This chapter is a brief summary of the report. Details and background are in the subsequent chapters and appendixes.
In 1898, 3 years after the discovery of X-rays, 17 years before Einstein derived the notion of stimulated light amplification, and 62 years before the first laser was demonstrated, H.G. Wells created the enduring popular image of a tool that projects enormous energy as intense and invisible beams of light:
It is still a matter of wonder how the Martians are able . . . to generate an intense heat . . . in a parallel beam against any object they choose, by means of a polished parabolic mirror . . . much as the parabolic mirror of a lighthouse projects a beam of light . . . Heat, and invisible, instead of visible, light. Whatever is combustible flashes into flame at its touch, lead runs like water, it softens iron, cracks and melts glass, and when it falls upon water, incontinently that explodes into steam.1
Thus the case for high-intensity laser science was first made more than a half-century before the first laser ever fired.
1 H.G. Wells, War of the Worlds; Series: Dover Thrift Editions Paperback: 160 Pages Publisher: Dover Publications (January 10, 1997) Language: English ISBN-10: 0486295060 ISBN-13: 978-0486295060, n.d.Reprint, Dover, New York, 1997.
Presently, high-intensity laser science and engineering employing petawatt-class lasers2 is a component of a broad range of science, technology, and industries that use lasers to make, image, and analyze materials and processes for physics, chemistry, medicine, manufacturing, and national security. The recent National Academies’ report Optics and Photonics: Emerging Technologies for the Nation3 describes the multi-billion dollar laser industry and the trillion-dollar economy that depends on it.
During the first half of laser history, the 1960s to the 1980s, there were several areas of application for the H.G. Wells version of lasers as tools for delivering energy at a distance in cutting, in welding, or for weapons technology. These mostly involved lasers that could deliver a lot of total energy but not necessarily deliver the energy rapidly. High-intensity laser science evolved following further advances in laser technology and engineering in the 1990s that made it possible to concentrate the energy of a laser into a short pulse and focus it to a small area. These led to new applications in fundamental and applied research and in commerce, which continue to expand in the current decade. These applications are evolving still more rapidly due to recent large investments in new science facilities in Europe and Asia (Figure 1.1), as well as some important technology breakthroughs.
The committee’s topic is high-peak-power short-pulse lasers for high-intensity applications, where large amounts of energy are concentrated into short pulses and small areas to reach the highest peak power and intensity. Therefore, this report will not discuss the early history of long-pulse or continuous lasers. Nor will it discuss some applications for long-pulse laser radiation such as melting materials or laser weapons. There is no strict lower limit for intensity-enabled science, however, because the effects of intensity also depend on other factors such as wavelength, and this study will report on science involving high-intensity lasers from the infrared to X-ray wavelengths. Therefore, the committee’s threshold criterion is that the peak intensity of the laser pulse is driving the process of interest.
This report is intended primarily for its sponsors, but their purpose is only well-served if its findings, conclusions, and recommendations are useful for a broader and less technically specialized audience, including policy makers and the general public. The Summary and this introductory chapter should be accessible to all. Chapter 2, on Stewardship, discusses U.S. leadership and policies that have shaped the nation’s role in this science and technology. This should also be of interest to program plan-
3 National Research Council (NRC), 2013, Optics and Photonics: Essential Technologies for Our Nation, The National Academies Press, Washington, D.C.
ners as well as the science funding community. Following this are separate chapters that delve into the details of the science and technology and contain more technical descriptions and analyses: Chapter 3 describes the laser technology and its current limitations. Chapter 4 discusses the international landscape and development of large-scale multi-national laser research institutes. Chapter 5 looks at the major science opportunities opened up by high-intensity lasers, and Chapter 6 discusses applications. Chapter 7 summarizes the conclusions and recommendations of the study. Finally, there are several appendixes to provide further details, including a glossary, and references to help readers who want to delve into greater detail on any of these subjects.
The historical background leading to this report is discussed in detail in Chapter 2 and in the Preface to this study. High-intensity laser science was under way at the terawatt level in the 1980s, but a real advance was the first petawatt laser built at Lawrence Livermore National Laboratory in 1996, in order to assist with the Nova program on laser fusion. In 2002, a group of U.S. scientists from universities and national laboratories assembled a report entitled Science and Applications of Ultrafast, Ultraintense Lasers: Opportunities in Science and Technology Using the
Brightest Light Known to Man, also known as the “SAUUL” report.4 The report was a consequence of a grassroots effort by the U.S. scientists to examine the science opportunities and technological needs and propose new models for stewardship that would leverage interagency investments. The report also proposed that a new paradigm for funding high-intensity laser science based on multi-institutional networks and mid-scale facilities was imperative for exploiting scientific opportunities. The notion of networks to organize the community was also a recommendation of the National Academies’ Harnessing Light report carried out the previous decade.5
The recommendations of the SAUUL report began to receive attention, particularly in Europe, which has a thriving high-intensity laser community, and has been engaged in developing means to connect researchers in different European Union (EU) Countries. Europe developed programs through its economic and cultural “Framework” funding that included several networks devoted to short pulse and strong field research. The response in the United States was more modest but included plans for science user programs at the large lasers at Department of Energy (DOE) weapons laboratories, as well as smaller university projects funded by DOE or the National Science Foundation (NSF) to build petawatt-class lasers at the University of Michigan, University of Texas at Austin, and University of Nebraska. User networks were not created in the United States.
In the intervening years, additional National Academies’ studies in related areas have continued to emphasize the science opportunities enabled by high-intensity lasers, the benefits of network organizing within the community, and the larger benefits to society from leadership in laser science and engineering. These reports include AMO 2010: Controlling the Quantum World,6Frontiers in High Energy Density Physics,7 and the recent report entitled Optics and Photonics: Essential Technologies for our Nation.8
4 P. Bucksbaum, T. Ditmire, L. Di Mauro, J. Eberly, R. Freeman, M. Key, W. Leemans, D. Meyerhofer, G. Mourou, and M. Richardson, 2002, The Science and Applications of Ultrafast Lasers: Opportunities in Science and Technology Using the Brightest Light Known to Man, presented at the SAUUL Workshop, Washington, D.C., June 17-19. “SAUUL_report.Pdf,” accessed October 16, 2016, http://science.energy.gov/~/media/bes/csgb/pdf/docs/Reports%20and%20Activities/Sauul_report_final.pdf.
6 NRC, 2006, Controlling the Quantum World: The Science of Atoms, Molecules, and Photons, The National Academies Press, Washington, D.C.
7 NRC, 2002, Frontiers in High Energy Density Physics: The X-Games of Contemporary Science, The National Academies Press, Washington, D.C.
8 NRC, 2013, Optics and Photonics.
The European research community has created the ELI project (Figure 1.2) in the last decade to propel them towards the development goal of an exawatt class laser, 1,000 times more powerful than current petawatt lasers, achieved by concentrating kilojoules of energy into 10 fs.9 If this power is focused to a micrometer diameter spot it will spur research relevant to nuclear physics, high energy physics, and related fields accessible at intensities up to 1025W/cm2. The current ELI project is building nearly a dozen petawatt-class lasers in three new laser facilities in Eastern Europe. The EU has determined that ELI will have broad benefits for society in areas such as improved clinical cancer therapy, biomedical imaging, and nuclear materials and waste processing. Furthermore, ELI will aid the European photonics industry and will provide educational and training opportunities for new scientists and engineers in photonics and laser-enabled areas of research.
9 G.A. Mourou, G. Korn, W. Sandner, and J.L. Collier, 2011, Extreme Light Infrastructure Whitebook: Science and Technology with Ultra-Intense Lasers, Thoss Media, Berlin, http://www.eli-beams.eu/wp-content/uploads/2011/08/ELI-Book_neues_Logo-edited-web.pdf.
Although the United States led innovations in high-intensity lasers throughout the 20th century, leadership is rapidly moving to Europe, and in some cases, to Asia as well. This is seen in the area of laser manufacturing as well as applications. The situation is described more fully in Chapter 2 of this report.
In addition, there is broad interest in high-intensity laser science and technology across multiple U.S. science agencies, but the efforts are not well coordinated. Historical trends in federal funding for laser science and its effect on high-intensity laser research is also described in Chapter 2. Several contributions to the loss of U.S. leadership are summarized below:
First, the committee found that important scientific and economic studies of the benefits of laser science, such as the 1998 National Academies’ report Harnessing Light, had far greater impact on policies in Europe than in the U.S. Significant rapid coordinated strategic investments were subsequently made in Europe, Japan, and later in China and elsewhere, but not here.
Second, the U.S. research reliance on mission-based large-scale national efforts and small “single-investigator” funding across different agencies with no motivation or established protocol to coordinate them led to lack of stewardship strategy for this field. Thus the United States lost out on creation of research collaborative networks, mid-scale instrument development, and also has few means to manage awards for commercial development of advanced laser hardware. This has also led to a decrease in academic and commercial participation.
Third, the decline of corporate investment in long-term research in the United States has hit this field especially hard, since many of its 20th century innovations came from Bell Laboratories, IBM Labs, Ford Labs, GE Labs, and others. Europe has responded with incentives for industrial participation in research through vehicles such as the Frauenhofer Institutes. The United States has no comparable programs.
Fourth, a decade of flat federal funding has eliminated the budget flexibility needed to seed new discoveries, while economic expansion overseas has been accompanied by significant growth in foreign research infrastructure and program funds.
The recommendations laid out in the Summary and Chapter 7 of this report all address these points by urging the creation of a cross-agency multiscale broad participant program to re-establish leadership-level research in high intensity laser science in the United States.
Appendix A summarizes the most important technical terms for the context of the study and defines mathematical connections between them when needed. The properties of the light are summarized in Box 1.1, where the committee stresses the physical qualities and magnitudes and avoids equations.
Petawatt lasers and the associated fields of high-intensity science are enabled by the technology of optical power compression that can concentrate joules of optical energy into a single packet only tens of microns in each dimension. The history of laser technical advances that led to this is described in Chapter 3 and in the associated Appendixes A and B. These advances use the special properties of electromagnetic radiation.
A laser uses a mirrored cavity to circulate light through an optical amplifier medium to convert the excitation in the amplifier into additional light. High-intensity laser light uses several key additional elements. The “Q-switch” is a device inside the laser cavity that turns the light circulation on and off so that the amplifier medium can become highly energized before lasing depletes its excitation. A “mode locker” is another device inside the laser cavity that compresses the circulating optical power into pulses that are much shorter than the cavity. The third key element is “chirped-pulse amplification” (CPA), which is outside the laser cavity. This disperses the short pulse like the colors of a rainbow, enabling much higher amplification. Together these innovations have led to the laser light described in the previous section. These components are described more fully in Chapter 3, together with many figures to help the reader. There is also a glossary of technical acronyms in the Appendixes of this report.
Some additional technologies bear mentioning because of their promise to extend to higher intensities. “Optical parametric chirped-pulse amplification” (OPCPA) is a parametric amplifier, not a laser. It converts energy from the excitation source directly into the output laser in a single step. The host medium is just a converter; it does not need to store the energy, and this has advantages for scaling to higher powers. “Free-electron lasers” (FELs) are wholly different kinds of lasers that can produce comparable high intensities to petawatt lasers, but at X-ray wavelengths. They utilize high-energy electron accelerators located at national laboratories in the United States, Europe, and Asia. Several are currently operating, and more are scheduled for the next decade.
The “Moore’s Law” analog in high-intensity lasers is shown in Figure 1.3.
Since the invention of CPA, the power has increased about an order of magnitude every 4 to 5 years. The current limits are due to the optical elements with the lowest damage thresholds, which are the special dispersion optics required for pulse compression in CPA or OPCPA systems. More details can be found in Chapter 3 and in Appendix A4.
The worldwide high-intensity laser science community includes laser engineers, scientists who need the high-intensity environments for their science, and manufacturers. A number of websites and international conferences support and track their activities. These are described in Chapter 2 of this study. The committee concludes that there is a large, active, diverse, international collection of scientists and engineers interested in high-intensity lasers as tools for working on science and technology problems in fundamental physics, imaging science, accelerator science, plasma physics, planetary and astrophysics, ultrafast chemical and cluster dynamics, and others. However, in the United States there are few opportunities for these different areas to coalesce into a single community. The situation is quite different in Europe.
A remarkable and key example of the organization of the ultrafast and high-intensity laser community is Laserlab-Europe, a confederation of 33 organizations in 16 countries that describes itself as an “Integrated Initiative of European Laser Research Infrastructures.”10 The committee heard time and again about the importance of Laserlab-Europe to organize the community, which had a great influence on science policy and created the conditions necessary to bring about the ELI program.
The largest community-driven project in high-intensity laser science today is the ELI program, located at three sites in the EU—Czech Republic, Hungary, and Romania. The European Union and member states have committed $1 billion over the next 10 years to this project, which will provide the science community with multiple laser facilities tailored to the broad needs of the different science areas.
The science and applications chapters 5 and 6 summarize the extensive case for the continued development and deployment of high-intensity lasers for research and applications. H.G. Wells’ imagined futuristic application of intense lasers falls far short of a reality that includes research in particle physics, cosmology, nuclear physics, planetary astrophysics and geophysics, chemistry, atomic physics, materials science engineering, manufacturing science, and medical science. This summary chapter includes only a few highlights. The unifying theme is that high-intensity lasers provide an extreme environment that cannot be replicated more easily or at all with other laboratory techniques; therefore, these facilities are required for further progress in several areas.
The fundamental science underpinning laser fusion is but one of the intellectual drivers for research into the interactions of intense laser radiation with high-density matter. Planetary astrophysics, geophysics, and materials science all have a need for experiments in these environments. The electric field in the laser focus in laser-matter interactions can oscillate electrons to wiggle energies over 100 MeV. The ponderomotive pressure exceeds one billion atmospheres. At these extreme conditions even the fundamentals of the laser absorption mechanism is not understood, so this is truly a new regime of plasma physics.
Relativistic electron–positron (e+e−) pair plasmas are present in the early universe and also exist in exotic astrophysical objects such as blazars, pulsar winds, and gamma-ray bursts, which are of great interest in studies of the evolution of the violent events that affect the structure of the universe.11 Intense laser-plasma interactions are the one place where these can be explored in the laboratory. Dense production of positrons (1015 cm–3) in high-intensity experiments have been reported recently in the focus of petawatt lasers exceeding 1021 W/cm2.12 The magnetic fields produced in these plasmas exceed 109 G, far larger than fields created using other means.
Several sections of both the science and applications chapters of this report are devoted to the use of high-intensity laser pulses to make secondary sources of particle beams or radiation. X-rays, gamma rays, protons, electrons, positrons, and neutrons have all been created using high-intensity lasers interacting with material targets. One of the ELI nuclear physics beamlines will be devoted to a brilliant beam of gamma rays produced by scattering of a petawatt laser from an electron beam. Particle beams created by intense lasers can be more effective sources for clinical medical therapies, and so this is also an important focus.
11 E. Liang, 2010, Intense laser pair creation and applications, High Energy Density Physics 6(2): 219-22.
12 E. Liang, T. Clarke, A. Henderson, W. Fu, W. Lo, D. Taylor, P. Chaguine, et al., 2015, High e+/e−ratio dense pair creation with 1021W.cm−2 laser irradiating solid targets, Scientific Reports 5. doi:10.1038/srep13968.
High-intensity lasers are a well-developed route to advanced particle accelerators. They accomplish high-gradient acceleration through the low-density plasma process of plasma wakefield generation. Major projects to advance this technology have been funded in the United States by the Department of Energy Office of High Energy Physics, and currently the only approved petawatt laser for the DOE Office of Science is intended to demonstrate length-scaling of GeV-class electron accelerators using cascaded sections of plasma wakefield acceleration (Figure 3.1). The DOE scaled back most of its research into superconducting radiofrequency (SRF) for the next linear collider. It funds some SRF research at a level that has been described on its own program solicitation page, quoting an HEPAP report, as “seems inadequate given the need for basic understanding of the physics. . . .”13 Laser-based acceleration schemes may provide one of the most viable possible reentry points for U.S. leadership in advanced technology for future linear colliders.
The vacuum is not empty at the quantum level, but rather it is filled with matter-antimatter pairs of particles that are created and annihilated in extremely short time, on the order of Planck’s constant ħ divided by the rest energy 2mc2, where c is the speed of light and 2m is the mass of the particle pair. This time is about 2.5 zeptoseconds (2.5×10–21 s) for e+e- pairs. Many years ago Schwinger, Heisenberg, Euler, and others proposed that light of sufficient intensity could heat the quantum vacuum such that matter-antimatter pairs could boil out of it. The minimum intensity required is called the Schwinger intensity, equal to 2×1029 W/cm2. A possible way to study this regime is to collide intense lasers with relativistic electron beams. This is one of the most startling and exciting new frontiers using high-intensity lasers, described extensively in Chapter 5.
High-intensity lasers are gateways to the attosecond (as, 10–18 s) time scale. Times below one femtosecond (fs, 10–15 s) are important because that is the natural time scale for electron motion within a molecule following a sudden event like a fast particle collision or radioactive decay. The strong fields in high-intensity lasers
13 B. Strauss, “Fundamental Research In Superconducting Rf Cavity Design | U.S. DOE Office of Science (SC),” quoting “Report of the HEPAP Subpanel on the Assessment of Advanced Accelerator Research and Development, August 21, 2006,” accessed July 1, 2017, https://science.energy.gov/hep/funding-opportunities/fundamental-research-in-superconducting-rf-cavity-design/.
can create attosecond radiation by means of high harmonic generation (HHG) in bound atoms and molecules, the non-relativistic analog of the Schwinger process described in the previous paragraph. X-ray free-electron lasers have also demonstrated attosecond pulses. There are also proposals for plasma-based HHG that could conceivably get to zeptosecond (zs) pulses.
The first commercial application of lasers employing CPA was in approximately 2002 for use for tissue cutting in LASIK vision correction (see Chapter 6); since then the market for industrial/medical high-intensity lasers has reached several hundred million dollars per year, primarily for precision micromachining. For example, ultraprecision machining of hard materials such as for cell phone faceplates is in many cases done with high-intensity femtosecond lasers. An emerging area with a potential enabling impact in the >$100 billion range is in the use of high-intensity lasers to implement “tabletop X-ray lasers” through coherent upconversion.14 More information about this and other applications in medicine and manufacturing are in Chapter 6.
14 A laser is upconverted by a nonlinear element inserted into its beam, which generates an output laser frequency that is greater than the input laser frequency. A common example is a frequency doubler, which, as its name implies, doubles the output photon frequency.