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Nanophotonics Accessibility and Applicability 1 Introduction SCOPE OF THE STUDY The purpose of this study is to determine the accessibility of nanophotonics technology in a 10-to-15-year time frame and to identify the nations that control these technologies, both currently and throughout the time frame of the study, reviewing the scale and scope of offshore investments and interests in nanophotonics. Further, this study aims to identify feasible nanophotonics applications; their potential relationship to military systems; associated vulnerabilities of, risks to, and impacts on critical defense capabilities; and other significant indicators and warnings that can help avoid and/or mitigate surprise related to technology application. Finally, this study recommends priorities for future action by appropriate departments of the intelligence community (IC), the Department of Defense (DOD) research and development (R&D) community, and other government entities. In this study, the National Research Council’s Committee on Nanophotonics Accessibility and Applicability (see Appendix A for biographical information) addresses the following questions (see Box 1-1 for committee’s statement of task): Can emerging nanophotonics technology lead to disruptive capabilities that could threaten U.S. national security? Will nanophotonics introduce a paradigm shift that could potentially alter the balance of power? Can a country accomplish meaningful results that would enable significant applications in nanophotonics with a modest effort? If so, what indicators exist to gauge milestones on the way to the achievement of such results? By adopting nanophotonics as a technology, is the United States unknowingly leaving itself vulnerable from a national security perspective? In answering these questions, the committee examines the threats, opportunities, and vulnerabilities posed by emerging applications of nanophotonics. It also examines the underlying capabilities required to develop a strong nanophotonics technology base. Although the focus of this study is defense applica-
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Nanophotonics Accessibility and Applicability BOX 1-1 Statement of Task The NRC will: Study the accessibility and potential applicability of nanophotonics in the 10-15 year timeframe and identify who controls these technologies in the areas of photonic crystals, plasmonics, metamaterials, and negative index materials.a Review the scale and scope of off-shore investments and interest in nanophotonics. Identify feasible nanophotonic applications, their potential relationship to military systems, associated vulnerabilities, risks, impacts to critical defense capabilities, potential alternate technologies that could compete with nanophotonics, and other significant indicators and warnings to help avoid and/or mitigate technology application surprise.b Suggest priorities for future action by appropriate departments of the Intelligence Community, Department of Defense R&D community, and other Government entities. a“The charge to determine ‘who’ controls nanophotonics is interpreted as ‘which nations’ control nanophotonics. This precludes the notion that specific research institutions or industries, smaller in scale than a nation, may control facets of nanophotonics. These entities may possess fundamental knowledge, trade secrets, or capacities for innovation that may be hidden from the academic community or that may be so broadly networked (even across international boundaries) that their latent ability for break through discovery may not be obvious. However, because global multi-institution and multi-national research collaborations are abundant the bias to categorize the threat of technological surprise by nation may be limiting.” bBy the definition of basic research, nanophotonics is still at an early stage with many possibilities and ultimate applications are still undefined. Discussing alternatives to as yet undeveloped technologies is clearly folly. The committee did identify underlying themes that characterize nanophotonics (see summary). Most of these have to do with surmounting wavelength limitations and enabling optical functionality at sub-wavelength scales. This by definition is “nanophotonics” and anything that allows it will be called “nanophotonics,” whether it is one of the classes of objects we have identified or an as yet undiscovered direction. In this sense the question is moot. Manifestly there are advantages of optical functionality at sub-wavelength scales, and just as manifestly the enabling technology will be called nanophotonics. It is not as if we are considering a single system such as a biosensor and trying to decide between an optical (nanophotonic) approach and, for example, gas chromatography coupled with mass spectroscopy. In this specific case the question is well posed. In a global sense, however, it is not. tions, commercial applications are also described and discussed, since much of the infrastructure and many of the capabilities required for defense applications will be commercially driven. BACKGROUND The domain of nanoscale science and technology lies between the familiar classical world of macroscopic objects and the quantum mechanical regime of atoms and molecules. Nanostructures can have unique, controllable, and tunable optical properties that arise from their nanoscale size and from the fact that they are smaller than the wavelength of light used to observe them. Both the properties of
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Nanophotonics Accessibility and Applicability the nanostructures and their organization into large-scale materials, which may be ordered on the scale of the wavelength, are important for determining the optical response. Indeed, the optical properties of nanomaterials can be tailored for important commercial and defense applications, such as compact photoelectric power sources; efficient and tunable light sources, detectors, filters, waveguides, and modulators; high-speed all-optical switches; environmental (both chemical and biological) sensors; next-generation classical and quantum computation; and biophotonic medical diagnostics and therapeutics. This area of nanoscience, called nanophotonics, is defined as “the science and engineering of light-matter interactions that take place on wavelength and subwavelength scales where the physical, chemical or structural nature of natural or artificial nanostructured matter controls the interactions.”1 One-dimensional nanoscale structures, such as multilayer optical coatings and distributed Bragg reflectors, have long been staples of optical design and engineering. Form-birefringence is an example of a well-known two-dimensional structural optical functionality with pattern scales below the wavelength. This report is restricted to the new developments that arise from the ability to control structures at the nanoscale in multiple dimensions (two- and three-dimensional photonic crystals, reduced dimensionality, and quantum confinement), to control both the magnetic and the electrical response of materials (metamaterials), or to manipulate nanoscale structures for enhanced field concentration (plasmonics). For purposes of this study, the committee divided the areas of nanophotonics to be discussed in terms of the different physical nanoscale phenomena, driven by physics that is determined by the size scale (relative to a wavelength) of the modulation of the index of refraction in the nanoscale material or system. The resultant four areas of nanophotonics discussed in this report are as follows: Photonic crystals—in which the spatial index modulation is on the order of a wavelength; Metamaterials—in which the structural elements are much smaller than the wavelength, permitting an effective medium approach to the optical properties; Plasmonics—in which manipulation of light at the nanoscale is based on the properties of surface plasmons arising from metal free-electron response (negative permeability); and Confined semiconductor structures—whose physics is driven by reduced dimensionality and quantum confinement. Photonic crystals are optical materials engineered using periodic dielectric structure with spatial periodicity of the order of the wavelength of the light that enables the tailoring of the propagation of light through the control of the photonic crystal structure (John, 1987; Yablonovitch, 1987). A key idea for photonic crystal structures is the periodicity of the structure giving rise to dramatic changes in the optical properties and possibly to the formation of a forbidden gap in the electromagnetic spectrum, a “photonic” band gap, thus altering the properties of the light passing through the structure (Ho et al., 1990, 1991). The photonic band gap defines a set of frequencies for which light cannot propagate in the crystal: the tunability of the band gap, through control of the dimensions and symmetry of the photonic structure, provides exquisite frequency control of the propagation properties through the crystal. Alternatively, the perfect translational symmetry of the photonic crystal can be disrupted in a controlled manner, providing a localized photonic state within the photonic band gap, making possible the localization of photons (Yablonovitch et al., 1991). As with surface plasmons, extremely high field densities can be achieved within these photonic crystal “defects” or resonators, leading to a wide range of opportunities for nonlinear operation and highly sensitive detection. Another consequence of the structure of the photonic band 1 See http://www.phoremost.org/about.cfm. Last accessed April 9, 2007.
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Nanophotonics Accessibility and Applicability gap is the dispersion behavior near the band edge, and the possibility of group velocities approaching zero, thus “slowing” light in the photonic crystal, or the velocity can be negative as in opposed phase and group velocities (Notomi et al., 2001; Vlasov et al., 2005). Electromagnetic metamaterials are created from individual nanostructures that are fabricated on a scale much less than a wavelength and that respond resonantly to either electric or magnetic fields (Pendry et al., 1999, 2004), as shown in Figure 1-1. To first order, the subwavelength dimensions of such resonant structures allow for treating the composite structures in the effective medium limit, enabling the construction of materials composed of arrays of these particles with specifically designed electromagnetic properties. Such composite materials can be described in terms of the constitutive relations appearing in Maxwell’s equations, meaning that the electrically resonant response is characterized by the electrical permittivity ε(ω) and the magnetically resonant response is characterized by the magnetic permeability μ(ω). This new ability to design metamaterials with tunable ε and μ enables many applications hitherto unattainable. In particular, it is possible to design metamaterials with a magnetic response at optical frequencies that no known natural material exhibits. To date, the primary goals of metamaterials research have been the extension of the wavelength range to the near infrared and visible regions; increasingly, attention is turning to the novel optical properties that can be achieved with spatial control of the refractive index over wide, and including negative, ranges. Although most of the metamaterials community has focused on the demonstration of a negative index at optical frequencies, a host of other exciting and relevant possibilities exist, including high-sensitivity detection as well as switching and modulation. The frequency of the response of metamaterials can be scaled from the microwave to the near infrared by decreasing the dimensions of the nanostructures (Dolling et al., 2006; Shelby et al., 2001; Yen et al., 2004; Zhang et al., 2005b). There FIGURE 1-1 (a) Array of nanoscale resonators (gold loops, inductors, atop a dielectric/metal film stack, capacitors). The resulting structure has a resonance evident in the normal incidence reflection spectrum in the mid-infrared at approximately 5 µm and exhibits a negative permeability; (b) staple-shaped nanostructures. SOURCE: Reprinted with permission from Zhang et al. (2005a). Copyright 2005 by the American Physical Society.
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Nanophotonics Accessibility and Applicability are well-established fields using subwavelength structures for antireflection coatings, form-birefringent polarization manipulation, and other optical elements. This report is restricted to the new developments arising from the ability to control both the magnetic and the electric response of materials. Reduced dimensionality and quantum confinement occur when a structure’s extent in one or more dimensions becomes comparable to the Fermi wavelength of the charge carrier (i.e., electron or hole). See Figure 1-2 for a visual depiction of the latest developments in increasing the resonance frequency in metamaterials. Very recently, these results have been extended throughout the visible spectrum using wire pairs (vertically stacked metal-dielectric-metal structures similar to the structure at the top right in Figure 1-2) (Cai et al., 2007). In this case, the allowed energy levels of the charge carriers become significantly modified, increasing in energy as the structure dimensions are decreased and the confinement becomes more severe. The double heterostructure—which eventually came to be called a quantum well—enabled the charge carriers to be concentrated into a thin layer of material having a smaller band gap than the material surrounding it (Ho et al., 1994). This suppressed electron-hole recombination outside the active region, and increased optical confinement owing to the higher index of refraction in the quantum well. In such two-dimensional semiconductor structures, the fundamental concept of quantum confinement uses quantum wells to localize excitons, increase oscillator strengths, enhance radiative recombination efficiencies, and control charge-carrier transport. One-dimensional quantum confined structures, quantum wires, and zero-dimensional quantum structures—quantum dots or nanocrystals—also yield size-controlled optical and electronic properties of importance to nanophotonics. Indeed, quantum dots can be considered tunable “artificial atoms” whose optical properties can be engineered for a particular application. Based on the properties of surface plasmons, plasmonics is a subfield of nanophotonics concerned primarily with the manipulation of light at the nanoscale. Plasmons are the collective oscillations of FIGURE 1-2 Latest developments in increasing the resonance frequency of metamaterials. The four insets show fabricated structures in different frequency regions. SOURCE: Soukoulis et al. (2007). Reprinted with permission from AAAS.
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Nanophotonics Accessibility and Applicability the electron gas in a metal or semiconductor. The bulk plasmon is a strictly longitudinal excitation that does not couple to transverse field photons. The coupling to photons is possible only in the presence of a surface or a boundary. There are both longitudinal and transverse parts to the surface plasma wave fields on both sides of the interface. Surface plasmon polaritons, also called surface plasmons, represent an electromagnetic wave bound to the surface of a metal film surrounded by a dielectric. This excitation can be considered as a charge density wave combined with an electromagnetic field. The surface plasmon has a propagation vector parallel to the interface, while its amplitude decays exponentially in the direction orthogonal to the surface (see Figure 1-3) (Barnes et al., 2003). Unlike pure electromagnetic (optical) waves, surface plasmons can be localized to subwavelength dimensions in the plane perpendicular to the propagation direction, providing a viable route to nanoscale optics. Much of today’s research is aimed at structures that provide additional localization in multiple dimensions; examples are surface plasmons localized to single metal particles and to the interstices between metal particles. Localization of the electromagnetic fields at the nanoscale also yields a dramatic increase in the field intensity, thus suggesting the use of surface plasmons in nonlinear applications, such as optical switching, Raman spectroscopy of single molecules and atomic clusters, and even coherent control of a single molecule’s quantum dynamics. Surface plasmons are supported by structures at all length scales and largely determine the optical properties of metal-based nanostructures. The field of plasmonics is based on the use of surface plasmons for a large variety of tasks, through the design and manipulation of the geometry of metallic structures, and consequently their specific plasmon-resonant or plasmon-propagating properties. FIGURE 1-3 Surface plasmons (SPs) at a metal-dielectric interface: (a) electromagnetic field vector and charge distribution diagram, (b) field amplitude dependence orthogonal to the interface, and (c) dispersion curve for SPs (solid line) and photons (dashed line) showing the momentum mismatch. SOURCE: Reprinted by permission from Macmillan Publisher’s Ltd: Barnes et al. (2003). Copyright 2003, Macmillan Publisher’s Ltd.
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Nanophotonics Accessibility and Applicability METHODOLOGY The scope of interest of this study is global in terms of developments in nanophotonics technology, but it emphasizes the following: Speculation on future advances in nanophotonics relevant to U.S. military applications; Identification of vulnerabilities of, risks to, and impacts on critical defense capabilities from feasible nanophotonics applications and alternatives; and Assessment of offshore research and development in nanophotonics. Data and other information for the study included the following: Relevant prior studies (see Appendix C for excerpts from selected studies), State-of-the-art reviews, Current research and development reviews, and Presentations to the committee (see Appendix B for the list of presentations). The committee’s review considered historical trends, notable domestic centers of excellence, offshore competition, and related enabling technology developments. The report includes projections of future developments and the identification of plausible applications and threats related to such developments. The scope of coverage of technological developments also includes nonoptical frequencies (as exemplified by terahertz phenomenology and technology, where the concepts are similar but the size scale is appropriately extended). Anticipating Threats and Projecting Threat Levels In a generic sense, a difficulty with anticipating threats is the latency between innovation, application, and emerging threat. This latency makes translating nanophotonics technology innovations into specific long-term military spin-offs problematic. An additional challenge is that the assessment also depends on the state of the industrial and enabling technology base. Commercial trends in this report are increasingly important and are further enabled by globalization trends, which can be become somewhat convoluted. Potential threats to U.S. military systems from outside the United States may be leveraged or reverse-engineered from U.S. technology and may even be a driver for subsequent U.S. technological developments. While the danger of threats could be prioritized, for example, in terms of possible geographic extent (city to nation) and impact (nature of disruption, monetary losses, and fatalities), this study attempts to assess nanophotonics-related threats in terms of the IC’s technology watch and warning framework, which correlates projected threat levels with technology development phases or milestones over a progressive time line, as illustrated in Figure 1-4. But this is not the only paradigm, since it can be short-circuited—that is, competitors may enter the development chain at any point. Ideally an assessment should identify the utility or observables to monitor, track, and quantify in terms of the expected benefits—for example, gains in performance (decibels). Type I and Type II errors (false positives and false negatives) will be unavoidable. In addition to projecting threats, results of this will help ensure technological superiority and influence investment strategies for U.S. programs, especially within the context of globalization and a perceived decline in a U.S. science and engineering advantage.
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Nanophotonics Accessibility and Applicability FIGURE 1-4 Time line for technology development common in commercial, defense, and space sectors of the “blue world” (domestic and other Western countries). Two related “landscapes” exist: one delineated by threat and one by the technology. In general, threats can be broken down by the scope of their geographic coverage and their credibility. Supporting evidence must eventually be weighted by the possible strategies for conflict resolution: direct military conflict, economic competition, or terrorism. Technology can be broken down by technology subareas directly (e.g., metamaterials, photonic crystals, and so on.), by enabling technologies (fabrication versus modeling and simulation), and by physics constraints (fundamental limits). The evidence supporting the committee’s findings could conceivably just include weighting the findings by analogy with common practices (“blue world” R&D methodologies) and in terms of the credibility of a competing (foreign) R&D technology base. But not all such threats follow this model. Important exceptions include leveraging or reverse-engineering of commercial-off-the-shelf technology and their implementation as so-called asymmetric threats. Matrix of Critical Technologies The committee formulated a matrix of critical technologies to “roll up” more detailed deliberations into a top-level assessment: the matrix shows the major areas of nanophotonics (photonics, metamaterials, negative index materials, and plasmonics) versus a notional critical technologies list, as shown in Figure 1-5. This assessment is described in more detail in Chapter 6. The committee considered four levels of probability in estimating the likelihood of a technology’s impacting U.S. strategic and critical capabilities (extremely high, high, medium, and low) and the case of not applicable (none). In arriving at these probabilities, the committee implicitly took into account several levels of maturity:
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Nanophotonics Accessibility and Applicability FIGURE 1-5 Matrix of critical technologies developed by the committee for the top-level assessment of nanophotonics. Is the technology in a conceptual feasibility phase? Is it in hardware development? Is it ready to embed in a system? Is it in product development? Is deployment imminent? Technological applications were also assessed by the committee in terms of their perceived potential for immediate application. The two distinctive domains of this aspect of the assessment are (1) envisioned or pending applications, and (2) enabling technology, as delineated below. Examples of applications of the first domain include the following: Embedded sensors fabricated by replication or self-assembly and scalable to large areas; Stealthy nanoscale taggants for warning or for the tracking of security perimeter incursions; Metamaterials enabling enhanced antennas for sensing, and for communications with covertness and/or stealth at selected frequencies; Reverse-engineered, remotely activated nanoparticles; and Micro- and nano-optical elements for ultraminiature hybrid optoelectronic processors. Examples of applications of the second domain, enabling technology, include the following: Fabrication, both top down (classic semiconductor techniques) and bottom up (self-assembly, growth, and synthesis);
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Nanophotonics Accessibility and Applicability Characterization of nanophotonic materials and devices; Modeling and simulation; and Packaging and integration. STRUCTURE OF THE REPORT Following this introductory chapter, the four areas that comprise nanophotonics are described in more depth in Chapter 2; supporting material in Appendix D describes a representative sampling of research efforts in the area of plasmonics. The enabling technologies for nanophotonics, including nanomaterials growth, synthesis and fabrication, characterization of nanophotonic materials and devices, nanoscale device integration, nanophotonic packaging, and modeling and simulation, are discussed in Chapter 3. Chapter 4 presents potential applications of nanophotonics, emphasizing those of interest to the defense and intelligence communities. The focus of Chapter 5 is international capabilities and investments in nanophotonics. Finally, Chapter 6 discusses the relevance of nanophotonics to major strategic and critical military technologies and summarizes the committee’s conclusions and recommendations. REFERENCES Barnes, William L., Alain Dereux, and Thomas W. Ebbesen. 2003. Surface plasmon subwavelength optics. Nature 424 (6950):824-830. Cai, W., U.K. Chettiar, H-K. Yuan, V.C. de Silva, A.V. Kildishev, V.P. Drachev, and V.M. Shalaev. 2007. Metamagnetics with rainbow colors. Optics Express 15:3333. Dolling, Gunnar, Christian Enkrich, Martin Wegener, Costas M. Soukoulis, and Stefan Linden. 2006. Low-loss negative-index metamaterial at telecommunication wavelengths. Optics Letters 31 (12):1800-1802. Ho, K.M., C.T. Chan, and C.M. Soukoulis. 1990. Existence of a photonic gap in periodic dielectric structures. Physical Review Letters 65:3152-3155. Ho, K.M., C.T. Chan, and C.M. Soukoulis. 1991. Comment on “Theory of photon bands in three dimensional periodic dielectric structures.” Physical Review Letters 66(3):393. Ho, K.M., C.T. Chan, C.M. Soukoulis, R. Biswas, and M. Sigalas. 1994. Photonic band gaps in three dimensions: New layer-by-layer periodic structures. Solid State Communications 89:413. John, Sajeev. 1987. Strong localization of photons in certain disordered dielectric superlattices. Physical Review Letters 58 (23):2486-2490. Notomi, M., K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama. 2001. Extremely large group-velocity dispersion of line-defect waveguides in photonic crystal slabs. Physical Review Letters 87(25). Pendry, J.B., A.J. Holden, D.J. Robbins, and W.J. Stewart. 1999. Magnetism from conductors and enhanced nonlinear phenomena. IEEE Transactions on Microwave Theory and Techniques 47(11). Pendry, W. J., John B. Smith, and David R. Smith. 2004. Reversing light with negative refraction. Physics Today 37-43. Shelby, R.A., D.R. Smith, and S. Schultz. 2001. Experimental verification of a negative index of refraction. Science 77-79. Soukoulis, Costas M., Stefan Linden, and Martin Wegener. 2007. Negative refractive index at optical wavelengths. Science 315 (5808):47-49. Vlasov, Yurii A., Martin O’Boyle, Hendrik F. Hamann, and Sharee J. McNab. 2005. Active control of slow light on a chip with photonic crystal waveguides. Nature 438:65-69. Yablonovitch, E. 1987. Inhibited spontaneous emission in solid-state physics and electronics. Physical Review Letters 58 (20):2059-2063. Yablonovitch, E., T.J. Gmitter, R.D. Meade, A.M. Rappe, K.D. Brommer, and J.D. Joannopoulos. 1991. Donor and acceptor modes in photonic band structure. Physical Review Letters 67(24):3380-3383. Yen, T.J., W.J. Padilla, N. Fang, D.C. Vier, D.R. Smith, J.B. Pendry, D.N. Basov, and X. Zhang. 2004. Terahertz magnetic response from artificial materials. Science 303(5663). Zhang, Shuang, Wenjun Fan, B.K. Minhas, Andrew Frauenglass, K.J. Malloy, and S.R.J. Brueck. 2005a. Midinfrared resonant magnetic nanostructures exhibiting a negative permeability. Physical Review Letters 94(3):037402. Zhang, Shuang, Wenjun Fan, N. C. Panoiu, K. J. Malloy, R. M. Osgood, and S. R. J. Brueck. 2005b. Experimental Demonstration of Near-Infrared Negative-Index Metamaterials. Physical Review Letters 95(13):137404.