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The Laser: Still Young at 25? Anthony E. Siegman The first laser device was operated just over 25 years ago. In the subsequent two-and-a-half decades, laser devices of many di- verse types have produced an enviable record of accomplish- ments in fundamental science, applied technology, medicine, and even home entertainment. Although not yet of the eco- nomic importance of conventional electronics, the laser industry is significant and growing. This paper will provide a brief review of the history of the laser; describe some of the characteristics and performance capabilities of different types of laser devices; give a short introduction to the breadth and diversity of laser applications; and finally, summarize some of the exciting current accomplish- ments and possible future advances in laser technology. To begin with the very early history or, perhaps, the mythology of the laser, the following passage from H. G. Wells's famous 1896 novel The War of the Worlds, about an invasion of Earth by Martians, gives a reasonably accurate description of how one might make and then use a laser. In some way they fthe Martians] are able to generate an intense heat in a chamber of practically absolute nonconductivity.... This intense heat they project in a parallel beam against any object they choose, by means of a polished parabolic mirror of unknown composition.... The second part of this quotation then says: L00K~NG BACK 25 YEARS
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2 ANTHONY E. Sit EGMAN 1 1 However it is done, it is certain that a beam of heat is the essence of the matter. What is combustible flashes into flame at its touch, lead runs like water, it softens iron, cracks and melts glass, and when it falls upon water, that explodes into steam.... To anyone who has seen the materials processing effects pro- duced by even a medium-power laser beam, Wells's description, written at the turn of the century, will seem a remarkably accurate account of the effects of the beam from a modern carbon dioxide (CO2) laser of perhaps a few kilowatts of power output. It should also come as no surprise that we now know, from our own space probes, that the natural atmosphere of the planet Mars consists primarily of carbon dioxide, and that, in fact, natural laser action, pumped by sunlight, occurs in the Martian atmosphere. We of course do not really believe that Martian invaders brought CO2 lasers with them to Earth a century ago, but the basic principles of stimulated emission from atoms or molecules were first recognized by Einstein, Ladenburg, and others in the 1930s. The first man-made device to use these principles came only in 1955, when Charles H. Townes of Columbia University operated the first ammonia beam maser the acronym coined by Townes to stand for microwave amplification by stimulated emission of radiation. This was closely followed by similar developments by N. G. Basov and A. M. Prokhorov in the Soviet Union. Townes's first maser was, of course, not an optical device, but a weak microwave oscillator at 24 GHz. In the years that followed, many researchers gave much thought to the possibility of optical masers, or lasers. This process of development was much assisted by the concept of the continuously pumped three-level microwave maser developed by N. Bloembergen at Harvard University in 1956. In 1958 Townes and Arthur L. Schawlow of Bell Laboratories published a paper and patent application that gave a theoretical recipe for laser action. The first successful optical-frequency laser device, the pulsed ruby laser, was actually developed in 1960 by an industrial researcher, Theodore H. Maiman, in the Hughes Research Laboratories in Malibu, California. Maiman produced this first laser action by placing silver mirrors directly on the end of a synthetic crystal of ruby, and then pumping or exciting this crystal with an intense flash of light from a standard photographic flash lamp. Maiman's pio- neering advance was rapidly followed by the development of a number of other laser devices by the IBM and Bell Telephone Laboratories; in particular, the first continuously running, elec- trically pumped gas lasers were developed in the same year at
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TH E LASER: STI LL YOU NG AT 25? 3 FIGURE 1 Arthur L. Schawlow demonstrating that a flash of red light from a small ruby laser breaks the dark-colored inner balloon without damaging the transparent outer balloon. This procedure exactly mimics the way laser light can be used to repair a detached retina inside the eye- ball, make a weld inside a closed vacuum chamber, or trigger a chemical reaction inside a closed chemical cell. Photograph by Frans P. Alkemade. Bell Labs. The laser field has been characterized ever since by the continual emergence of new and ever-surprising laser sys- tems, a process that is still going on today. Schawlow, one of the most distinguished scientists in the laser field, was a corecipient of the 1981 Nobel Prize for his work on laser spectroscopy. He was also the pioneer of the first edible laser: a glass cell full of unflavored gelatine, which he first operated as a laser and then ate. Figure 1 shows another of Schawlow's distinctive demonstrations. HOW LASERS WORK The construction of a laser begins with a collection of atoms. These atoms or molecules can be gaseous, liquid, or solid in form, but they are characterized, as are all atoms, by a set of discrete and distinctive quantum energy levels. Next, some form of pumping process must be applied to these atoms. This pumping process can be accomplished in a great
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ANTHONY E. SIEGMAN many ways. In all cases, however, its essential function is to excite or lift some of the laser atoms out of their lowest quantum energy level and into upper energy levels (see Figure 2~. If atoms can be excited into upper energy levels and, more importantly, a condition of population inversion can be achieved, in which more atoms are excited into some upper atomic level than into some lower atomic level, then laser action can occur. If a beam of light tuned to the transition frequency between those two levels is sent through the collection of atoms, that light beam will be amplified through the process of stimu- lated emission. Stimulated emission means simply that the electromagnetic fields in the light beam cause the atoms to drop down from the more heavily populated upper level into the less heavily populated lower level, giving up their energy to the light beam in the process, in phase (i.e., coherent) with the exciting field. Next, a carefully aligned laser mirror is added at each end of the collection of atoms to form an optical resonator, in which the lightwave can bounce back and forth many times. If the round- trip gain in this resonator exceeds the round-trip losses due to absorption and finite mirror reflectivity, then the light signal in this laser cavity can build up to a coherent optical oscillation, exactly the same as in an audio frequency oscillator or a radio ~ . frequency transmitter. The optical signal in this laser cavity can be highly monochro- matic, or of a single frequency, or temporally coherent because it is a true coherent oscillator. It can also be highly directional (collimated) or spatially coherent because of the directional control produced by the two mirrors. THE PRESENT STATUS OF LASERS An enormous number of widely different types of laser devices have now been discovered (Table 11. They cover the wavelength range from shorter than 1,000 A in the vacuum ultraviolet to longer than 800 ,um in the far infrared or millimeter wavelength range. The list of laser materials covers essentially every form of matter, from simple gases and solids to liquids, plastics, flames, jet engine exhausts, and interplanetary space. Indeed, Schaw- low's law, yet to be disproved experimentally, says that anything will lase (i.e., generate a beam of laser light) if it is hit hard enough. Of course, if something does not lase, then it was not hit hard enough. The list of atoms in which laser action has been obtained 1 1
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- THE LASER: ST~L YOUNG AT 25? 5 (a) (b) (C) FIGURE 2 (a) A collection of laser atoms and their quantum energy levels. (b) The laser pumping process. (c) Stimulated emission and laser oscillation.
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6 ANTHONY E. Sit EGMAN TABLE ~ Present Status of Lasers Laser Characteristic Present Status Wavelength range Laser materials Laser atoms Laser transitions Peak powers Average powers Frequency stability Pulsewidths ~ - 1 unlng ranges From longer than 800 ,um to shorter than 150 Gases: atoms, ions, molecules, excimers Liquids: organic dyes, H2O solutions, Scotch whisky Solids: crystals, glasses, plastics, semiconductors, plasmas, flames, jet engine exhausts, interstellar space, planetary atmospheres More than 100 individual atoms and ions, innu- merable molecular species More than 106 discrete laser lines Greater than 10'3 W Greater than 1 MW Few parts in 10~4 Less than 8 fs (8 x 1o-~5 S) Greater than 200 ~ (about 24,000 GHz) covers essentially the entire periodic table, in both atomic and ionized forms, in addition to a virtually unlimited list of molec- ular species. The number of individual laser transitions is practically uncountable there are, for example, more than 200 individually identifiable laser transitions between different quantum energy level pairs in the neutral neon atom alone. Therefore, the number of possible laser transitions is almost surely in the millions. AN ELECTRONIC OLYMPIC GAMES? One way of dramatizing the extraordinary capabilities of lasers might be through a sort of "Electronic Olympic Games," a set of competitive events to see which electronic devices transistors, integrated circuits, vacuum tubes, or lasers- set the current performance records in generating the highest powers, the shortest pulses, the greatest frequency stability, the lowest noise figure, and other limits. How many gold medals might the laser, in particular, win in such a competition? FREQUENCY RANGE, TUNING RANGE, AND BANDWIDTH Laser devices of many different kinds will clearly win all the available gold, silver, and bronze medals for frequency range, tuning range, and instantaneous bandwidth. The frequency or wavelength range over which different kinds of lasers and
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THE LASER: STILL YOUNG AT 25? masers can operate extends from the subaudio to the x-ray regions. The fractional tuning range of most individual lasers is relatively small, limited by the atomic linewidth of the laser transition used. In absolute terms, however, the bandwidths or tuning ranges of many lasers still extend over tens to hundreds of gigahertz. So-called organic dye lasers, along with semicon- ductor lasers, can, in fact, be continuously tuned over linewidths of hundreds of angstroms; commercially available dye lasers with multiple dyes can be tuned continuously over essentially the entire visible and near-infrared spectral range. The applications of such lasers in chemistry and chemical diagnostics can be easily . . . Imagined. Without going into further detail here, it should also be noted that an ordinary dye laser with a 200-A spectral width has a frequency bandwidth sufficient to transmit the equivalent of one simultaneous telephone channel for every person on earth. PEAK POWER For setting peak power output records, laser devices also stand absolutely supreme, in large part because of their ability to generate very short pulses. Indeed, rather modest mode-locked lasers of tabletop size can easily produce optical pulses with instantaneous peak optical powers in excess of 10~3 W. or several times the total installed electrical generating capacity of the United States though these pulses last for only a few trillionths of a second. These same laser pulses can then be focused into spots only a few optical wavelengths in diameter to produce peak power intensities of billions of watts per square centimeter sufficient to tear atoms apart, break molecular bonds, produce intense nonlinear optical effects, and melt and vaporize any material. Even a small pulsed solid-state laser, for example, can readily drill or cut through steel, ceramic, diamond, or any other material. CONTINUOUS POWER OUTPUT The continuous or average powers available from certain laser devices are also impressive, although it is uncertain whether they exceed those available from all other high-power electronic devices, including klystrons, high-power triodes, and even mo- tor generator sets. Of particular interest here, however, is the enormous diversity of pumping or excitation methods that can 1 _~_
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~ ANTHONY E. Sit EGMAN 1~1 be used for producing laser action (Table 2~. These include not merely electrical discharges of all kinds and optical pumping methods using almost any conceivable light source but also laser action in flames, plasmas, chemical reactions, focused sunlight, nuclear reactions, and nuclear explosions. Especially striking is the fact that several types of natural laser and maser action also occur (without mirrors) both in interstellar space and in plane- tary and solar atmospheres. Particularly impressive in the context of high-power lasers are the chemical and gas-dynamic laser systems, which can convert the energy of a chemical reaction, or pure heat energy in gases, directly into coherent laser radiation with extremely high energy output. Figure 3 shows, for example, a large gas-dynamic laser built in the early 1970s. This laser burned a chemical fuel (cyanogen) in the lower chamber, and then sent the hot gases upward through supersonic expansion nozzles into the laser region to produce some hundreds of kilowatts or more of laser power. The water-cooled mirrors and mirror mounts are in the boxes at the end, and the hot gases are exhausted through the deflectors at the top. This is one of the few lasers that must be bolted down because it has thrust. The rule of thumb for such chemical lasers is that the combustion of 1 kg of fuel can typically produce a sufficient number of excited molecules to provide several hundred kilo- joules of coherent (though multiwavelength) laser output en- ergy. A fuel supply of a few kilograms per second suffices, there- fore, to power a 1-MOO laser oscillator. Israeli scientists have TABLE 2 Laser Pumping Methocis Pumping Method Laser Action Optical pumping Laser materials pumped by flash lamps, arc lamps, tungsten lamps, exploding wires, light-emitting di- odes, flames, focused sunlight, other lasers Gas discharges Direct electron and collisional excitation in glow dis- charges, arc discharges, hollow cathode discharges Chemical reactions Laser action following chemical mixing, flash photoly- sis, flame photolysis Direct electrical Direct electrical excitation in semiconductor injection lasers, electron beam-pumped solids and gases Gas-dynamic lasers Laser action derived from hot gases, supersonic expan- sions, shock fronts Plasmas Laser action in plasma pinches, laser-induced plasmas Nuclear reactions Fission fragment pumping of gas lasers Nuclear explosions Atomic bomb-pumped x-ray lasers Natural lasers Sunlight, interstellar radiation, particle beams
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THE LASER: STILL YOUNG AT 25? 9 FIGURE 3 A large high-power gas-dynamic laser. even developed a gasoline-fueled gas-dynamic laser, with the initial fuel mixture being ignited by an automobile spark plug. EFFICIENCY To be conservative, the laser should probably receive only a silver medal in the category of average power. Many commonly used lasers are also, unfortunately, far less efficient in the use of electrical input energy than would be desirable. The common small helium-neon laser has a typical operating efficiency of only a small fraction of a percent, although there are also useful types of gas and semiconductor lasers that have efficiencies of 60-70 percent from direct electrical input. In this category the laser would receive a bronze medal. PULSEWIDTH In the competition for generating the shortest possible pulses, however, the laser is second to none. The units for expressing the duration of a pulse scale downward in jumps of 1,000 from seconds to milliseconds, microseconds, nanoseconds, picosec- onds with 1 ps already shorter than any form of electronics can go—and finally down to femtoseconds, or units of 1o-~5 S.
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0 ANTHONY E. SIEGMAN One of the most astonishing recent accomplishments in laser technology has been the generation of fully coherent optical pulses as short as 8-10 fs. These have provided truly extraordi- nary new scientific capabilities for exciting, probing, and mea- suring internal processes in atoms and molecules, chemical reactions, biological processes, and solid-state physics far be- yond the time resolution that can be achieved electronically, now or in the near future. FREQUENCY STABll l TY AND SPECTRAL PURI TY Lasers may also be judged with respect to absolute frequency stability and spectral purity. For many decades the international standard for measurements of length has been not the historic meter bar but a visible wavelength derived from an incoherent, or nonlaser, gas-discharge light source. The international stan- dard of time has been a microwave atomic clock, which is not quite but is almost a maser. Certain laser oscillators can, however, have a spectral purity and absolute frequency stability so extraordinary that just in the past few years the international standards for both frequency and time have been, by definition, unified into a single laser device. That is, the new standards for both distance and time are now provided by a certain ultrastable laser oscillator transition located in the middle infrared region of the spectrum. It is more than a little disconcerting to realize what this means: The goal of Albert A. Michelson and so many other distin- guished physicists in the past to make ever more precise measurements of the velocity of light will no longer even be meaningful. Now that the basic standards of time and length are one and the same laser transition, the velocity of light, c, is reduced to a mere defined quantity: the numerical relationship between one wavelength and one period of this laser frequency. One can never again measure c. ANTENNA BEAMWIDTH The antenna properties or the antenna beamwidth of a laser beam are also important. The beam traveling back and forth inside a laser cavity is extraordinarily parallel or well collimated, and the beam outside the laser retains these extremely direc- tional properties, spreading only slightly as it propagates (see Plate 1~. Such a laser beam thus provides a kind of "weightless string" that neither sags nor blows in the wind and so is extremely useful 1
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THE LASER: STILL YOUNG AT 252 ~ ~ as an alignment tool for building construction, tunneling, pipe laying, and many other civil engineering works. This is, in fact, one of the simplest but most significant commercial applications for the laser, and the fortunes of some laser companies in the past have risen and fallen with the construction market. In technical terms, the beam angle in radians for a collimated electromagnetic wave coming from an antenna is more or less equal to the number of wavelengths across the antenna aper- ture. For a visible laser beam and a diffraction-limited, 10-cm- diameter aperture which is not at all difficult to achieve in practice this means a beamwidth of 10 microradians; this means, in turn, that a laser easily has the ability to illuminate a spot not much more than a mile wide on the face of the moon. A microwave antenna with the same beamwidth would have to be several kilometers in diameter. At present, laser antennas up to a meter in diameter are commonplace, for example, in satellite and lunar ranging sys- tems. Given the narrow beams and high peak powers of lasers, laser radar echoes are routinely obtained from optical reflectors located on the moon, with accumulated range accuracies of a few centimeters in the distance to the moon. By making such laser-ranging measurements to a cooperative orbiting satellite simultaneously from multiple stations, it is possible first to determine the satellite orbit and its perturbations with great accuracy and then to determine the relative positions of the laser stations on earth, and thus to map the earth with comparable accuracy even across seas and oceans. COMPUTING CAPABlilTY Laser light certainly will be already is—of enormous impor- tance in fiber-optic communications generally, including fiber- optic computer networks and communications between and within computers. Less well established, however, are the pure computing capabilities of lasers the so-called nhotonic logic possibilities that may come in the future. For sheer power in general computing, all the gold medals will probably continue to go to the silicon chip and its derivatives. The silver medals may well go to gallium arsenide or other new forms of ultrafast electronics rather than to any kind of "pho- tonic computers." Lasers can offer unique capabilities in certain specialized techniques that are more or less computational in nature, such as holography and certain kinds of image processing. However, the laser would receive at most a bronze medal in this area.
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12 ANTHONY E. SIEGMAN Out of a dozen or so major Olympic events, therefore, laser devices of various kinds will win at least seven or eight gold medals, half a dozen silver, and many bronze a record that no other class of electronic devices can approach. LASER APPLICATIONS To date, the applications of the laser in all fields of scientific measurement have been diverse, extraordinary, and unique. Measurements have been and are being made that simply could not be made in any other fashion. Chemistry, biology, and physics laboratories use many types of lasers to probe, measure, and modify the fundamental properties of matter. Mechanical and aeronautical engineering laboratories are equally well equip- ped with lasers. For example, laser beams are projected into huge wind tunnels to measure local flow velocity and turbulence. Lasers are extraordinary tools for identifying materials as well. A medical researcher, by focusing a weak laser beam on an experimental object, can painlessly vaporize a minuscule sample of animal or human tissue for spectroscopic diagnosis of atomic composition, trace elements, and other components. Similarly, an archaeologist can vaporize a tiny sample of a suspect artifact to see if its chemical makeup agrees with its alleged origin. Outside of pure science and engineering research and devel- opment, however, lasers have also been applied in equally diverse and often unanticipated ways in many fields of com- merce, manufacturing, industry, and medicine. Lasers are used in bar code scanners for retail stores, inven- tory control in warehouses, and supermarket checkout stands. Lasers are found in nearly every elementary surveying instru- ment these days for example, lasers mounted on transits are used to obtain contours for leveling of rice paddies and to control dredging barges and automated bulldozers. Laser rang- ing instruments are used for highway surveying and housing . . . . su tic .lvlslon construction. In industry, laser beams offer both measuring and manufac- turing tools that are flexible and versatile; can be precisely controlled; are well adapted to robotics and numerical control; can be adapted to almost any material or environment; are clean, reliable, and economical to run; and never grow dull. The simplest industrial applications of high-power lasers come, of course, in the straightforward cutting of materials. Examples include the cutting of armor plate; the cutting of cast iron, without heating or annealing the surrounding material; the cutting of complex patterns in plywood, glass, plastic, or .
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TH E LASER: STY LL YOU NO AT 25? ~ 3 cardboard; and the cutting of cloth for clothing, under com- puter control, with self-sealing of the fabric edges and minimal waste of material. But beyond this, there is laser heat treating; laser surface hardening; laser scribing, annealing, debarring, soldering, and resistor trimming; the cutting and repairing of integrated circuits; and the drilling of precise holes in turbine blades (Plate 2), as well as in plastic irrigation pipes and rubber baby bottle nipples. The laser has also already truly revolutionized the document- scanning, typesetting, newspaper platemaking, and printing industries, at both the high and low ends of the scale. Today, one can print either a newspaper or an interoffice memo with all the information, including the typefaces, stored in a computer memory and printed out by a computer-controlled scanning laser beam. Similar concepts can also be used for the fast and easy marking and labeling of complex mechanical parts made from almost any material. Lasers are now appearing even in consumer products, such as laser video recorders and audio compact disc players. All of these commercial, industrial, and home applications of laser devices, however diverse and ingenious, thus far have been only preliminary. Indeed, the worldwide sales of laser devices in 1985 totaled only about $400 million-$500 million—or about a quarter of the sales of small computers in the same period by Apple Computers alone. The worldwide sales of laser systems for all purposes- scientific, engineering, industrial, medical, and military, includ- ing ancillary equipment, controls, and materials handling- is believed to have totaled about $4 billion-$5 billion in 1984. Both of these sales figures have been growing annually by 30 percent or more in recent years, although with much higher growth rates in some areas. In the opinion of most observers, the real boom in the use of lasers in industrial and manufacturing processes is just begin- ning. For example, large machine tool manufacturers and small laser companies are only now beginning to merge on a wide scale. Beyond industrial applications, one of the fastest growing areas of laser application is in medicine. Laser surgery is now being used not only to remove skin tumors and conduct other external surgery and to treat many eye diseases but also in ear and throat surgery and gynecology. In addition, using fiber- optic delivery systems, laser surgery is done even inside the intestinal tract and blood vessels. A form of cancer therapy with lasers is also the subject of much investigation and hope in the medical field. In this 1 1
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~ 4 ANTHONY E. Sit EGMAN NEW DEVELOPMENTS 1~1 1 therapy, laser light of the proper wavelength activates a photo- sensitive chemical within malignant human tissue, releasing singlet oxygen that destroys the cancer cells. There are, of course, also many military applications of the laser. Some of the most successful include laser-guided bombs, laser communication links, and laser range finders and aiming devices for guns. Lasers of all types have proved extraordinarily useful devices for bettering the human condition in nearly every area of life, and they will become even more so in future years. But beyond these practical applications, fundamental new basic research advances in the laser field are, even after 25 years, still emerging nearly as rapidly as in the laser's early years. FREE ELECTRON LASERS The last few years have seen, for example, the emergence of the so-called free electron laser, in which coherent oscillation at visible or infrared wavelengths is generated by passing the beam from a high-quality electron accelerator through a suitable wave-propagating structure. This device is not really a laser at all, but it is nonetheless a revolutionary development in coherent optical sources whose capabilities are still only in the infant stage. These devices can provide marvelously tunable, efficient, high-power sources in the far infrared region, from 50 ,um out to a few millimeters in wavelength, where laser sources are still somewhat limited. Scientific applications of such sources are manifold; unforeseen industrial applications are equally certain to emerge. The free electron laser may, with further develop- ment, also provide a similarly useful source in the visible and ultraviolet regions of the spectrum. FEMTOSECOND OPTICAL PUlSES, BISTABILITY, CHAOS, AND SOLITONS The incredible advances made in femtosecond laser pulses during the past few years represent one example of progress in the field. Other fundamental developments have also come within the past few years in the basic understanding of new concepts of optical bistability, instabilities, and chaotic optical
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THE LASER: STILL YOUNG AT 252 15 behavior in lasers. Unexpected developments also occurred in optical solitons and soliton lasers that emerge when such pulses propagate through optical fibers. TRAPPING AND COOLING OF SINGLE ATOMS A/~/D IONS Laser researchers are only now succeeding in trapping individ- ual atoms and ions, or small clouds of atoms, and then cooling them to temperatures in the millikelvin range. Once trapped in this fashion, these atoms can be examined and interrogated with laser beams in ways never before possible. This will extend the ultimate precision of physical measurements and laser standards far beyond the few parts in 10~° that is now achievable to ultimate accuracies of a few parts in 10~3 or 10~4 or even better. MUL TlPlE QUANTUM WEll STRUCTURES . . Another fundamental development of the past few years, im- portant for both electronics and lasers, has been the so-called multiple quantum well structures, or artificial layered materials. It has now become possible, using several different techniques such as molecular beam epitaxy or metal-organic chemical vapor deposition, to prepare layered synthetic materials by depositing under precise control a discrete number of atomic layers, first of one material for example, gallium arsenide—and then an- other such as aluminum arsenide in an alternating sequence. The result is an essentially perfect artificial crystal with an adjustable period or layer thickness in the range of a few hundred angstroms. Because the properties of the electrons in these multiple quantum well structures can differ greatly from ordinary materials, the electronic and optical properties of these materials offer remarkable new capabilities, including much faster forms of conventional electronic devices. In optics, the result has already been much more efficient and shorter wavelength diode lasers for use in fiber-optic communi- cations or audio compact disc players, as well as improved photodetectors, light modulators, and other electro-optic de- vices. By using these diodes as pumps for other laser materials, ultraminiaturized lasers of many different types can be pro- duced. X-RAY lASERS Another topic of continuing interest is the extension of laser techniques to the x-ray region. This will always be a difficult task
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1 6 ANTHONY E. Sit EGMAN for several reasons, one of which is the difficulty of providing mirrors at these wavelengths. Another basic barrier derives from the fundamental equations of laser theory that say that obtaining laser action becomes more difficult at somewhere between the third and the fifth power of the inverse laser wavelength. Lasers in the x-ray region, if they ever become common, are thus unlikely to be similar to lasers in the optical region. Nonetheless, Lawrence Livermore Laboratories has recently announced the observation of stimulated emission and laser amplification in the far-ultraviolet or soft x-ray region, at wave- lengths of 150-200 A, in a target plasma pumped by a high- power laser beam. The same lab has also produced a true, if short-lived, one-shot x-ray laser in the few-angstrom region by pumping a suitable laser material directly with a small nuclear explosion. Given all these recent advances, therefore, it seems clear that the laser field truly is still young, vigorous, and exciting, even as it passes the mature age of 25.
Representative terms from entire chapter: