Case Studies in AMO Science
In this chapter the panel presents three case studies that illustrate the close coupling between basic research, development, and application typical of AMO science. New ideas and discoveries made in the research laboratory are incorporated, sometimes rapidly, into new technologies and products and frequently enable advances in other areas of science. This diffusion of ideas maximizes the return on the initial research investment and is such that today the products of AMO science are an important contributor to the national economy.
The case studies also illustrate the different phases in the evolution from basic research to marketable products. The case study on lasers shows how basic research and development in AMO science has resulted in products with numerous commercial applications. That on optical manipulation of atoms highlights an area that has come into being only in recent years but in which commercial products are already beginning to appear. The case study on carbon nanostructures describes an exciting new area in which potential economic impacts are in the speculative stage.
LASERS: FROM BASIC RESEARCH TO NEW TECHNOLOGIES AND NEW INDUSTRIES
Lasers, one of the most remarkable products of twentieth-century science and technology, evolved directly from basic AMO research concerning light and its interaction with matter, including atoms, molecules, and solids. The laser has revolutionized many fields of science and is a device whose applications today touch all our lives. For example, long distance telephone calls are now routinely
carried by laser beams traveling through optical fibers thinner than a human hair. Lasers have become indispensable weapons in the arsenal of medical therapeutic and diagnostic procedures, frequently providing attractive lower-cost alternatives to conventional surgery. Low-cost lasers have been developed, allowing their application in a wide variety of consumer products, including compact-disk (CD) players and laser printers. The total annual revenue of U.S. laser manufacturers is now approximately $1.1B (D. Kales, "Review and Forecast of Laser Markets: 1993," Laser Focus World 29 (January), 70-88, 1993), but the true economic impact of the laser is at least 100 times greater because lasers are a critical component of many consumer products and services.
The widespread application of lasers is a result of the unique characteristics of the radiation they provide. The output beam from a laser can be ultradirectional. Thus small objects can be selectively illuminated, even at large distances. This is key to the development of laser rangefinders, now widely used in surveying and in military applications, and of laser target designators. The pinpoint accuracy of "smart" weapons guided to their targets by scattered laser light was graphically demonstrated during Desert Storm. Laser beams may also be focused to extremely small spots, resulting in high energy densities sufficient to melt or vaporize many materials. This capability has resulted in numerous applications in, for example, industrial processing. Today lasers are used to heat, cut, or weld a wide variety of materials, including metals, ceramics, plastics, wood, and cloth. Radiation from a laser also can be highly monochromatic; that is, it encompasses a narrow range of frequencies and wavelengths. The output wavelengths available and their range of tunability vary with laser type. This wavelength choice makes possible spectroscopic analysis of atomic and molecular species and is the basis of many techniques used in remote sensing of atmospheric pollutants and of species present in environments such as combustion chambers.
Since the initial invention of the laser, research and development activities have resulted in the discovery of many different classes of lasers that, taken together, provide an enormous range of output wavelengths, pulse lengths, and power levels. A variety of lasers based on electrical discharges in gases have been developed, a good example of which is the helium-neon laser that provides the red beam observed in the point-of-sale scanners at the checkout stands in supermarkets and other stores (Figure 1.1). Argon and krypton ion lasers are widely used in ophthalmology and laser light shows, and carbon dioxide lasers find application in laser surgery. Many laser systems employ solid-state gain media including the ruby laser, which was the first system to demonstrate successful lasing. Recently, solid-state laser systems have been developed at infrared wavelengths that are "eye-safe," that is, that are absorbed by the fluid in the eye without damage to the retina, and these are finding applications in, for example, rangefinders and remote sensing. Laser systems based on organic dyes also provide tunable radiation but at wavelengths extending down into the ultraviolet.
Dye lasers have enabled major advances in spectroscopy and metrology. Semiconductor diode lasers are now available that are compact (typically the size of a grain of sand), simple to operate, and inexpensive. These are employed, for example, in optical communications and in CD players. Chemical, gas dynamic, and free-electron lasers provide powerful sources of infrared radiation with military applications. X-ray laser systems are also under development that promise advances in imaging and lithography.
Much effort has been directed toward the generation of high powers and very short pulse lengths. The search for alternate sources of energy has led to the development of lasers with pulse energies in excess of 1 kilojoule as part of the effort to harness the energy of nuclear fusion through ignition of thermonuclear fuel by laser implosion of small pellets. Such high-power lasers are also used as drivers for X-ray lasers and to produce extremely high temperature plasmas. Research has resulted in lasers that produce output pulses with durations as short as a few millionths of a billionth of a second. These enable measurements of processes occurring on time scales inaccessible using any other approach. It is, for example, now possible to monitor a chemical reaction as it occurs. The generation of ultrashort-duration pulses also holds the promise of extremely high rate communications and information processing systems. Major improvements in laser output beam quality, efficiency, stability, and reliability have been realized in recent years.
The many advances in laser systems and technology achieved in recent years have resulted from close interactions between the AMO sciences, condensed matter physics, electrical engineering, materials science, and traditional optics. Much research has been directed toward development of lasers with operational properties optimized for specific applications. These development projects, however, have frequently resulted in spin-offs into other areas. For example, eye-safe near-infrared solid-state lasers were initially developed for Department of Defense (DOD) and National Aeronautics and Space Administration (NASA) applications but now play an important role in medicine because their radiation is strongly absorbed by tissue.
The benefits provided by laser technology will continue to increase as new applications are discovered and as new highly reliable, efficient, easy-to-use, and moderately priced devices become available. In this regard, semiconductor diode lasers and solid-state lasers pumped by semiconductor lasers appear particularly attractive because of their modest size and cost and because of the ruggedness of solid-state technology for applications outside a laboratory. The last decade has witnessed the discovery and practical realization of solid-state lasers containing transition metal ions, including the titanium:sapphire laser, which is continuously tunable over a broad range in the near infrared and has become the laser of choice for the generation of ultrashort pulses. This development has been made possible in part by the high-quality crystals now available in the United States. The wavelength domain of semiconductor lasers continues to
expand with the recent demonstration of lasers in the blue/green region. Indium gallium arsenide (InGaAs) and indium gallium arsenic antimonide (InGaAsSb) lasers have also been fabricated that extend wavelength coverage farther into the infrared.
With further research and development, solid-state lasers should evolve to the point that they will be efficient sources of laser radiation over the entire optical spectrum from the ultraviolet to the middle infrared. Their becoming efficient sources over this regime will require a combination of new materials, new devices, and new optical configurations and the use of nonlinear optics for frequency conversion. Semiconductor diode laser technology is also rapidly advancing on several fronts. Diode laser arrays are now being developed that provide high average powers and that can be used either alone or as pump sources for solid-state lasers. One interesting recent development is the creation of large arrays of laser diodes using techniques employed in very-large-scale integrated (VLSI) circuit technology. It may be possible to coherently lock the arrays, and if this is accomplished, diode lasers could eventually replace optically pumped solid-state lasers for many applications. Advances in semiconductor materials technology are making possible layered semiconductor systems (quantum wells) whose properties might be specifically tailored to optimize laser operation. Of particular interest are arrays of surface-emitting vertical cavity diode lasers. Unlike conventional, edge-emitting diode lasers, the surface-emitting lasers can be fabricated close together on a substrate. In the past few years, it has become possible to make arrays of about 1 million surface-emitting quantum-well lasers on a single 1-cm2 GaAs substrate, each individual laser being only a few micrometers across and a few micrometers high. One of the long-term goals of semiconductor laser technology is the full integration of lasers and detectors with electronic circuitry, all on the same chip, to form optoelectronic integrated circuits.
Applications of optical fibers rely on their ability to guide a beam of light. A serious practical consideration is the attenuation of the light beam as it propagates along the fiber, an effect that becomes increasingly important as the fiber length increases. Recently, it has become practical to dope optical fibers and to pump them by injection of light from a diode laser in such a way that they amplify light by stimulated emission. Thus laser action has been realized in fibers. The erbium-doped silica fiber laser offers high gain together with the advantages of single-mode guided-wave fiber optics, making it extremely valuable as an amplifier or repeater for fiber-optic communications systems.
From their beginnings as a scientific breakthrough, lasers have become a prominent component of a large commercial market. They provide vital enabling technology in many areas critical to the nation's economic productivity, competitive position, and security. The applications noted above, and described in other chapters of this report, speak to their many contributions in areas such as manufacturing, materials, environmental monitoring, medicine, and communications.
It is difficult to assess the economic impact of the laser. Although the laser manufacturing business is relatively small, the economic activity that results from direct application of lasers in consumer products and in equipment that generates other goods and services is much greater. For example, sales of laser printers amount to $6.3B per year, and sales of CDs to $3.9B per year. The telecommunications market, in which the laser is a key player (together with other optical technologies), is now $161B per year. It is thus clear that the laser has had a substantial economic impact, and it is certain that as new laser systems and applications are developed this impact will continue to increase.
The story of the laser illustrates how investment in basic and applied research can lead to new technologies of great benefit to the nation and its people.
MANIPULATING ATOMS: NEW TECHNOLOGY FOR TODAY AND TOMORROW
Traditionally, neutral objects are held and moved by mechanical means. In the last few years, however, it has become possible to manipulate atoms and neutral microscopic particles with astonishing facility using light. What distinguishes this approach from other manipulation techniques is that it permits neutral particles to be positioned and moved without physical contact. The story of optical manipulation illustrates the interplay between basic research and the development of techniques with potential for widespread application in many areas, including ultraprecise atomic clocks, structural engineering at the atomic level (nanotechnology), and the study of deoxyribonucleic acid (DNA). The basic research was driven initially in large part by the desire to eliminate uncertainties caused by atomic motion and thus allow precise measurements on atomic systems. The goal was to be able to hold or trap an atom at rest in free space, unperturbed by contact with other atoms. The techniques developed to accomplish this grew out of a basic understanding of optical interactions with atoms that had been gained through decades of fundamental research.
Laser Trapping and Cooling of Atoms and Ions: Particle Optics
In 1978, it was suggested that atoms could be trapped at the focus of a laser beam. The trapping forces, however, were predicted to be very feeble and insufficient to overcome the normal random thermal motion of an atom. In order for trapping to be possible, the atoms would have to be cooled to a temperature of only a few thousandths of a kelvin, that is, only slightly above absolute zero. A technique to cool atoms with light was suggested in the seventies and had been demonstrated with charged atomic ions, but it was not until 1985 that neutral
atoms were first successfully cooled to less than one-thousandth of a kelvin with a configuration of laser beams termed ''optical molasses." Because of the Doppler shift resulting from the atom's motion, the light in this configuration acts as a viscous medium and atoms imbedded in the "molasses" are slowed to very low velocities. Once such cooling was obtained, trapping became fairly straightforward, and a variety of optical, magnetic, and magneto-optic traps were quickly demonstrated. Currently, scientists are able to cool trapped atoms to an effective temperature of less than 10-5 kelvin.
Concurrent with the development of laser cooling and trapping, neutral particle optics equivalent to lenses, mirrors, diffraction gratings, and beam splitters have been devised. Atom optics are especially powerful when coupled with laser cooling techniques because methods that would normally be rejected as too feeble to manipulate atoms at room temperature become feasible with slowly moving atoms. It is now possible, for example, to focus an atom beam by reflection from a curved surface (mirror) or to diffract and deflect a beam using a standing lightwave. The application of atom optics in metrology and lithography is already being explored.
The ability to manipulate cold atoms has also enabled scientists to develop devices that have no optical counterpart. For example, an "atomic fountain" in which laser-cooled and trapped atoms were tossed upward and shown to return in a ballistic trajectory due to gravity has been demonstrated. Unperturbed atoms in free fall make possible measurement times that are much longer than can be achieved by using conventional thermal atomic beams, and this achievement has stimulated work to build an improved atomic clock based on an atomic fountain. Work with laser-cooled ions, or even a single ion, in an ion trap also shows great promise for improved atomic clocks. The expectation is that the time standard used by the world today can be improved at least 100-fold. This refinement will affect many areas of science and technology. For example, the high-speed computers used in telecommunications are synchronized by atomic clocks. The most accurate means of global positioning, the Global Positioning System, depends on a set of satellite-based atomic clocks, and the highest-resolution radio images of distant galaxies yet obtained depend on the synchronization of signals received simultaneously by several radio telescopes.
Another device that has been made possible by a combination of atom manipulation techniques is the atom interferometer, which takes advantage of the wavelike nature of atomic particles. Conventional optical interferometers are used as extremely sensitive measuring devices, and interferometers based on atoms promise new measurement capabilities and increased precision. The first successful atom interferometers were reported only recently. One of these employed laser-cooled atoms in an atomic fountain. The long measurement time afforded by the fountain enabled measurement of the local acceleration due to gravity to high precision. Indeed, with further refinement, the interferometer
may be comparable to the best absolute gravity meter available. Applications of this instrument include oil exploration, the measurement of land height changes due to the motions of continental plates that cause earthquakes, and monitoring of sea height changes due in part to global warming and the melting of the polar ice caps. A slight modification of the geometry of the gravity meter may also allow construction of an extremely sensitive atom gyroscope that could greatly increase the accuracy of inertial navigation systems.
Optical Tweezers and the Biosciences
The work on atom trapping stimulated renewed interest in the manipulation of microscopic neutral particles. Several years prior to demonstration of optical trapping, micron-sized glass spheres were levitated and trapped in air by laser beams, but it was not until much later that it was realized that a similar beam could be used to trap the spheres in water. The trapping forces are strong enough to overcome thermal motion at room temperature. The great advantage of using a single beam is that it can be used as an "optical tweezers" to manipulate small particles. The optical tweezers can be easily integrated into a conventional microscope by introducing laser light into the body of the microscope and using the microscope objective to focus the light beam (Figure 1.2). A sample can be viewed and simultaneously manipulated by simply moving the focused light beam.
Shortly after the demonstration of the light trap, it was discovered that bacteria could be captured in the trap, moved at will, and then released. If the trapping light is in the near infrared (where compact and inexpensive lasers are available), the bacteria can be held without apparent optical damage. Since that discovery, scientists have moved rapidly to apply this tool to a variety of problems. Scientists have reached inside living cells and moved organelles in the cytoplasm (Figure 1.3), studied the mechanical properties of the molecular motor that propels Escherichia coli through water, and used a laser beam to cut chromosomes within the nucleus of a cell undergoing mitosis, the fragments subsequently being moved into different locations using the optical tweezers.
Only a few years after laboratory demonstration, commercial versions of optical tweezers are already available.
On a still finer scale, individual molecules of DNA can be manipulated by two optical tweezers. Although the molecule is too small to be held by an optical trap at room temperature, it is possible to attach micron-sized polystyrene spheres to the ends of the molecule, which act as tiny handles that can be held by the optical tweezers. The elastic properties of the molecule have been measured with this technique, testing basic theories of polymer physics. Work is under way to stretch out and then pin down the molecule so it can be examined by an atomic force microscope. The ability to manipulate the molecule while viewing it in an optical microscope (DNA can be stained with a fluorescent dye) opens up a number of exciting possibilities to study the function of gene expression, regulation, and repair.
Optical manipulation is an example of how basic research on the interaction of radiation with matter and the trapping and cooling of atoms has resulted in powerful new tools for physics, chemistry, geology, and the biosciences with many applications.
BUCKYBALLS AND CARBON NANOTECHNOLOGY: SURPRISING NEW MATERIALS FROM SMALL SCIENCE
Since the 1800s, students have learned that elemental carbon naturally occurs in two forms: graphite and diamond. In graphite, carbon atoms are bonded together in large sheets, and in diamond carbon is bonded in a three-dimensional tetrahedral crystal structure. However, recent and unexpected discoveries have shown how to make carbon in a number of other structural forms with a wide variety of different and potentially useful mechanical and electrical properties. AMO research has revealed a broad new class of materials with major importance in materials science, solid-state physics, and chemistry.
The remarkable discoveries described here grew out of research programs designed to understand the physical properties of microscopic nanometer-sized particles and clusters. A cluster is a molecule or particle consisting of approximately 10 to several hundred atoms. Theory had predicted that many physical properties would be sensitive to the size of a cluster, but experimental studies became possible only with the invention of new techniques. To form clusters of refractory elements normally found only as bulk solids, laser ablation methods were developed, and laser spectroscopic methods were devised to characterize their structure. A major fraction of this effort was devoted to clusters of metallic and semiconducting compounds because it was recognized that the strong bonding of these materials might cause the clusters to adopt different, previously unseen, structures and types of bonding. For example, it was quickly found that
clusters of alkali metals, where bonding is essentially isotropic (that is, nondirectional), could be described by a simple electron shell model partially analogous to the nuclear shell model. As a result, clusters containing certain numbers of electrons have unusual stability and abundance.
Carbon and, to a lesser extent, silicon represent the opposite chemical extreme from alkali metals in that the bonding is strongly directional in space. Simple theoretical considerations and early free radical experiments suggested that small carbon clusters would exist as chains and rings, but there were no reliable predictions of how large a carbon cluster would have to be to exhibit graphite or diamond structure, or what intermediate-sized clusters might actually look like. Carbon, then, was expected to be an intriguing cluster research problem, and in 1985 it was observed quite unexpectedly that a carbon cluster with 60 atoms is far more stable against growth to larger species than are clusters of neighboring numbers of atoms. It was proposed, without direct structural evidence or a macroscopic sample of the material, that this C60 cluster had the now famous truncated icosahedral structure. This icosahedral structure, and its popular name "buckyball," was inspired by the geodesic domes of R. Buckminster Fuller. The connected structure of the figure implies that C60 is a stable molecule with strong bonding on the surface of the icosahedron. Conceivably, other molecules and atoms could be chemically bound to the surface, or could even be trapped inside the icosahedron.
This proposal was revolutionary, yet, as occurs in most significant discoveries, it is simple and elegant in retrospect. It immediately suggested that C60 would be a sufficiently stable molecule to be the building block of new materials, essentially a new form of elemental carbon. The probable existence of C60 became widely known in chemistry, materials science, and solid-state physics, and for 5 years scientists tried without success to find a practical method to make macroscopic amounts of C60.
A practical method came from a totally unexpected source. Researchers studying light scattering from graphitic particles recognized that C60 might account for the curious optical spectra associated with "soot" made for decades by graphite rod vaporization in helium. C60 was crystallized using chemical methods from such soot and proved to have an icosahedral structure. As C60 is present in an astonishing ~ 25% yield, a practical synthesis was achieved.
Synthesis of C60 triggered a rapid series of further discoveries that continues today. Materials chemists studying organic conductors recognized that crystalline C60, when doped with extra electrons, might be an organic metal with nearly isotropic conductivity. Organic metals are currently being developed for a number of materials applications in electronics and optics. To their great surprise, they found that alkali-doped C60 crystals are superconductors at low temperature. Theory has not yet advanced to the point that new superconductors can be predicted, and so the discovery of a new type of superconductor, along with the fundamental understanding of the microscopic mechanisms responsible for the
superconducting, is important. Superconducting materials have important applications in integrated circuits, magnetic sensors, large magnets for imaging systems, and power transmission systems.
Buckyballs can now also be made with metal atoms inside the cage. The interior atoms donate their valence electrons to the cage and thus change its electrical properties. Organic molecules can be bonded to the outside of the cage, and this fact allows buckyballs to be incorporated into polymeric "plastics." The possibilities of both outside and inside doping of buckyballs in solids should give electrical properties we cannot now anticipate. The C60 structure, however, is now known to be the smallest member of a family of similar, larger icosahedral structures, the next member of which is C240. Remarkably, these structures can occur nested in an "onion skin" arrangement, and their properties are only just beginning to be examined.
In the past year another dimension of carbon nanotechnology was revealed
when cigar-shaped, capped tubes of 1- to 2-nanometer diameter, and even extended "nanotubes" of essentially macroscopic lengths, were experimentally discovered. Nanotubes are, in essence, sheets of graphite that have been rolled into tiny tubes. It is predicted that they can be either semiconductors or metals, depending on the angle of the bonding with respect to the tube axis. Nested nanotubes have also been recently observed.
More broadly, the discovery of buckyballs and the carbon variants has stimulated researchers in many other fields to consider a wide range of useful nanostructures that might be built from curved graphite sheets. Researchers are now actively pursuing the growth of continuous carbon nanotubes. When grown in length to many meters, these nanotubes would constitute a fiber of incredible tensile strength. Multifilament cables of such pure carbon nanofibers are expected to be the strongest (and toughest) cables that could ever be constructed of any material—roughly 100 times stronger than a steel cable of the same diameter and 400 times stronger than steel per unit weight. Nanofibers, doped with metal atoms down the hollow inside cavity, are expected to have electrical conductivity at room temperature substantially higher than pure copper and could provide an attractive replacement for use in electrical power transmission lines worldwide. Furthermore, their large thermal conductivity suggests they may replace diamond and copper in many applications requiring heat sinks. AMO science was critical to the genesis of these ideas and will be critical in bringing them to life.
The importance of C60 is extending into research areas beyond those associated with new materials. For example, in biomedical research, recent theoretical and experimental tests have shown that C60 derivatives interact with the active site of HIV-1, removing much of the active area of this deadly virus (Figure 1.4). Thus it is conceivable that this recently discovered particle may offer some help in the worldwide battle against AIDS.
Carbon, already important in biology and organic chemistry, now offers the possibility of new materials, devices, and products with numerous potential applications.