Applications of high-intensity lasers stem from direct laser beam interactions with matter and from interactions with matter of the secondary particle and photon sources they drive. The most common applications are motivated by scientific, commercial, medical, and security needs. The division between this chapter and Chapter 5 is somewhat artificial: Science is clearly a main application of high-intensity lasers, and all applications of high-intensity lasers rely on the fundamental science of high-intensity laser-matter interactions.
The use of lasers in applications has economic and practical motivations. In manufacturing, for example, robotic lasers have been programmable in a way that mechanical cutting tools had not been, with the same factory floor laser station capable of cutting, drilling, measuring, and in some cases welding and peening. High-intensity short pulse lasers, in particular, have unique capabilities for precision, mainly due to minimal thermal energy deposition in materials, resulting in negligible collateral damage beyond the desired interaction volume. This can yield high aspect ratio holes and precisely imprinted patterns unrealizable with long pulse or continuous wave (CW) lasers. For medical uses, lasers have reduced the need for sterilization or anesthetics, and intense laser pulses can be delivered to internal tissues via optical fibers. In security, lasers can accelerate charged particles to relativistic energies, and beams of these particles can be used as probes of concealed materials in ways that complement the penetrating power and contrast available with X-rays. And in science, the high intensity and short duration laser
pulses make possible new techniques for ultrafast imaging of a wide range of transiently evolving matter.
The direct photonics economy, which includes manufacturing and deploying all kinds of lasers and components, is estimated in the annual range of $300 billion globally.1 This feeds a multi-trillion dollar economy that depends on laser products and services in myriad ways.2 The contribution of high-intensity short-pulse lasers to this economic activity is increasing rapidly. Even in areas that do not directly use high-intensity lasers, such as communications, developments motivated by high-intensity laser design, such as dispersion and pulse shape control, will increasingly play a major role.
Lasers occupy a major economic footprint in our national defense and security. The United States invests a significantly larger share of its tax revenues in defense than any other nation. It is expected that lasers will become an increasingly important element of this effort, owing to applications in communications, remote sensing, directed energy, and the production and diagnosis of materials in extreme environments. The utility of high-intensity short-pulse lasers spans a significant portion of this security space.
Laser material processing is now a major component of the manufacturing process. Lasers accomplish tasks ranging from heating for hardening, melting for welding and cladding, and the removal of material for drilling and cutting. Typical intensities required for such tasks include heat treating at 103 – 104 W/cm2, welding and cladding at 105 – 106 W/cm2, and material removal 107 – 109 W/cm2 for drilling, cutting, and milling. Figure 6.1 depicts laser processing activities as a function of the laser pulsewidth.3
Femtosecond laser processing for materials manufacturing is expanding as robust commercial lasers become available. Typical operating parameters for commercial lasers used for manufacturing include pulsewidths of 100-200 fs, peak
1 Photonics21, “2020 Photonics Roadmap,” accessed January 8, 2017, http://www.photonics21.org/download/Brochures/Photonics_Roadmap_final_lowres.pdf.
2 T. Baer, 2010, Lasers in Science and Industry: A Report to OSTP on the Contribution of Lasers to American Jobs and the American Economy, http://www.laserfest.org/lasers/baer-schlachter.pdf; National Research Council, 2012, Optics and Photonics: Essential Technologies for Our Nation, The National Academies Press, Washington, D.C.; The White House, “President Obama Announces New Manufacturing Innovation Institute Competition,” last update October 3, 2014, https://www.whitehouse.gov/the-press-office/2014/10/03/fact-sheet-president-obama-announces-new-manufacturing-innovation-instit.
3 William M. Steen and Jyotirmoy Mazumder, Laser Material Processing (London: Springer London, 2010), http://link.springer.com/10.1007/978-1-84996-062-5.
energies of 50-150 µJ, average powers of 100-150 W, and pulse repetition rates up to 1 MHz.4Table 6.1 shows a list of 20 global industrial laser companies whose short-pulse lasers range in pulsewidth from <6 fs to ~1 ns.
High-intensity femtosecond laser processing is considered a “cold” process since the substrate does not heat during the interaction.5 The physical mechanism used in material removal is plasma formation leading to ablation rather than melting. High-value commercial applications include surface processing, where the laser may be used to clean surfaces,6 or may be used in texturing of surfaces to decrease
4 F. Korte, S. Nolte, B.N. Chichkov, T. Bauer, G. Kamlage, T. Wagner, C. Fallnich, and H. Welling, 1999, Far-field and near-field material processing with femtosecond laser pulses, Applied Physics A 69(1): S7–S11.
5 X. Liu, D. Du, and G. Mourou, “Laser Ablation and Micromachining with Ultrashort Laser Pulses,” IEEE Journal of Quantum Electronics 33, no. 10 (October 1997): 1706–16, doi:10.1109/3.631270.
6 P. Pouli et al., “Femtosecond Laser Cleaning of Painted Artefacts; Is This the Way Forward?,” in Lasers in the Conservation of Artworks (Springer, Berlin, Heidelberg, 2007), 287–93, doi:10.1007/978-3-540-72310-7_33.
TABLE 6.1 A Selection of Industrial Ultrafast Laser Companies with Typical Parameters
|AMPHOS GmbH||400 W||100 fs|
|Amplitude Systemes||> 100 W||< 500 fs|
|Calmar||< 4 W||< 100 fs|
|Clark MXR||20 W||<150 fs|
|Coherent||< 100 W||< 15 ps|
|EKSPLA||~ 1 W||~ 100 fs|
|ESI||25 W||~ 1 ns|
|FemtoLasers||~ 0.5 W||< 6f s|
|Fianium||> 5 W||< 200 fs|
|IMRA America||> 20 W||< 400 fs|
|JENOPTIK Laser GmbH||~ 5 W||~ 500 fs|
|KM Labs||1.4 W||< 12 fs|
|Laser Quantum||> 1.8 W||< 15 fs|
|Light Conversion Ltd.||~ 20 W||< 290 fs|
|PolarOnyx||> 1 W||100 fs|
|Rofin Sinar Laser GmbH||10 W||700 fs|
|Spectra Physics||> 16 W||< 400 fs|
|TOPTICA Photonics AG||~ 0.5 W||< 150 fs|
|TRUMPF Scientific Lasers GmbH+Co.KG||Up to hundreds of W||Sub-ps to ns|
|R.P.M.C. Lasers||50 W||< 1 ps|
SOURCE: Compiled by the Committee.
7 A. Y. Vorobyev and Chunlei Guo, “Enhanced Absorptance of Gold Following Multipulse Femtosecond Laser Ablation,” Physical Review B 72, no. 19 (November 21, 2005): 195422, doi:10.1103/ PhysRevB.72.195422.
8 Max Groenendijk, “Fabrication of Super Hydrophobic Surfaces by Fs Laser Pulses,” Laser Technik Journal 5, no. 3 (May 1, 2008): 44–47, doi:10.1002/latj.200890025.
9 E. Stratakis, “Ultrafast Laser Micro/Nano Processing for Microfluidic and Tissue Engineering Apllications,” in 2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference (CLEO EUROPE/EQEC), 2011, 1–1, doi:10.1109/CLEOE.2011.5943318.IEEE Photonics Society, European Physical Society, and the Optical Society, Munich, Germany.
lasers is the ability to drill clean, small, deep holes in materials without damaging the surrounding material.10
The technology is now commonly used in the medical industry for fabricating high quality surgical stents.11 The medical industry has need for micron size feature such as 1µ diameter holes with a large length to diameter ratio. Single-mode fs laser technology is proving the best tool for these needs.
Femtosecond laser ablation depths from a single laser pulse can be more precise than material removal with conventional laser melting, as shown in Figure 6.2.12 Cracks due to thermal damage are present at picosecond to femtosecond pulses but nearly disappear when the pulse duration is reduced to 5 fs.13 Since thermal damage and stress-induced cracking depends on the average power of the femtosecond laser source, absorbed laser power leads to melting or thermal shock even with picosecond or femtosecond pulse duration.
Other ultrashort pulse laser applications that have been studied and reported for industrial processes include higher peak power femtosecond pulses to help to improve the resolution of the laser-induced breakdown spectroscopy (LIBS) process14
10 Sergei M. Klimentov et al., “The Role of Plasma in Ablation of Materials by Ultrashort Laser Pulses,” Quantum Electronics 31, no. 5 (2001): 378, doi:10.1070/QE2001v031n05ABEH001958; Lan Jiang et al., “Femtosecond Laser High-Efficiency Drilling of High-Aspect-Ratio Microholes Based on Free-Electron-Density Adjustments,” Applied Optics 53, no. 31 (November 1, 2014): 7290–95, doi:10.1364/AO.53.007290; G. Kamlage et al., “Deep Drilling of Metals by Femtosecond Laser Pulses,” Applied Physics A 77, no. 2 (July 1, 2003): 307–10, doi:10.1007/s00339-003-2120-x; V. N. Tokarev et al., “Optimization of Plasma Effect in Laser Drilling of High Aspect Ratio Microvias,” Laser Physics 25, no. 5 (2015): 056003, doi:10.1088/1054-660X/25/5/056003.
11 Amplitude Systems, “Ultrafast Lasers for Manufacturing Surgical Stents,” News | Amplitude Systèmes, accessed January 8, 2017, http://www.amplitude-systemes.com/headlines-ultrafast-lasersfor-stent-manufacturing.html.
12 Beat Neuenschwander et al., “Optimization of the Volume Ablation Rate for Metals at Different Laser Pulse-Durations from Ps to Fs,” vol. 8243, 2012, 824307-824307–13, doi:10.1117/12.908583; O. Utéza, “Surface Ablation of Dielectrics with Sub-10 Fs to 300 Fs Laser Pulses: Crater Depth and Diameter, and Efficiency as a Function of Laser Intensity,” Journal of Laser Micro/Nanoengineering 5, no. 3 (December 2010): 238–41, doi:10.2961/jlmn.2010.03.0011; Gerard A. Mourou et al., Method for controlling configuration of laser induced breakdown and ablation, US5656186 A, filed April 8, 1994, and issued August 12, 1997, http://www.google.com/patents/US5656186.
13 M Lenzner et al., “Photoablation with Sub-10 Fs Laser Pulses,” Applied Surface Science 154–155 (February 2000): 11–16, doi:10.1016/S0169-4332(99)00432-8.
14 W. Wessel et al., “Use of Femtosecond Laser-Induced Breakdown Spectroscopy (Fs-LIBS) for Micro-Crack Analysis on the Surface,” Engineering Fracture Mechanics, International Conference on Crack Paths 2009, 77, no. 11 (July 2010): 1874–83, doi:10.1016/j.engfracmech.2010.03.020; Timur A. Labutin et al., “Femtosecond Laser-Induced Breakdown Spectroscopy” 31, no. 1 (December 23, 2015): 90–118, doi:10.1039/C5JA00301F.
and ultrafast laser-generated X-rays to assess rotary equipment.15 Laser peening for generating compressive surface stresses to mitigate crack initiation and growth is an important use of nanosecond pulsed laser-generated shocks. It has been investigated
15 V. Raspa et al., “Plasma Focus as a Powerful Hard X-Ray Source for Ultrafast Imaging of Moving Metallic Objects,” Brazilian Journal of Physics 34, no. 4B (December 2004): 1696–99, doi:10.1590/ S0103-97332004000800034.
using femtosecond pulses as well, where the hardening effect is dominated by ablation of the surface layer rather than shock formation.16
The mission of the Stockpile Stewardship Program (SSP) of the U.S. Department of Energy17 is to maintain a safe and reliable stockpile of nuclear weapons and to support the non-proliferation missions of the agency. To support this effort, leading-edge technologies must be developed in coordination with a robust high energy density science program. The main application of high-intensity lasers to SSP science is to produce bright penetrating high-energy X-rays for radiography of high-energy-density matter. Short-pulse high-energy (kilojoule range) petawatt lasers can deposit a large amount of energy on a picosecond time scale, and this makes them a unique tool to probe inertial confinement fusion (ICF) implosions and high-energy-density physics occurring on a much longer (nanosecond) time scale. Examples of research that utilizes such X-ray sources include measurements of the equation of state for hot dense matter, which uses X-ray diffraction to observe structure when materials are compressed to high pressures. X-ray sources for radiography of ICF implosions and dense high-Z materials need picosecond-duration and require energetic X-rays in the range of 10 to 100 keV. X-ray backlighters driven by kilojoule class petawatt lasers can achieve a brightness exceeding 1010 photon/um2 at X-ray energies of 50-100keV, well above the capability of conventional long-pulse (nanosecond) high-energy lasers whose brightness falls below 109 photons/um2 for energies above 10 keV. Scattered radiation from the drive process and the matter itself are sources of background, so several kilojoules of high-intensity laser light at ~ 1018-1019 W/cm2 are required for creating broadband backlighter sources brighter than the emission from the matter being probed (Figure 6.3).
Multibeam petawatt lasers are preferable since the beamlets can be staggered in time onto backlighter targets and produce a temporal sequence of radiographic images (including diffraction). For example, temporally resolved radiography could be used to diagnose the evolution of the fuel compression of an inertial fusion implosion, strong shock propagation in materials, evolution of hydrodynamic instabilities in accelerated targets, and other high energy density physics experi-
16 Dongkyun Lee and Elijah Kannatey-Asibu, “Experimental Investigation of Laser Shock Peening Using Femtosecond Laser Pulses,” Journal of Laser Applications 23, no. 2 (March 31, 2011): 022004, doi:10.2351/1.3573370.
17 National Nuclear Security Administration, “Maintaining the Stockpile,” Maintaining the Stockpile, 2017, https://nnsa.energy.gov/ourmission/maintainingthestockpile.
ments. The first facility to use multibeam backlighters driven by high-intensity laser pulses is the Advanced Radiographic Capability (ARC)18 at Lawrence Livermore National Laboratory (LLNL). ARC provides short (1-50 picoseconds) high-power (> 1 Petawatt) laser pulses. ARC’s short, wide bandwidth, 1-µm light laser pulses propagate down the existing National Ignition Facility (NIF) beamlines. The ARC pulses are amplified before being redirected through large aperture gratings performing chirped pulse compression. Each of the eight ARC beams is pointed at up to one of eight backlighter foils. Each backlighter produces a short X-ray flash—staggered in time—to provide an eight-frame backlit movie of the target imploded by NIF (Figure 6.4). The Omega-EP laser at the University of Rochester19 is also used to develop experimental and diagnostic techniques for such applications. In
18 D.D. MDiMauro et al, “SAUUL Presentation,” 2002, http://www.lle.rochester.edu/pub/viewgraph/PDF/PR/PRMHFRSAUUL.pdf.
19 Laboratory for Laser Energetics, “About OMEGA EP,” http://www.lle.rochester.edu/omega_facility/omega_ep/, accessed March 2, 2017.
addition to peak power and pulse duration, features such as contrast ratio (described in Appendix A1) are important for such applications.
In addition to radiographic images, high-intensity lasers can create near monoenergetic K-shell emission over a wide range of frequencies (depending on the target material) to be used for diffraction experiments where the internal spatial structure of the compressed material can be determined and related to macroscopic thermodynamic properties such as equation of state and heat capacity.
In addition, high-intensity short-pulse lasers can be used to create radiation (both photon and particle) that can be used to interrogate systems and identify the presence of nuclear or chemical materials of concern for proliferation or homeland security.20 Such lasers could have some advantages over alternative technologies, but must be capable of both high intensity and average power.21–23 Programs
20 Sudeep Banerjee et al., “Compact Source of Narrowband and Tunable X-Rays for Radiography,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 350 (May 1, 2015): 106–11, doi:10.1016/j.nimb.2015.01.015.
21 “UNL | Novel X-Ray Method Could Detect Nuclear Materials | Office of Research & Economic Development,” accessed June 30, 2017, http://research.unl.edu/blog/novel-x-ray-method-could-detect-nuclear-materials/.
22 Andrew J. Gilbert et al., “Non-Invasive Material Discrimination Using Spectral x-Ray Radiography,” Journal of Applied Physics 115, no. 15 (April 15, 2014): 154901, doi:10.1063/1.4870043.
and sponsorship of the laser science and technology to achieve the necessary performance (nominally kHz rep rates, 1-10 joule energy per pulse and sub 100 femtosecond pulses are required) are yet to be identified.
Many kinds of laser surgery are now available that utilize ultrafast high-intensity laser processing of tissue. Particularly well known is Laser-Assisted In-Situ Keratomileusis (LASIK), which uses ultrafast laser scalpels to make incisions in the eyeball as part of a laser sculpting protocol to improve eyesight.24 These laser methods use some of the materials-processing benefits of high-intensity laser-matter interactions, such as reduction of collateral damage due to heating; but the lasers themselves are small-scale instruments because peak power is limited by the microscopic nature of this kind of surgery. This section will have a primary focus on medical applications that require the use of high peak power lasers that are capable of ultrahigh intensities. Those applications result from the promised ability of ultra-high-intensity laser pulses to create different kinds of high energy particles and radiation through interaction with a variety of sources.
The science and some materials applications of high power laser generation of auxiliary sources of particles and photons are reviewed in Section 6.4.3 and Section 6.3. Among the envisioned applications of these auxiliary laser-driven sources are X-ray and γ-ray imaging, therapies using high energy X-rays and γ-rays, therapies using laser-accelerated electron beams, therapies using laser-accelerated ion beams (mostly protons), and transmutation to create radioactive sources of positrons for positron emission tomography (PET).
The medical applications reviewed in this section are based on both imaging and cancer therapies based on particles generated from these ultra-high-intensity lasers. The challenge for many clinical medical applications is far greater than simply demonstrating the technology, since translating it into a clinical setting requires a sequence of research from in vitro to in vivo experiments, and then on to clinical trials.25
Ultra-high-intensity lasers can provide hard X-rays that emanate from an almost-point source, which provides spatially coherent X-rays. These can be trans-
24 Tohru Sakimoto, Mark I Rosenblatt, and Dimitri T Azar, “Laser Eye Surgery for Refractive Errors,” The Lancet 367, no. 9520 (May 5, 2006): 1432–47, doi:10.1016/S0140-6736(06)68275-5.
25 U. Linz and J. Alonso, 2007, What will it take for laser driven proton accelerators to be applied to tumor therapy? Physical Review Accelerators and Beams 10(9): 094801.
mitted through biological media and diffract, creating phase-contrast X-ray images at high resolution and contrast.26 Computer-based tomography can turn these into high quality 3D images.27 These hard X-rays have been able to produce high-resolution 3D views of human bone,28 and the phase contrast imaging provides enough contrast so tumors can be separated from healthy tissue.29 The laser-generated coherent hard X-rays offer imaging features that cannot be achieved with ordinary X-rays emitted from tubes. In a fully integrated high-intensity laser system, these advanced imaging technologies will be used just before and after radiotherapy with laser-accelerated particles, discussed next.
Radiotherapy seeks to selectively kill cancer cells by breaking off the caps at the end of their deoxyribonucleic acid (DNA) chains without doing too much damage to healthy cells. High energy electrons have been shown to be more effective than hard X-rays because they can be more easily focused to tumor locations. High energy electrons created in bunches by ultra-high-intensity lasers using laser wakefield acceleration in plasmas are expected to replace cyclotrons and linacs for this application. After early demonstrations of effectiveness,30 laser-accelerated femtosecond bunches of electrons were shown to have the same biological effectiveness as those from cyclotrons.31 They have presented the first evidence that such electrons are more effective than X-rays in radiotherapy. Laser-driven electron beams have been evaluated to establish a path toward cost-effective delivery of therapeutic doses, thereby replacing conventional particle accelerators such as linacs, which can be large and costly (see footnote 38).
26 R. Toth, J.C. Kieffer, S. Fourmaux, T. Ozaki, and A. Krol, 2005, In-line phase-contrast imaging with a laser-based hard x-ray source, Rev. Sci. Instrum. 76(8): 083701.
27 J. Wenz, S. Schleede, K. Khrennikov, M. Bech, P. Thibault, M. Heigoldt, F. Pfeiffer, and S. Karsch, 2015, Quantitative X-ray phase-contrast microtomography from a compact laser-driven betatron source, Nature Communications 6: 7568.
28 J.M. Cole, J.C. Wood, N.C. Lopes, K. Pode, R.L. Abe, S. Alatabi, J.S.J. Bryant, et al., 2015, Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone, Scientific Reports 5: 13244.
29 S. Fourmaux, S. Corde, K. Ta Phuoc, S. Buffechoux, S. Gnedyuk, A. Rousse, A. Krol, and J.C. Kieffer, 2011, Initial steps towards imaging tumors during their irradiation by protons with the 200TW laser at the Advanced Laser Light Source facility (ALLS), Proc. of SPIE 8079: 80791.
30 E. Beyreuther, W. Enghardt, M. Kaluza, L. Karsch, L. Laschinsky, E. Lessmann, M. Nicolai, et al., 2010, Establishment of technical prerequisites for cell irradiation experiments with laser-accelerated electrons, Med. Phys. 37(4): 1392-1400.
31 M. Oppelt, M. Baumann, R. Bergmann, E. Beyreuther, K. Brüchner, J. Hartmann, L. Karsch, et al., 2015, Comparison study of in vivo dose response to laser-driven versus conventional electron beam, Radiat Environ Biophys 54(2): 155–166.
38 D. Habs, P.G. Thirolf, C. Lang, M. Jentschel, U. Köster, F. Negoita, and V. Zamfir, 2011, Medical applications studies at ELI-NP. Proc. SPIE 8079: 1H.
High energy ion beams are an important cancer therapy because of their well-defined stopping distance in human tissue, the so-called Bragg peak, where most of the ion-induced tissue damage is concentrated. Ion beams thereby minimize tissue damage along the path to a deeply located tumor and concentrate the damage at the tumor. At the present time, therapy with high energy ions (protons, etc.) requires synchrotrons or cyclotrons. Creating ion beams with ultra-intense lasers will reduce the size, cost, and undesirable radiation generation (such as that accompanying cyclotron operation). Early laser-proton radiotherapy research began in 1998 in a collaboration between Stanford and LLNL;32 it was 10 years before beams containing high-current, ultra-short pulses of protons accelerated by ultra-short, ultra-intense lasers, produced biological effects.33 Experiments showed that ultrashort bunches of high-energy protons produced by lasers were equally efficient in killing cancer cells as synchrotron-produced proton beams.34
The well-known medical procedure of PET currently requires the nearby presence of a synchrotron/cyclotron to create the short-lived radioactive sources that provide the positrons. Ultra-intense lasers show promise of creating the necessary radioactive sources—although none have yet been demonstrated in practical quantities. Initial studies at Strathclyde, UK,35 with additional experiments carried out on the Titan laser at LLNL,36 allowed modeling to predict that the 10 Hz Extreme Light Infrastructure (ELI) facilities should be able to create practical levels of useful positron-emitting sources.37
32 R.A. Snavely, M. Key, S. Hatchett, T.E. Cowan, M. Roth, T.W. Phillips, M.A. Stoyer, et al., 2000, Intense high energy proton beams from Petawatt Laser irradiation of solids, Phys. Rev. Lett. 85(14): 2945-2948.
33 A. Yogo, K. Sato, M. Nishikino, M. Mori, T. Teshima, H. Numasaki, M. Murakami, et al., 2009, Application of laser-accelerated protons to the demonstration of DNA double-strand breaks in human cancer cells, Applied Physics Letters 94(18): 181502.
34 A. Yogo, T. Maeda, T. Hori, H. Sakaki, K. Ogura, M. Nishiuchi, A. Sagisaka, et al., 2011, Measurement of relative biological effectiveness of protons in human cancer cells using a laser-driven quasimonoenergetic proton beamline, Applied Physics Letters 98(5): 053701.
35 K.W.D. Ledingham, P. McKenna, T. McCanny, S. Shimizu, J.M. Yang, L. Robson, J. Zweit, et al., 2004, High-power laser production of short-lived isotopes for positron emission tomography, J. Phys. D: Appl. Phys. 37(16): 2341–2345.
36 S. Kimura and A. Bonasera, 2011, Deuteron-induced reactions generated by intense lasers for PET isotope production, Nuclear Instruments and Methods in Physics Research A637: 164–170.
37 E. Amatoa, A. Italiano, D. Margarone, B. Pagano, S. Baldari, and G. Korn, 2016, Study of the production yields of 18F, 11C, 13N and 15O positron emitters from plasma-laser proton sources at ELI-Beamlines for labeling of PET radiopharmaceuticals, Nucl. Instr. & Methods in Phys. Res. Sect. A 811: 1–5.
The expected hardware for each of these applications will be highly complex. Enough is known of the science behind the processes used that in many cases the feasibility of medical applications can be simulated, saving time and cost compared to preliminary experiments. Detailed modeling and design of the equipment required to carry out medical applications are being carried out at a number of institutions: In Germany, at LMU in Munich and at ONCORAY in Dresden; in France, at Institut Laue Langevin; and in Romania, at the National Institute of Physics and Nuclear Engineering, where they predict that “developing these techniques and applications is a promising task of ELI-NP with a strong societal component.”38 Considerable modeling for biomedical applications is going on while the facility is being built.
In 2014, a group of pioneers (from the United Kingdom, Germany, Japan, and the United States) in laser-accelerated particle radiotherapy reviewed the status of the field: “Reaching medically relevant particle kinetic energies is obviously essential, yet equally important is the demonstration of repetition-rated, well-controlled beamlines at these energies with suitable bunch parameters that are highly reproducible. A systems mindset is necessary to incorporate, optimize and even exploit the multiple technologies that must be combined. . . . The full system components include the laser driver, the laser target, instrumentation for diagnostics and control and beam line (ion) optics.”39 Issues under consideration include the following: Reducing the energy spread and increasing the energy of the ion beam; controlling the angular divergence; beam transport and delivery; understanding the radio-biological basis of radiation therapy, including DNA damage, dose rate, oxygen, and nitrogen effects. They conclude, “As the relevant laser, laser-plasma and target science and technologies mature, the committee remains optimistic that cost/size comparisons will become increasingly favorable. . . .”
Researchers in Germany understand the complexity of the challenge that includes the development of transport and delivery (including gantry) systems, as key to achieving a compact laser-ion beam radiotherapy facility: “Enroute it is critical now to demonstrate the comprehensive need for laser-driven energetic ions; including applications that might require only the emergent ion ‘spray’ (or have minimal ion optics requirements). A balanced variety of targeted doable applications (medical and nonmedical especially for near term) can help to sustain the vision, the multidisciplinary collaboration, the cooperation of multiple communities and incremental success paths over the long term that are essential
38 D. Habs, P.G. Thirolf, C. Lang, M. Jentschel, U. Köster, F. Negoita, and V. Zamfir, 2011, Medical applications studies at ELI-NP. Proc. SPIE 8079: 1H.
39 K.W.D. Ledingham, P. R. Bolton, N. Shikazono, and C.-M. Ma, 2014, Towards laser driven hadron cancer radiotherapy: A review of progress, Appl. Sci. 4(3): 402-443.
ingredients; first for ultimately realizing integrated laser drive ion acceleration systems and second for maturing it multi-faceted capability for laser-driven ion beam radiotherapy.”40
Fundamental High Energy Density science (HED) and some of the successful research using high energy lasers such as NIF and OMEGA was discussed in some detail in Chapter 5. Here the committee introduces another possible application of high-intensity lasers to that topic. Because of their ability to accelerate charged particles to high energies, high power lasers have played an important role in the development of advanced ignition schemes for fusion energy via inertial confinement (ICF). A fusion scheme that has been proposed that uses ultrafast techniques in high power lasers is Fast Ignition (FI).41 In FI, the target is compressed to high density with a low implosion velocity and then ignited by a short, high-energy pulse of electrons or ions. Fast ignition has two potential advantages over conventional hot-spot ignition: higher gain, because the target does not need to be compressed as much (~300 versus ~ 600 g/cc), and relaxed symmetry requirements because ignition does not depend on uniform compression to very high densities. The fast-ignition concept for inertial confinement fusion was proposed with the emergence of ultrahigh-intensity, ultrashort pulse lasers using CPA.42 The target compression can be done by a traditional ICF driver (direct-drive by lasers or ion beams, or indirect drive from X-rays using a hohlraum driven by nanosecond lasers, ion beams, or a Z-pinch). The ignition is initiated by a short high-intensity laser pulse (the so-called “ignitor pulse”), which produces a high-energy electron or ion beam when it interacts with the target. The gain can be higher for fast ignition if the total energy of the compressor and ignitor drivers is less than that required for compression of a conventional target, as is suggested by numerical simulations.43 A number of different schemes for coupling a high-energy, short-pulse laser to a compressed core have been proposed. The “hole-boring”44 scheme assumed that there would be two short-pulse laser beams, one having an ~100-ps duration to create a channel
40 P.R. Bolton, 2016, The integrated laser-driven ion accelerator system and the laser-driven ion beam radiotherapy challenge, Nucl. Instruments and Methods in Phys. Res. A 809: 149–155.
41 M. Tabak, J. Hammer, M.E. Glinsky, W.L. Kruer, S.C. Wilks, J. Woodworth, E.M. Campbell, and M.D. Perry, 1994, Ignition and high gain with ultrapowerful lasers, Physics of Plasmas 1:1626.
42 D. Strickland and G. Mourou, 1985, Compression of amplified chirped optical pulses, Opt. Commun. 56: 219.
43 R. Betti, A.A. Solodov, J.A. Delettrez, and C. Zhou, 2006, Gain curves for direct-drive fast ignition at densities around 300g∕cc, Physics of Plasmas 13(10): 100703.
44 A. Pukhov and J. Meyer-ter-vehn, 1997, Laser hole boring into overdense plasma and relativistic electron currents for fast ignition of ICF targets, Phys. Rev. Lett. 79(14): 2686-2689.
in the coronal plasma through which the high-intensity laser pulse that generates energetic electrons would propagate. An alternative design uses a hollow Au cone inserted in the spherical shell.45 The fuel implosion produces dense plasma at the tip of the cone, while the hollow cone makes it possible for the short-pulse-ignition laser to be transported inside the cone, without having to propagate through the coronal plasma, and enables the generation of hot electrons at its tip, very close to the dense plasma. A variant cone concept uses a thin foil to generate a proton plasma jet with multi-MeV proton energies.46 The protons deliver the energy to the ignition hot spot—with the loss of efficiency in the conversion of hot electrons into energetic protons balanced by the ability to focus the protons to a small spot. The minimum areal density for ignition at the core (ρR ~ 0.3 g/cm2 at 5 keV) is set by the 3.5-MeV α-particle range in D-T and the hot-spot disassembly time. This must be matched by the electron-energy deposition range. This occurs for electron energy in the ~1- to 3-MeV range. The minimum ignition energy of the particle beam Eig is independent of target size and scales only with the density of the target as Eig ~ ρ–1.85.47
The optimum compressed-fuel configuration for fast ignition is an approximately uniform-density spherical assembly of high-density DT fuel. High densities of large fuel masses can be achieved by imploding thick cryogenic-DT shells with a low-implosion velocity and low entropy. Such massive cold shells produce a large and dense DT fuel assembly, leading to high gains and large burn-up fractions. Experimental investigations of the fast-ignition concept are challenging. The fast-ignition concept involves extremely high energy density physics: ultra-intense lasers (intensities >1019 W cm–2) produce a >100-Mbar pressure, a magnetic field in excess of 100 MG, and electric fields >1012 V/m. These laser fields generate massive currents (~GA in tens of microns diameter) at the critical surface. These currents can propagate through a variety of plasma conditions, from cold, nearly solid systems to hot (~keV), dense (~g/cc) plasmas. The sheer scale of the problem, e.g., the generation of a large current pulse of tens of picoseconds time duration that traverses ~100 µm requires the investigation of this concept and inherently requires high-energy and high-power laser facilities that are currently available (e.g., OMEGA EP, NIF-ARC, etc) to study the principles of fast ignition.
45 R. Kodama, P.A. Norreys, K. Mima, A.E. Dangor, R.G. Evans, H. Fujita, Y. Kitagawa, et al., 2001, Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition, Nature 412: 798-802; R. Kodama, H. Shiraga, K. Shigemori, Y. Toyama, S. Fujioka, H. Azechi, H. Fujita, et al., 2002, Nuclear fusion: Fast heating scalable to laser fusion ignition, Nature 418: 933-934.
46 M. Roth, T.E. Cowan, M.H. Key, S.P. Hatchett, C. Brown, W. Fountain, J. Johnson, et al., 2001, Fast ignition by intense laser-accelerated proton beams, Phys. Rev. Letts. 86(3): 436.
47 S. Atzeni, 1999, Inertial fusion fast ignitor: Igniting pulse parameter window vs the penetration depth of the heating particles and the density of the precompressed fuel, Physics of Plasmas 6: 3316-3326.
For an ignition scale target, the minimum fast particle energy required for ignition is about 20 kJ for a monoenergetic collimated beam with beam radius ~ 20 um and time duration ~ 10 ps. For a Maxwellian energy distribution and a reasonably low beam divergence, the minimum beam energy for ignition is about ~40 kJ.48 Assuming conversion efficiency from laser to particle beam energy of ~20 percent, fast ignition ICF requires very large high power lasers of tens of petawatts with energies of hundreds of kilojoules.
The following applications of ultra-short-pulse lasers have been considered by the Navy:49
- Long range directed energy, exploiting nonlinear focusing to overcome diffraction. This could include both direct target damage or target “softening” for a high-energy laser attack.
- Long-range target impairment/disruption. An example is high-power RF generation at the target.
- Long range detection and composition probing of an atmospheric area of interest. Examples include detection of harmful aerosols and bio-agents from a safe distance, where one could distinguish between bio-aerosols and natural background aerosols.
- Filamentation for triggering and/or guiding of energy or electromagnetic waves.
To this list the committee notes that, primarily for Homeland-Security applications, there has been discussion of the use of laser Compton scattering of a high-peak-power laser from a high-energy electron beam to generate tunable, narrow-linewidth gamma rays.50 These in turn can be used as a probe to detect, say, fissionable materials in transport vehicles. An example is the Department of Homeland Security’s FINDER project for high-confidence detection of nuclear materials to enhance port security.
48 A.A. Solodov, R. Betti, J.A. Delettrez, and C.D. Zhou, 2007, Gain Curves and hydrodynamic simulations of ignition and gain for direct-drive fast ignition targets, Physics of Plasmas 14: 101063.
49 Ryan Hoffman, “USPL Applications for Navy,” n.d.
For light of intensity high enough to generate nonlinear effects such as self-focusing, any applications requiring remote delivery of energy suffer significant limitations due to the atmosphere. In particular, for current-generation near-infrared PW-class solid-state lasers at ~800-1000 nm, self-focusing and resultant generation of atmospheric filaments occurs for critical peak powers Pcr ~ 2-10 GW range, depending on air’s ultrafast nonlinear response as a function of laser pulsewidth.51 This means that pulses of peak power P ~ 1 TW – 1 PW can break up into hundreds to many thousands of filaments, with that number approximately given by the ratio P/Pcr.52 Such beam breakup leads to beam dissipation far short of its intended range, becoming a limitation on directed energy or remote sensing applications. However, there are possible strategies under development to mitigate filamentation breakup, or even find potential uses for it, both leveraging high-intensity short-pulse lasers. Because Pcr scales as λ2, high energy ultrashort pulses in the mid-infrared to long wavelength infrared range can be much more resistant to filamentation than optical beams.53 This is one of the factors spurring development of high-intensity infrared lasers using technologies such as OPCPA54 and a revisiting of CO2 laser technology. In another development, it was shown that rather than carrying significant beam energy itself, a multi-filamenting ultrashort pulse laser could imprint long-lasting waveguides in the atmosphere that can guide auxiliary high energy and high average power laser beams.55
An area of high-intensity laser science with high potential impact is the generation of coherent short-wavelength (extreme ultraviolet [XUV] to soft X-ray) light through the process of high-order harmonic generation (HHG). In HHG, an intense femtosecond laser focused into a gas results in coherent upconversion of visible to infrared laser beams into laser-like EUV and soft X-ray beams. In gases
51 J.K. Wahlstrand, Y.-H. Cheng, and H.M. Milchberg, 2012, Absolute measurement of the transient optical nonlinearity in N2, O2, N2O, and Ar, Phys. Rev. A 85(4): 043820.
52 A. Couairon and A. Mysyrowicz, 2007, Femtosecond filamentation in transparent media, Phys. Rep. 441(2-4): 47-190.
53 P. Panagiotopoulos, P. Whalen, M. Kolesik, and J.V. Moloney, 2015, Super high power mid-infrared femtosecond light bullet, Nat. Phot. 9: 543-548.
54 G. Andriukaitis, T. Balčiūnas, S. Ališauskas, A. Pugžlys, A. Baltuška, T. Popmintchev, M.-C. Chen, M.M. Murnane, and H.C. Kapteyn, 2011, 90 GW peak power few-cycle mid-infrared pulses from an optical parametric amplifier, Opt. Lett. 36(15): 2755-2757.
55 N. Jhajj, E.W. Rosenthal, R. Birnbaum, J.K. Wahlstrand, and H.M. Milchberg, 2014, Demonstration of Long-Lived High-Power Optical Waveguides in Air, Phys. Rev. X 4(1): 011027.
these high harmonics are generated through a “recollision” process where an atom or molecule is ionized by the strong electric field of the laser, makes a “boomerang” excursion, and re-encounters its parent ion. During the re-encounter, the interaction of the high-energy electron with the parent ion can result in what is essentially sub-femtosecond Bremsstrahlung emission of a short wavelength photon. The HHG light is emitted as a directed, coherent, and collimated beam.56
Coherent HHG emission is a universal response of atoms, molecules, and even solids to an intense femtosecond laser field. The deBroglie wavelength of the active electron is comparable to interatomic spacing in molecules, so that HHG emission is sensitive to rotations, vibrations, and chemical dynamics.57 HHG employs femtosecond-duration pulses so it can resolve dynamics in molecular systems on time scales from picoseconds to few-fs/attosecond time scales. More recent work has observed HHG from solid materials and has shown that this emission is sen-
56 A. McPherson, G. Gibson, H. Jara, U. Johann, T.S. Luk, I.A. McIntyre, K. Boyer, and C.K. Rhodes, 1987, Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gasses, Journal of the Optical Society of America B 4: 595-601; M. Ferray, A. Lhuillier, X.F. Li, L.A. Lompre, G. Mainfray, and C. Manus, 1988, Multiple-harmonic conversion of 1064-Nm radiation in rare-gases, Journal of Physics B-Atomic Molecular and Optical Physics 21(3): L31-L35; B.W. Shore and K.C. Kulander, 1989, Generation of optical harmonics by intense pulses of laser-radiation, Journal of Modern Optics 36(7): 857-875; J.L. Krause, K.J. Schafer, and K.C. Kulander, 1992, High-order harmonic-generation from atoms and ions in the high-intensity regime, Physical Review Letters 68(24): 3535-3538; P.B. Corkum, 1993, Plasma perspective on strong field multiphoton ionization, Physical Review Letters 71: 1994-1997; M. Lewenstein, P. Balcou, M.Y. Ivanov, A. L’Huillier, and P.B. Corkum, 1994, Theory of high-harmonic generation by low-frequency laser fields, Physical Review A 49(3): 2117-2132; J. Zhou, J. Peatross, M.M. Murnane, H.C. Kapteyn, and I.P. Christov, 1996, Enhanced high harmonic generation using 25 femtosecond laser pulses, Physical Review Letters 76(5): 752-755; Z. Chang, A. Rundquist, H. Wang, I. Christov, H.C. Kapteyn, and M.M. Murnane, 1998, Temporal phase control of soft-x-ray harmonic emission, Physical Review A 58(1): R30-R33; A. Rundquist, C.G. Durfee, Z.H. Chang, C. Herne, S. Backus, M.M. Murnane, and H.C. Kapteyn, 1998, Phase-matched generation of coherent soft X-rays, Science 280(5368): 1412-1415.
57 J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pepin, J.C. Kieffer, P.B. Corkum, and D.M. Ville-neuve, 2004, Tomographic imaging of molecular orbitals, Nature 432(7019): 867-871; N.L. Wagner, A. Wuest, I.P. Christov, T. Popmintchev, X.B. Zhou, M.M. Murnane, and H.C. Kapteyn, 2006, Monitoring molecular dynamics using coherent electrons from high harmonic generation, Proceedings of the National Academy of Sciences of the United States of America 103(36): 13279-13285; H. Niikura, N. Dudovich, D.M. Villeneuve, and P.B. Corkum, 2010, Mapping molecular orbital symmetry on high-order harmonic generation spectrum using two-color laser fields, Physical Review Letters 105(5); S. Baker, J.S. Robinson, C.A. Haworth, H. Teng, R.A. Smith, C.C. Chirila, M. Lein, J.W.G Tisch, and J.P. Marangos, 2006, Probing proton dynamics in molecules on an attosecond time scale, Science 312(5772): 424-427.
sitive to band structure in materials.58 The full implications of HHG as a useful method for probing matter are still under active investigation.
Another area of recent investigation is the control of the electron recollision process itself.59 Recent work has used recollision control to change the spectrum and polarization, and even create novel circularly polarized VUV light.60 The temporal structure of high-order harmonic generation is intrinsically in the form of a sequence of sub-femtosecond bursts of radiation, separated in time by half of an optical cycle. When driven with a few-cycle light pulse, the emission can be confined to a single isolated attosecond pulse.61 Attosecond science makes use of this to observe very fast processes. The HHG process itself is the first example where attosecond dynamics are clearly responsible.62 Research in this new ultrashort time domain is discussed in Chapter 5.
HHG radiation is a quintessential application of the interaction of high intensity laser radiation with matter. Currently it is a compact affordable laboratory-scale
58 S. Ghimire, A.D. DiChiara, E. Sistrunk, P. Agostini, L.F. DiMauro, and D.A. Reis, 2011, Observation of high-order harmonic generation in a bulk crystal, Nature Physics 7(2): 138-141.
59 P.B. Corkum, N.H. Burnett, and M.Y. Ivanov, 1994, Subfemtosecond pulses, Optics Letters 19(22): 1870; X. Feng, S. Gilbertson, H. Mashiko, H. Wang, S.D. Khan, M. Chini, Y. Wu, K. Zhao, and Z. Chang, 2009, Generation of isolated attosecond pulses with 20 to 28 femtosecond lasers, Physical Review Letters 103(18): 183901.
60 W. Becker, B.N. Chichkov, and B. Wellegehausen, 1999, Schemes for the generation of circularly po-larized high-order harmonics by two-color mixing, Physical Review A 60(2): 1721-1722; A. Fleischer, O. Kfir, T. Diskin, P. Sidorenko, and O. Cohen, 2014, Spin angular momentum and tunable polarization in high-harmonic generation, Nature Photonics 8(7): 543-549; T. Fan, P. Grychtol, R. Knut, C. Hernandez-Garcia, D.D. Hickstein, D. Zusin, C. Gentry, et al., 2015, Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism, Proceedings of the National Academy of Sciences of the United States of America 112(46): 14206-14211 (2015); D.D. Hickstein, F.J. Dollar, P. Grychtol, J.L. Ellis, R. Knut, C. Hernandez-Garcia, D. Zusin, et al., 2015, Non-collinear generation of angularly isolated circularly polarized high harmonics, Nature Photonics 9(11): 743-750.
61 I.P. Christov, M.M. Murnane, and H.C. Kapteyn, 1997, High-harmonic generation of attosecond pulses in the ‘’single- cycle’’ regime, Physical Review Letters 78(7): 1251-1254; I.P. Christov, M.M. Murnane, and H.C. Kapteyn, 1998, Generation and propagation of attosecond x-ray pulses in gaseous media, Physical Review A 57(4): R2285-R2288; A. Baltuska, T. Udem, M. Uiberacker, M. Hentschel, E. Goulielmakis, C. Gohle, R. Holzwarth, et al., 2003, Attosecond control of electronic processes by intense light fields, Nature 421(6923): 611-615.
62 Z. Chang, A. Rundquist, H. Wang, I. Christov, H.C. Kapteyn, and M.M. Murnane, 1998, Temporal phase control of soft-x-ray harmonic emission, Physical Review A 58(1): R30-R33; R. Bartels, S. Backus, G. Vdovin, I.P. Christov, M.M. Murnane, and H.C. Kapteyn, 2000, Sub-optical-cycle coherent control in nonlinear optics, Optics and Photonics News 11(12): 23; R. Bartels, S. Backus, E. Zeek, L. Misoguti, G. Vdovin, I.P. Christov, M.M. Murnane, and H.C. Kapteyn, 2000, Shaped-pulse optimization of coherent emission of high-harmonic soft X-rays, Nature 406(6792): 164-166.
coherent vacuum ultraviolet light source has broad applications, particularly in areas where synchrotrons are inconvenient or impractical. In addition, this source has some superior features compared to synchrotrons. These include high spatial (diffraction limited) coherence and broad bandwidth femtosecond-scale temporal coherence. This makes HHG sources particularly useful for femtosecond dynamics studies.
Current areas of broad impact of HHG sources are mostly in materials and nanoscience. These include photoemission using high-harmonic light sources;63 time- and angle- resolved photoemission (TARPES);64,65 excited electron lifetime studies in the attosecond range;66 transient reflectivity and absorption, including Magneto-Optic Kerr Effect (MOKE)67 ultrafast demagnetization;68 and laser-induced spin currents.69,70,71,72 Achievements in this area include the ability to image
63 R. Haight and P.F. Seidler, 1994, High resolution atomic core level spectroscopy with laser harmonics, Applied Physics Letters 65: 517.
64 S. Eich, A. Stange, A.V. Carr, J. Urbancic, T. Popmintchev, M. Wiesenmayer, K. Jansen, et al., 2014, Time- and angle-resolved photoemission spectroscopy with optimized high-harmonic pulses using frequency-doubled Ti:Sapphire lasers, Journal of Electron Spectroscopy and Related Phenomena 195: 231-236.
65 T. Rohwer, S. Hellmann, M. Wiesenmayer, C. Sohrt, A. Stange, B. Slomski, A. Carr, et al., 2011, Collapse of long-range charge order tracked by time-resolved photoemission at high momenta, Nature 471(7339): 490-493; L.X. Yang, G. Rohde, T. Rohwer, A. Stange, K. Hanff, C. Sohrt, L. Rettig, et al., 2014, Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons, Physical Review Letters 112(20): 207001.
66 Z. Tao, C. Chen, T. Szilvási, M. Keller, M. Mavrikakis, H. Kapteyn, and M. Murnane, 2016, Direct time-domain observation of attosecond final-state lifetimes in photoemission from solids, Science 353(6294): 62-67.
67 C. La-O-Vorakiat, M. Siemens, M.M. Murnane, H.C. Kapteyn, S. Mathias, M. Aeschlimann, P. Grychtol, et al., 2009, Ultrafast demagneti-zation dynamics at the m edges of magnetic elements observed using a tabletop high-harmonic soft x-ray source, Physical Review Letters 103(25): 257402-1.
68 S. Mathias, C. La-O-Vorakiat, P. Grychtol, P. Granitzka, E. Turgut, J.M. Shaw, R. Adam, et al., 2012, Probing the timescale of the exchange interaction in a ferromagnetic alloy, Proceedings of the National Academy of Sciences of the United States of America 109(13): 4792-4797.
69 D. Rudolf, C. La-O-Vorakiat, M. Battiato, R. Adam, J.M. Shaw, E. Turgut, P. Maldonado, et al., 2012, Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current, Nature Communications 3: 1037.
70 J.W. Miao, P. Charalambous, J. Kirz, and D. Sayre, 1999, Extending the methodology of X-ray crystal-lography to allow imaging of micrometre-sized non-crystalline specimens, Nature 400(6742): 342-344; J. Miao, T. Ishikawa, I.K. Robinson, and M.M. Murnane, 2015, Beyond crystallography: Diffractive im-aging using coherent x-ray light sources, Science 348(6234): 530-535.
71 R.L. Sandberg, A. Paul, D.A. Raymondson, S. Hadrich, D.M. Gaudiosi, J. Holtsnider, R.I. Tobey, et al., 2007, Lensless diffractive imaging using tabletop coherent high-harmonic soft-x-ray beams, Physical Review Letters 99(9): 098103-098104.
72 M.D. Seaberg, B. Zhang, D.F. Gardner, E.R. Shanblatt, M.M. Murnane, H.C. Kapteyn, and D.E. Adams, 2014, Tabletop nanometer extreme ultraviolet imaging in an extended reflection mode using coherent Fresnel ptychography, Optica 1(1): 39-44.; B. Zhang, D.F. Gardner, M.D. Seaberg, E.R. Shanblatt, H.C. Kapteyn, M.M. Murnane, and D.E. Adams, 2015, High contrast 3D imaging of surfaces near the wavelength limit using tabletop EUV ptychography, Ultramicroscopy 158: 98-104.
buried layers in reflection and to characterize the diffusive properties of the interface.73 These results suggest that HHG may have a role in the emerging industry of EUV light for nanoelectronics lithography.74
Time-resolved diffraction of HHG light enables studies of the nanoscale thermal and acoustic properties of materials75 and measurements of thin film properties.76 HHG light sources have also been used for studies of dynamics in molecular systems.77,78,79
The properties of HHG radiation vary quite dramatically with the parameters of the driving laser. Longer wavelength drive lasers produce coherent soft X-ray emission at shorter wavelengths. Using longer wavelength laser, coherent light at
73 E. Shanblatt, C. Porter, D.F. Gardner, G.F. Mancini, R. Karl, C. Bevis, M. Tanksalvala, M. Murnane, H. Kapteyn, and D. Adams, 2015, “Reflection Mode Tabletop Coherent Diffraction Imaging of Buried Nanostructures,” presented at the Frontiers in Optics 2015, San Jose, California, October 18.
74 European Semiconductor Industry Association, Japan Electronics and Information Technology Industries Association, Korean Semiconductor Industry Association, Taiwan Semiconductor Industry Association, and United States Semiconductor Industry Association, 2013, International Technology Roadmap for Semiconductors, http://www.itrs.net/Links/2013ITRS/Home2013.htm, accessed March 14, 1017.
75 M.E. Siemens, Q. Li, R.G. Yang, K.A. Nelson, E.H. Anderson, M.M. Murnane, and H.C. Kapteyn, 2010, Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams, Nature Materials 9(1): 26-30.
76 K. Hoogeboom-Pot, J. Hernandez-Charpak, T. Frazer, X. Gu, E. Turgut, E. Anderson, W. Chao, et al., 2015, Mechanical and thermal properties of nanomaterials at sub-50nm dimensions characterized using coherent EUV beams, Proc. SPIE 942: Metrology, Inspection, and Process Control for Microlithography XXIX (J.P. Cain and M.I. Sanchez, eds.), San Jose, Calif., February 22.
77 E. Gagnon, P. Ranitovic, A. Paul, C.L. Cocke, M.M. Murnane, H.C. Kapteyn, and A.S. Sandhu, 2007, Soft x-ray driven femtosecond molecular dynamics, Science 317(5843): 1374-1378; A.S. Sandhu, E. Gagnon, R. Santra, V. Sharma, W. Li, P. Ho, P. Ranitovic, C.L. Cocke, M.M. Murnane, and H.C. Kapteyn, 2008, Observing the creation of electronic feshbach resonances in soft x-ray-induced O2 dissociation, Science 322(5904): 1081-1085.
78 I. Thomann, R. Lock, V. Sharma, E. Gagnon, S.T. Pratt, H.C. Kapteyn, M.M. Murnane, and W. Li, 2008, Direct measurement of the transition dipole for single photoionization of N2 and CO2, Journal of Physical Chemistry A 112 (39): 9382–9386.
79 Z.H. Loh, M. Khalil, R.E. Correa, R. Santra, C. Buth, and S.R. Leone, 2007, Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy, Physical Review Letters 98(14): 143601.
>1 keV photon energy has been demonstrated.80 Higher conversion efficiency can be generated using intense-field lasers at ultraviolet wavelengths.81 A frequent limitation to wider applications is the limited average power of HHG sources, and so this is an area of active technology development.
80 T. Popmintchev, M-C Chen, D. Popmintchev, P. Arpin, S. Brown, S. Ališauskas, G. Andriukaitis, T. Bal iunas, et al., 2012, Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers, Science 336(6086): 1287-1291.
81 D. Popmintchev, C. Hernandez-Garcia, F. Dollar, C. Mancuso, J.A. Perez-Hernandez, M-C Chen, A. Hankla, et al., 2015, Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas, Science 350(6265): 1225-1231.