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8 Applications of Atomic, Molecular, and Optical Physics Atomic, molecular, and optical (AMO) physics lies at a confluence of basic science, applied science, and technology. The applications of AMO physics to the needs of society and to our nation's goals are extensive. They play a conspicuous role in the field, contributing to its vitality and its diversity; they represent a visible return to society for its support of basic science. From a large list of these applications we have chosen to describe precision measurements, fusion, national security, fiber-optics com- munication, materials processing, manufacturing with lasers, data-base services, and medicine. This list, however, is far from comprehensive. For lack of space, or because the information may not be publicly available, we have omitted a number of other major activities. Subjects that are entirely omitted, or are only discussed in passing, include environmental monitoring, optical data processing and optical comput- ing, laser isotope separation, inertial confinement, photochemical processing, and laser weapons systems. PRECISION MEASUREMENT TECHNIQUES The art of high-precision measurement and the application of preci- sion measurement techniques to basic science and to technology is strongly entrenched in AMO physics. The tradition can be traced to Michelson, who invented the Michelson interferometer to search for 151
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152 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS the ether drift and realized that he could use it to measure machinists' gauge blocks and length standards to unprecedented precision. The tradition is very much alive today. Within the past decade there have been major advances in precision measurements, the most dramatic of which has culminated in the redefinition of one of the four independent basic units mass, time, electric current, and length which are gen- erally considered to make up the cornerstone of physical measurement. Laser metrology became so precise that the accuracy with which the speed of light could be measured was limited by the primary standard of length. As a result, the speed of light was recently fixed by convention, removing the need for an independently defined standard of length. Formerly, the meter was defined as a certain multiple of the wavelength of the red spectral line of krypton; now it is defined as the. distance that light travels in a certain fraction of the second. (See Figure 8.1.) Of all the quantities in physics, time is by far the most accurately measured. The primary time standard in the United States is an atomic clock basically an atomic-beam magnetic resonance apparatus lo- cated at the National Bureau of Standards in Boulder, Colorado. It has an accuracy of 1 part in 10~3, approximately 3 seconds in one million years. Atomic clocks play an essential role in very-long-baseline interfer- ometry in radio astronomy. Antennas at distant positions in the world observe radio waves emitted by distant radio galaxies and quasars. Hydrogen maser atomic clocks at each antenna synchronize recordings of the phase and amplitude of the signals to a fraction of a microsecond. The recordings are then taken to a central location where the interfer- ence patterns formed from the signals are studied. The resultant data correspond to an angular resolution of 0.0001 arc second, far beyond the resolution of an optical telescope. (0.0001 arc second is the angle subtended by 20 centimeters at the distance of the moon.) Atomic clocks are routinely used to synchronize radio and TV communication signals, and they are an essential component of a global system of navigational satellites. The gravitational red shift of time has been measured with a rocketborne atomic clock that was stable to parts in 1O~5 over a period of hours, as described in Chapter 4 in the section on Elementary Atomic Physics. (See also Figure 1.1.) The ultimate precision of a cesium atomic-beam frequency standard is limited by the brief time the cesium atoms spend in the atomic-beam apparatus. Appreciably longer interaction times have been obtained by
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APPLICATIONS OF AMO PHYSICS 153 using electronic traps to store ions, permitting the creation of clocks of much greater precision. A combination of laser and light radio- frequency field is used to observe the hyperfine transition of the stored ion. A technique called laser cooling (described in Chapter 6 in the section on Laser Spectroscopy) has been successfully applied to reduce the temperature of the trapped ion, an important advantage for precision time measurement. Temperatures in the millikelvin range have been achieved, and still lower temperatures appear to be possible. Not only will the spectral lines be narrower, but the detectability of a small number of ions will be enhanced by their localization in microm- eter-size regions. These trapped ions can be used to create an optical frequency standard, that is, an atomic clock operating at an optical frequency rather than a microwave frequency. Neutral atoms have recently been slowed with laser light, even brought to rest in free space. It may be possible to trap these atoms and employ them to create yet another new type of optical atomic clock. Stable laser sources based on atomic and molecular transitions opened the way to measuring directly the frequency of light in the visible spectrum. (The frequency of light is about 106 times higher than the frequency of microwaves.) This feat was performed with nonlinear optical devices used to multiply and compare the signals of a series of lasers operating at successively higher frequencies. The first laser in the chain was related directly to the cesium frequency standard. The measurement provided an absolute reference standard for wavelengths in the visible part of the spectrum; it is this advance that led to the redefinition of the meter described above. The ability to measure optical frequencies accurately represents an important advance in the transfer of electronic techniques from the radio-wave and microwave regions to optical regions. Advances in optical communications and optical data processing are already emerg- ing as part of this revolution. Another advance in metrology has been made possible by combining laser interferometry with x-ray diffraction. The technique has been used to measure directly the ratios of wavelengths of selected x-ray transitions to precisely known optical wavelengths. (See Figure 8.2.) These measurements provide calibration lines in the x-ray spectrum and make it possible to use x-ray measurements on heavy muonic atoms to make precision tests of quantum electrodynamics (QED). Furthermore, they have eliminated the need for the X unit formerly used to link x-ray wavelengths to standards units of length.
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154 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS I2 | Counter: Photodiode [~3 x 2 T He-Ne Laser I ~ 1 | Counter Em) Cry ~ x2 Color-Center Laser Photod lode _ l | Counter ~ ~ x5 Diode CO2 Laser 1 | Counter - ($) f x7 1 Diode Infrared Laser (alcohol) , - 1 | Klystron ~ Diode X7 [ Very-Far-lnf fared Laser l T x7 | Klystron Diode 6) Diode X7 -
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APPLICATIONS OF AMO PHYSICS 155 FUSION The states of reactant material in controlled thermonuclear fusion devices are varied and extreme. Inertially contained plasmas are thousands of times more dense than normal solids; magnetically confined plasmas are a million times less dense than air. Plasma temperatures can reach 108 degrees. Atomic and molecular physics provide vital expertise and essential data needed for designing thermo- nuclear fusion reactors. Fusion research also contributes to atomic physics, for the processes that occur under fusion conditions provide opportunities to observe at close range atomic phenomena that are rarely found on Earth, though they may be common elsewhere in the universe. Controlled thermonuclear devices burn isotopes of hydrogen to form an ash of helium and neutrons, with a prodigious energy release. (The currently envisioned reaction uses tritium and deuterium as fuel.) In the interior of stars the containment of such a ball of fire is performed effortlessly by gravity, but containment on Earth presents a major challenge. Today's efforts focus on two approaches: magnetic confine- ment, which uses magnetic fields to curb the escape of charged particles from the plasma, and inertial confinement, in which burning occurs too quickly for the material to escape. Atomic problems are involved in each approach. FIGURE 8.1 The Frequency of Light and the New Meter. The frequencies of radiowave or microwave signals can be measured quickly and accurately using high- speed electronic methods to count the number of oscillations in a fixed time interval, for instance, 1 second. In the optical regime, however, one normally measures wavelength, using a spectrometer, not frequency. This is because in the past there was no way to count the high-frequency oscillation of light, typically one million times faster than microwave signals. Now this has changed. The drawing shows how the frequency of light has been measured, specifically the frequency of one of the absorption lines in molecular iodine. A series of highly stable lasers of increasing frequency was compared, using nonlinear elements to generate harmonics (exact multiples of the frequency) to "jump" from one frequency to the next. Frequency counters were used to measure the small differences between the multiples of one laser frequency and the frequency of the next. The final laser was locked to a narrow absorption line in molecular iodine. Measuring the frequency of light represents an important advance in metrology, for the frequency of light can be measured much more accurately than its wavelength. One result of this measurement is that the meter has been redefined: the meter is no longer defined as a certain number of wavelengths of a spectral line in krypton; the meter is now defined as the distance that light travels in exactly 1/299,792,458 of a second. The experiment marks an important step toward transferring electronic techniques to optical frequencies. (Courtesy National Bureau of Standards.)
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156 A TOMIC, MOLECULAR, AND OPTICA ~ PHYSICS ~ OPTICAL FRINGE X-RaY SOURCE OPTICAL ~ ~:~ LIGHT E jE X-RaY FRINGES ,\JW' at/ (a) ,....... ~;~1 ( b ) X-RAY FRINGE (C) -t. J- ~ (d) : ,& · · _ , . , i. %~ 1.9 ANGSTROMS - .~ ; ~ ~ - ,. : .' . . . . . . . ; . . . DISTANCE ~ FIGURE 8.2 Measuring the Distance Between Atoms. It is relatively straightforward to measure distance on the scale of 1 meter to 1 part in 106 or better. The wavelength of light is less than a millionth of a meter and with modern laser interferometry the number of wavelengths between two surfaces can be counted directly. Accurately measuring distance shorter than the wavelength of light, however, is a much more challenging problem. The distance between atoms in a crystal is less than a thousandth of the wavelength of light, but this distance can be measured to high accuracy using an apparatus that simultaneously combines x-ray and optical interferometry shown in (a). Once calibrated, the crystal can then be used to measure the wavelength of x rays to about the same accuracy. The heart of the device is an x-ray interferometer made of a single crystal of silicon. (The creation of such crystals is one of the important scientific advances made possible by the semiconductor device industry.) The interferometer, which is composed of three slabs, can be seen at the top of (a). One of the slabs slides laterally, translated by a mechanical stage composed of a series of levers and hinges formed by the holes and slots in the block. Every time it moves by a distance equal to the spacing between atoms in the crystal, the x-ray interference pattern creates a new fringe. The movement is governed by a feedback signal from a laser-driven optical interferometer. The arrangement of the two interferometers is shown in (b). Data showing a single spike from the optical interferometer superimposed on the rapidly varying x-ray fringe are shown in (c). The
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APPLICA TIONS OF AMO PHYSICS 157 Magnetic Confinement The major problem in magnetic confinement is to insulate the plasma from the vacuum vessel. Bremsstrahlung and line radiation from the capture of electrons into excited states by medium- and high-, impurity ions, such as iron and tungsten, are important cooling processes, for the plasma is so tenuous that the radiation flows unhindered to the walls. Electron recombination processes play a crucial role in governing this energy loss, for they determine the charge states of the ions. The dominant process is resonant or dielectronic recombination. For tungsten under typical tokamak conditions, for example, at a temperature of 1 keV, if dielectronic recombination is neglected the average charge state is +50; when it is included, the charge state is +28 and the radiation loss is drastically decreased. Until recently, dielectronic recombination had never been observed directly, and plasma modeling had to rely solely on theoretical rates. Experiments are now possible, and four independent measurements of this process have been made, as described in Chapter 4 in the section on Atomic Dynamics. The results are startling: discrepancies between the experimental values and the theoretical predictions have been as large as a factor of 5. Understanding this process quantitatively is a vital problem for the fusion plasma modeling effort. Line radiation from impurities in the plasma is a valuable diagnostic probe of the reactant material. Forbidden transitions to the ground states of elements whose ionization energies are near the electron temperature are particularly useful. These lines reveal the ion temper- atures by their Doppler profiles and provide ion transport information by their relative line intensities. Unfortunately, the energy levels for highly stripped species are not commonly known: much of the neces- sary spectroscopic work has had to be carried out on the fusion plasma itself. Such studies are expensive, and they cannot provide the data needed for the next generation of fusion devices. This information can, however, be provided from sources such as laser-produced laboratory plasmas, fast-ion beams, and ion-beam-pumped gases. The experimen- measurement depends on being able to move the interferometer with a resolution much finer than the size of an atom. The enormous resolution of the interferometer is shown by the blowup of one of the x-ray fringes (d). By comparing the x-ray and optical fringe patterns, the distance between the atoms in the silicon crystal can be found directly in terms of the wavelength of light to an accuracy of about 1 part in 107. Crystals calibrated by this method can be used to measure x-ray and gamma-ray wavelengths to about the same accuracy. (Courtesy of the National Bureau of Standards.)
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158 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS tat studies need to be accompanied by theoretical research, particularly on many-electron systems in which relativistic and QED effects are large. The most successful heating method for several fusion devices is to inject fast neutral atoms of hydrogen, which are ionized and trapped in the reactor. Premature ionization of the neutral beam by collisions with highly stripped impurities on the plasma sheath is a potential problem. New data on electron loss from molecular hydrogen in collisions with highly charged impurity species suggest that early estimates were overly pessimistic and that adequate penetration into the core of today's devices is possible. Little direct data for atomic hydrogen is yet available, however. For the very large machines planned more ener- getic neutral beams are needed. These beams currently are created by accelerating singly charged ions, H+ or D+, to the desired energy and then neutralizing them by charge exchange in a gas such as H2 or D2. Unfortunately, at high energy the electron capture cross section is too small for the method to be practical. This problem could be avoided by using high currents of negative ions of light elements. Such ions can easily be stripped of their outer electrons, even at high energies. The physics of single-electron detachment methods for millielectron-volt negative ions remains to be explored. Hydrogen and light impurities in the center of the plasma are generally fully ionized. However, a neutral hydrogen concentration as low as 1 part in 105 can drastically change the ionization state distribution in the plasma and the energy loss due to radiation from impurities. The radiation loss is governed by charge exchange into excited states of the highly stripped impurities. Experimental total cross sections for these processes have become available, but cross sections into particular final states need to be known, and these are so far available only theoretically. Inertial Confinement Many of the collision and spectroscopic parameters of concern to magnetic-containment fusion are also important in inertially contained plasmas. One problem that is specific to inertial confinement is development of an efficient driver for compressing the reactant mate- rial to ignition conditions. For example, development of a high-pow- ered short-wavelength laser with a well-chosen pulse shape could provide a more efficient driver than existing CO2 or Nd: glass lasers: the KrF excimer laser is one candidate. An ion beam is a possible alternative to laser light for compressing the reactants. A central
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APPLICA TIONS OF AMO PHYSICS 159 question in ion-beam compression is how fast ions stop in material whose density and temperature are high. A vast number of ion-electron and ion-ion collision processes must be understood. The spectroscopic needs of inertial confinement present an ad- ditional complication: the plasma is so dense that it can cause signifi- cant changes in the energy-level structure of the ions. For example, shifts of tens of electron volts are expected in the Lyman-alpha radiation from argon. X radiation from the plasma core is the major diagnostic signal that can penetrate the compressed material, but to interpret the signal it is essential to understand the structure of highly relativistic systems, including energy levels, transition rates and fluo- rescence yields, and satellite structures. Extremely high Z ions occur in ablative pusher targets, which use thin shells of, for example, gold (Z = 79), for which QED shifts to the energy-level structure are impor- tant. In addition to this spectroscopic information, diagnostic interpre- tation of the spectra require that the inner-shell ion-ion and ion- electron collision processes, which are responsible for production of the needed inner-shell vacancies, be well understood. The very fast decay of inner-shell vacancies through radiationless transitions pro- vides a femtosecond or even attosecond time scale that makes it possible in principle to utilize these Auger processes to probe the dynamics of the target compression. A thorough theoretical under- standing of the de-excitation of deep vacancies in multiply ionized atoms is required for this purpose. Although much information on ion-atom collisions has become available over the past decade, exper- imental work on inner-shell vacancy production in ion-ion collisions is rare. Considering all these problems, it is evident that the fusion energy program will continue to demand intensive efforts from atomic physics. NATIONAL SECURITY AMO physics contributes to the national security by providing basic information and devices that are vital to our defense systems. The challenge of accurate navigation offers one example. Atomic clocks and frequency standards are at the heart of our modern navigational systems. The accuracy of these devices is prodigious clocks with an accuracy approaching parts in 10'5 have been developed, and even higher accuracy appears to be possible. Atomic clocks are used in a precise positioning system that makes it possible to determine where one is anywhere on Earth to within 10 meters. The system can be used
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160 A TONIC, MOLECULAR, AND OPTICA f PHYSICS for civil as well as military navigation. The creation of the atomic clocks and frequency standards that make this system possible drama- tizes the interplay between basic and applied science in atomic physics. These devices grew out of basic research in spectroscopy and the structure of matter. In addition to their role in navigation, they are crucial to the operation of secure communications systems, and they have applications in areas of science such as very-long-baseline interferometry, determination of the fundamental constants, and tests of relativity. Atomic clocks in use include the hydrogen maser, the cesium atomic beam clock, and the optically pumped rubidium cell. The precision of these devices is ultimately limited by the second-order Doppler effect, the time dilation due to the motion of the atoms. To overcome this barrier, the motion of the atoms must be eliminated. Stored ion spectroscopic techniques provide one solution. During the last 5 years enormous strides have been made in designing ion traps and in cooling trapped ions to a small fraction of a kelvin. Recently a beryllium-ion frequency standard has been operated within a stability that is within a factor of 3 of high-performance cesium-beam devices. Future work using mercury ions is expected to improve this by several factors of 10. Another technique that can reduce the second-order Doppler effect is the cooling and trapping of neutral atoms. An atomic beam of sodium has been cooled to 4 percent of its original velocity. This advance has catalyzed interest in atom traps, for if the atoms are sufficiently slow they can be trapped and stored in a neutral-particle trap. The problem of communicating with submarines provides a second example of how AMO research can play an important role in the national defense. Our nuclear fleet is the least vulnerable of all of our defense systems. Communicating with the submarines, however, is difficult because radio waves are strongly absorbed by seawater. Laser communication is a promising technique. Ocean water has a spectral window in the blue-green region. A number of laser systems are being studied for generating blue-green light for this application and also for underwater surveillance, for illumination, and for bathymetry. The excimer lasers, which are described in Chapter 6 in the section on Lasers The Revolution Continues are particularly promising. Be- cause these lasers operate in the violet and ultraviolet, their light must be downshifted to the blue-green. Several atomic gases are promising candidates, including barium, bismuth, and lead. These vapors, how- ever, require high temperatures, and they are corrosive. Raman- shifting techniques using the molecules hydrogen and deuterium pro- vide a second approach for shifting the light from the ultraviolet to the
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APPLICATIONS OF AMO PHYSICS i61 blue-green. Work continues toward developing a system that satisfies all the requirements of wavelength, efficiency, power, and lifetime. A highly sensitive frequency-selective detector is needed for most of the applications of the blue-green laser. One proposed device is based on atomic absorption and fluorescence. A narrow-band, wide-field-of- view detector can be created by enclosing an atomic gas between two filters. The first filter transmits the to-be-detected radiation, while the second filter blocks it. The light excites a resonance transition in the gas. The fluorescence occurs at a different wavelength, which is transmitted by the second filter. Very high quantum efficiency has recently been achieved with this scheme. Highly sensitive frequency-selective detectors have military appli- cations in the optical, infrared, and millimeter-wave regions. One scheme for a millimeter/submillimeter detector exploits the high ab- sorption cross sections of highly excited Rydberg atoms for very-long- wavelength radiation. A pair of Rydberg states is chosen to be resonant with the incident radiation. By putting the system in an electric field so that the higher Rydberg state is field ionized while the lower is not, one produces, on absorption of a photon, a free electron, which is readily detected. The system behaves like a phototube for millimeter waves and microwaves, with the additional advantage of being highly fre- quency selective. The high power and high efficiency of carbon dioxide lasers devel- oped over the past decade suggest the potential for application for high-resolution radar. Unfortunately, their radiation at a wavelength of 10 micrometers can be attenuated under certain atmospheric condi- tions. Many atmospheric windows exist in the millimeter and submil- limeter regions, however, and although efficient lasers are not available at these frequencies, the radiation can be generated by pumping various organic molecules with a CO2 laser. The free-electron laser has been demonstrated in the past decade. This device is potentially capable of generating highly efficient, tunable short-wavelength radiation. Radiation at 1.5 micrometers has been generated with almost 10 watts of power, and the device has been operated in the visible. Chemical lasers are another class of high-power lasers of molecular interest. These, too, are quite efficient, though restricted with respect to wavelength. High-power laser, microwave, and particle-beam devices require high-power switches that can operate at rates greater than 1 kilohertz, pass more than 10 kiloamps, and hold off more than 50 kilovolts. Spark-gap and diffuse discharge switches are two possible gaseous devices. There is no difficulty in closing such switches rapidly, but
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164 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS MANUFACTURING WITH LASERS Industrial applications for lasers were evident when the first ruby laser punched a hole through metal. The earliest applications were all highly specialized, but lasers are now also employed for numerous routine manufacturing tasks. (See Figure 2.2.) Their use is spreading rapidly, and lasers can be expected to play major roles in wide areas of the U.S. industrial enterprise in years to come. Foremost among the advantages of lasers for materials processing and manufacturing are these: Lasers can deliver energy at far greater density than is possible by any other technology. The ability to achieve high temperature in short times permits laser processing of almost any material and opens the way to new machining, welding, annealing, heat treating, and chemical procedures. —Laser light is effectively inertialess and it can be focused with optical precision. As a result, lasers are ideally adapted to automatic control techniques, to robotics, and to high-speed processing of intricate shapes. Thus, lasers are expected to play an increasingly important role in the development of new flexible manufacturing plants. Laser Drilling: Lasers are used to drill both exotic and ordinary material. Tasks include highly specialized applications such as drilling diamond-wire pulling dies and creating multitudes of air-cooling holes in jet engines and routine jobs such as for drilling holes in baby-bottle nipples. Laser drilling is practical for all the metals, plus ceramics, gemstones, semiconductors, plastics, wood, and rubber. Laser Cutting: Laser cutting employs the light to heat the workpiece to its melting temperature and a jet of gas to remove the vaporized or molten material. The focused laser beam acts as an effective point tool, a tool that never makes physical contact with the material. Applications range from cutting out suit patterns to cutting sheet-metal parts. In addition to textiles and metals, the list of materials that can be cut includes ceramics, quartz, glass, composites and exotic aerospace materials, plastics and fiber-reinforced plastics, wood, leather, and fiberboard. Laser Welding: Because laser light can be accurately controlled and directed, laser welding is a direct competitor to electron-beam welding. However, laser welding has an important advantage. It does
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APPLICATIONS OFAMO PHYSICS 165 not require the workpiece to be under vacuum; room conditions are sufficient. Most applications include metals and metal alloys, though some nonmetals can be welded. In some cases, it is practical to weld dissimilar material that could not otherwise be joined. One-half-inch steel plate is routinely welded; even thicker materials can sometimes be handled. Other Applications: Lasers are used in the semiconductor industry for annealing, resistor trimming, and silicon scrubbing. Lasers are used for marking, soldering, and surface treatments including hardening, cladding, and alloying. For example, one factory that manufactures power-steering units currently uses 20 laser systems to heat treat the guiding surfaces. The Future: The factory of the future is expected to be highly automated. The laser, which lends itself naturally to operation under control systems and to automation, provides an ideal technology. The Japanese government has recently initiated a $60 million, 5-year joint university/industry program to study flexible manufacturing systems with lasers, including the integration of lasers with control systems. Approximately 6000 lasers are currently used in factories around the world. Altogether more than 1000 lasers were sold for material processing in 1982, with prices ranging from $5000 to $500,000. The market for laser material-processing equipment has been growing at 20 percent per year over the past several years, and this rate is expected to be maintained over the next decade. In 1982, the sales were $110 million; in 1992 they are expected to be $700 million. At the end of the decade lasers will be commonplace in manufacturing plants ranging from small shops to heavy industrial plants and automated factories. The economic benefit to the United States from the use of lasers in manufacturing and materials processing is expected to be enormous: one survey estimates that lasers will lead to the creation of 600,000 new jobs in these areas (Newsweek, Vol. 108, p. 78, October 18, 1982). The question of manpower is a potentially serious problem for the orderly development of laser-based manufacturing in the United States. There is already a shortage of qualified personnel in the optics and electro-optics industries. As discussed in Chapter 2, the total production in the United States of physicists with Ph.D. degrees in optics is less than 50 a year; the production at the bachelor's level is also small. Unless this problem is successfully addressed, our manu- facturing capability may be limited by a personnel shortage.
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166 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS MATERIALS PROCESSING Laser-Induced Surface Chemistry Laser-induced surface chemistry provides new processing tech- niques with applications in semiconductor electronics and electro- optics. Submicrometer features have been produced in a single proc- essing step, and novel microstructures have been made. In one process, a focused laser beam drives a small-scale chemical reaction at the surface, either in the gas or liquid phase. Submicrometer patterns can be produced by doping, etching, and deposition all based on laser-controlled chemistry. The deposition rate can be much larger than possible by any other means. In a second process, radiation from a high-power laser creates radicals in gas-phase parent molecules that then react at the surface. The exact reaction can be specified by selecting the proper laser wavelength. Since the dissociation occurs i the gas phase, substrate heating is negligible. Lasers can be used to dope surfaces and to etch them. Typically, a pulsed excimer laser is used to photodissociate a gas-phase compound and simultaneously to melt a nearby substrate. A small amount of doping can be incorporated with each pulse. Both Si and GaAs have been doped using this technique. Dry etching of dielectrics, semiconductors, and polymers can be achieved by photodissociating methyl-containing compounds. By illu- minating solid polymers with high-intensity, pulsed, UV light, photo- chemical decomposition of the polymer cross-linking can cause well- defined surface etching. Ion Implantation The bulk properties of solids can be altered or completely trans- formed by the introduction of small amounts of impurity atoms. Accelerator-based atomic physics has created techniques for introduc- ing these impurities in highly novel fashions. Using a small accelerator, ion-implantation methods allow impurities to be added free from the usual material constraints of diffusion and solid solubility. Ion bom- bardment can create electrically insulating layers or new surface alloys; ion-induced defects can induce material diffusion or ion-induced segregation. Near-surface modification by ion bombardment has pro duced amorphous metals and led to the creation of surfaces that are tough and relatively free from corrosion and oxidation. Ion-implanta-
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APPLICATIONS OF AMO PHYSICS 167 tion methods have been applied to the epitaxial growth and doping of diamonds and to the creation of ion-implanted solar cells. Ion- and molecular-beam techniques are now used for the production of integrated circuits. Ion beams can be controlled so precisely that it is possible to construct integrated circuits directly on a silicon wafer. Using molecular-beam epitaxy, solids can be produced in layers of precisely controlled thickness. The layers can be only a few atoms thick. High-power lasers provide a unique tool for annealing the surface of solids and providing diffusion in a limited region. A very short pulse-probe laser can be used to monitor the state of the surface and study the dynamics of the melt. DATA-BASE SERVICES AMO physics provides atomic and molecular data that are essential to wide areas of science and technology and to our nation's energy, military, and environmental programs. An enormous growth of raw data has been made possible by advances in laser spectroscopy, in neutral- and charged-particle scattering, and in theoretical and compu- tational methods. To be useful, it is essential that the data be accurately evaluated, systematically compiled, and efficiently disseminated. Here is a summary of some of the applications. Laser Physics and Development: Electron and photon collision cross sections and potential energy curves are essential for modeling laser plasmas and designing high-power lasers. Atomic-energy levels, wave- lengths, transition probabilities, and atomic lifetimes are needed to model lasing action and to assess probable population inversions. Recombination cross-section data are required for the design of gas-discharge lasers. Nuclear Fusion Energy: Requirements include collision rates and cross sections for electron impact excitation, ionization, and dielectronic recombination; atomic-energy levels and wavelengths for identification of impurity elements in fusion plasmas, for plasma diagnostics and modeling, and for calculating plasma-cooling effects due to radiation from impurity atoms; photon mass attenuation coef- ficients and electron stopping powers are needed to design reactor blankets. Isotope Separation: Atomic-energy levels, transition probabilities,
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168 ATOMIC, MOLECULAR, AND OPTICAL PHYSICS and lifetimes are all needed in order to design a laser isotope-separation process. Atmospheric Monitoring: Photon and electron interaction data are essential for understanding such problems as radio-wave propagation in the ionosphere, the theory of the aurora, cosmic-ray-induced cascade showers and carbon-14 production in the atmosphere. Molec- ular spectral data are needed for remote sensing of the complex chemistry of the polluted atmosphere. Medical Physics: Photon attenuation data are required for cancer therapy using accelerator and isotopic sources of radiation and for the imaging studies and dosimetry of internal nuclear medicine. Industrial Applications: The needs are broad. To cite two examples: electron and photon attenuation and cross-section data are required to determine the effectiveness of ionizing radiation for food processing, the sterilization of medical supplies, and chemical processing such as the polymerization of plastics; atomic-energy levels, lifetimes, and transition probabilities are needed for laser development, surface- property studies, and the design of lamps for street lighting and other applications. National Security: Atomic transition probability data are essential for modeling nuclear explosions in the atmosphere; photon attenuation data are needed to design shielding against ionizing radiation from nuclear weapons. Atomic-energy level, lifetime, and transition proba- bility data are all needed for x-ray laser development. Astrophysics: The needs include extensive electron excitation, ion- ization; and recombination data on astrophysical ions. Wavelengths, atomic-energy levels, and transition probabilities are needed for iden- tifying spectral lines and determining the elemental composition of astrophysical sources. Most atomic and molecular data centers in the United States are associated with the National Standard Reference Data System, man- aged by the Office of Standard Reference Data at the National Bureau of Standards. This program develops important data bases of physical and chemical properties needed by industry, academia, and govern- ment. The program coordinates the activities of 23 continuing Data Centers and 31 shorter-term Projects. Each major Data Center monitors an important disciplinary area and develops and maintains a major data base that is subsequently made available in published, computer-readable, and on-line formats. The Projects answer the need for specialized data bases in particularly important areas. The scientists associated with the Data Centers and
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APPLICA TIONS OF AMO PH YSICS 169 Project monitor the literature in their fields of expertise, compile and evaluate data, mathematically predict data in difficult to measure regions, and prepare major compilations and data bases for the U.S. technical community. Computer-based data networks can now be created. Such networks are essential to meet the nation's growing needs for AMO data and data from other disciplines. MEDICAL PHYSICS AMO physics contributes to the basic science of medicine and to the creation of new techniques for medical practice. We describe here two activities: laser surgery and NMR body imaging. Laser Surgery Laser surgery is becoming a standard practice as lasers replace scalpels in more and more surgical procedures. Fields include general and cardiovascular surgery, urology, dermatology, dentistry, plastic surgery, and oral surgery. A recent conference on medical and surgical application of lasers attracted 1300 physicians and surgeons from 31 countries. One type of cervical cancer that formerly required a hysterectomy can now be treated in a doctor's office or on an outpatient basis using CO2 laser light. A simple, painless procedure provides a high cure rate for a condition that otherwise demands a major operation. The magic of the laser light is in its being able to remove the malignant cells without disturbing the underlying tissue. Lasers are used by eye surgeons for procedures that include repair of a detached retina, treatment of diabetic retinopathy, glaucoma, and iridectomy. All of these procedures capitalize on the ability to control and deliver the laser's energy with extremely high precision. Treatment of diabetic retinopathy, for example, involves destroying cells on the outer layer of the retina without damaging the underlying layer. Iridectomy requires making a tiny hole in the iris, which the laser can do without damaging any of the nearby tissue. Laser surgery is intrinsically sterile. The radiation tends to cauterize a wound, reducing the bleeding and scarring. Optical fibers can deliver the light precisely where it is needed. For example, fatty deposits that clog coronary arteries can be vaporized by laser light delivered through a fiber-optic endoscope. The method holds the promise of revolution- izing coronary arterial surgery.
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170 ATOMIC, MOl ECULAR, AND OPTICAL PHYSICS
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APPLI CA TI ONS OF AMO PH YSI CS 1 7 1 The wide range of wavelengths available from different lasers is an important asset to laser surgery. Infrared radiation from CO2 lasers is most commonly used to remove tissue. The radiation is absorbed rapidly by water and is effective in coagulating blood vessels. Eye surgery generally employs green light from argon-ion lasers, since this is transmitted freely by the lens and the aqueous humor. Argon-ion laser light is also used to treat crimson birthmarks: the light passes through the skin and is absorbed by the underlying blood capillaries that are responsible for the unsightly stain and congeals them. In certain applications, laser light can be used to induce therapeutic photochemical reactions. For instance, tumors can be treated by injecting the patients with the chemical HPD, which is selectively retained by malignant cells. Red light from a tunable dye laser excites the HPD, which transfers the excitation to the cell, killing it. The technique provides a new field of therapy for cancer. Magnetic-Resonance Whole-Body Imaging Using nuclear magnetic resonance (NMR) it is now possible to peer into the human body as gently and clearly as we view its surface. (See Figure 8.3.) Magnetic-resonance imaging (MRI) is nonperturbing, noninvasive, and free of ionizing radiation. It can achieve spatial resolution of 1 mm and can often clearly distinguish different types of tissue. As far as is known, the method is without biological hazard. Recent trials using first-generation equipment have demonstrated MRI to be useful for detecting diseases in the brain, spinal column, heart, thorax, abdomen, liver, and kidneys. MRI has aroused the intense FIGURE 8.3 Magnetic Resonance Imaging. Nuclear magnetic resonance provides a new way to form images of the body's interior using a noninvasive procedure that is believed to be without hazard. In contrast to x rays, magnetic resonance imaging can display the differences between soft tissues. Tumors or lesions and differences between diseased and healthy tissue can often be identified precisely and rapidly. The upper picture shows a cross-sectional image through the midplane of the head. The spinal nerve is clearly visible. The lower image (facing right) is through a parallel plane passing through the eye. The orbit lens and optic nerve are easily discerned. The data collection time for each of these images was slightly over 3 minutes. Magnetic resonance imaging is expected to have a major impact on medical diagnosis. It is made possible by contributions from many fields: magnetic resonance from AMO physics, tomographic techniques developed by mathematicians and medical physicists, and superconducting magnets from low-temperature research. Minicomputers and modern data-processing techniques also play a vital role. (Courtesy of General Electric Research and Develop- ment, Schenectady, New York.)
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172 A TOMIC, MOLECULAR, AND OPTICAL PHYSICS interest of both the medical community and the commercial sector: some practitioners believe that it provides the biggest single advance in medical diagnosis since the discovery of x rays. Nuclear magnetic resonance spectroscopy has been used in chemis- try and biochemistry laboratories for over 30 years as an analytical tool to study the conformation and dynamics of biological molecules. Within the last 5 years two new variants of the MRI technique have emerged: proton imaging, which makes it possible to produce cross- sectional pictures of the human body, and phosphorous imaging, which makes it possible to study physiological function in vivo over volumes of 50 cm3. For example, phosphorous imaging can distinguish between healthy and unhealthy tissue, providing information that can be crucial to cardiologists and peripheral vascular physicians. Applications for NMR body imaging include the following. Thorax: Blood produces a strong NMR signal, which reveals the major vessels and the cardiac chambers. NMR can be used to assess tissue perfusion, to measure cardiac function, and to examine the myocardium (i.e., the middle and the thickest layer of the heart wall composed of muscle). The absence of NMR signals from air-filled lungs makes it easy to observe a hemorrhagic lung, which might be caused by pulmonary embolus, pleural effusion, or tumor. Abdomen: NMR is superior to all the previous methods for diag- nosing lesions of the liver and for detecting many liver diseases. Cystic lesions in the urinary tract are easily differentiated from solid lesions. Aneurisms blood-filled sacs formed by the dilation of the walls of the abdominal aorta—are clearly demonstrated. The presence of arterio- sclerosis with degenerative changes in the wall of the descending aorta can be observed. Pelvis: NMR scanning of female patients shows the extent of malignant new growth that infiltrates the ovary, uterus, and cervix. In males, it can provide a definitive diagnosis of malignant growth before it invades through the prostate capsule. Musculo-Skeletal: Observations in patients with rheumatoid arthri- tis have shown that the presence of inflammation can be imaged, making it possible to assess the response of the inflammation to antiinflammatory drug therapy. Imaging can also reveal how a tumor shrinks as a result of chemotherapy. Companies in the medical equipment business have been quick to grasp the commercial potential of MRI technology. At least 15 manu- facturers are said to have imaging systems available for controlled
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APPLICA TIONS OF AMO PH YSICS 173 clinical research or under development. As of 1983, it was estimated that investigational MRI systems were in use in 35 U.S. sites and 12 sites abroad. The average cost of this equipment is about $1.3 million per unit. A total of about $800 million of internal funds has been spent on MRI systems development by 13 member companies of the National Electrical Manufacturers Association. The market for MRI systems by the year 1990 is assessed at about $2 billion to $3 billion. The development of MRI whole-body imaging required advances from many areas of science. Understanding of the magnetic resonance properties of nuclei in various media comes from studies in physical chemistry and biology; the superconducting magnets, which are essen- tial parts of the imaging apparatus, are a product of low-temperature materials research. Minicomputers and modern data-processing tech- niques also play an essential role. The basic idea, however, magnetic resonance, is due to Felix Bloch at Stanford University and Edward Purcell at Harvard University, who developed NMR in 1946 in order to understand the motions of nuclei in matter. It is difficult to think of an argument more eloquent than this for the benefit to mankind of basic . . researc ~ In science.
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Representative terms from entire chapter: