, microwave spectroscopy became the basis of development of quantum electronics. In the USSR work on the microwave spectroscopy of gases was begun in the oscillation laboratory of the Institute of Physics of the Academy of Sciences of the USSR (A. M. Prokhorov), where, in addition to the solution of a number of problems of spectroscopy, research was conducted on the use of SHF spectral lines to create frequency standards.
The accuracy of a frequency standard based on measuring the position of a resonance absorption line depends on the width of the spectral line. The narrower the line, the higher the accuracy. Gases have the narrowest lines because the particles in a gas interact weakly with one another. In addition, the chaotic thermal motion of the gas particles brings about as a result of the Doppler effect the Doppler broadening of the spectral lines. An effective method of avoiding such a broadening effect is the shift from chaotic to ordered motion—for example, from gases to molecular beams. However, in this case the capability of a microwave spectroscope is severely limited by the low intensity of the resonance lines. There are few particles in the beam, and consequently the difference between the number of excited and unexcited particles is negligible. At this stage of development the idea of artificially changing the ratio between the number of excited and unexcited particles arose, making possible a substantial increase in the sensitivity of microwave spectroscopes. In addition, if a population inversion is produced in a beam, amplification of the radio-frequency waves, rather than their absorption, becomes possible. If a certain system amplifies radio-frequency radiation, it can then generate it by means of suitable feedback.
The theory of generation was well developed in radio physics. Oscillatory circuits are the basic elements in radio oscillators. In the SHF region the role of circuits is played by cavity resonators, which are particularly suitable for operation with beams of particles. Thus, it was precisely in radio physics that all the necessary elements and prerequisites for the creation of the first quantum generator were available. In the first quantum electronic device—the maser developed in 1955 simultaneously in the USSR by N. G. Basov and A. M. Prokhorov and in the USA by J. Gordon, H. Zeiger, and C. Townes—the active medium was a beam of molecules of ammonia, NH3. The method of electrostatic spatial selection was used to produce a population inversion. The more excited molecules of NH3, were selected from the beam, and those with lower energy were driven aside. The beam of selected molecules was passed into a cavity resonator, in which generation occurred when the conditions for self-excitation were fulfilled. The generator frequency matched that of the emission from the excited NH3 molecules to a high degree of accuracy and was therefore extremely stable: the relative frequency stability is 10−11−10−12. The appearance of masers opened up new possibilities for ultraprecise clocks and accurate navigation systems. Their error is about 1 sec in 300, 000 years. Hydrogen generators operating on similar principles, which were developed later, have an even greater frequency stability, about 10−13.
The origin of quantum electronics in the radio-frequency region explains the emergence of the term “quantum radio physics,” which is used at times instead of “quantum electronics,” a term with a more general meaning that embraces the optical range.
Population inversion cannot always be achieved, particularly in solids, by selection of excited particles. In addition, if the temperature is not very high, there are virtually no excited particles at high optical levels. Therefore, as early as 1955, N. G. Basov and A. M. Prokhorov proposed a new method of creating population inversions, in which the excited particles are produced rather than selected from the number available. In this method, which is called the three-level method, particles having an energy spectrum with three levels ε1, ε2, and ε3 (Figure 3, a) are subjected to powerful auxiliary radiation (pumping), which, upon being absorbed by the particles, “pumps” them from level ε3 to level ε3. The pumping must be sufficiently intense so that enough particles are raised to the highest level ε3 from the lowest to maintain virtually identical numbers of particles in each state (Figure 3, b). In this case there can then be more particles in level ε2 than in level ε1 (or more in level ε3 than in level ε2)—that is, a population inversion will occur for levels ε2 and ε1 (or ε3 and ε2). The frequency of the pumping radiation matches the resonance conditions of absorption, that is,
Vp = (ε3 – ε3)/h
The three-level method was applied as proposed by N. Bloembergen (1956, USA) to make quantum amplifiers in the radio-frequency range using paramagnetic crystals. The quantum amplifiers usually operate at the temperature of liquid helium (4.2°K), where virtually all the particles are at the lowest energy level. During pumping half of all the particles available in the crystal are transferred to a higher level ε2 and take part in coherent amplification. Whereas the maser met the requirement for a high-stability source of monochromatic oscillations in electronics, the quantum amplifier solves the other very important problem of radio physics—the problem of drastically reducing the noise (that is, increasing the sensitivity of SHF receivers). Consequently, quantum mechanical amplifiers have come to be used in radio astronomy, radar, and global and space communications.
The success of quantum electronics has raised the question of its progress toward the shorter wavelengths, in which the development of resonators presents the main difficulty. Closed cavities with conducting walls, whose dimensions are comparable to the wavelength, are used in the SHF region. However, in the optical range it is impossible to produce resonators of this type. An open type of resonator was proposed by A. M. Prokhorov in 1958. In the submillimeter region the resonator consists of two parallel, highly reflective metal disks between which a system of standing waves is set up. In the case of light, this resonator becomes two parallel mirrors similar to a Fabry-Perot interferometer.
The first quantum electronics achievement in the optical range was the development of the laser by T. Maiman of the USA in 1960. The working substance was a single crystal of ruby, and the three-level method was used to produce a population inversion. The reflecting mirrors of the resonator were the well-polished and silvered ends of the ruby crystal. The pump source was a flash lamp. Ruby lasers, along with neodymium-doped glass lasers, yield record-breaking energies and powers. In the free oscillation mode with powerful pumping, large ruby crystals will provide a pulse of energy of up to 1,000 joules (J) and a power of up to 106 watts (W). Another mode of ruby lasers is produced by switching on the resonator mirrors at the moments when a population inversion reaches its maximum value. Then all the particles accumulated in the metastable level emit at practically the same time and the generator will produce a giant radiation pulse of very short duration (10−8–10−9 sec) and comparatively low energy (about 3 J). However, since this energy is emitted in a very short time, the peak power of the pulse attains values of 3 × 106 to 3 × 109 W.
Very soon after the ruby laser came the development of the first gas laser (A. Javan, W. Bennett, and D. Herriott; USA, 1960) using a mixture of neon and helium atoms. It was followed by the semiconductor injection laser (R. Hall, and also W. Dumke and collaborators; USA, 1962). In gas lasers, a population inversion is achieved not by pumping with light but rather as a result of collisions of the atoms or molecules in the working gas with electrons or ions provided by an electrical discharge. Among the gas lasers the helium-neon laser and the laser using a mixture of carbon dioxide, nitrogen, and helium (the CO2 laser) are distinguished, since they can operate in both the pulse and continuous modes. The light oscillations produced from a helium-neon laser are very stable (ℏ 10−13) and highly monochromatic (Δv = 1 Hz at a frequency of 1014 Hz). Although the efficiency of this laser is very low (0.01 percent), the high mono-chromaticity and directivity of its emission (which are due particularly to the homogeneity of its active medium) have made it indispensable in all types of alignment and leveling work. A powerful CO2 laser (C. Patel, USA, 1964) generates infrared radiation (λ = 10.6μ). Its efficiency, which reaches 30 percent, exceeds that of all existing lasers operating at room temperature. The gas dynamics laser is particularly promising because it can produce a power of several dozen kilowatts in the continuous mode. Its monochromaticity, directivity, and high power make it promising for a number of technological applications.
In semiconductor lasers a population inversion is attained mainly by injecting current carriers through a p-n junction into a suitable doped semiconductor. There are a number of semiconductor materials from which lasers can be made for an extensive range of wavelengths. The most common is gallium arsenide (GaAs), which at the temperature of liquid nitrogen can radiate continuously in the near infrared region at a power of up to 10 W with an efficiency of 30 percent. The power generated by injection lasers may be controlled virtually without lag by changing the injection current, suggesting their use in high-speed computers and communication systems.
Population inversions are produced in paramagnetic amplifiers and in ruby, gas, and semiconductor lasers by using completely different physical phenomena, but the common and important factor for all methods of creating a population inversion is the need to overcome the processes that tend to reestablish an equilibrium population. The processes of reestablishing a population equilibrium can be inhibited only by expending energy derived from an external source. As a rule, only a small part of the pumping energy is converted into laser emission. In the free generation mode the efficiency of a ruby laser is less than 1 percent, and for giant pulses it is even lower. However, the “loss” in the emitted energy is compensated in quantum electronics by the gain in its “quality,” the monochromaticity and directivity of the radiation, which are due to the nature of stimulated emission.
Monochromaticity and high directivity make it possible to focus all the energy radiated by a laser into a spot having dimensions close to the wavelength of the radiation. In this case the electric field of a light wave attains values close to those of intra-atomic fields. When such fields interact with a substance, completely new phonemena take place.
Applications. Quantum electronics has revolutionized SHF radio physics and optics and has produced very large transformations in optics. In radio physics the development of masers denoted the appearance of radio devices that were new in principle but at the same time had properties familiar to the radio engineer. Even before the development of quantum electronics, coherent amplifiers and monochromatic generators existed in radio physics. Quantum electronics merely made a marked improvement in the sensitivity of amplifiers (by a factor of 103) and the stability of frequency generators (by tens of thousands of times). In optics, on the other hand, before the appearance of lasers all light sources had neither any appreciable directivity nor monochromaticity. The creation of lasers marked the appearance of light sources with completely new properties. This provided an opportunity previously unknown in optics to concentrate radiant energy both spatially and within a narrow frequency band.
Industry produces various types of lasers, which are used not only as effective research instruments but also in solving various types of practical problems. The principal advantages of the laser effect are the small area of diffusion of heat, the absence of electric charge transfer and mechanical contact, and the feasibility of operating inside vacuum flasks and in corrosive gases. One of the first applications of lasers was in measuring the distance to the moon with greater accuracy than had been possible using the radio-physics method. After a corner reflector had been set up on the moon, the distance to it was measured to an accuracy of 1.5 m. A laser radar service for the earth-to-moon distance now exists.
The use of lasers in optical communications has opened up new possibilities. The development of optical communications links, with their problems of modulation of oscillations, detection, heterodyning, and converting the frequencies of the light oscillations, made necessary the transfer into optics of methods used in radio physics and the theory of oscillations.
Nonlinear optics, which deals with nonlinear optical effects whose nature depends on the light intensity (self-focusing of light, generation of optical harmonics, induced light scattering, and self-brightening and self-darkening of light), has emerged. Methods of nonlinear optics have been used to create a new class of frequency-tunable sources of coherent radiation in the ultraviolet region. Phenomena of nonlinear optics occur only in a narrow intensity range of the laser radiation. At low intensities no nonlinear optical effects appear, but as the intensity is increased they appear and build up, but even at flux intensities of 1014 W/cm2 all known substances are destroyed by a laser beam and are converted into plasma. The production and investigation of the laser plasma is one of the most interesting uses of the laser. Thermonuclear fusion initiated by laser radiation has been achieved.
Because of the high spectral and spatial concentration of electromagnetic energy lasers have come to be used extensively in microbiology, photochemistry, chemical synthesis, dissociation, and catalysis. Quantum electronics has led to the development of holography—a method of producing three-dimensional images of objects by reconstituting the structure of the light wave reflected from an object.
N. G. Basov and A. M. Prokhorov of the USSR and C. Townes of the USA received the Nobel Prize in physics in 1964 for their work on quantum electronics.
REFERENCESKvantovaia elektronika: Malen’kaia entsiklopediia. Moscow, 1969.
Fabrikant, V. “Klassika, kvanty i kvantovaia elektronika.” Nauka i zhizn’ 1965, no. 10.
Prokhorov, A. M. “Kvantovaia elektronika.” Uspekhi fizicheskikh nauk, 1965, vol. 85, fasc. 4.
Basov, N. G. “Poluprovodnikovye kvantovye generatory.” Ibid.
Shavlov, A. “Sovremennye opticheskie kvantovye generatory.” Uspekhi fizicheskikh nauk, 1963, vol. 81, fasc. 4.
Townes, C. “Poluchenie kogerentnogo izlucheniia s pomoshch’iu atomov i molekul.” Uspekhi fizicheskikh nauk, 1966, vol. 88, fasc. 3.
N. V. KARLOV
A loosely defined field concerned with the interaction of radiation and matter, particularly those interactions involving quantum energy levels and resonance phenomena, and especially those involving lasers and masers. Quantum electronics encompasses useful devices such as lasers and masers and their practical applications; related phenomena and techniques, such as nonlinear optics and light modulation and detection; and related scientific problems and applications, such as quantum noise processes, laser spectroscopy, picosecond spectroscopy, and laser-induced optical breakdown.
In one sense any electronic device, even one as thoroughly classical in nature as a vacuum tube, may be considered a quantum electronic device, since quantum theory is accepted to be the basic theory underlying all physical devices. In practice, however, quantum electronics is usually understood to refer to only those devices such as lasers and atomic clocks in which stimulated transitions between discrete quantum energy levels are important, together with related devices and physical phenomena which are excited or explored using lasers. Other devices such as transistors or superconducting devices which may be equally quantum-mechanical in nature are not usually included in the domain of quantum electronics. See Laser, Maser, Optical detectors, Optical modulators