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Atomic, Molecular, and Optical Science: An Investment in the Future 2 Recent Major Advances and Opportunities in AMO Science and Applications to the Needs of Society The intellectual excitement and rate of progress in the field of AMO science are at an all-time high. The discovery and development of the laser and of other innovative techniques make this a time of unparalleled scientific opportunity, and these discoveries have led to scientific and technological advances that once could only be dreamed of. AMO science has enabled major technological advances in manufacturing, materials, communications, space, defense, energy, the environment, health, and transportation that have had a major impact on the nation's economic productivity, competitive position, security, and technological infrastructure and on the general well-being of its people. This chapter provides examples that highlight recent advances in AMO science and its applications and that illustrate the promise of the field. Some of the examples included below have been discussed in a recent National Research Council (NRC) report, Research Briefing on Selected Opportunities in Atomic, Molecular, and Optical Sciences (National Academy Press, Washington, D.C., 1991). More detailed discussions of many of these topics can be found in the report Future Research Opportunities in Atomic, Molecular, and Optical Physics (PUB-5305, Lawrence Berkeley Laboratory (LBL), Berkeley, California, 1991), sponsored by the Department of Energy (DOE). Here, as in the earlier reports, it is only possible to touch on a few of the accomplishments and opportunities because their number is so large. THE NATION'S SCIENTIFIC KNOWLEDGE BASE New and revolutionary discoveries of a fundamental nature continue to be made in AMO science, and, as demonstrated by the citation analysis presented in
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Atomic, Molecular, and Optical Science: An Investment in the Future Appendix C, the United States is a world leader in this area. To identify scientific forefronts, technological opportunities, and windows of future opportunity, broad input was sought from the AMO science community through letters to individual scientists, open sessions at professional society meetings, and other forums. That input identified the areas presented below as those meriting special discussion. Clearly, not all areas of significant progress can be included in such a brief presentation. Nevertheless, this summary will provide a glimpse of some of the scientific frontiers in AMO science. Recent Discoveries and Future Opportunities in AMO Science Fundamental Laws and Symmetries One unique aspect of AMO physics, distinguishing it in all of science, is the capacity to make measurements with extraordinarily high precision. In suitably chosen systems, such precision measurements can probe physics far beyond the confines of what is customarily considered AMO science. For example, precision AMO measurements are testing our basic concepts of space and time, revealing new details about nuclear structure, probing the existence and properties of elementary particles, and exploring our fundamental understanding of the forces of nature. In this way, AMO science provides the unusual opportunity to explore the frontiers of physics without leaving the proverbial "tabletop." A common theme in much of this work is that one tests accepted theories at increasingly higher levels of precision until, at some point, a discrepancy is observed, which leads to important new insights. One example is the testing of the ideas of space and time that are embodied in the theory of special relativity. The optical experiments of Michelson and Morley and of Kennedy and Thorndike provided important early tests of the isotropy of space and the speed of light. Recently, laser versions of these experiments have tested the isotropy hypothesis at a precision many orders of magnitude higher. Similarly, recent laser spectroscopy experiments have provided dramatically improved precision for the confirmation of the time dilation formula of special relativity. The area of precision measurements that has made perhaps the largest contribution to the basic understanding of physics is the detailed examination of atomic structure. Historically, precision measurements of atomic and molecular spectra laid much of the groundwork for the development of quantum mechanics, and the high precision of these data provided an exceptionally rigorous testing ground for new theories as they were put forth. The microwave technology developed in World War II led to the precise measurement of the Lamb shift in hydrogen, which stimulated the modern development of quantum field theory. The rather radical concepts of renormalization and vacuum fluctuations gained quick acceptance because of the remarkable agreement between theoretical predictions
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Atomic, Molecular, and Optical Science: An Investment in the Future and experiment. The development of quantum electrodynamics has advanced to the point that calculated and measured values of the magnetic moment of the electron agree to 1 part in 109. This same spirit underlies the ongoing efforts to find and study new, non-Coulomb forces through their effects on atomic structure. Particular attention has been given to forces that violate time (T) and/or parity (mirror) (P) reversal symmetries. At present the only evidence for a time-reversal-violating force is in the neutral kaon system, and there is much speculation, but no solid arguments, as to its origin and relationship with other forces. An electric dipole moment (EDM) of a fundamental particle could only exist if there were a T-violating force, so there has been a long history of experiments that have searched for EDMs of neutrons, electrons, atoms, and nuclei. Over the years, these measurements have improved enormously and now have achieved extraordinary levels of sensitivity. For example, the present limit on the EDM of an atom is equivalent to a displacement of the positive and negative charges of about 10-26 cm. The fact that EDM searches have yet to yield a nonzero result places severe constraints on theoretical models. Indeed, the standard model is one of the few that have been put forth that can simultaneously explain the presence of T-violation in kaons and its absence at the level currently set by experiments in atomic systems. It is generally believed, however, that the standard model is only a part of a larger scheme, such as supersymmetry. EDM searches are one powerful means to test many of the ideas put forth in these larger schemes. The study of parity nonconserving forces in atoms is, in a historical sense, positioned between the Lamb shift and the search for the electric dipole moments. The Lamb shift was at one time a radical new discovery but is now accepted without question, whereas the search for EDMs is a quest (as yet unfulfilled) to observe a new phenomenon. Throughout this century, there have been numerous experiments that set limits on parity nonconservation (PNC) in atoms, but it has only been in the last 10 to 15 years that experimental precision reached the necessary level to detect this effect. Although atomic PNC is at the extremely small level of only 1 part in 1011 mixing of atomic states, it is now possible to measure the size of this mixing, for example in a cesium atom, to 2% accuracy. PNC in atoms is understood to be a manifestation of the neutral weak interaction that was predicted as a result of efforts to unify the weak and electromagnetic forces. The study of the neutral weak force remains one of the best methods of testing the standard model of electroweak unification and probing the many unexplained features of this model. By themselves, the experimental atomic PNC data are not sufficient to probe the nature of the neutral weak force. In order to interpret the data, it is also necessary to precisely calculate the structure of many-electron atoms such as cesium. In the past few years, dramatic advances in computational techniques have made this possible. This "marriage" of theory and measurement of atomic PNC now provides one the most precise tests of the standard model and complements the many high-energy tests of the model,
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Atomic, Molecular, and Optical Science: An Investment in the Future because only the atomic experiments are sensitive to the values of two of the four basic electron-quark neutral current coupling constants. A substantial number of new theories have been put forth to avoid the problems of the standard model, and their primary observable effects involve these two coupling constants. Because of this, the atomic PNC results set the most stringent constraints on much of this so-called ''new physics." As these experiments and the atomic structure calculations are improved, they will further probe the nature of the electroweak unification. Only a few other experiments have comparable potential to address basic questions in elementary particle physics, and these are generally being conducted within the high-energy physics community. Precision AMO measurements of the sort discussed here drive, and are driven by, the technology of the entire AMO field. For example, the progress in the search for EDMs has directly reflected improvements in radio frequency resonance and, more recently, in laser spectroscopic techniques. Work is now under way to incorporate laser cooling techniques into these searches, which promises to lead to dramatic improvements. Efforts to improve special relativity tests and atomic PNC experiments have led to developments in laser stabilization and optics that are now spreading throughout the field of AMO science and, in particular, have had a substantial impact on techniques for laser cooling and trapping of atoms. This mutually beneficial relationship will continue to advance the "tabletop frontier," as well as further drive the technology that has made AMO science such a vital field. Cavity Electrodynamics and Micromasers Cavity quantum electrodynamics (QED) deals with the modification of free-space atomic radiation processes by cavities and other structures. Although it has been nearly 50 years since such effects were first considered, it is only in the past decade that experimental techniques, especially the use of Rydberg atoms and superconducting cavity walls, have become available to study such effects with single atoms. These techniques have allowed the observation of the exchange of energy between an atom and a single mode of the electromagnetic field in a cavity, which has been successfully modeled theoretically. When this treatment is extended to allow for the coupling of the atom to an arbitrary number of modes, the sinusoidal exchange of energy is generally replaced by an effectively irreversible transfer of energy from an excited atom to the field. The effect of the cavity is then to modify the atom's spontaneous emission in a way that depends on such things as the position of the atom within the cavity and the reflectivities of the cavity walls. Spontaneous emission can be inhibited if there are no allowed cavity modes at the emission frequency. Various experiments have verified the predicted modifications of radiative lifetimes by cavities. Recent developments have also opened the possibility of experimental studies of the transition between small and large systems. Such studies promise to
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Atomic, Molecular, and Optical Science: An Investment in the Future shed light on the few-body problem and classical/quantum correspondence. In so-called micromasers a low-density beam of Rydberg atoms is injected into a single-mode microwave cavity at such a low rate that at most one atom at a time is in the cavity. As such, micromasers are dynamically driven systems: both the cavity mode and the atoms are dynamical systems, so that the cavity mode in particular is an open system that can evolve from mixed states to pure states. Since good photon detectors in the microwave range are not available, one studies instead the state of the Rydberg atoms as they exit the cavity. The atoms play the dual role of pump and detector. This measurement scheme makes the micromaser a particularly attractive test system to investigate a number of aspects of quantum measurement theory. In addition to the issues in quantum dynamics and measurement theory, micromasers are theoretically attractive because the amplifying medium is relatively simple and an accurate quantum treatment is possible. Quantum fluctuations play an important role in these systems, since the mean photon number in the cavity is extremely low. Laser action is possible in the micromaser because the field amplification by a single atom is sufficient to overcome the tiny loss of the cavity. It is possible using micro-optical, non-superconducting cavities to achieve laser action in many-atom systems with nearly arbitrarily low pumping levels, and without any distinct pumping threshold. This laser action occurs because in a micro-optical cavity, where one dimension is of the order of half a wavelength long, photons are emitted into a single mode, without spontaneous emission into all nonlasing modes, as in a conventional laser. Increased pumping then results in a gradual transition from predominantly spontaneous to predominantly stimulated emission, without a sharp threshold. Thresholdless lasing has been demonstrated with microcavities containing dye solutions and should be possible also with semiconductors. The ultralow power consumption of such devices makes them interesting for various applications. Microcavity lasers also offer response rates exceeding 100 gigabits (100 billion bits) per second, which cannot be realized with more conventional lasers. Efficiencies of such important semiconductor devices as laser diodes and solar cells are strongly determined by electron-hole radiative recombination rates. Radiative decay rates can be suppressed in geometries having no allowed electromagnetic modes at the radiative wavelengths, as already noted. Inhibition can also be realized by forming structures in such a way as to produce photonic band gaps, that is, regions in a transmission versus wavelength curve where transmission is forbidden, analogous to forbidden energy bands of electrons in crystals. Recently, a cubic lattice with a photonic band gap having a width of about 20% of the central (optical) frequency has been fabricated. Such structures, which are an extension of the ideas of cavity QED, offer the possibility of dramatically improving the efficiencies of various electronic devices. Although photonic band gap structures are currently in a basic research stage, it is not difficult to imagine possible practical applications. For instance,
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Atomic, Molecular, and Optical Science: An Investment in the Future they might be used in lasers to inhibit radiative decay into nonlasing modes. This could be commercially important because it might lead to a substantial reduction in pumping requirements of diode lasers and contribute to the development of thresholdless lasers. Highly Perturbed Atoms in Intense Laser and Microwave Fields The interaction between laser light and atoms has been an active area of experimental and theoretical research since the discovery of the laser. By the early 1980s, these interactions were thought to be reasonably well understood, based on well-characterized perturbation theory. At about that time, however, improvements in laser technology led to intensities approaching 1013 watts per square centimeter (W cm-2), where the ponderomotive, or "quiver," energy of the electron in the field becomes comparable to the photon energy. Intensities a few orders of magnitude higher are now possible, providing laser fields comparable to the strength of the electric fields that hold an atom together. At these intensities, where the effect of the laser light on the atom is not a small perturbation, entirely unexpected phenomena began to be observed. Experiments measuring the energy distribution of photoejected electrons showed large peaks corresponding to the absorption of many more photons than were necessary to ionize the atom. Such processes, now known as above-threshold ionization (ATI), had previously been thought to be of marginal importance but were found to dominate the spectrum at high laser intensities. Other unanticipated phenomena were also discovered, such as inexplicably high probabilities for multiple ionization of atoms by strong laser fields. Unexpected results had been observed earlier in studies of multiphoton ionization of atoms in high-lying Rydberg states by strong microwave fields. It is now clear that these various experiments were signaling the entry of atomic physics into the realm of strongly coupled systems, where perturbation theories no longer can be depended on to provide descriptions of atomic behavior. Entry into this new realm pushed atomic theory in two different directions. The first was away from simpler perturbative approaches and into extensive computer analyses of the detailed quantum mechanics of these problems. Because these become extremely complex and difficult, a complete analysis is not yet possible even with present computational capabilities. However, calculations of appropriate model problems have now qualitatively reproduced most of the experimentally observed features. One intriguing prediction of these calculations is that with increasing field intensity in the strong field regime atoms can actually be stabilized against ionization. This counterintuitive prediction suggests that ionization probabilities are not, in all cases, monotonically increasing functions of field intensity. The second theoretical approach to describing the behavior of atoms in intense laser fields has been through the broad field of nonlinear dynamics and
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Atomic, Molecular, and Optical Science: An Investment in the Future chaos and, more specifically, through the investigation of quantum dynamics in systems with chaotic classical limits. These investigations, often referred to as "quantum chaos," mark a paradigm shift in atomic physics and include the study of statistics of energy-level spacings, the phenomenon of "dynamical localization" representing the suppression of diffusive behavior seen in the classical limit, and "scarring," the peaking of eigenstates of the time-evolution operator or quasi-energy states on unstable classical invariant structures. Because studies of nonlinear dynamics and chaos tend to emphasize universal aspects of the phenomena, this suggests that atomic physics might be an important testing ground for the development of new ideas having application throughout the physical world. Microwave experiments involving Rydberg states of hydrogen have provided evidence for scarring and dynamical localization. These mechanisms provide the most complete and compact explanation of the experimental observations, while a more conventional interpretation is either exceedingly cumbersome or not possible. Recent efforts to extend the same approach to problems of ATI have led to the suggestion that scarring is relevant to the interaction of ground state atoms with intense lasers. The universality of the ideas of chaos and nonlinear dynamics links these studies to recent investigations of such seemingly unrelated areas of atomic physics as the spectrum of the diamagnetic hydrogen atom, the doubly excited spectrum of the helium atom, Rydberg charge transfer, and the motion of charged particles in traps. These well-characterized and controlled studies in atomic physics can then serve as paradigms for higher-dimensional problems in other areas, including atomic or molecular collisions that are not weak and cannot be considered slow or fast, driven quantum wells, and other mesoscopic solid-state systems. Transient States of Atomic Systems and Collision Dynamics The key to understanding a vast array of complex atomic collision phenomena involving the transfer of energy, angular momentum, and charge is, in many cases, the accurate description of the transient intermediate states of the collision complex. Improved experimental capabilities are making possible far more complete analyses of such states than have been heretofore possible, thereby permitting stringent tests of theory. In addition, a host of novel physical processes are being studied experimentally for which a theoretical understanding is only beginning to be developed. The scope of such processes encompasses all the various interactions between photons, electrons, positive and negative ions, neutral atoms, and even antiparticles. Dynamics of Three-Body Systems. While systems that comprise two interacting particles may be described analytically, systems having three or more interacting particles can be described only approximately. Three-body systems are thus the
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Atomic, Molecular, and Optical Science: An Investment in the Future prototypes of many-body systems. Experimental and theoretical progress in their description is a key to much of the physics of the everyday world. Indeed, the physics of many complex processes is governed by the interactions of three particles (one or more of which is often a composite particle). Important progress has recently been made in several areas. High-resolution experimental measurements of the photodetachment cross section for the negative hydrogen ion have uncovered an extremely rich spectrum of doubly excited states, and this has been accompanied by commensurate theoretical advances. Experimental measurements of photo double ionization of atoms have revealed an intriguing empirical relationship between this process and electron impact ionization of the corresponding singly charged ions. The measurements are so precise that they have also tested theoretical understanding of the threshold laws for three-body breakup and, indeed, have led theory to predict alternative modes for three-body breakup applicable in different energy regions above threshold. Finally, the experimental measurements at high photon energies are providing severe tests of theoretical estimates of the ratio of double to single photoionization at asymptotically high photon energies. Resonances in low-energy electron-atom and electron-molecule collisions provide detailed information on transient states of negative ions. Many such states are doubly excited, and electron-correlation effects are of paramount importance in their theoretical description. Several years ago, experimental data on electron-cesium scattering were used to make a semiempirical prediction of a stable 3P state of the negative cesium ion. The existence of this state has more recently been predicted by ab initio theoretical calculations and confirmed by direct experimental observation. Collisions at Ultralow Temperatures. Recent advances in laser cooling and manipulation of alkali, alkaline earth, and rare-gas metastable atoms have opened up many new opportunities in atomic collision studies. In particular, these advances now permit the study of inelastic energy transfer and associative and Penning ionization reactions at temperatures below 1 millikelvin (mK). The study of such collisions is important for both practical and fundamental reasons. Collisions limit achievable trap densities and give rise to difficulties in intended applications, such as preventing the realization of Bose-Einstein condensation of cold trapped atoms or causing pressure shifts in high-precision atomic clocks. Low-temperature collisions also display a number of unusual characteristics. Two distinctly different classes of collisions can occur. Collisions of ground or metastable states can be described by conventional scattering theory. In the near-threshold regime, only s-wave collisions have nonvanishing inelastic rates, and the rate coefficients are sensitive to the spin statistics of the colliding atoms and are subject to manipulation by external magnetic fields. In contrast, if laser radiation tuned near the cooling transition is present, collisions involving excited states will occur. These collisions are controlled by an extremely long range
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Atomic, Molecular, and Optical Science: An Investment in the Future resonant dipolar interaction and enable a new kind of molecular spectroscopy, which probes the long-range potentials of the excited molecule formed from the two atoms and can be used to study molecular bound states having turning points of up to many hundreds of angstroms. Collision rates can be manipulated optically by varying the detuning and intensity of the radiation. If the detuning from resonance is only a few natural line widths, the atoms can be excited only when they are far apart, and they undergo optical pumping and spontaneous decay as they interact during their slow approach to one another. Conventional scattering theory no longer suffices, and new theoretical methods must be developed to account for the fluctuations and dissipation during the long-range part of the collision. Numerous experimental and theoretical studies of various aspects of the physics of ultracold collisions are now beginning, and this subject promises to have an exciting future. Highly Charged Ions. Highly charged ions dominate hot plasmas such as those encountered in nuclear fusion reactors, X-ray laser research, and stars. Accurate spectroscopic and collision data are required to model the behavior of such plasmas. The technology for making and handling highly charged ions has advanced dramatically during the past few years, opening a whole new class of phenomena for study. Highly charged ions provide a critical test bed for our fundamental understanding of atomic structure and interactions. The enhanced long-range Coulomb forces between highly charged ions and other charged particles give rise to large cross sections for some processes. The electronic potential energy carried by the ion can overshadow kinetic effects in slow collisions. For highly charged ions, the reduced nuclear screening increases the binding energy of the outer valence electrons. Thus, processes involving inner-shell electrons often dominate as the ionic charge increases, for example, in electron-impact ionization. Electron-electron correlation effects may also be enhanced and elucidated in interactions of highly charged ions, for example, in multiple electron capture. New Insights into Molecular Dynamics Recent advances in laser technology have enabled scientists to examine phenomena with femtosecond time resolution. This capability has triggered many developments in AMO science. It is now possible, for example, to observe atoms moving in response to chemical forces, to monitor the flow of energy out of individual chemical bonds, and to contemplate optical control of the outcome of particular reactions (Figure 2.1). Complementary to this experimental understanding of molecular motion are numerous conceptual theoretical breakthroughs permitting, for example, the analysis of static spectra using time-dependent wave packets or the observation of flux propagation through curve crossing regions.
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Atomic, Molecular, and Optical Science: An Investment in the Future FIGURE 2.1 In 1872, Eadweard Muybridge developed a camera with a shutter speed of 1/500th of a second to help Leland Stanford settle a bet that all four of a horse's hooves leave the ground at some time while the horse is galloping. (Reprinted, by permission, from Eadweard Muybridge (1830-1904), "Annie G." Cantering, Saddled, 1887, Philadelphia Museum of Art, City of Philadelphia, Trade and Convention Center, Department of Commerce (Commercial Museum).) The bottom panel indicates that we can now follow the motion of individual atoms in molecules within times of a few quadrillionths of a second—or more than 100 billion times faster than in 1872! (Reprinted, by permission, from Ahmed H. Zewail, "The Birth of Molecules," Sci. Am. 263 (December), 76-82, 1990. Copyright © 1990 by Scientific American, Incorporated. All rights reserved.)
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Atomic, Molecular, and Optical Science: An Investment in the Future The ability to excite and probe molecules on very fast time scales has greatly enhanced the understanding of energy transfer and relaxation in solids and liquids and of the dynamics of solvation. This is a particularly valuable endeavor because much of the important industrial chemistry occurs in solution or on catalysts. For example, these experiments have begun probing the key steps in solvation and reaction by monitoring the fate of an excited molecule in solution or as individual solvent molecules are added to clusters. Ultrafast techniques have also opened new possibilities for studying interfaces. In some of the first experiments, the energy flow out of surface-adsorbed molecules has been observed directly. These approaches provide new details about charge transfer in molecules adsorbed on electrode surfaces, and nonlinear spectroscopy using the high fields created by ultrashort pulse lasers permits entirely new measurements that reveal the nature of molecules adsorbed on surfaces. Efforts to understand atomic and molecular interactions have produced fundamental new insights into reactive and nonreactive events. A key new direction involves the complete solution of the "vectorial" nature of collision dynamics problems, requiring elaborate angular momentum analyses and ingenious polarization and imaging technologies. Another is "coherent" control dynamics using preparation of nonstationary states. Many of the recent experimental insights come from the application of lasers to quantum state probing, the development of which has involved atomic, molecular, and optical sciences together. Computer technology has vastly expanded the scope of the accompanying theory, enabling the calculation of potential energy surfaces and dynamics through quantum and classical methods. By applying lasers to a variety of elementary processes, researchers are exploring atomic and molecular interactions in unprecedented detail. In these experiments, atoms or molecules are prepared in selected initial states, even including selected orientations or alignments in space, and the states are analyzed after reaction or inelastic scattering. This preparation and analysis extends to the control of chemical reactions in a few prototypical cases. The measured orientation dependences provide the underlying basis for an understanding of reaction stereochemistry. One approach to controlling reactions is preparation of eigenstates that have special reactivity, and the other is preparation of nonstationary (coherent) states that allow laser control of the chemistry. In the future, it is envisioned that individual molecular bonds could be broken or formed selectively by the injection of modulated pulses of light with specific frequencies. Rapid advances in theory, driven by both computational technology and new ideas, are being made. The implementation of time-dependent quantum mechanical methods for practical calculations is one example. The evolution of a quantal wave packet, properly analyzed, provides in one calculation the answers to numerous questions concerning the absorption spectrum and the time evolution on dissociative pathways. Other methods enable very large scale calculations of the structure of complex molecules and of the potential energy surfaces
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Atomic, Molecular, and Optical Science: An Investment in the Future are used to treat a wide variety of medical problems and frequently provide an attractive alternative to surgical intervention. With further research into treatment methods and development of low-cost laser systems tailored to medical needs, it is clear that lasers will become even more important in medicine in the future. The use of lasers in medicine and surgery has expanded rapidly for two reasons. First, a broad range of lasers have become available, allowing matching of the wavelength and temporal characteristics of the radiation to each particular clinical problem. Second, there has been considerable progress in understanding laser-tissue interactions. Laser energy deposition results in localized heating that can, for example, cause blood vessels to coagulate or lead to tissue ablation. At very high pulsed laser intensities, optical breakdown occurs, resulting in plasma formation and almost complete absorption of the laser pulse in a tiny volume. Research is under way using low-intensity laser radiation to detect and treat cancer. Tumors tend to collect and store certain body pigments such as porphyrins, and cancerous tumors will take up and retain exogenously administered pigments and certain fluorescent dyes. Thus an early-stage cancerous mass can be detected through its concentrated fluorescence under blue or near-ultraviolet laser irradiation during, say, a fiber-optic catheter examination. A similar procedure can also be used to selectively kill cancerous cells. Illumination of porphyrin molecules with red light can result in the production of singlet oxygen, which is highly toxic. Thus, porphyrin-laden cancer cells can be destroyed by irradiation with red laser light, whereas interspaced normal cells are essentially unaffected. Another laser application in cancer therapy is laserthermia. Cancerous tumors are poorly supplied by blood vessels compared to normal tissues and cannot readily disperse heat. Thus, it is possible to locally heat and destroy a tumor using laser radiation introduced by an optical fiber. Lasers are finding numerous applications in ophthalmology. Photocoagulation using an argon ion laser is now the standard treatment for diabetic retinopathy, although diode lasers are also now being considered for this application. Various techniques involving small laser perforations are being used to treat glaucoma. A common problem following cataract surgery is that the membrane holding the implanted intraocular lens becomes opaque. Optical breakdown induced by a Q-switched Nd:YAG laser is used to cut the membrane, removing it from the optical path. Studies of the cutting process have revealed a complicated series of events initiated by laser-induced breakdown, which can cause cavitation, shock waves, and liquid jets and create unwanted damage. These studies have led to an understanding of the scaling laws for such damage and have prompted investigation of the use of less energetic picosecond pulses for initiating optical breakdown. Picosecond pulse microsurgery has application in cutting unwanted structures near the retina, which would be damaged if nanosecond pulses were used. Lasers are also used in correcting vision defects, most of which are due to small anomalies in corneal curvature (the curved corneal-air
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Atomic, Molecular, and Optical Science: An Investment in the Future FIGURE 2.13 CO2 laser being used to treat endometriosis. The operating laparoscope delivers the laser radiation directly to the tissue. The laser radiation serves as a scalpel, cutting adhesions and endometrial tissue with minimum damage to surrounding healthy tissue. (Courtesy of Coherent, Inc., Palo Alto, Calif.) interface provides ~70% of the total refractive power of the eye). Research is under way using argon fluoride (ArF) excimer lasers to reshape imperfect corneas through selective ablation. In addition to reducing the need for eyeglasses, this technique can also be used to remove corneal scars. Lasers are also used as a scalpel (Figure 2.13). If the laser parameters and focusing are chosen such that the beam is intense enough to ablate and cut through the material directly in its path, and provide sufficient heating of neighboring tissue to cause coagulation, incisions can be made with minimal bleeding, which is especially valuable for surgery on vascular organs. If a short-pulse (=1 msec) laser is employed that is strongly absorbed in the target, the absorbing volume ablates before there is time for appreciable outflow of heat, and consequently each pulse precisely removes a thin layer of material. For example, lasers have had a dramatic positive impact on the treatment of gynecological diseases in women. The precise application of intense laser energy permits destruction of only damaged tissue while maximizing preservation of the normal reproductive tract. A number of new laser technologies are finding their way into clinical use. The pulsed Er: YAG laser (with a wavelength of 2.9 µm) has been shown to precisely cut both bone and soft tissue. The pulsed Ho: YAG laser (with a wavelength of 2.1 µm) is not as strongly absorbed by tissue water, resulting
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Atomic, Molecular, and Optical Science: An Investment in the Future in greater residual tissue damage, but has the advantage that it can be transmitted by conventional quartz fibers, allowing its use in procedures requiring fiber-optic delivery of laser light. Thermal and acoustical effects of pulsed lasers are being actively modeled, and deleterious effects, such as the generation of gas bubbles, are being studied. Diode laser systems are also now reaching output power levels suitable for use in tissue cutting and welding. Both Ho:YAG and xenon chloride (XeCl) lasers have been used in clinical trials as an alternative to balloon angioplasty. In laser angioplasty, pulsed-laser ablation is used to clear obstructing arterial plaque. However, this requires precise removal of as much diseased tissue as possible without puncturing the vessel wall. Diagnostic feedback methods such a ultrasonography and tissue spectroscopy are being studied for this role. The acoustic shock waves that result from pulsed-laser ablation are used to clinical advantage in the treatment of kidney stones. Stones lodged in the urinary tract are often fragmented by shock waves when illuminated with visible pulsed dye laser light. Such laser procedures provide an attractive alternative to surgery. Medical Imaging. Medical imaging frequently employs X-rays with the attendant risk of cancer induction. As an alternative, researchers are exploring the possibility of imaging using light. This is a difficult problem because strong multiple scattering occurs in tissue, destroying the image contrast and making it difficult to extract spatially resolved information from measurements of the transmitted or backscattered radiation. However, multiply scattered photons travel a greater distance in the target before emerging than do those transmitted directly, and recent work has demonstrated that illumination with picosecond laser pulses, in combination with the use of an ultrafast optical gate, provides a means to selectively discriminate against multiply scattered light. Numerous variations of this simple idea are currently being explored. Time-gating has been used for optical transillumination of tissue, and the images obtained have been investigated for applications such as the early detection of excess blood in the brain (detection of strokes) using multiple source-detector pairs to locate the source of absorption by blood. Knowledge of the absorption spectrum of blood can be used to guide the selection of wavelengths optimal for detection of oxyhemoglobin or deoxyhemoglobin. Picosecond lasers are also being used in the early diagnosis of breast cancer. Initial imaging experiments used mode-locked dye lasers as sources and streak cameras or fast microchannel devices as detectors. Such time-domain imaging involves expensive lasers and detectors and is quite complex. As a consequence, a number of laboratories are now investigating the use of simpler frequency-domain techniques in imaging. Femtosecond laser pulses have been used to obtain optical echoes from structures within the eye and from different layers in skin. Such optical ranging has demonstrated better spatial resolution than is currently available using ultrasound techniques; however, the need for ultrafast pulsed lasers and gated detection
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Atomic, Molecular, and Optical Science: An Investment in the Future limits the clinical utility of the technique. Recent work has shown that ranging measurements can be made by using a continuous wave low-coherence-length superluminsescent diode in an interferometer system. Measurements have been made on eyes and arteries in vitro, and a number of ophthalmic applications, such as early detection of damage to the optic nerve due to glaucoma, are being investigated. Femtosecond pulse technology is also being used to study the rapid processes involved in vision. Cell Manipulation. Optical trapping (see Chapter 1) has a myriad of potential research and therapeutic uses in medicine, and commercial optical trapping systems have recently become available. Specifically, optical traps are now being used to study the forces involved in the locomotion of biological macromolecules and to manipulate and position cells ("optical tweezers"). Cells can be held for fusion or perforation with a second laser to facilitate genetic manipulation or in vitro fertilization. Monitors. Measurements involving isotopic tracers are widely used in both clinical medicine and basic research. For example, this approach is being used to monitor the metabolism of calcium in the body, a process of interest in many areas, ranging from the study of osteoporosis to the feasibility of long-term spaceflight in zero gravity. Isotopically labeled calcium is consumed in the form of milk or other foods and subsequently measured in bones or blood. Traditional techniques that make use of radioactive tracers are clearly not desirable, especially in studies involving growing children and pregnant women. A better approach is to use a stable calcium isotope, such as 48Ca. The 48Ca atoms present in a blood or bone sample can be monitored with high sensitivity using high-resolution mass spectroscopy or laser spectroscopy. (These spectroscopic techniques can also be used to monitor heavy metals and other species present in the blood.) Studies using isotopic tracers require the availability of large amounts of isotopically enriched material, but these can be obtained by using laser isotope separation. Radiation and Health Physics Health physics deals with understanding the interaction of ionizing radiation such as alpha particles, beta rays, gamma rays, and neutrons with living systems and with the design of instrumentation to measure sources of radiation and estimate possible harmful effects on living systems. Such studies rely heavily on AMO science and are vital to human society because ionizing radiation occurs in the environment in the form of natural radiation and cosmic rays and also because it is introduced by human activities such as medical and industrial uses of radiation and nuclear energy technology. Once energy is deposited in living tissue by ionizing radiation, a complex sequence of events occurs starting at the atomic level, continuing through a
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Atomic, Molecular, and Optical Science: An Investment in the Future chemical phase, and ending with the observed biological and medical effects. Microscopically, collisions of energetic particles with atoms and molecules result in the production of excited atoms, ions, and secondary particles, most importantly electrons. Analyzing the subsequent chain of events and their possible effects requires a detailed spectroscopic knowledge of the pertinent excited and ionized states and the cross sections for all the major collision processes operative, remembering that the predominant chemical reactions in ground and excited-state collisions may be different. Further research is required to establish a more complete database of cross sections for collisions of electrons, protons, and heavy ions with molecules, especially polyatomic molecules of biological importance such as water and the hydrocarbons. AMO science is also essential in the design of dosimeters to measure radiation fields and sources. Because many such instruments are dependent on excitation or ionization of some medium, research on the interaction of radiation with matter is required to calibrate and interpret the readings of such instruments. One outgrowth of dosimeter research was the development of resonance ionization spectroscopy, which can provide isotopically selective trace element detection and is now used to monitor accidental emission of long-lived radio isotopes and other species. Design of Bioactive Molecules (Pharmaceuticals) Molecular theory is making numerous contributions to the design of bioactive molecules, including drugs for treatment of disease, and herbicides and pesticides for agricultural use. The principal "tools" of molecular theory encompass quantum chemical techniques and a variety of methods that make use of empirical potential-energy functions, such as molecular dynamics and Monte Carlo simulations. The goal of most applications of these methods in the design of bioactive molecules is a determination of the geometric and electronic structural properties of the molecules of interest. Two overriding concerns in this regard are the electronic characteristics (e.g., charge distribution, dipole and quadrupole moment, and molecular electrostatic potential) and the nature of the bioactive conformation(s) of a molecule. Quantum chemical methods are playing an important role in dealing with these concerns for a wide variety of small molecules of biological interest. Due, however, to the significant number of torsional degrees of freedom possessed by a substantial number of these molecules, it is necessary in many cases to employ a "hybrid" procedure, which uses empirical potential-energy-function-based conformational searches followed by some form of quantum chemical calculation to determine the required electronic properties of appropriate conformers. In cases in which suitable potential-energy functions are unavailable, semiempirical quantum chemical methods have been used, but, as noted above, these methods are restricted in the size and conformational complexity of the molecules that can be treated.
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Atomic, Molecular, and Optical Science: An Investment in the Future The development of transferable potential-energy functions, coupled with the rapid growth in computational power of today's high-performance computers, has extended the range of problems that can be treated by molecular theory methods to include bio-macromolecular systems such as proteins and nucleic acids. Of especial interest here are a number of molecular dynamics and Monte Carlo-based techniques (e.g., thermodynamic integration and perturbation methods) for calculating thermodynamic properties such as the free energy of ligand-protein binding—a property that is notoriously difficult to calculate. These techniques have also been employed in studies of, for example, the solvation and conformational free energies of small molecules in aqueous and other solvents. Advances in ab initio electronic structure theory in combination with new computing capabilities now permit approximate but realistic calculations on sizable molecules. In particular, such calculations can guide the synthesis of new drugs and can suggest strategies for creating new bioactive molecules. However, improved quantum mechanical methods for treating larger systems by semiempirical or ab initio techniques are required, together with development of improved parallel molecular dynamics algorithms to allow for more realistic simulations and for treatment of supramolecular systems such as biological membranes. AMO science is important in a number of health-related areas, especially medicine. With the development of new low-cost lasers and delivery systems, laser-based procedures will become more widely accessible in the future. Also, new optical techniques promise to reduce the reliance on X-rays in medical imaging. Molecular theory is making significant contributions to drug design. Space Technology The U.S. space program has sizable efforts in astrophysics, space science, and atmospheric and environmental science as well as in a number of other areas, and the impact of AMO science in these areas has already been emphasized. In addition, AMO science, particularly through lasers, plays a strong role in space technology. Measurement and Sensing Until a few years ago, virtually all sensing from space was done passively. Active remote sensing systems using space-based lasers (LIDAR) are expected to play an increasingly important role, providing information for meteorology and pollution monitoring as well as long-term studies of global climate change.
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Atomic, Molecular, and Optical Science: An Investment in the Future LIDAR is a form of optical probing by light scattering that can be used to measure the total aerosol distribution and the concentration profiles of aerosols and trace gases, as well as wind vectors in the atmosphere. For the past several years, NASA has been developing the Laser Atmospheric Wind Sounder (LAWS) LIDAR system for the worldwide mapping of winds from space. Another program, LIDAR In-space Technology Experiment (LITE), is aimed at range-resolved studies of cloud top heights, a parameter important in weather forecasting. LITE II, a follow-on system, is being designed using a tunable Ti:sapphire laser for vertical profiling of the atmospheric temperature and water vapor content. Lasers are used to accurately measure and control baselines in coherent interferometry, to obtain high angular resolution in the microwave region. These uses may be for astronomical applications or for tracking satellites at great distance. Research is under way to connect several microwave antennas with lasers by means of fiber-optic systems, but achieving this goal requires a detailed understanding of the noise properties of semiconductor lasers modulated by ultrastable microwave sources and the effects of fiber-optic transmission. Spacecraft Navigation and Communication Satellite-based laser communication links are being explored because they can be made very directional. This capability offers security, of considerable military significance, and the potential for very long distance communications, as needed for the deep-space network. Laser systems are projected to require lower power, weight, and volume than comparable microwave links. The military plans to fly diode-pumped Nd:YAG lasers in the first generation satellite-to-satellite communications technology, but these should ultimately be superseded by semiconductor diode lasers. The challenge in satellite communications is to achieve ultrareliability over long periods not only in laser performance but also in pointing and tracking. The resolution of these technological issues promises spinoffs into commercial applications. For example, diode-pumped solid-state lasers can provide ultrareliable sources with a wide array of uses in manufacturing and medicine. The introduction of deep-space satellite missions using radio-frequency signals for communication was the impetus for a field known as radio science, in which the radio communication signals are used to extract scientific information about the intervening medium. The development of communication systems using optical signals enables an entirely analogous field of "light science." This field includes the study of planetary atmospheric absorption at optical wavelengths, fine-scale scattering from planetary ring systems, and integrated forward scattering over interplanetary distances in the solar system dust field. Furthermore, many previous radio-frequency experiments that were contaminated by charged particle density fluctuations from the solar wind, like gravitational body bending of electromagnetic waves, can now be performed without those
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Atomic, Molecular, and Optical Science: An Investment in the Future disturbances, because the effects of charged particle fluctuations fall off as the square of the carrier frequency. The development of solar-pumped lasers is being pursued for space power transmission and propulsion. New potential applications of solar lasers in space are emerging. These include earth, ocean, and atmospheric sensing from space; detecting, illuminating, and tracking hard targets in space; and deep-space communications. Gas, liquid, and solid lasers have all been considered as candidate solar lasers, and successful lasing has been achieved with the use of a number of such systems. It is predicted that solid-state lasing materials with broadband absorption characteristics such as alexandrite or Nd:Cr:GSGG might yield greater than 10% conversion efficiency. High-performance satellites need massive on-board information-processing capability. Fiber-optic communication systems are expected to take their place naturally in the computers of spacecraft, as they do in ground-based systems. Optical information processing for on-board analysis and synthesis of sensor data is becoming increasingly important in satellites. NASA has programs in optical implementations of on-board signal processing, image processing for robot vision systems, and spatial light modulators as devices needed to achieve these objectives, as well as in fiber-optic communications. Long-range plans for lunar colonization and Mars exploration point up the need for these advanced optical technologies. Optical sensors are needed in a variety of satellite applications. The fiber-optic rotation sensor, particularly in its integrated optics configuration, will be important in high-sensitivity, low-weight inertial navigation systems. Optical systems are being developed to assist in docking between satellites. Fiber optics and integrated optics sensors are also being considered for use in monitoring the health of a spacecraft. Many spacecraft require accurate on-board clocks. For example, the cesium atomic clock is used on the Global Positioning System satellites. Recent developments in the field of atom-ion trapping and cooling suggest that with further research greatly improved spacecraft clocks can be developed that will offer much higher accuracies. The space shuttle glows in the dark, and so do orbiting satellites. The glow is accompanied by erosion of the surfaces of the spacecraft and the instruments it carries. The glow complicates the design and operation of optical and infrared instruments, and the erosion limits their effective lifetime. The glow and erosion are produced by the impact of atmospheric ions and atoms with the spacecraft at velocities that correspond to energies of about 5 eV. This phenomenon results in a complex array of surface and gas-phase reactions that depend on the nature of the surface. Much further AMO research is, however, required in order to fully understand the phenomenon and minimize its effects. One increasing problem in the space program is the growing amount of debris in orbit. Lasers have been suggested both as optical monitors of space
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Atomic, Molecular, and Optical Science: An Investment in the Future debris (through a form of laser radar) and as a means for elimination of small amounts of debris (either by vaporizing it or by deflecting it out of the satellite path). In either case, lasers must be placed in the satellites. Further research and development are required to determine the optimal design for such applications. AMO science plays an important role in spacecraft operations and applications that will increase in the future with the advent of sophisticated atmospheric remote sensing systems and laser satellite communications. Transportation The pace of our nation's commerce continues to rely heavily on an aging, yet critically important, transportation infrastructure. Unlike other segments of our national economy, the responsibility for this infrastructure rests heavily with federal, state, and local governments. At a time of increasing demands on scarce governmental resources, it is crucial that cost-effective technologies be available to improve the safety and effectiveness of transportation systems throughout the nation. Although AMO research is not usually considered central to transportation issues, it nonetheless is playing an important role in air, land, and sea transportation through sensing and control of vehicle movement (navigation), through improvements in safety, and through increased fuel efficiency. Aviation In aviation, lasers are being used to detect wind shear, clear air turbulence (CAT), and wake vortices. These phenomena can cause severe injury to passengers and damage to aircraft. For example, the 1987 crash of a Delta airlines jet at the Dallas–Fort Worth Airport was attributed to severe wind shear. High-sensitivity light scattering techniques developed by AMO researchers are being applied to the detection of wind shear. For instance, a LIDAR system can measure wind velocity through minute frequency shifts of laser light scattered from moving aerosol particles in the atmosphere. Wind shear velocities are in the range of 10 to 30 m sec-1 and give frequency shifts of 1 to 3 megahertz (MHz) for 10-µm carbon dioxide lasers and 5 to 15 MHz for 2-µm solid-state laser sources. LIDAR systems can determine wind shear and turbulence out to a range of 10 km in front of aircraft and give pilots sufficient warning to take appropriate avoidance action. Measurement of air turbulence has implications for the efficiency of use of airport resources. Airport operators need information on air turbulence coming from wake vortices created during takeoff and landing of large transport aircraft.
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Atomic, Molecular, and Optical Science: An Investment in the Future Because of the danger from this turbulence, aircraft must be spaced sufficiently far apart that the wake of the preceding aircraft does not disturb the next flight; such spacing can produce a severe bottleneck at congested airports. Likewise, collision avoidance through automated systems, or through a warning system activated by laser radars, would be a considerable boon in skies near airports. Lasers could also be used to detect damaging levels of particulates from the large, widespread ash clouds caused by erupting volcanos. In a different application, the use of laser bar code readers has been suggested as a means for automated optical identification of aircraft near to or on the ground. This could assist controllers in keeping track of air traffic. There are numerous other ways in which lasers and electro-optics are becoming enabling technologies in aviation. Commercial and military aircraft have used laser gyros for a number of years. In addition, optical fiber communications and optical sensors will play an increasing role in the aircraft of the future and provide pilots with more information than they currently have, which will be fused into intelligent optical displays. The infrastructure of quantum electronics has provided building blocks for many of these advances. Ground Transportation In ground transportation, optical techniques for detecting speeders are coming into use. A laser-based system can unambiguously discriminate a particular car and give a rapid and precise reading of its speed. The system is compact, efficient, lightweight, and safe to use. Several programs are under way to develop ''smart highways" and automated rail systems. Optical sensors identify the position of vehicles, and a systems analysis of a real-time optical display, suggesting alternate routes. Already used in a rudimentary form on highways, such systems would be dramatically improved by less expensive and more efficient display technologies. Future generations of automobiles may use lasers and optical science in several places. Fiber optics has been used to demonstrate that the lighting systems required in an automobile all can be powered by a single light source. Inexpensive and localized laser radar systems may be used to assist in collision avoidance. Optical fiber sensors are being developed to monitor combustion within the cylinders of an automobile engine. Such sensors make it possible to optimize combustion within an operating automobile, improving efficiency. Although currently used only for research on improved automobile engines, someday such monitors may be implemented in all vehicles. Laser-based CD systems are currently entering the market as sources of onboard maps for land vehicles and boats (and aircraft). When combined with inexpensive global positioning receivers, they can provide the operator of almost any vehicle reliable, detailed information on location and local terrain, road
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Atomic, Molecular, and Optical Science: An Investment in the Future FIGURE 2.14 The extremely accurate timing made possible by atomic clocks has made the Global Positioning System a reality. An affordable receiver gets timing signals from satellites, and a built-in computer triangulates from the satellite locations to give locations on (or above) Earth within about one meter. Already these systems are coming into popular use for cars, trucks, boats, and planes. networks, and so on (Figure 2.14). Such systems are being tested, for example, in rental cars in Florida. AMO science is making a significant contribution to improvements in transportation systems and safety. The many new ideas currently under development suggest that this contribution will increase in the future
Representative terms from entire chapter: