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Harnessing Light: Optical Science and Engineering for the 21st Century (1998)

Chapter: 7 Optics Research and Education

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Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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7
Optics Research and Education

Research in optics has a long and distinguished history, dating back even further than the work of Galileo and Newton. In recent decades, optics research has blossomed with the invention of the laser, an increasing interaction between optics and electronics, the development of new materials with unique optical properties, and other extraordinary advances. The first part of this chapter highlights some examples of research areas that hold special promise for further discoveries. This is a time of great excitement for all optics researchers, whether in universities, industry, or government laboratories.

The second part of the chapter discusses the state of optics education. Combining this report's discussion of research and education issues in a single chapter is only appropriate. The creation of research universities, which combine research and education endeavors to create a synergy between the discovery of knowledge and the education of students, has been one of the key institutional developments of the past century for research and education in optics as in other fields.

Introduction

Both basic and applied research are motivated by a deeply rooted human instinct: the desire to know and understand nature. Over hundreds of years, humans have gradually learned to explore nature in a systematic way known as the scientific method—the use of experiments and observations, guided initially by intuition and hypothesis and later by theory. The resulting understanding of nature is the hallmark of the scientific process. Roger Bacon, a multitalented thirteenth-century scientist and philosopher whose work in optics included a description of a telescope

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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(more than 300 years before anyone actually built one) and the first application of geometry to the study of lenses and mirrors, is often considered the forerunner of experimental science and the scientific method.

Applied research, which is often indistinguishable from basic research, is motivated by the need to understand how as well as the need to understand why. Louis Victor de Broglie, recipient of the 1929 Nobel Prize in physics, explained that ''the two aspects of science [pure and applied] correspond to the two principal activities of man: thought and action. They are inseparable if human science is to progress as a whole and fulfill with increasing success its high and twofold task." Decades of basic and applied research are often needed to lay the foundations for key discoveries, such as the invention of the laser. The most important discoveries often arise at boundaries between established fields—in the case of the laser, at the interface between physics and electrical engineering.

The ultimate impact of research is rarely predictable. For example, as the earlier chapters of this report demonstrate repeatedly, the invention of the laser has had a major economic and social impact, with remarkable applications in areas that range from communications to the environment to medicine. Yet Arthur Schawlow, the laser's coinventor, once commented that "if I had set out to invent a way to improve eye surgery, I certainly would not have invented the laser."

Development, often guided by understanding gained through basic and applied research, is motivated by a need to make something that works and is of commercial value. However, development—especially the development of commercial products—usually requires a much larger financial investment than most research. Such resources are often unavailable at universities; therefore, in the United States, development activities are generally concentrated in the commercial sector. The exception is in certain areas of special interest to the nation, such as defense, space, and the environment. Military needs in particular have historically motivated substantial support for long-term optics R&D by the Department of Defense—support that has been very important over the years but that is being reduced, leading to concern about a gap emerging between basic conceptual research and commercial hardware development. Development often involves multiple scientific and engineering disciplines, multiple approaches to problem solving, and the unique techniques and resources of large public and private organizations.

Education at all levels is critical to the future of optics. Optics is a natural tool for a visual approach to the education of K-12 students in many aspects of science and mathematics. Post-high school education, including 2-year (associate) and 4-year degree programs, is important for meeting the future needs of this rapidly growing area of

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

knowledge. There is a need for graduate education, as discussed below, as well as special courses of study in optical engineering provided by universities with an historical focus on optics. Jobs are available for those trained in optics.

It is often asked whether optics constitutes a separate discipline, and this question requires critical attention. Optics is certainly recognized as an integrated field of knowledge. Its teaching extends across disciplinary boundaries and includes science departments, engineering departments, and departments in schools of medicine.

Because the field of optics is not sharply defined by a job title or membership in a single professional society, it can be difficult to develop a quantitative picture of the size and breadth of the optics research community. One indicator is participation in professional conferences. About 6,000 scientists and engineers usually attend the annual U.S. Conference on Lasers and Electrooptics (CLEO). The Optical Fiber Conference attracts more than 7,000 participants each year. Other major conferences in optics include the annual meeting of the Quantum Electronics and Lasers Society, the International Quantum Electronics Conference, the annual meeting of the Optical Society of America, the Lasers and Electrooptics Society annual meeting, and conferences organized throughout the year by SPIE, such as Photonics West and Photonics East. Altogether, the field involves at least 30,000 active scientists and engineers worldwide, and many areas of the optics industry are growing rapidly.

Another indicator of the strength and growth of the field is its impact on the economy. Optoelectronics is now a major component of U.S. import and export trade. The rate of formation of optics-related businesses has grown rapidly during the 1990s.

The need for greater investment in research and education in optics was recognized in a 1994 National Research Council (NRC) study, Atomic, Molecular, and Optical Science (the FAMOS study). Although this study focused on a narrow segment of the field of optical science and did not address the engineering aspects, it nevertheless found that this field is ripe for breakthroughs, growing rapidly, and having an impact on sciences beyond optics, including biology, chemistry, and medicine. Today, the United States invests approximately 0.5% of its gross domestic product (GDP) on research in its research universities. This is the lowest rate of such investment by any major industrialized country.

As a result of a workshop in May 1994,1 the National Science Foundation (NSF) in 1995 announced a multidisciplinary research initiative in optical science and engineering. The program attracted more than 600 pre-proposals and 70 full proposals. Ultimately only

1

The workshop produced a report; see NSF (1994).

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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18 projects could be funded. These numbers indicate both the vitality and the competitive nature of optics research and the strong interest nationwide in the opportunities it presents.

In recent years an important feature of optics research has been the growing interaction between optical physics and electrical engineering. The first hint of this link was the discovery by Heinrich Hertz in 1883 that radio waves and light waves are described by the same theory of electromagnetism and are distinguished only by their different frequencies. Marconi soon demonstrated the transmission of information by pulsed radio waves, and by the 1920s there were more than 30 million radios in use in the United States alone. From the development of electrical devices for radio and other applications came a new research discipline, electrical engineering. In this same period, physicists were exploring newly discovered properties of light. Max Planck found that light is quantized in units called photons. Einstein used this discovery to explain the emission and absorption of light quanta by atoms and predicted that it should be possible to use light to stimulate an atom to emit more light. Other physicists—most of them engaged in basic research with little thought about potential applications—explored electrical discharges, the properties of atoms in solids, the properties of semiconductors, and atomic and molecular spectroscopy. The two communities—physicists, with their emphasis on quantum mechanics and the basic understanding of nature, and electrical engineers, with their understanding of electronic circuits and wave propagation—were brought together in 1960 by the invention and demonstration of the laser (named for light amplification by stimulated emission of radiation). Today, work at the interface of physics and electrical engineering is a key element of optics research, producing such important developments as the semiconductor diode laser.

Optics is a multidisciplinary field that cuts across many of the traditional academic disciplines. The funding of initiatives in such crosscutting areas is often hindered by the structure and organization of the federal agencies that support research and development. In an effort to overcome these difficulties, the government has created the National Science and Technology Council (NSTC) and charged it with addressing the following goals: to maintain leadership across the frontiers of scientific knowledge; to enhance connections between fundamental research and national goals; to stimulate partnerships that promote investments in fundamental science and engineering and in the effective use of physical, human, and financial resources; to produce the finest scientists and engineers for the twenty-first century; and to raise the scientific and technology literacy of all Americans. These goals serve as guideposts for NSTC in making recommendations and setting priorities for investment in new crosscutting research and development

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

opportunities that involve multiagency and multidiscipline collaboration and support. NSTC works to identify important crosscutting science and technology programs and works with multiple agencies to fund new initiatives. For example, an initiative on high-performance computing and communications involves nine agencies and an investment of $1 billion (National Science Board, 1996). As demonstrated throughout this report, the broad area of optics is multidisciplinary and growing rapidly. It is ripe for support as a crosscutting initiative at a level comparable to the earlier investment made in high-performance computing and communications.

Multiple government agencies should form a working group to collaborate in the support of optics, in a crosscutting initiative similar to the earlier one for high-performance computing and communications systems.

NSF should develop an agency-wide, separately funded initiative to support multidisciplinary research and education in optics.

The Department of Commerce should explicitly recognize optics as an integrated area of knowledge, technology, and industry and should structure its job and patent databases accordingly.

Research Opportunities

Opportunities for research in optics have the potential for significant benefit to society in the next decade. Many new developments, with a high leverage for return on the investment of increasingly scarce research dollars, have been identified. However, research by its nature returns benefits on a portfolio of many possible avenues of investigation. A few examples of the many areas of research in optics that show promise and offer high leverage for future return are presented in this section. It is impossible to cover all areas of research in the extensive and multidisciplinary areas of optics. A more inclusive set of research opportunities in optics is presented each year at large annual meetings such as CLEO.

Areas of optics research selected for discussion include control of atoms by light, fundamental quantum limits of measurements, and light in biology. Recent advances in optical microscopy are highlighted. Femtosecond laser technology and its application to ultrafast physics, chemistry, and engineering is identified as a particularly promising opportunity. Advanced laser sources and frequency conversion of lasers using nonlinear optical devices offer significant potential for applications in semiconductor processing, reprographics, and image display. Semiconductor lasers and solid-state lasers are promising sources of coherent optical radiation at ever-decreasing cost. Advances

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

in optical materials are recognized to play an important role in the next generation of laser sources, nonlinear frequency converters, and new optical elements such as gradient index optics and media for optical storage of information. Finally, progress in the generation of coherent radiation progressed from radio frequencies in 1900 to microwave frequencies in 1930 and to optical frequencies in 1960. We are now entering the era of extreme ultraviolet (UV) and, in the future, coherent x-ray sources with applications that extend from lithography to advanced x-ray imaging.

Quantum, Atomic, and Biological Optics

Control of Atoms by Light

Light continues to be the principal method of probing matter. Powerful spectroscopic techniques continue to be developed as light sources extend into new regimes of the electromagnetic spectrum and as optical sources with extremely short pulses such as femtosecond lasers, synchrotron sources, and free electron lasers become more widely used.

In the past decade, light has begun to be used to detect and to control matter, particularly the position and velocity of atoms, molecules, and small particles. Currently, atoms in the gas phase can be laser-cooled to microkelvin temperatures where their velocities are on the order of a centimeter per second. Once cold, atoms can be manipulated with relative ease. Atoms tossed upwards in a ballistic trajectory as in a fountain form the basis of a new generation of atomic clocks. Prototype atomic fountain clocks already have short-term stability an order of magnitude better than atomic clocks based on thermal beams of atoms. Alternatively, ions trapped in magnetic and electric fields could lead to atomic clocks based on optical transitions in which the stability would be improved another thousandfold. (Since ion clocks will have a low signal-to-noise ratio, they will need stable "flywheel" frequency references, such as ultrastable hydrogen masers or cryogenic microwave oscillators, to take full advantage of this potential for superior accuracy.) Atom traps have also been used to store radioactive isotopes for nuclear physics studies and for tests of fundamental symmetries of nature such as parity and time.

Atom optics has emerged as a new field. Atom lenses were used to deposit 0.05-µm lines and dots on a surface. These structures can be written over large areas and may be used to pattern a surface for high-density optical storage. One promising approach uses the optically guided atoms to react chemically with a resist with better than 0.1-µm spatial resolution. Atom beam splitters, mirrors, and diffraction gratings have been used to construct atom interferometers. Atom interferometers have already proven to be sensitive accelerometers and gyroscopes.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

BOX 7.1 BOSE CONDENSATION

In 1925, Einstein made a dramatic prediction. If the atomic density n increases to nλDeB3= 2.612, where λDeBis the DeBroglie wavelength, a sizable fraction of the atoms will condense into a single quantum state. Examples of Bose condensation media include superfluid helium, paired electrons in a superconductor, and paired atoms in superfluid 3He.

In 1995, 70 years after Einstein's prediction, research groups at the University of Colorado and the Massachusetts Institute of Technology achieved Bose condensation of a dilute gas of rubidium and sodium atoms by using a laser to cool the atoms. A year later, a group from Rice University was able to show that 7Li atoms also form a Bose condensate.

Figure 7.1 shows the momentum distribution of a gas of (Rb or Na) atoms in the vicinity of the Bose condensation threshold. Just above the required phase space density (left), the atomic energy is distributed in all directions equally among the many occupied quantum states, in accord with the principles of statistical physics. As the threshold condition is crossed (center), a central peak at v = 0 begins to form, signifying the onset of a condensation. With further cooling (right), most of the atoms condense into the ground quantum state of the system.

The properties of dilute Bose gases are being explored, and it is too early to predict how this field will develop. Perhaps new physics will be discovered. Perhaps Bose condensates will be used as intensely bright sources of atoms analogous to photons from a laser. The one certain prediction is that the future is hard to predict: The inventors of the laser had no idea of the myriad of ways in which their invention would contribute to science and technology.

Freely falling atoms have been used to measure the acceleration of gravity with a precision of one part in a billion after 100 seconds of averaging time, and an atom interferometer gyroscope has exceeded the sensitivity of the best commercial laser gyroscopes. Atom gravity gradiometers currently being developed may eventually be used for oil exploration.

The control of atoms permits the creation of new states of matter. For example, standing wave interference patterns caused by interfering laser beams create periodic potential wells in which atoms can be trapped. These so-called optical lattices offer a unique model system of "condensed matter" concepts in which the characteristics of either fermionic or bosonic particles can be probed. Atoms have been cooled with sufficient density that they have condensed into a single quantum state known as Bose-Einstein condensation (Box 7.1), predicted by Einstein (see Figure 7.1). Once the atoms are in a single quantum state, the concept of temperature is no longer valid. The properties of this new state of matter are now being studied with great fervor, and important applications will likely result from these studies. Photons, which share the same quantum statistical-mechanical properties as atomic bosons, have been made to "condense" into laser light. As the atom analogue to the

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.1 The momentum distribution of atoms during Bose condensation of a laser-cooled dilute gas. (Courtesy of M. Matthews, JILA.)

laser, the Bose condensate may also revolutionize the way atoms are used just as the advent of the laser revolutionized the way light is used.

Fundamental Quantum Limits of Measurement

In addition to providing some of the most refined measurement tools, optics is and will continue to be the primary means of studying the fundamental limitations of any measurement. The measurement of a physical quantity is limited by the size of the signal relative to the noise in the measurement. Quantum mechanics defines the fundamental noise floor. For example, the uncertainty principle tells us that the simultaneous measurement of position and momentum with uncertainties Δx and Δp is limited to an accuracy ΔxΔp ≤ h, where h is Planck's constant. Similarly, the simultaneous measurement of the amplitude and phase of an electromagnetic wave is also limited.

Although these fundamental limitations were known since the early days of quantum mechanics, we have only recently shown that the partition of quantum noise can be altered. For example, in a laser interferometer measurement of distance, the phase of the light in the interferometer is the quantity of interest. The phase noise due to the vacuum can now be "squeezed" into the amplitude sector, allowing more precise measurement of the phase of the interferometer. Before this refined measurement sees wide application, the methods of altering noise at the quantum limit have to be made easier.

Quantum mechanics also tells us that any measurement of a physical system necessarily alters the system. This effect of the measurement on the system being studied can be tailored to influence different aspects of the system. To measure the intensity of a pulse of light, it is now possible to make a "nondemolition" measurement in which the number of

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

photons in the pulse is not altered; the unavoidable perturbation of the light pulse manifests itself with a change in the phase of the optical pulse.

The quantum dynamics of strongly coupled quantum systems allows precise coherent control of both the internal state and the external dynamics of an atom. This type of quantum state engineering has already been pursued in order to control chemical dynamics. More recently, it was shown that it may be possible to engineer correlated quantum states where the sensitivity of an optical interferometer could be improved by the square root of the number of photons used in the measurement. Since the number of photons used in a measurement can be very large, the gain in sensitivity could be staggering.

The study of highly correlated quantum states may also offer fundamentally new ways of transmitting information and computing. Unlike a classical computer based on irreversible, dissipative transitions between distinct states such as 0 and 1, a quantum computer based on the reversible quantum evolution of linear superpositions of quantum states could in principle greatly reduce the time needed to solve certain problems. For example, on a conventional computer the time required to determine a number's prime factors (a problem with important applications in cryptography) grows exponentially with the size of the number, so any computer can easily be overloaded. On the other hand, a factorization algorithm based on the logic of quantum computing necessitates only a polynomial growth in computing time. A practical quantum computer may never be realized because of the difficulty of maintaining the phase coherence of a system with many degrees of freedom, but this area of research will no doubt lead to greater facility in handling complex quantum systems and eventually to new technologies. The laser, the transistor, and magnetic resonance imaging are examples of how a deeper appreciation of quantum mechanics has led to technological revolutions. Optics will continue to provide some of the best tools for the exploration of quantum mechanics and quantum systems.

Light in Biology

Not only are optical techniques driving a revolution in the manipulation of atoms and the refinement of physical measurements, but they are also generating a resurgence in biological fields. Even the venerable optical microscope, invented in 1625 and "perfected" at the beginning of this century, is having a renaissance because of the confluence of several technological developments (see Figure 7.2).

Fluorescence methods made possible with high-performance interference filters, laser illumination, and the synthesis of new dyes resistant to photobleaching have contributed to this resurgence. The fluorescence from single molecules can now be detected with detectors such as microchannel plate image intensifiers or avalanche photodiodes.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.2 ADVANCES IN OPTICAL MICROSCOPY

Technical innovations in the optical microscope have often been followed by major advances in biology. The invention of the first compound microscope in 1625 was followed quickly by the discovery of microscopic organisms such as yeast, algae, and protozoa. The construction of good-quality, high numerical aperture microscope objectives, the development of a method of achieving uniform sample illumination (Kohler illumination), and the introduction of cell-staining methods elevated microscopy to a general-purpose diagnostic tool in medicine. The introduction of phase contrast methods for observing unstained biological samples opened up the possibility of observing live cells.

We are in the midst of a renaissance in the development of the microscope, generated by the integration of other technologies such as video cameras, image intensifiers with single-photon counting sensitivity, and computer pattern recognition and image deconvolution. Research into laser trapping techniques has led to "optical tweezers" that allow one to micromanipulate samples with light while simultaneously viewing them. The scanning confocal microscope, coupled to a computer, gives three-dimensional images of samples. Near-field optical microscopes have achieved molecular resolution. Molecular biology techniques now permit the staining of specific proteins expressed in living cells.

What are the current technological challenges? Near-field optical techniques have to be improved so that molecular resolution can be routinely achieved for biological samples in water. Fluorescent tags with greater resistance to photobleaching and additional colored protein markers to complement the green protein marker will allow biologists to track protein-protein interactions in living cells. The most important ingredient is the invention of methodologies that will exploit these new tools. History has shown repeatedly that new discoveries in basic science have always followed significant technological advances.

Proteins manufactured in cells can be tagged with a green fluorescent protein (GFP) by adding instructions to the protein DNA that result in a protein-GFP molecule. The fluorescent protein can then be seen as it moves in a live cell. The development of alternate color tags (such as red and blue dyes) will enable the observation of protein-protein interactions in a live cell in real-time. Two-photon excitation from inexpensive diode-pumped femtosecond lasers will permit the observation of the intrinsic fluorescence of biological molecules and molecules in the interior of tissues that could not otherwise be detected. Fluorescent dyes can also be used to monitor their local biological environment since the fluorescence is affected by conditions such as temperature, electric fields, and pH.

New laser methods such as confocal microscopy for three-dimensional imaging and near-field optical microscopy with nanometer resolution are adding to the set of powerful, noninvasive optical diagnostic tools (Box 7.2). Lasers have also been used to control micron-sized

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.2 Four hundred years of progress in the history of the optical microscope. Rapid progress over the past 40 years is illustrated on the expanded scale.

objects at room-temperature in aqueous solution. These so-called optical tweezers have given us the ability to manipulate individual cells, organelles within the cell, and even molecules in an optical microscope. For example, muscular contraction at the molecular level has been reduced to the interaction of a myosin molecule on an actin filament. Optical tweezers have been used to measure, in real-time, the force and displacement of the myosin molecule during hydrolysis of a single ATP (adenosine triphosphate) molecule. Just as powerful new insights have come from the study of single, isolated quantum systems, the study of biology at the level of a single molecular event will undoubtedly reveal unexpected behavior.

The National Science Foundation should recognize the dramatic new opportunities in fundamental research in atomic, molecular, and quantum optics and should encourage support for research in these areas.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.3 Ultrafast laser techniques have shown that the first step in vision, a rapid structural change in the rhodopsin molecule, occurs in only 200 fs.

Femtosecond Optics

Throughout history, short flashes of light have provided a means of capturing and studying high-speed events. High-speed electronic flashes can do this with accuracies of nanoseconds. With lasers, the potential quickness and precision of measurement has been sharpened by almost a factor of a million. We now have the capability to observe, on a time scale of femtoseconds, some of the most fundamental interactions among the atoms, electrons, and molecules that compose materials (Figure 7.3). These interactions determine the effectiveness of important chemical and biological processes as well as the ultimate speeds of electronics and optoelectronics. Observing previously unseen phenomena and understanding them better will have a major impact on the development of a wide range of technologies.

In addition, the use of femtosecond pulses is expected to go beyond fundamental measurement to new ways of controlling and changing matter. Femtosecond pulses (see Figure 7.4) capture the full bandwidth of optics coherently. Thus, they can be sculpted by frequency-domain methods to produce the unique optical waveforms required to manipulate selected quantum states of atoms and molecules. The short duration of femtosecond pulses also makes it possible to amplify them to extremely high powers with relatively modest energy. Today's power levels (which produce electric fields 100 times greater than those holding the hydrogen atom together) are already leading to the discovery of new phenomena and the creation of effects similar to those in high-energy accelerators.

High-Intensity Laser-Matter Interactions

At optical intensities greater than 1013 W/cm2, the electric field of the focused light beam exceeds that found in the interior of atoms by many orders of magnitude, and atoms fly apart or ionize during one optical cycle. In this regime, the ponderomotive wiggle energy of the emitted electron as it oscillates in the laser field becomes comparable to the photon energy, and atomic energy levels and widths can no longer be calculated with perturbation theory. The subject of high-intensity laser-matter interactions has important applications, which range from the understanding of laser-induced inertial confinement fusion to the investigation of laser-plasma instabilities at high field strengths. Many new phenomena have already been uncovered,

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.4 The femtosecond time domain was pioneered in the 1970s and 1980s with dye laser technology. The development in the 1990s of femtosecond solidstate lasers is opening up a wide range of new applications.

such as above-threshold ionization (ATI), new pathways to direct multiple ionization, fine intensity-dependent resonances due to large ac (alternating current) Stark shifts in the bound-state spectrum, and bond softening and above-threshold dissociation in molecules. Yet the intensity range over which these phenomena occur is only a small fraction of what will be achievable with amplified femtosecond solid-state lasers. Several lasers are now capable of focused intensities a million times higher than the threshold for atom ionization. At these intensities, relativistic effects are important. Two laboratories have recently reported generating relativistic electrons from low-density plasmas using high peak power lasers. Further exciting developments can be expected during the next decade, including laser acceleration and laser-driven inertial confinement fusion.

Intense femtosecond pulses have also been found to have some dramatic advantages in materials processing. By optimizing pulse duration for the specific material being processed, one can greatly improve the reliability and localization of the desired damage or ablation. One important application is laser surgery in the transparent vitreous fluid of the eye without damage to the retina. Micromachining in solid materials with a resolution of better than 1 μm could open up new possibilities for high-technology fabrication.

Lasers in Chemistry

With the rapidly improving ability to control all aspects of ultrashort laser pulse shapes, optics is poised to have a dramatic impact on progress in chemistry and biochemistry. Developing an improved, predictive understanding of chemical reaction dynamics has always been a major goal of research in physical chemistry. New excitement has been generated in this area recently by novel laser-based capabilities for studies of transition states by photoelectron spectroscopy and femto-chemistry, for control of reaction pathways, and for bond-selective chemistry. The potential for optical control of chemistry in particular is made possible by the combination of advances in femtosecond lasers, sophisticated pulse shaping methods, theory, and computational capability. Experimental demonstrations of the control of reaction products, vibrational dynamics, and population transfer have already been

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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demonstrated. This research is generating new knowledge and has significant potential for experimental discovery.

A new frontier also exists in efforts to understand, at a microscopic level, processes that occur in the solution phase. Most chemical reactions, especially those of commercial significance, occur in solutions. Yet only recently have femtosecond laser developments facilitated direct studies of these important reactions. Some of the most exciting recent results concern reactions that have coherent excited states as products. The key to chemistry lies in understanding the influence of the solvent on the dynamics of these transition states. Small molecules are permitting the first clear observations of such reactions in solution, but similar phenomena are also beginning to be seen in complex biological molecules. Emerging from these experiments are new paradigms of condensed-phase reactivity. Here, in particular, is where advances in optical science are the rate-limiting step in advancing the understanding of chemistry. A further step could be direct visualization of the nuclear dynamics during such reactions by means of x-ray spectroscopy, and optics would play an enabling role in that as well. The first femtosecond x-ray pulses have recently been produced by Thomson scattering of femtosecond laser pulses by an accelerated electron beam. Efforts to apply these pulses to questions of microstructural dynamics are under way. Other possible applications include the study of protein folding using time-resolved x-ray scattering.

Technology of Femtosecond Lasers

The current rapid advance in femtosecond technology is made possible by recently invented methods for ultrashort pulse generation with solid-state lasers. Early pioneering work with dye lasers opened the femtosecond time domain to science and laid many of the foundations for present technological advances. Solid-state technology now brings greater reliability, shorter pulses, and higher powers to scientific applications. It also opens the door for more widespread commercial application by making femtosecond sources more efficient, more compact, and less expensive.

Key to the all-solid-state femtosecond revolution has been the invention of nonlinear optical pulse-forming methods and devices, such as additive pulse modelocking (APM), Kerr lens modelocking (KLM), soliton modelocking, antiresonant Fabry-Perot saturable absorbers (AFPSAs), and saturable Bragg reflectors (SBRs). Each is important for a somewhat different type of application. What they have in common, which sets them apart from previous technologies, is that they are no longer simply determined by the properties of basic materials. They can be optimized artificially, engineered, and enhanced by synthetic design techniques. Opportunity still exists for the discovery of new enabling

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
×

BOX 7.3 LASER-DRIVEN PARTICLE ACCELERATORS

The first particle accelerators, developed in the 1930s, explored the properties of the atomic nucleus. The accelerator is basically a transformer, in which electrons gain energy as they traverse the voltage applied across the structure. A key breakthrough in accelerator design was the use of voltage at radio frequencies in the cyclotron to accelerate electrons. This was followed by the use of microwave frequencies in a linear accelerator to reach electron energies of billions of electron volts. Today most accelerators operating in the world are driven by microwave radiation. Conceptual designs for the Next Linear Collider, based on high-frequency microwave radiation, call for a 20-km linear collider with 500-GeV colliding particle energy.

Livingston studied the evolution of particle accelerator technology and noted that each new step toward high-energy involved a new design approach or a new source of high peak power electromagnetic radiation. One new approach to accelerator technology is to use the laser to generate future tera-electron volt (TeV) accelerators. Recent success in experimental laser acceleration has been achieved by a group at the University of California, Los Angeles, using a plasma structure (Joshi and Corkum, 1995). However, not all researchers think that this approach will be practical.

Laser-driven TeV accelerators, if constructed, would allow probing of fundamental particles that were formed in the earliest few moments at the birth of the universe. Why the universe is formed of matter and why we are here are questions that could be studied with the next generation of high-energy particle accelerators.

Today, terawatt (1012W) peak power lasers operate on a table top. In the future, these lasers would operate at high-efficiency and high average power to drive kilometer-long TeV linear accelerators.

mechanisms, but now there are at least some direct pathways for technology development by improved design and manufacturing.

Research into the optical materials for these ultrafast lasers and into devices for femtosecond pulse manipulation continues to play a crucial role in the development of this emerging technology. New laser crystals for different wavelengths, improved nonlinear optical materials, novel active and nonlinear fiber devices, and a better understanding of light-matter interactions are all still needed to nurture the technology's success and widen its impact.

Technology Applications

To date, the principal beneficiaries of developments in femtosecond technology have been scientific applications, but the prospect of compact, practical, cost-effective femtosecond laser sources is now creating opportunities for a variety of mainstream commercial applications. Some take direct advantage of the ultrahigh-speed aspect of the technology. Others derive benefit from related aspects such as broad spectral width, high focused peak intensities, phase coherence, improved

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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detectability, and precise repetition rates. Optical communication systems have already benefited from advances in ultrafast optics, and they remain a fertile domain for new applications based on ultrashort pulses, such as soliton transmission, all-optical switching and networking, wideband wavelength-division multiplexing (WDM), ultrahigh-rate data processing, and clock distribution.

Femtosecond laser sources with high peak and average power open the possibility of using a laser to accelerate electrons in place of the microwave source traditionally used for this application. Laser sources with higher peak power and shorter wavelengths (Box 7.3) offer the possibility of achieving acceleration gradients of about 1 GeV per meter. If successfully developed over the next two decades, the laser-driven particle accelerator may open new avenues for studies of the physics of fundamental constituents of matter at the TeV energy scale (Figure 7.5).

Measurement technologies such as device and circuit testing, optical ranging, surface monitoring, and microscopy can take advantage of the peak powers and low temporal coherence of femtosecond pulses as well as their short durations. Low-coherence confocal microscopy and two-photon microscopy are examples of rapidly developing applications, as are biomedical imaging by optical coherence tomography and laser surgery. The feasibility of combining ultrafast microscopy with the measurement of ultrafast optical response will expand the list of possible applications.

A particularly novel and unexpected application of femtosecond light pulses is the generation of terahertz radiation, or T-rays. T-rays in the wavelength range of 0.05 to 1 mm provide a new technology for imaging hidden items and some invisible material properties (Figure 7.6).

FIGURE 7.5 (a) Evolution of laser peak power and focused intensity versus year and the physical phenomena that can be explored. Over the last 35 years, peak power has increased by 11 orders of magnitude. (Courtesy of P. Pronko, University of Michigan.) (b) A proposed dielectric-based laser accelerator structure with 1/3-mm repeat distance per stage. The average acceleration gradient is calculated to be 0.7 GeV/m.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.6 T-rays with wavelengths between 0.05 and 1 mm provide a clear image of this electronic circuit, still inside its manufacturer's packaging. (Courtesy of M.C. Nuss, Lucent Technologies.)

Successful development of this and other emerging applications will depend on the availability of cheap and versatile optical pulse power supplies. The availability of such supplies can also be expected to stimulate still other applications, as yet unforeseen.

Femtosecond optics and sources offer an opportunity for dramatic impact on science and technology. Agencies should focus attention on this opportunity and encourage innovative work in this cutting-edge field.

Semiconductor and Advanced Solid-State Lasers

Lasers are now essential to the national economy, enabling applications that extend from CD-ROM to the fiber information network; from the processing and fabrication of semiconductors to the cutting of cloth, plastic, and industrial materials; and from laser vision correction to theraputic medicine. Laser sources have made a transition from the gas discharge tubes associated with the ubiquitous red helium-neon lasers used in supermarket scanners and the blue-green argon ion lasers familiar in laser light shows to solid-state laser diodes (LDs) and advanced solid-state lasers. This transition to the solid-state will have a profound impact on the future growth of laser markets. Laser diodes and solid-state lasers can be produced at low cost and operate at high electrical efficiency with high reliability. One consequence of the transition from discharge tubes to the solid-state is lower production costs because of the increasing volume of production. Moore's law for the exponential increase in the number of transistors on a wafer, a consequence of the lithographic production techniques applied to silicon, is for the first time applicable to lasers. We can expect that laser diodes and solid-state lasers will be produced at ever-increasing volume and ever-decreasing cost as the markets continue to expand. This in turn will lead to a continued growth of many new laser applications. The markets for laser applications will grow from $3 billion today to tens of billions of dollars in the next decade.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Laser Sources
Semiconductor Laser Diodes

Lasers diodes based on the semiconductor gallium arsenide (GaAs), an extension of silicon technology, are widely used 30 years after the invention of the laser. GaAs laser diodes operate from the red to the near-infrared (IR) at power levels from milliwatts to multiple watts. Red laser diodes are now used in compact disk (CD) players, and more than 200 million of them are produced each year at a cost of less than $1 each. The GaAs material technology is more complex than silicon-based material technology, but lithography-based planar-processing technology developed for the silicon industry is applicable to GaAs laser diode production, so laser diodes can be manufactured in large volumes at low cost.

The cost of laser diode sources decreases rapidly with production volume. An annual market volume of $30 million is adequate to bring the production cost to less than $1. This low cost, along with reliable operation and small size (a grain of sand is larger than the diode laser source), opens many new markets for laser diode devices.

The market for optoelectronic equipment is expected to grow from $139 million in 1998 to $463 million in 2013. Applications of diode lasers include data storage, optical displays (Box 7.4), telecommunications, local area network communications, sensors, and medicine. Worldwide laser diode sales in 1996 were $1.92 billion. The worldwide market for all laser sources was $2.82 billion. Of course, the laser source is a critical and enabling element in a larger system. For example, the diode laser in the compact disk player enables a market for CD players that now exceeds $10 billion per year. The growth and market size of laser-based systems continue to track the growth and market size of computer-based systems with a delay of about 15 years.

BOX 7.4 ORGANIC AND SILICON-BASED LED DISPLAYS

Research into organic-based LEDs is now under way in many laboratories. Organic LEDs, manufactured from polymer films that are patterned and electrically excited directly, offer the promise of very low-cost optical displays that are like paper in flexibility, cost, and ease of use. Progress in research toward brighter, longer-life, organic-based LEDs continues to show promise (Kido et al., 1995). Porous silicon emits light that is broadband in nature and rather weak in output power. Nevertheless, silicon-based LEDs offer promise as inexpensive sources of light for speciality applications such as alphanumeric indicators directly on the silicon circuit based electronic component (Collins et al., 1997).

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Laser diodes based on GaAs are the key to the future growth of the laser markets. GaAs refers to not just one material system but a complex set of material systems based on extensions of GaAs. The progress in wavelength extension of diode lasers from the near IR to the blue depends critically on the development and understanding of the III-V semiconductor material systems. Today, commercially available laser diode products operate from the red to the near-infrared. Power levels range from 10 mW for reading compact disks to more than 20 W for pumping solid-state lasers. These lasers operate at greater than 40% electrical efficiency, which makes them among the most efficient sources of light.

The 45% operating efficiency of laser diodes is far greater than that of the tungsten bulb (~1%) and even greater than that of fluorescent bulbs based on mercury discharge (~25%). In the future, the application of laser diode sources to general lighting has the potential for saving significant electrical power usage in the country since 19% of generated electricity goes for lighting (see Chapter 3).

The laser diode is more efficient and can operate at higher output power than the light-emitting diode (LED; Box 7.5). Further, the laser diode has a coherent output beam that can be focused into an optical fiber or onto the surface of a CD. Thus, LDs complement LEDs, and both will have widespread applications in the future. Like LEDs, laser diodes are being improved and extended in wavelength. Today, the critical area of research is to extend the laser diode wavelength into the blue. The first realization of a blue laser diode based on GaN (gallium nitride), a material closely related to GaAs, was announced by the Japanese company Nichia in 1996. The GaN material system is not as well developed as that of GaAs. It took 10 years from the first demonstration of a room-temperature GaAs red laser diode to the introduction of commercial laser diode products. A similar time might be required before blue laser diodes are available commercially.

When introduced as a product, the blue laser diode will open major application areas including information storage and optical displays. Other applications include fluorescence microscopy for biological and medical applications. There is significant economic leverage for the development of blue laser diodes for application to the optical storage systems of the future.

Today companies in the United States lead the world in the manufacture of high-power LD sources. However, investments in R&D by companies in Europe and in Japan are challenging that lead. High-power laser diodes are used directly for medical and industrial applications and to pump advanced solid-state lasers. The leading producers of laser diodes for information storage, such as CD players, are located in Japan as is the first company to demonstrate the GaN blue laser diode.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.5 THE LIGHT-EMITTING DIODE

Light-emitting diode (LED) light sources, the incoherent versions of laser diodes, operate at near 10% efficiency and milliwatts of output power. LEDs are less efficient and less powerful than laser diodes but are also much less expensive. LEDs are familiar as indicator lights in electronics and as red brake lights in some automobiles. However, they will soon be found in signal lights, road signs, exit signs, and lighting displays (see Chapter 3). The market for LEDs is larger than the laser market and is growing rapidly because of the small size, long operating life, high visual brightness, and low cost of LEDs. As blue LEDs become commercially available, three-color displays will become possible and will add to the growth of the multibillion-dollar LED market.

Blue LEDs have been introduced to complement the already available green and red LEDs. Applications for blue LEDs include color displays, specialty lighting, and medicine and biology where blue LEDs induce fluorescence in molecules for detection. For example, an array of 100 blue LEDs has been used for research on the treatment of jaundice in newborn babies.

Visible laser diodes are used for information storage, bar-code readers, and laser alignment in construction.

High-power laser diodes are used for materials processing, soldering, xerographic image generation, and medical applications. Laser diodes coupled to optical fibers enable direct processing of materials through cutting, welding, and annealing. High-power laser diode arrays are also widely used today to pump advanced solid-state laser sources.

Advanced Solid-State Lasers

Advanced solid-state lasers play a unique, significant, and growing role in scientific and industrial applications. Solid-state lasers offer a combination of performance characteristics that lend themselves to a wide variety of tasks, from the frontiers of research to applications in reprographics, medicine, and defense. High-power solid-state lasers enable high-value manufacturing processes in the automotive and aerospace industries.

Some of the favorable characteristics of solid-state lasers are the ability to store optical energy for periods up to 1 ms and to extract the stored energy in a short time, yielding a high peak power pulse of a few-nanosecond duration (Q-switching); the ability to operate with a large gain bandwidth for ultrashort pulse generation by modelocking; and the ability to emit optical radiation in a single spectral and spatial mode with extreme spectral purity and stability. When extended in wavelength by nonlinear optical frequency converters, solid-state lasers offer the prospect of high peak and average power across a wide wavelength range from the ultraviolet to the infrared.

In 1995 the sales of solid-state lasers reached $394 million. Sales were projected to reach $500 million in 1997. The fastest-growing

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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segment of this technology is diode-laser-pumped solid-state lasers (DPSSLs). Here advantage is taken of the high operating efficiency of the laser diode compared with that of the traditional flashlamp pump source. The DPSSL allows the power from multiple laser diodes to be summed and extracted in a single high-power beam (Anderson, 1997).

Figure 7.7 shows a schematic of a high-power laser-diode-pumped solid-state laser. Using this approach, solid-state lasers now operate at power levels of 1,000 W, and 5,000-W lasers are under development. The electrical operating efficiency of DPSSLs is greater than 10%, which is a significant improvement relative to traditional lamp-pumped solid-state lasers. Further, reliability is improved from the 200-hour lifetime of lamps to the greater than 7,000-hour lifetime for laser diodes. These factors, coupled with the small size of the DPSSL, have opened new areas of application in reprographics and medicine and have extended earlier applications in laser radar, remote sensing, semiconductor processing, and industrial materials processing such as welding, cutting, and surface hardening.

The United States has recognized the importance of DPSSLs to industry. A major program is now under way involving a consortium of companies to develop and utilize kilowatt-class DPSSLs for industrial materials processing applications in the automobile and aircraft industry. If continued, this program should enable the introduction of this new technology into industrial processing applications in the United States.

Internationally, both Germany and Japan have explicitly recognized the potential and the value of DPSSLs to industry and have established assertive national programs to capture the projected $10 billion market for this technology. Germany's Laser 2000 program emphasizes

FIGURE 7.7 Schematic of a laser-diode-pumped solid-state laser that operates at output power levels in excess of 1 kW.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.6 LASER INTERFEROMETER GRAVITATIONAL-WAVE OBSERVATORY (LIGO) PROJECT

Einstein predicted in his general theory of relativity that accelerating massive objects radiate gravitational waves. Taylor and Hulse earned the Nobel Prize in 1994 for their precise measurements of the period of a pulsar that indicated a slow loss of energy by gravitational radiation. This was the first indirect evidence of the existence of this extremely weak form of radiation. The goal of LIGO and related projects is to detect gravitational waves directly by laser interferometry.

The LIGO project (for more information, visit the Web site at www.ligo.caltech.edu) was established in the United States as a research project to detect and study the very weak gravitational waves emitted by co-orbiting neutron stars, by binary black holes, or perhaps by exploding supernova stars. Similar projects have been initiated in France and Italy (VIRGO), in Germany and Scotland (GEO), and in Japan (TAMA). Detection of the extremely weak gravitational waves requires the highest degree of sensitivity. The detector is a 4-km-long Michelson/Fabry-Perot interferometer housed in an ultrahigh vacuum and illuminated by a 10-W Nd:YAG solid-state laser. Gravitational waves propagating through space and matter (including Earth) cause space to ''warp" a very small amount. The change in space-time induced by gravitational waves is detected by a very small change in the optical path length of the 4-km interferometer—one part in 1021. This is equivalent to measuring the diameter of an atom at the distance of the Sun.

The interferometer detectors for LIGO are being constructed at two sites: Hanford, Washington, and Livingston, Louisiana. First measurements are expected to be initiated in 2002.

DPSSLs for manufacturing applications and is funded at more than $20 million per year for the next 5 years. Japan's larger Advanced Photon Research Program is sponsored by the Science and Technology Agency (STA) and is being carried out by the Japan Atomic Energy Research Institute (JAERI) as the lead organization. A new laboratory is being constructed in the Kansei region near Kyoto. Complementary programs are being carried out by MITI and by the Ministry of Education at Osaka to develop high peak power class lasers for inertial-fusion energy generation. Collectively these programs have been funded at a level of $500 million over 5 years. The program was initiated in April 1997.

Several big science projects are in progress in the United States that depend on advances in solid-state laser technology. These projects serve as grand challenges for the coming decade and will drive the performance of laser sources. The Laser Interferometer Gravitational-Wave Observatory (LIGO) project (Box 7.6), a research project supported by NSF, requires a solid-state laser with extreme frequency stability, high average power, and extended operational lifetimes (Figure 7.8).

The DOE National Ignition Facility (NIF) will use advanced glass lasers in an array of 192 beam lines to focus 1.8 MJ of energy in approximately 3 ns onto a millimeter-size target containing hydrogen

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.8 The LIGO site at Hanford, Washington, showing the 4-km arms stretching from the main building. The interferometer mirrors, or test masses, are isolated from ground vibrations to allow extremely sensitive measurements for the detection of gravitational waves. (Courtesy of LIGO.)

isotopes. The target will be compressed and heated to temperatures that may ignite a laboratory-scale fusion reaction, thus releasing more energy than used to ignite the target (Box 7.7). In the future, DPSSLs may provide the 10 MJ of energy at the required 100-MW average power for the generation of electricity using the laser-driven inertial confinement fusion process.

The prospect for scaling the output power of advanced solid-state lasers from 5 to 250 kW is now being explored for possible defense applications. The recently announced Airborne Laser project is based on a chemical laser source with solid-state lasers used for target location and tracking. However, the compact size, overall electrical efficiency, ease of long-term storage, and supply of prime power to the solid-state laser make it a possible candidate for future megawatt-power lasers. One application of such a laser is to destroy short-range missiles in flight, as demonstrated in February 1996 in a test at White Sands, New Mexico (see www.boeing.com/airbornelaser).

Given the substantial progress in advanced DPSSLs in the past decade and the rich prospects for continued advances with the potential

FIGURE 7.9 (a) A Hubble Space Telescope image of supernova SN 1987A. (b) A simulation of the plasma instabilities expected to be found in supernovas. (c) A two-dimensional experiment on the NOVA laser showing plasma instabilities and a three-dimensional computer simulation of plasma instabilities.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.7 ASTROPHYSICS ON THE NOVA AND NIF LASERS

The existing NOVA laser and the next-generation National Ignition Facility (NIF) lasers are capable of irradiating matter to achieve densities and temperatures equal to those found in stars. At these extreme temperatures and densities, matter exists as a plasma. Further, matter at high-density and temperatures can undergo fusion such that hydrogen isotopes burn to create helium and in doing so liberate fusion energy. The NIF laser is designed to reach the temperatures and densities required to ignite a fusion bum in the laboratory.

However, these plasmas exhibit instabilities that are representative of the instabilities found in exploding stars or supernova. A striking example is the Hubble Space Telescope image of supernova SN1987A in the Large Magellanic Cloud, shown in Figure 7.9 (a). A simulation of the instabilities expected to be observed is shown in (b). Experiments on the NOVA laser show a striking resemblance to the instabilities thought to exist in stars. Thus, the high peak power NOVA and, in the future, NIF lasers will allow astrophysics studies to be done in the laboratory.

The next generation NIF laser will enhance our ability to study and to understand the astrophysical plasmas exhibited by stars in space. The same laser will enable the grand challenge of demonstrating fusion in the laboratory to be realized. With further research and development, fusion in the laboratory may lead to a source of fusion energy in the future.

for enormous impact in the coming decade, continued and even increased funding of R&D in this area is justified. This conclusion was stated previously in the FAMOS study (NRC, 1994), which recommended that "the third (and final) priority is to promote research that promises new and improved lasers and other advanced sources of light for a broad range of applications and for furthering studies of the properties of light and its interaction with atoms and molecules."

R&D and applications of solid-state lasers are cross-disciplinary and should be supported by a special initiative involving multiple agencies.

Advanced Materials for the Generation and Control of Light

Materials for Nonlinear Frequency Conversion

A few well-developed lasers have made an impact in the commercial market. These include the helium-neon, argon ion, and carbon dioxide gas discharge lasers, which operate in the red, blue-green, and infrared ranges. Solid-state lasers of commercial importance are the semiconductor laser diodes and the Nd:YAG solid-state laser. The difficulty and the cost of inventing and developing laser systems that operate at new wavelengths makes it important to extend existing well-developed lasers to new wavelengths by nonlinear frequency conversion.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Nonlinear frequency conversion was first demonstrated in 1962 shortly after the invention of the laser. In less than a decade, hundreds of new nonlinear materials were identified, tested, and demonstrated for converting lasers to new frequencies. Thirty years after this early research, which earned N. Bloembergen of Harvard the Nobel Prize, there are less than a dozen commercially available nonlinear crystals. These crystals are produced in small volumes and require hand polishing; they are therefore expensive, costing more than $1,000 per crystal. The 1980s saw the introduction of new nonlinear crystals based on extensive research programs under way in China and Japan. These new crystals such as barium borate (BBO) and lithium triborate (LBO) opened the ultraviolet wavelength region where there are important applications such as UV lithography, CD master production, and materials processing by laser ablation.

To be of commercial interest, nonlinear crystals must meet the physical requirements of adequate nonlinear coefficient, good optical quality and transparency, ease of growth and fabrication, and chemical stability. In addition, they must meet demanding cost objectives. For research applications the cost must be less than $1,000 per crystal. For original equipment manufacturing (OEM) applications, the cost must be less than $200. For widespread consumer applications, the cost must be less than $1.

Recently, a breakthrough in the technology of nonlinear materials has led to a reduction in cost by a factor of 1,000. By applying lithographic planar processing techniques developed for the mass production of silicon integrated circuits, it is now possible to fabricate nonlinear crystal "chips." Since thousands of nonlinear crystal chips can be fabricated from a standard 4-inch-diameter wafer of lithium niobate costing $300, the cost per chip is less than $1, thus meeting the cost objectives for widespread consumer applications. Further, the nonlinear interaction can be "engineered" to optimize conversion efficiency by using the spatial modulation of the ferroelectric domains to achieve quasi phase matching (QPM). QPM has allowed the generation of wavelengths from the ultraviolet to the infrared. For example, a Nd:YAG pumped tunable optical parametric oscillator can generate output from 1.4 to 4.5 μm in the infrared. In the latter case, more than 3 W of continuous-wave (cw) output power was demonstrated at the important 3.5-µm wavelength, a wavelength where hydrocarbon molecules absorb and can be detected remotely in the atmosphere. The conversion efficiency of the tunable parametric oscillator exceeded 90% and approached the theoretical maximum of 100%. This promising work requires continued support to make the transition from the research laboratory to commercial and defense application areas.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.8 ENGINEERED NONLINEAR FREQUENCY CONVERSION BY QUASI PHASE MATCHING

In 1962, in their seminal paper on nonlinear frequency conversion, Bloembergen and colleagues suggested that efficient nonlinear frequency conversion might be accomplished by periodically changing the sign of the nonlinear response to reset the phase (Armstrong et al., 1962). This approach to phase velocity matching is called quasi phase matching (QPM). In 1968, Bloembergen patented the idea.

Twenty-five years later, in 1988, after the patent had expired, the first engineered QPM interaction was demonstrated in the ferroelectric crystal lithium niobate using spatially modulated chemical diffusion and crystal growth to achieve periodic inversion of the ferroelectric domains and periodic change in the sign of the nonlinear coefficient. In 1993, room-temperature electric-field poling was used to invert the ferroelectric domains, and by 1996, wafer-scale processing of 3-inch wafers of lithium niobate was demonstrated. Today this technology is being transferred to industry for use in commercial applications of nonlinear frequency conversion. Interest in this technology stems from the tremendous cost reduction possible by using the lithography and planar processing technology developed for silicon. Nonlinear crystals are now being fabricated by lithographically printing patterns on the wafer, electric-field poling, and then dicing the wafer into chips. Waveguides can be implanted on the chip for enhanced nonlinear conversion at lower power levels.

The technological know-how for processing nonlinear crystal chips is being transferred to industry (Box 7.8). In the United States a number of companies are pursuing product development using this breakthrough. However, the importance of this technology has been recognized internationally. Research to take advantage of this breakthrough is also under way in Europe, Japan, and China. Continued investments are necessary to extend our knowledge and to maintain our technology lead.

Future applications of nonlinear crystal chips may include the frequency doubling of laser diodes to generate blue radiation for information storage. Prototype products are being investigated at this time. This approach directly competes with the GaN-based blue laser diode source that is now a subject of extensive research. Other applications include the generation of red, green, and blue output for laser color projection displays. The laser-based projection display provides a brighter, crisper image than existing technology and may be the future choice for the projection of high-definition color images in theaters.

Future applications of tunable parametric oscillator technology include remote sensing, coherent laser radar, and medical applications that require control of the wavelength. The tunable solid-state laser is a direct replacement for the tunable free-electron laser source that has been under research and development for medical applications for more than a decade.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Over the long term, nonlinear materials will enable applications such as optical switching of fiber-to-the-home wideband communication (see Chapter 1). In these applications, the nonlinear devices will amplify and channel-shift or band-shift the optical radiation. It is likely that new materials will be needed for these applications, such as polymer-based nonlinear materials. A significant challenge remains to develop these new materials, especially in the United States, where research funding is short-term and is not consistent with the decade time frame required to develop a new material to the degree necessary to make the transition to commercial products.

Semiconductor Quantum-Well Materials

The ability to modulate the properties of a semiconductor at periods of tens of nanometers allows control of the quantum states of the conduction electrons and holes. In particular, if the bandgap of the semiconductor is modulated periodically, electrons are found in well-defined potential wells in the solid and the electron is confined to these quantum-wells. Quantum wells are used to engineer the density of states of semiconductor diode lasers to provide both higher gain and wavelength control. Most laser diodes, and especially high-power laser diodes, are constructed by the use of quantum wells to control the electron states.

In laser diodes, electrons and holes combine and emit a photon at the bandgap energy. The output wavelength of the laser diode is thus controlled by the bandgap energy. In 1995 a new type of quantum-well laser was demonstrated for the first time, the quantum-cascade laser. In this laser the electrons remain in the conduction band and cascade down a quantum-well ladder emitting photons at a high rate along the way. The quantum-cascade laser emits radiation in the transparent region of the semiconductor at energies determined by the cascade's quantum energy levels. Quantum-cascade lasers open the midinfrared to laser diode radiation and are a promising new type of laser source.

Recently, voltage controlled quantum-well absorption has been used in conjunction with a Fabry-Perot interferometer to provide a high-speed optical switch. A small change in the absorption within the quantum-well band induces a large change in the reflectance or the transmittance of the interferometer.

The ability to control the depth of a quantum well allows control of the quantum-well states and the nonlinear response of the quantum-well semiconductor. Progress in quantum-well nonlinear materials (exhibiting a nonlinear response that is 1,000 times greater than the typical nonlinear response of the bulk semiconductor crystal) has been impressive. The large nonlinear response has allowed IR generation by frequency mixing and by harmonic conversion. In the future, the combination of guided-wave structures with the quantum-well enhanced

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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nonlinear response should allow efficient nonlinear conversion at very low power levels across the infrared region of the spectrum.

Photonic Bandgap Materials

Artificially structured materials are already used effectively in many areas of optical science and technology. Multilayer dielectric coatings provide the ultrahigh reflectances needed for laser resonators and eliminate unwanted surface reflections. Semiconductor quantum wells and other nanostructures make a variety of novel optoelectronics devices possible. For example, photolithographically produced distributed feedback corrugations control the frequency of advanced semiconductor laser diodes for use in communications.

Recently, a new concept in dielectric structuring has emerged that could lead to an even more widespread revolution in optical device functionality and practicality. Theory and microwave model experiments have shown that strong three-dimensional structuring of dielectric materials can be used to control light (photons) in a manner similar to that used by semiconductors to control electrons. Emission of light can be suppressed or enhanced, greatly improving the efficiency of lasers and LEDs; new devices created from optical waveguide circuits can be concentrated at structural defects to produce miniature wavelength filters and create novel nonlinear optical behavior. The dielectric structures with these novel optical properties are referred to as photonic bandgap materials.

To realize practical photonic bandgap devices will require breakthroughs in device technology. New concepts in materials processing of three-dimensional structures must be sought, with pattern dimensions of the order of one-tenth of an optical wavelength. The materials processing challenges are great, but so is the payoff if photonic bandgap crystals can be realized.

Materials for Shaping and Focusing Optical Radiation

Lenses have been important in the development of optical devices and other technologies for centuries. However, new approaches to focusing and controlling light have emerged that offer revolutionary potential. These new approaches are based on materials advances and on the application of planar processing technologies developed for the silicon industry.

The gradient index (GRIN) lens is an example of an advance in materials that impacts approaches to the control and focusing of optical radiation. GRIN lenses are formed by diffusion of ions into glass, thus altering the index of refraction. One-dimensional arrays of GRIN lenses are now an important technology that has enabled low-cost fax and photocopying machines. This lens market alone is approximately $100 million per year and enables a market worth in excess of $10 billion per year.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Diffractive optical elements, or holographic optical elements (HOEs), are another example of a new approach to controlling optical radiation. HOEs are now widely used in optical systems along with standard lenses to focus laser radiation. There is a significant leverage in supporting research to improve diffractive optical elements for applications that include telescopes, cameras, and medical imaging devices.

It has been known for more than a century that the use of aspheric surfaces in optical systems would provide better imaging quality. Only in the past decade has it become possible to produce aspheric surfaces by pressing glass or plastic into a mold (see Chapter 6). Virtually all video cameras sold today use pressed aspheric plastic lenses. Thus, this technology offers tremendous leverage in opening new markets to lower-cost but high-performance optical systems. For large-scale manufacturing of aspheric optics, the use of the new technique magnetorheological polishing is promising. This technique is early in the development stage and offers an opportunity for leverage if supported by R&D funding.

Materials Research and Development Opportunities

Research opportunities in lasers and engineered optical devices that offer significant leverage for research investment are based often on a better understanding and control of materials (Box 7.9). In laser diode sources, progress in the understanding and synthesis of III-V semiconductor materials will enable new laser diodes and blue laser diodes. In solid-state lasers, the development of laser host crystals with low optical loss and high optical quality will enable more efficient, high-output-power solid-state laser sources. In optics, advanced materials will enable improved optical storage devices, possibly leading to the reinvention of the printing press as an optical disk printer the size of a small copying machine.

The challenge is to sustain research and development through the decade time frame that it takes to fully develop a new material. Research and development activities in the United States are often biased toward the immediate payoff. If devices are not demonstrated in a short time, the research effort is redirected. Often it takes more than 20 years for a new material to mature and gain wide acceptance. In the early 1960s, the Advanced Research Projects Agency (ARPA) took the bold step of establishing 13 interdisciplinary centers for materials research. (The program was transferred to NSF in 1972 as the Materials Research Laboratories program. Since 1994, the centers have been part of NSF's Materials Research Science and Engineering Centers program.) During the next 25 years, these centers produced a body of materials knowledge that is sustaining our industry today. However, they are now reduced in size and are being closed under the stress of reduced research funding. The challenge is to renew our commitment to long-range materials research that will enable new devices and new industries in the future.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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BOX 7.9 MATERIALS RESEARCH AS AN ENABLER FOR OPTICS

The discovery and optimization of new materials is vital to the exploitation of optical phenomena and the development of optical devices and systems. Consider the role that materials research has played in the development of three optical technologies of vital economic importance: semiconductor lasers, optical fibers, and optical amplifiers. See Chapter 1 for more details about the applications of these devices.

SEMICONDUCTOR LASERS

The process that makes lasers possible was first postulated as early as 1917, but the first working laser, based on a single-crystal synthetic ruby rod, was demonstrated only in 1960. By 1962, helium-neon gas lasers and gallium arsenide (GaAs) semiconductor lasers had joined the ruby laser in this new class of device.

The first semiconductor lasers suffered badly from overheating. It was soon suggested that confining the laser action to a thin active layer—creating a semiconductor "sandwich" with the active layer set between layers of another material—would reduce the required current and keep the heat output manageable. In 1967, researchers proposed to add small amounts of aluminum to GaAs, growing confining layers of AlGaAs on either side of the GaAs active layer. Adding aluminum would change the atomic spacing of the crystal by less than one part in 1,000. Research on layer-by-layer growth of high-purity crystals, on defects and dopants, and on the effects of heat on the stability of compounds led to the demonstration in 1969 of an AlGaAs laser able to operate continuously at room-temperature. Semiconductor lasers are now ubiquitous in optical communications and optical data storage.

OPTICAL FIBERS

As early as 1910, the equations had been determined that govern the theory of light transmission along what were then known as glass wires. By 1964, hair-thin strands of glass were carrying light over short distances, but up to 99% of the light was lost when it passed through as little as 30 feet

Progress in materials science and engineering is critical to progress in optics. The committee recommends that the Defense Advanced Research Projects Agency (DARPA) coordinate and invest in research on new optical materials and materials processing methods with the goal of achieving breakthrough capability through engineered semiconductor, dielectric, and nonlinear optical materials.

Extreme Ultraviolet and X-Ray Optics

Our common experiences involve visualizing the world around us with natural light, or optical radiation, to which our eyes respond. Visible light covers the wavelength range from blue (450 nm) to yellow (590 nm) to red (650 nm). To the shorter-wavelength side of the visible spectrum lie the ultraviolet, deep ultraviolet, extreme ultraviolet (EUV),

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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of fiber. In 1966, a landmark theoretical paper asserted that these high losses were caused by minute impurities in the glass—primarily water and metals—rather than any intrinsic limits of glass itself. The paper predicted that eliminating impurities could reduce light loss from 1,000 decibels per kilometer of fiber to less than 20. This would allow signal-boosting amplifiers to be spaced at intervals of miles rather than yards, intervals comparable to those between the repeaters that amplified weak electrical signals along conventional telephone lines. Such predictions spurred a prodigious effort in materials research, especially in the area of materials processing, resting heavily on a knowledge of chemical thermodynamics, kinetics, gas dynamics, and polymer science. By 1980 the best fibers were so transparent that a signal could pass through 150 miles of fiber before becoming too weak to detect.

OPTICAL AMPLIFIERS

Despite these extraordinary advances in transparency, periodic amplification is still necessary. By the early 1980s, the transmission capacity of optical fiber communications systems was becoming limited by the capabilities of the amplifiers. The amplifiers of the day converted an optical signal to an electrical current, electronically amplified the current, and then used this to drive a laser that re-created the optical signal with greater intensity. The transmission capacity of these electronic amplifiers was considerably less than that of the lasers and optical fibers to which they were connected and thus limited the overall capacity of the system.

It was discovered that a short strand of erbium-doped glass, spliced into the main fiber and supplied with energy from an external source, could act as a laser in its own right, amplifying a weak optical signal without electronics. Key to turning this discovery into practical and effective optical amplifiers was mastering the materials chemistry of erbium doping, especially the appropriate reactions for volatilization. All-optical systems have now been demonstrated with transmission capacities 100 times that of any system built using electronic amplifiers.

and vacuum ultraviolet (VUV, so called because the radiation is not transmitted by air). Beyond the VUV are the soft x-ray and finally the x-ray wavelength region. It is x rays that are used for medical and dental x-ray imaging. The spectrum continues to gamma radiation at very short wavelengths or very high photon energies.

The history of progress in the study of the electromagnetic spectrum has been one of steady progress in the generation of radiation beginning 100 years ago at radio wavelengths. Coherent sources were extended to the microwave region and, with the invention of the laser, to the infrared and visible wavelengths. This progress continues today with research aimed toward the generation of ever shorter wavelengths or higher frequencies. In recent work, coherent laser-like sources have been demonstrated in the deep ultraviolet, extreme ultraviolet, and soft x-ray spectral regions. The development of synchrotron sources has

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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opened both the soft x-ray and the x-ray spectral regions for science and applications. Although synchrotron radiation is very intense, it is largely incoherent in nature. Research continues toward the possibility of generating coherent, laser-like, x-ray sources.

X-Ray Microscopy

The infrared and visible spectral region is the domain of light interactions with molecules and atoms. It is also the region where optical instruments, such as the microscope, allow us to extend our vision to very small objects. With microscopy, we are constantly striving to resolve ever smaller structures. It is well known that the limit of resolution of a microscope is set by the wavelength of the light used to produce the image. Thus, there is a great interest in microscopes that operate in the UV, EUV, and even x-ray regions for enhanced image resolution.

Recent research has led to the first images using soft x-ray microscopy. For example, soft x-ray images of a malaria-infected red blood cell show finer details than could be observed using a light microscope (see Figure 7.10). Microscopy in the EUV and x-ray regions offers the ability to tune the wavelength of the radiation to the atomic resonances of various elements—for example, carbon or oxygen—so that these elements, if present in the object, can be highlighted in the image. Although so far only preliminary, EUV and x-ray microscopic images give a glimpse of the future potential for higher-resolution microscopy with significant advances in areas such as nanometer materials science and subcellular structure of intact biological material.

EUV and X-Ray Lithography

The limiting resolution of the microscope also applies to writing features using advanced lithography. The smallest feature that can be written by lithography is determined by the illumination wavelength and the numerical aperture of the optics. Lithography is now moving from conventional mercury discharge lamps with output in the ultraviolet region, to excimer lasers with output in the UV and the deep UV.

FIGURE 7.10 X-ray microscope images of a red blood cell show features of a malaria infection site observed at a resolution not obtainable with an optical microscope. The left image shows a new infection. The right image was taken after 36 hours. (Courtesy of C. Magowan and W. Meyer-Ilse, Lawrence Berkeley National Laboratory.)

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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These lithography systems are under development and are just now moving to the production line.

The requirements for yet higher resolution, to meet the demands of semiconductor devices beyond the turn of the century, will force the move to even shorter-wavelength lithography. Research is under way to define approaches to both EUV and x-ray lithography, with potential feature sizes of the order of hundreds of atoms. Progress has been steady but certainly not rapid since research was initiated 15 years ago. Research for new sources and for a practical EUV and x-ray lithography process is driven by the enormous market demand for the next generation of semiconductor devices. Although no consensus has yet been reached as to what steps lie beyond current lithography based on UV excimer lasers, it is clear that shorter wavelengths must be used to obtain the required feature sizes. Shorter wavelengths demand that new EUV and x-ray sources and optics be developed. For example, an industrial consortium has been formed to invest $200 million in the development of EUV lithography. One promising approach with research under way in the United States, Japan, and Europe is to use high peak and average power solid-state lasers to generate a laser-produced plasma that radiates EUV and x-ray output. This is an active area of research and one that demands attention both for the laser drivers and for the control and focusing of the generated radiation.

X-Ray Optics and Sources

Plans are well under way to extend lithographic capability to the EUV and eventually to the soft x-ray region. Here research is needed to develop not only new sources of radiation but also optics and reflective coatings for the optical elements. This work is breaking new ground because of the lack of fundamental knowledge of material parameters in the EUV and x-ray regions. Sources of EUV and soft x-radiation include synchrotron sources, laser-produced plasmas, and laser-pumped soft x-ray lasers. Synchrotron sources are the most advanced but require a large facility and thus are located only in national laboratories and large corporations. There is a need for ''granular" EUV and x-ray sources that can be accommodated in research and production laboratories and are of modest cost and size.

The progress in x-ray optics has been significant over the past decade. Today mirrors can be produced with greater than 60% reflectivity using high-Z and low-Z coatings such as molybdenum-silicon layers (see Figure 7.11). An alternative approach is to use diffractive optics such as a Fresnel zone

FIGURE 7.11 An EUV mirror with a multilayer coating of molybdenum-silicon. (Courtesy of J. Underwood, Lawrence Berkeley National Laboratory.)

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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FIGURE 7.12 Diffractive-optic x-ray zone plate lens used to focus x-ray radiation. (Courtesy of E. Anderson, Lawrence Berkeley National Laboratory.)

plate (see Figure 7.12) to focus the x rays. The short x-ray wavelength demands very high-precision in the fabrication of optical elements. Fortunately, metrologies are now available with accuracies measured in fractions of a nanometer, which permit fabrication of x-ray components with greatly reduced wavefront distortion. Considerable research remains to be done to fully develop high-quality x-ray optics suitable for lithography, microscopy, and other emerging applications.

Multiple agencies with interest in the crosscutting science and technology of EUV and soft x-ray optics and techniques should encourage research in this area because of the substantial potential economic payback in the near future.

Education in Optics

Formal university-level education in optics started in the United States in 1927, after considerable national debate. The debate arose from the experience of World War I when "in all the allied countries, requirements of the armed forces had revealed woefully inadequate production facilities and an abysmal lack of trained engineers to design and build optical devices of every kind" (Kingslake and Kingslake, 1970). As a result, optics centers and institutes were founded in France, England, and eventually, the United States.

The debate at the time was in some ways similar to discussions that continue today. Was there no tradition of optics in the United States before World War I? Indeed there was. Jackson's An Elementary Treatise on Optics was published in 1848 and was followed by a whole series of books, pamphlets, and handbooks. However, the debate, then as now, was about recognizing the existence of an integrated body of optics knowledge, that can and should be taught in a unified way. The recognition of optics as an integrated body of knowledge is the premise for this section's discussion of education. (This does not necessarily mean its recognition as a "discipline," however, or the creation of separate university departments. The field's diversity and multidisciplinarity are great strengths.)

The broad picture for science and engineering education is reviewed periodically by the National Science Board and published in Science and Engineering Indicators (1996): The 500,000 degrees

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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earned in science and engineering in 1993 included 25,000 doctorates, 86,000 master's degrees, 366,000 bachelor's degrees, and 23,000 associate's degrees; in that year, 2.2 million people were employed in science and engineering jobs. Models of future employment demand are famously uncertain, of course, but only in the NSB's most aggressive growth scenario for the period 1994-2005, in which science and engineering employment expands by 62%, is there a strong demand for new scientists and engineers. The supply side has flexibility to respond to increased demand in a variety of ways, including the actions of individuals who have choices about education, retraining, postponed retirement, and moving to other technical fields. Small shifts in the career paths of employed scientists and engineers can have major effects on the supply and demand of scientists and engineers across fields.

Yet because optics is a new field that is undergoing rapid growth, demand may not be met by such adjustments and additional students may be needed. This is already the case for many subfields of optics. In some subfields, the demand for trained employees already exceeds the supply.

U.S. Optics Education Programs

U.S. colleges and universities offer a wide variety of educational programs in optical science, technology, and engineering. Some of these programs have broad curricula that span the field of optics. Others have more limited curricula that are devoted to specific subfields. Over the years, there have been several attempts to compile directories of these programs. Two major guides are produced by the professional societies. The most recent is Optics Education 1996: Annual Guide to Optics Programs Worldwide, produced by the International Society for Optical Engineering (SPIE). The Optical Society of America (OSA) published a three-volume guide to optics courses and programs in 1992.

Gathering such data is complicated, however, by the lack of a universally agreed-upon definition of the field. It should be noted that the major fields of engineering are recognized, in part, by the accreditation standards used by the Accreditation Board on Engineering and Technology (ABET) in evaluating university engineering departments. An accreditation program for optical engineering—similar to the ABET program for electrical engineering, for example—would provide several benefits, including an official government employment classification. This would help to identify prospective employees with specific experience and expertise; it would also help to provide a clear picture of the field's status by improving the basis for gathering statistics. The topic of accreditation was studied a few years ago by committees of OSA and SPIE. Their studies included discussions with ABET, but at the time, ABET concluded that there were too few formal programs to merit its attention and both

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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societies decided against seeking an accreditation program. Continued rapid growth in the field would be likely to change this conclusion.

The 1996 Annual Guide lists some 114 U.S. institutions that offer programs in optics. Of these, 93 offer one or more degree programs at the bachelor's level or above. A majority (55%) of these 93 institutions offer all three degrees: B.S., M.S., and Ph.D. About 14% offer the B.S. and M.S. only, and about 25% the M.S. and Ph.D. only. A few offer only a single degree (this is not to say that a stand-alone bachelor's program cannot be successful; there are excellent examples to the contrary). It is important to note, however, that although the 1996 Annual Guide is quite useful, it is the result of self-reporting and should be evaluated carefully with this fact in mind. The professional societies should evaluate educational programs in optics and jointly produce an annual guide.

Because of the strong correlation between education and scholarly publishing, the major optics research journals are useful indicators of optics education, especially when comparing the level of activity in different countries. For example, the 450 papers published in Optical Engineering in 1995 involved 1,333 authors, 64% of whom were associated with academic institutions. A total of 33 countries were represented, corresponding closely with the 31 countries known to have educational programs in optics; 155 of the 450 papers (and 432 of the 1,333 authors) were from the United States. Other optics journals (of which there are more than 50, less than one-third of them published in the United States) appear to present similar statistics. The high representation of academic institutions in these journals attests to the quality of academic scholarship, which is of course tied closely to the quality of educational programs, particularly at the graduate level (Thompson, 1996, 1997).

Approaches to Academic Programs in Optics

The term optics, as used this report, encompasses an extraordinarily wide range of study, including such subfields as imaging and image processing, photonics, electro-optics, acousto-optics, fiber optics, optoelectronics, and lasers. The fundamentals of geometrical optics, physical optics, and quantum optics are the foundations of many of the teaching programs in these areas. Major educational institutions here and abroad offer structured programs that cover fundamentals, applied topics, and engineering applications from components to systems. Thus, unlike traditional departmental disciplines, optics cuts across traditional educational boundaries, involving the full range from very basic academic science to applied science to technology to systems engineering. For example, Chapter 1 notes the importance of education in systems issues for employment in the communications, display, and information storage industries. Chapter 6 notes that students educated in optics manufacturing are in high demand.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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One reason optics educational programs achieve success in teaching across these boundaries is that educators in optics have an excellent network to help them with their work. Key to this network are the two major professional societies, OSA and SPIE, and a number of other societies that have sections devoted to optics or have optics as a parallel track to their main thrust. Recognizing the breadth of optics, the societies have formed the Coalition for Photonics and Optics to serve the broader community of optics professionals and to inform government agencies, educational institutions, and industry about the future importance of optics. The professional societies should work to strengthen optics as a recognized crosscutting area of science and technology through the recently established Coalition for Photonics and Optics. The society structure is supplemented by a number of international conferences on all areas of optics, including optics education.

There are two recognized approaches to the teaching of optics. One is a broad program of study, at an institute that recognizes optics as a separate discipline, covering the field with components in geometrical, physical, and quantum optics. The other model is an extended set of course sequences that bridge departments and schools and offer study in subfields of optics, such as lasers, image processing, electronic imaging, or electro-optics. The multidisciplinarity of the latter approach is a great strength, but the narrowness of some programs can be a disadvantage if graduates think they have been educated in optics when in fact they have been exposed only to certain subfields. No physics department would limit its program of study to solid-state physics or to quantum mechanics, and no electrical engineering department would teach courses only in circuits. Universities should encourage multidisciplinarity in optics education, cutting across departmental boundaries, and should provide research opportunities at all levels, from the bachelor of science to the doctorate and from basic science to applied technology.

These diverse optics programs are sufficient to meet current needs and probably also have the capacity and flexibility to meet future needs. The continued evolution of the field and increasing demand by industry may result in a need to extend optics education at the bachelor's or associate's degree level. There is no apparent need to create new programs of teaching in optics at the advanced level. The state of education and training in optics is generally healthy at present.

It has been traditional for many Ph.D. graduates in optics to go on to work in industry and for the government, unlike some other disciplines in which the "normal" track for a Ph.D. graduate is in academia. Thus, optics education is already meeting the goals stated in a recent report of the Committee on Science, Engineering, and Public Policy (COSEPUP, 1995).

The same can be said of most optics master's programs. These programs are styled after engineering programs but with some of the attributes

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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that promote "adaptability and versatility as well as technical proficiency," as called for by COSEPUP. Master's programs in optics are independent programs with specific curricular goals, not just a set of courses on the way to a Ph.D. Thus, the field is also meeting many of the goals suggested in a recent article "Mastering Engineering" (Fitzgerald, 1996).

Continuing Education

The field is fortunate to have an excellent set of continuing education opportunities provided by academic institutions and national societies. Courses in continuing education offered by the OSA, SPIE, and other societies cover a wide spectrum from basic science to engineering to technology. They also include professional development courses in such subjects as business, management, ethics, marketing, and patents. Courses are offered in conjunction with national meetings and are also available by videotape and broadcast. They are supplemented by excellent written materials prepared through the societies' publication programs. The professional societies should continue to expand their commitment to professional education in optics.

Summary and Recommendations

Broad Issues

Research in optics cuts across disciplinary boundaries and enables advances in many other areas of research as well as commercial applications that are important for the nation's future growth and prosperity. As indicated by the response to the NSF optical science and engineering initiative in 1995, there is high demand and strong competition for optics research funding. However, optics often has difficulty finding research support in the federal agencies, despite the scientific excitement in the field and its significant and growing economic importance, because of its multidisciplinary and enabling character, which contrasts with the agencies' structure by discipline or by mission. Optics involves multiple agencies and multidisciplinary collaboration and support. Optics is a rapidly growing crosscutting area of science and technology that is ripe for investment at a level comparable to the support of high-performance computing and communications.

In the United States the combination of research and education provides the joint benefit of the discovery of knowledge and the education of students. Support for research in the nation's universities is 0.5% of the GDP, the lowest investment rate of advanced industrialized nations. Fundamental scientific and engineering research laid the basis of understanding required for the discovery of the laser in 1960, a discovery that has ultimately led to significant economic and social benefits for

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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the nation. Continued investments in research and development have led to widespread applications of lasers and related technologies in basic science, engineering, medicine, communications, information storage, information display, and entertainment.

Multiple government agencies should form a working group to collaborate in the support of optics, in a crosscutting initiative similar to the earlier one for high-performance computing and communications systems.

The National Science Foundation should develop an agency-wide, separately funded initiative to support multidisciplinary research and education in optics.

The Department of Commerce should explicitly recognize optics as an integrated area of knowledge, technology, and industry and should structure its job and patent databases accordingly.

Research Opportunities

Quantum, Atomic, and Biological Optics

Light has historically been used to probe matter via spectroscopy. Important advances in spectroscopy techniques continue to be made. In addition, light is now used to manipulate atoms, molecules, and submicron particles. This control opens up possibilities for applications such as atomic-scale lithography and information storage. Laser-cooled atoms are being used to develop the next generation of atom clocks. Atom mirrors, beam splitters, and lenses have led to some of the most sensitive inertial sensors such as accelerometers and gyroscopes. Atoms laser-cooled to nanokelvin temperatures have formed a new state of matter, the Bose-Einstein condensate. Lasers have been used to create a form of light for which the fundamental noise limits of the Heisenberg uncertainty principle can be manipulated to enable more precise measurements. The unavoidable, quantum-limited perturbation of a physical system during a measurement can be modified. The disturbance of the quantum variable of interest can be minimized by shifting the effect of the measurement to another quantum degree of freedom. Quantum states with several degrees of freedom can be engineered so that the quantum phase of the entire system can be preserved. Although practical general-purpose quantum computers are unlikely to be developed, this research is at the forefront of the exploration of more complex quantum systems. Recombinant DNA techniques have been used to instruct cells to manufacture proteins with fluorescent tags. The intercellular motion of these labeled proteins can be tracked with an optical microscope. The optical microscope and new variations such as the confocal microscope and the near-field scanning microscope are undergoing rapid technological development.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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This progress is further aided by the introduction of sensitive photon detectors and computer processing of images. Laser-based optical tweezers can manipulate submicron-sized objects such as organelles within live cells. This tool has also been used to manipulate single biological molecules; molecular forces and displacements during an isolated molecular event can now be measured.

Light is now used to control atoms, which opens up possibilities for new applications from laser-cooled atomic clocks to gyroscopes to gravitometers. Laser control of atoms has created a new form of matter, the Bose-Einstein condensate, in which all atoms occupy the same atomic state. Light control of atoms allows quantum states to be engineered, which may open up new possibilities for applications of complex quantum systems. The new fluorescent dyes and dye-labeling techniques allow the detection of a single molecule, which has important applications in chemistry, biology, and medicine. With these techniques, coupled with recent advances in microscopy and sensitive light-based techniques for detection and control, measurement and control at the single-molecule level appear possible.

NSF should recognize the dramatic new opportunities in fundamental research in atomic, molecular, and quantum optics and should encourage support for research in these areas.

Femtosecond Optics

Optics has opened the femtosecond time domain to science and technology through the use of lasers that produce ultrashort pulses of light. Extremely intense optical fields, made possible by the amplification and compression of femtosecond laser pulses, are creating new opportunities in physics and medicine. Femtosecond sources of high average power offer dramatic advantages for laser processing of materials. Femtosecond laser pulses, because they are coherent optical waveforms, offer new and unique capabilities for manipulating molecules to produce new chemistry and biochemistry. Femtosecond lasers open the possibility of generating subfemtosecond x rays for time-resolved studies of biological structures. The rapid advance of femtosecond technology is being driven by the invention of new techniques for pulse generation and new all-solid-state laser systems. Low-cost femtosecond sources are key to the rapid application of these unique optical sources. Optical fiber communication technology is already a major beneficiary of short-pulse laser research and it should benefit further from rapid progress in the generation of ultrashort pulses. Many new applications for femtosecond lasers are based on properties other than ultrahigh speed. Laser acceleration benefits from the high peak power of focused femtosecond lasers. Terahertz imaging and optical coherence tomography (see Chapter 2) are two examples of new applications opened by femtosecond lasers.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Femtosecond optics is an important developing science and technology that is opening new opportunities for advances in physics, chemistry, biology, and medicine. New, unexpected commercial applications enabled by femtosecond lasers are beginning to emerge. These applications and others could be greatly stimulated by the development of highly reliable, low-cost femtosecond sources.

Femtosecond optics and sources offer an opportunity for dramatic impact on science and technology. Agencies should focus attention on this opportunity and encourage innovative work in this cutting-edge field.

Semiconductor and Advanced Solid-State Lasers

The growth and market size of lasers and applications track the growth and market size of computers with a 15-year delay, and the market for lasers and applications is expected to grow from about $3 billion per year to greater than $20 billion per year in the coming decade. Laser diodes are efficient, reliable sources of coherent light that are mass produced. Increasing production volumes with decreasing costs per laser diode will lead to expanding markets. Light-emitting diodes are lower in power and efficiency than laser diodes, but their very low cost enables applications in markets that exceed $3 billion. Organic and porous silicon-based LEDs offer the promise of a low-cost, paper-like display that can be addressed by a pocket-sized optical-based information storage unit. Companies in the United States lead the world as commercial suppliers of high-power laser diodes. However, R&D investments in Europe and in Japan are challenging that lead. The current market for solid-state lasers is $394 million and is growing at more than 20% per year. Advanced solid-state lasers now operate at power levels in excess of 1 kW and are finding applications in cutting, welding, hole drilling, surface hardening, reprographics, and medicine. Japan and Germany have initiated R&D programs for advanced solid-state lasers funded at levels of $500 million and $100 million over the next 5 years to capture the multibillion-dollar market projected for the next decade. Major science projects in the United States, such as the LIGO and NIF projects, are technology drivers for advanced solid-state lasers. Research on new and improved lasers is a priority recommended in the 1994 FAMOS report (NRC, 1994).

Semiconductor laser diodes, LEDs, and advanced solid-state lasers are important technologies that enable applications critical to the nation. Light sources based on solid-state technologies represent an important opportunity for R&D investment with very high leverage for return on investment. Investment in this technology has declined in the United States, whereas it has accelerated in both Germany and Japan. R&D investment in semiconductor and advanced solid-state lasers is essential if the United States is to retain technological leadership.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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R&D and applications of solid-state lasers are cross-disciplinary and should be supported by a special initiative involving multiple agencies.

Advanced Materials for the Generation and Control of Light

Frequency extension of laser sources by nonlinear crystals is critical to many applications that range from laser radar, to remote atmospheric sensing, to laser projection displays, to lasers for medical applications. The recent advances in engineered nonlinear materials and the breakthrough in cost reduction by a factor of 1,000 to less than $1 per nonlinear crystal "chip" open the possibility of commercial applications of frequency-shifted solid-state lasers. Structured materials have created new opportunities for advanced optical technologies. Future realization of structures with photonic bandgap properties would have major impact on optics and optoelectronics industries. New fabrication techniques allow the control of light using lenses that are mass produced at low cost using gradient index materials, diffractive optics, and aspheric lenses. New materials are critical to the future progress in the extension of laser capability by frequency conversion. Materials research often takes a decade or longer before the material is developed to the degree required for applications. The long lead time for materials research has to be recognized in the support of research programs.

Advances in materials enable new approaches to devices and open new application possibilities. Engineered nonlinear materials, structured dielectrics, and nanostructured quantum-well semiconductors are advanced materials capabilities that have significant impact on optical science and engineering.

Progress in materials science and engineering is critical to progress in optics. The committee recommends that DARPA coordinate and invest in research on new optical materials and materials processing methods with the goal of achieving breakthrough capability though engineered semiconductor, dielectric, and nonlinear optical materials.

Extreme Ultraviolet and X-Ray Optics

There is considerable interest in developing EUV and x-ray microscopes for higher-resolution imaging of biological and physical structures. X-ray microscopes allow atomic species to be detected by tuning the wavelength to the atomic resonance. Lithography is moving from the UV to the EUV and soft x-ray wavelengths to meet the demand for smaller feature size in future semiconductor devices. There is a need for EUV and x-ray sources that are laboratory scale for research and manufacturing activities.

EUV and soft x-ray optics are providing new opportunities in broadly based science and technology, with potential impact for the manufacture of turn-of-the-century semiconductor electronics devices through the application of EUV and soft x-ray lithography.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Multiple agencies with interest in the crosscutting science and technology of EUV and soft x-ray optics and techniques should encourage research in this area because of the substantial potential economic payback in the near future.

Education in Optics

There is a recognized integrated body of knowledge that constitutes optics. Educational institutions have recognized that this body of knowledge must be taught in an integrated and unified way. Professional societies have recognized that optics is a broad area of knowledge and practice by forming a Coalition for Photonics and Optics. Unlike traditional departmental disciplines in science and engineering, optics is involved in both science and engineering and hence cuts across traditional educational boundaries from very basic academic sciences to applied sciences to technology to engineering systems in an integrated manner. Academic educational programs thrive and are effective when students are exposed to and involved with research activity. This is true at the bachelor's level but vital at the master's and doctoral levels. Research activity in optics covers the full spectrum from basic to applied science to engineering to technology.

Students educated and trained in optics are in demand at this time in many fields of optics. The diversity of the U.S. educational system, coupled with the flexibility of scientists and engineers to change direction, allows the supply of students educated in optics to meet projected demands in the foreseeable future. The quality and diversity of continuing education courses offered by professional societies is first rate and should be continued. Excellent mainstream optics programs are in place that have the ability and flexibility to meet perceived national needs. A national program to establish new teaching centers is not required, but support for the existing structure should not be permitted to erode.

NSF should develop an agency-wide, separately funded initiative to support multidisciplinary research and education in optics. Opportunities include fundamental research in atomic, molecular, and quantum optics; femtosecond optics, sources, and applications; solid-state laser sources and applications; and EUV and soft x-ray optics.

The professional societies should work to strengthen optics as a recognized crosscutting area of science and technology through the recently established Coalition for Photonics and Optics. They should evaluate educational programs in optics and jointly produce an annual guide. The professional societies should continue to expand their commitment to professional education in optics.

Suggested Citation:"7 Optics Research and Education." National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, DC: The National Academies Press. doi: 10.17226/5954.
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Universities should encourage multidisciplinarity in optics education, cutting across departmental boundaries, and should provide research opportunities at all levels, from the bachelor of science to the doctorate and from basic science to applied technology.

References

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Optical science and engineering affect almost every aspect of our lives. Millions of miles of optical fiber carry voice and data signals around the world. Lasers are used in surgery of the retina, kidneys, and heart. New high-efficiency light sources promise dramatic reductions in electricity consumption. Night-vision equipment and satellite surveillance are changing how wars are fought. Industry uses optical methods in everything from the production of computer chips to the construction of tunnels. Harnessing Light surveys this multitude of applications, as well as the status of the optics industry and of research and education in optics, and identifies actions that could enhance the field's contributions to society and facilitate its continued technical development.

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