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Nanophotonics: Accessibility and Applicability (2008)

Chapter: 2 Nanoscale Phenomena Underpinning Nanophotonics

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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 36
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 39
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 40
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 41
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 42
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 44
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 45
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 46
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 47
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 48
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 49
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
Page 50
Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
×
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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Suggested Citation:"2 Nanoscale Phenomena Underpinning Nanophotonics." National Research Council. 2008. Nanophotonics: Accessibility and Applicability. Washington, DC: The National Academies Press. doi: 10.17226/11907.
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2 Nanoscale Phenomena Underpinning Nanophotonics This chapter explores the physical phenomena that distinguish nanophotonics from photonics. The chapter is organized in sections on photonic crystals (structures on the scale of the optical wavelength), metamaterials (structures much less than the optical wavelength), plasmonics (structures using the large, negative permittivity of metals to manipulate optical fields), and reduced dimensionality and quantum confinement (semiconductor nanostructures on the scale of electronic wave functions). Because many of the phenomena of nanophotonics are largely electromagnetic in origin, the discussion also includes applications to longer wavelengths (terahertz) to which the appellation “nano” no longer strictly applies. A very important caveat: the research areas discussed here are very active, with new developments being announced at a breakneck pace; the report provides a snapshot, frozen in time in the spring of 2007, of things that will inevitably have changed by the time the report is being circulated. Nonetheless, it is important to elucidate the fundamental concepts and to establish the vector along which the field of nanophotonics is progressing. spatial MODULATION at fractions OF A WAVELENgTH—PHOTONIC CRYSTALS Introduction In a paper published in 1987, Yablonovitch anticipated the possibility of inhibited spontaneous emission in solid-state materials through the formation of a three-dimensionally periodic dielectric structure with spatial periodicity on the order of the wavelength of the light considered (Yablonovitch, 1987). Such periodic structures can be formed from two materials that have different indices of refrac- tion—for example, air and SiO2. In the same time frame, S. John published a similarly visionary paper that speculated on strong localization of photons in “certain disordered superlattice microstructures of sufficiently high dielectric constant” (John, 1987). These papers formed the foundations of the tremen- dously fertile and productive research field of photonic crystals: this field involves engineered optical materials providing a multitude of ways to tailor the propagation of light through the control of the photonic crystal structure. While the first demonstrations of photonic crystal behavior were carried 19

20 Nanophotonics out at microwave frequencies in scaled structures of 6 millimeter (mm) Al 2O3 spheres (Yablonovitch and Gmitter, 1989) or drill/etched Stycast 12 (Yablonovitch et al., 1991), current research on photonic crystals truly embodies the concepts of “nanophotonics,” with spatial index modulation (etched holes or solid rods) at the 100 nanometer (nm) scale, allowing compact, highly integrable waveguides, filters, resonators, and high-efficiency lasers. The original predictions of Yablonovitch and John have been realized: first reports of photonic crystal lasers were made in 1999 (Painter et al., 1999), and localiza- tion of photons within photonic crystal “defects” was first observed in 1991 in the microwave regime (Yablonovitch et al., 1991a). Photonic Band Gap A key idea for photonic crystal structures is the periodicity of the structure giving rise to the forma- tion of a forbidden gap in the electromagnetic spectrum, thus altering the properties of the light passing through the structure. One-, two-, and three-dimensional photonic crystals, as well as a photonic band structure are described in Figure 2-1. 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 for multiple wavelength information processing (or wavelength division multiplexing, WDM). Various photonic crystal waveguides have been formed with deliberately engineered stop bands (e.g., Davanco et al., 2006; Fleming and Lin, 1999). Equally interesting, or perhaps more so, is the case in which the perfect translational symmetry of the photonic crystal is disrupted in a controlled manner. John (1987) alluded to these “certain disordered dielectric superlattices” in his 1987 paper, and Yablonovitch et al. (1991b) used the analogy of donor and acceptor modes in semiconductor crystals in defining these “defect” states: the disruption from symmetry providing a photonic state within the photonic band gap, making possible the localization of photons. a) b) FIGURE 2-1  (a) Simple examples of one-, two-, and three-dimensional photonic crystals. The different colors represent materials with different dielectric constants. (b) A notional dispersion diagram for a photonic crystal showing a band gap and regions of anomalous dispersion. SOURCE: Joannopoulos et al. (1995). Reprinted by permission of Princeton University Press.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 21 Defects in Photonic Crystals: Localization of Light A linear defect, in which the field propagates along the direction of the defect and decays exponen- tially in the transverse direction, can serve as an on-chip optical waveguide with some exceptional prop- erties. More-typically-fabricated on-chip optical waveguides confine optical modes through differential indices of refraction and can display radiation losses—for example, at the bends of curved waveguides. Appropriately designed photonic crystal waveguides are prohibited from radiating into the surrounding bulk material, even for a 90° bend in the waveguide (Meade et al., 1994) (see Figure 2-2). The first experimental demonstration was carried out for a photonic crystal comprising alumina rods with a lattice constant of 1.27 mm, evidencing 80 percent transmission around a 90° bend (Faraon et al., 2007; Lin et al., 1998; Scherer et al., 2005). Various photonic crystal waveguides have since been fabricated with much smaller lattice constants (<0.4 micrometer [µm]) (e.g., Chutinan et al., 2002), and controlled interactions and light exchange between two or more waveguides are possible (Chong and Rue, 2004; Fan et al., 1998). The Control of Dispersion and the Slowing and Storage of Light An interesting and powerful consequence of the structure of the photonic band gap is the dispersion behavior near the band edge and the possibility of group velocities approaching zero. Such slowing of light has been observed in photonic crystal slab waveguides, etched into semiconductor materials (Notomi et al., 2001; Vlasov et al., 2005). The slowing of light and the control of the dispersion properties of the material hold important implications for compact, on-chip processing systems in which controlled delay and storage of optical signals would form important components of any optical-­information- p ­ rocessing strategy. FIGURE 2-2  The field of a transverse-magnetic mode traveling around a sharp bend in a waveguide carved out of a photonic crystal square lattice. SOURCE: Joannopoulos et al. (1995). Reprinted by permission of Princeton University Press.

22 Nanophotonics High-Efficiency Optical Sources A “point defect” can localize photons, and the early predictions of possible high Q (low optical loss) have proven to be true (Meade et al., 1994). The combination of high Q and low modal volume possible with photonic crystal “defects” or cavities proves an extremely powerful one in producing ultralow-threshold lasers. Lasing was demonstrated in a quantum well (QW) gain medium in a photonic crystal structure (Painter et al., 1999) at low temperature, and subsequently in a dense quantum dot (QD) medium at room temperature (Yoshie et al., 2002). With lower density, QD and strategic matching of QD emission to photonic crystal cavity modal pattern, lasing has been observed at optical pump powers as low as 10s of nanowatts (nW), coupling to only 2 to 4 QDs (Strauf et al., 2006) (see Figure 2-3). Achieving lasing at such low thresholds is testimony to the control over spontaneous emission that formed the original vision for photonic crystals; numerous recent efforts have separately addressed these issues (Fujita et al., 2005; Lodahl et al., 2004; Ogawa et al., 2004). By changing the photon states accessible in the material, photonic crystal patterning of optical structures has also been shown to be an effective way of increasing the extraction efficiency of light- emitting diodes (LEDs), ideally converting optical guided modes within the device to extracted modes, with minimal loss. By designing the appropriate photonic crystal pattern for an LED structure, one can achieve efficient optical emission at particular wavelengths and angular directions (David et al., 2006; Oder et al., 2004; Orita et al., 2004; Wierer et al., 2004). The combination of high Q and low modal volume also makes photonic crystal cavities excellent testbeds for the validation of quantum computation schemes. Quantum dots or other emitters incorporated into the photonic crystal can be weakly or strongly coupled to the cavity: thus, control of the cavity (environment) can result in direct control of the emitters (qubits) within the environment (Badolato et al., 2005; Hennessy et al., 2007). (c) (a) (b) FIGURE 2-3  (a) Atomic force microscope showing ~5 quantum dots/μm2, mapped onto (b) simulation of mode strength in photonic crystal cavity, giving rise to (c) lasing characteristics with ultralow threshold. SOURCES: (a) Evelyn Hu, University of California at Santa Barbara; (b&c) Reprinted with permission from Strauf et al. (2006). Copyright 2006 by the American Physical Society. 2-3

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 23 Photonic Crystal Waveguides and Fibers A number of powerful photonic crystal elements currently will allow on-chip, fairly dense integration of optical processing components: waveguides and filters of exceptionally high frequency resolution, the possibility of optical storage and delay through photon localization and control of group velocity, and extremely low threshold optical sources, with narrow spectral outputs that can be sensitively directed in-plane or out of plane. Tuning the band structure of these photonic crystal elements allows photon gen- eration, transmission, and coupling with minimal loss. The majority of the applications described above have been fabricated in a planar geometry, forming two- or three-dimensional photonic crystal device elements on a planar substrate. Photonic crystal fibers represent a very powerful technology that applies many of the advantages previously described to the transmission and modulation of light propagating through optical fibers. These structures show a lateral periodic variation in the index of refraction (e.g., inclusion of air holes) along the entire length of the fiber. Examples of the cross sections of photonic crystal fibers are shown in Figure 2-4. From the initial demonstrations in the 1970s of low-loss (<20 decibels per kilometer [dB/km]) single- mode transmission, optical fiber technology has rapidly developed to become the predominant means of rapid, long-distance, low-loss transmission of optical signals. Conventional optical fibers employ stepped changes in the index of refraction to confine and guide light; the application of photonic ­crystal concepts allows the following: the engineering of index differences, beyond the choice of the fiber mate- rial alone; selective transmission of particular wavelengths; control of the dispersion ­properties of the FIGURE 2-4  Various photonic crystal fiber cross sections. SOURCE: Russell (2003). Reprinted with permission of AAAS. 2-04

24 Nanophotonics fiber; lower propagation loss; and lower loss from bending of the fiber. Additional optical properties, such as birefringence, can be engineered into the fiber, allowing the preservation of optical polarization information. Photonic crystal fibers can be formed with “hollow cores,” making possible a number of new appli- cations: very high power, ultrashort pulse propagation (Ouzounov et al. 2003), and nonlinear optical processes in gases that fill the hollow core (Knight, 2003; Russell, 2003). The relative ease of formation of photonic crystal fibers, its compatibility with existing optical fiber manufacturing techniques, and a natural scalability to the appropriate nanoscale modulation no doubt have all contributed to the rapid development of photonic crystal fibers between the time that the initial ideas were put forward in the 1990s and the current availability of commercial suppliers of such specialty fibers—for example, Crystal Fibres, Newport Corporation, and Corning International Corporation. Feasibility and Impact In a scant 20 years, the visionary predictions of the power of photonic crystal structures to modulate and control light have been dramatically proven to be accurate. Overcoming challenges of high-resolution fabrication, process imperfections, and materials loss, photonic crystal structures have shown the ability to filter and slow light, control spontaneous emission, and enhance optical efficiency. The impact of these structures is profound and wide-ranging, allowing top-down alteration of the fundamental optical prop- erties of the materials that are used as platforms for optical devices and systems. The challenges ahead with respect to photonic crystals lie in the achievement of superior performance of individual devices at reduced or equivalent cost and the ability to realize a major benefit of photonic crystal elements in the integration of multiple devices into high-performance, lightweight, compact systems. Further work will be required to achieve active electrical control and modulation of photonic crystal devices without loss. Much work needs to be done to improve understanding of long-term reliability and packaging issues associated with this technology. The link between potential benefits, feasibility, and impact of the photonic crystal technology can be demonstrated in the progress of photonic crystal fibers. With photonic crystals as vehicles for light transmission, the incorporation of photonic crystal modulation serves to make an inexpensive, outstand- ing technology even better, promising lower loss, control over dispersion, the possibility of optimization of transmission and various wavelengths (not just the wavelength determined by the core properties of the fiber), and the implementation of highly sensitive sensing and signal amplification. The benefits of the technology in this case are amplified and catalyzed by the existence of a manufacturable fabrica- tion strategy. Once similar technological challenges are met for planar dielectric photonic crystals, it is expected that their impact on optical information sensing and processing will be further realized. International Perspective The field of research in photonic structures has been an international endeavor from its very incep- tion, with substantial efforts taking place within the United States, Europe, Japan, and most recently China and Taiwan. Figure 2-5 illustrates some of the general trends in research as measured by publi- cations. Using the ISI Web of Knowledge and the Science Citation Index, all publications with any of the following topics: photonic band structure, or photonic crystal, or inhibited spontaneous emission, or localization of photons, were identified and separated into the time intervals 1986-1996, 1996-2001, and 2001-2007. The number of publications is plotted according to country or region and by time interval. “Europe” as used here refers to England, Germany, France, and Italy, which are generally the most pro-

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 25 lific European countries working in this area of research. The data in Figure 2-5 are intended simply to provide a general picture of the activity in the area of photonic crystal research by country or region and over time. Obvious trends are the accelerating activity in this area (comparing the number of publications in the 10-year period from 1986 through 1995 to the number in the roughly 6-year period from 2001 to the present) and the recent dramatic rise in publication activity in the People’s Republic of China. It would be interesting (but probably more difficult) to similarly monitor the changing patent port- folios in this area. At present, there are few examples of commercial products based on photonic crystal technology, with the exception of photonic crystal fibers, which are produced by companies in Europe and in the United States (Crystal Fibre in Denmark and Newport in the United States). Commercial opportuni- ties may give rise to photonic crystal technology for enhanced light extraction in LEDs in the nearer term, although increased manufacturing costs and as-yet not fully proven enhancements will prove to be formidable barriers. 1800 1986-96 1600 1996-2001 1400 2001-2007 1200 Number of Publications 1000 800 600 400 200 0 USA JAPAN EUROPE ISRAEL CANADA RUSSIA CHINA S. KOREA Region FIGURE 2-5  Analysis of photonic crystal research by country or region between 1986 and 2007, from the ISI Web of Knowledge and the Science Citation Index. NOTE: “Europe” as used here refers to England, Germany, France, and Italy. 2-5

26 Nanophotonics Metamaterials—spatial Index Modulation at a Scale Less than a Wavelength Electromagnetic radiation exists from below the radio frequency (rf) to x-rays and above. However, this committee takes the field of nanophotonics to apply to the much more restricted range of frequen- cies spanning the infrared (IR) (~20 terahertz [THz] to 350 THz), the visible, and the ultraviolet (UV) spectral regions. In these regions the scale of the wavelength ranges from tens of micrometers to hun- dreds of nanometers, and consequently the size of the structures devised to manipulate this radiation is commensurate with developing nanoscale fabrication and integration technologies. Background We are accustomed to describing electromagnetic interactions with materials in terms of continuum constitutive relations (electric permittivity, ε; magnetic permeability, µ). Materials, of course, consist of atoms and molecules with a spatial scale much less than the optical wavelength, so these continuum approximations are appropriate. With some important exceptions, the permittivity and permeability are related primarily to the density of the material constituents and are relatively independent of their organization. The emerging field of metamaterials is largely concerned with the fabrication of individual structures, on a scale much less than the wavelength, with localized electromagnetic resonances and their combination into macroscopic materials with novel electromagnetic responses, for which an effective permittivity and permeability are appropriate descriptors. Nature provides a wealth of materials with a wide range of electromagnetic properties. Dielectric (nonconducting) materials such as oxides exhibit dielectric permittivities (over their respective trans- parency ranges) from about 2 up to about 3. Semiconductors typically have larger permittivities; from approximately 5 up to about 20. Metals have by far the largest available permittivities, because the free electrons in metals respond to screen an applied electric field; the metal permittivity is negative below the plasma frequency (which is in the UV for most metals) and can be quite large. For example, for gold (Au) at 5 µm, ε = –433 + i37, where the imaginary part is a result of electron-scattering processes in the metal. The metal ε is also quite dispersive, following a Drude model 1/ω dependence across the infrared, with a complex behavior, with more losses, in the visible and ultraviolet as a result of contributions from bound transitions in addition to the free-electron contribution (Shelby et al., 2001). In contrast to the wide diversity of electrical permittivity, there is no magnetic response (i.e., µ = 1) for all known materials. At lower radio frequncy and microwave frequencies, magnetic materials (ferrites) are available, but they involve collective excitations and therefore have limited frequency response. Over the wavelength range being considered here, there are no naturally occurring magnetic materials. Therefore, the emphasis of the effort in metamaterials has been to construct materials with a mag- netic response. Since there are no magnetic monopoles, the building blocks of magnetic materials are magnetic dipoles (subwavelength current loops). In order to get a large magnetic response at specific frequencies, it is often necessary to provide resonant structures (inductor-capacitor tank circuits). The first of these structures was the split-ring resonator (Pendry et al., 1999). The current state of this research is reviewed in the next subsection. Status The field of spatial index modulation began in the late 1990s with the theoretical prediction and the first demonstration of split-ring resonators with a negative permeability in the rf (Pendry et al., 1999; Shelby et al., 2001). The frequency of operation has been steadily increased, first to 1.2 THz and

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 27 then to the IR (Linden et al., 2004; Yen et al., 2004; Yen et al., 2005). Initially, there was some skepti- cism that continued scaling from the lower frequencies would be possible because of the extremely small dimensions involved. At higher frequencies, the inductance is dominated by the inertia (mass) of the free electrons, and the geometric loop structure is no longer needed. This advance has led to a d ­ ramatic increase in the ability to fabricate metamaterials; a simple metal-dielectric-metal structure with transverse dimensions less than the relevant wavelength provides a simple, manufacturable route to n ­ egative-­permeability metamaterials. As an aside, this structure is closely related to a gap-mode surface plasma wave, and there is a strong connection between metamaterials and plasmonics (see the section on “Plasmonics” in this chapter). A major driver of this technology has been the development of negative-index materials ­(materials with both a negative permittivity and a negative permeability). The negative permittivity is easy to accomplish with metals; the negative permeability is the difficult part. Recently, three experimental groups (two in the United States and one in Germany) have demonstrated negative-index materials using negative-permeability metamaterials (Dolling et al., 2006; Shalaev et al., 2005; Zhang et al., 2005b). The wavelength has rapidly advanced from near infrared (2 µm) to visible (800 nm). At present, the best results have been obtained with a “fishnet” structure in a stacked metal-dielectric-metal film (Chettiar et al., 2006; Ku and Brueck, 2007). Spatial Index Modulation Metamaterials: Anisotropy The classical prescription of Pendry to realize negative-index metamaterials is to construct resonant elements with negative electric susceptibilities (χE < 0) and magnetic susceptibilities (χM < 0). If the magnitudes of the susceptibilities and the number densities r of the elements are sufficiently large, then the electric permittivity e = e0 (1 + rE χE) and the magnetic permeability m = m0 (1 + rM χM) will both be negative, and a negative refactive index n = εµ can be realized. To avoid scattering of radiation, the resonant elements must be much smaller than the wavelength at which the metamaterial is to operate. Since metals have negative dielectric permittivity at frequencies below the plasma frequency, metallic nanowires can provide negative susceptibility. The depolarizing factors arising from their shape introduce resonances in their response, with the result that their susceptibility is very different—possibly even in sign—for electric fields parallel and perpendicular to their length. Since all known natural materials have positive magnetic permeabilities, negative magnetic sus- ceptibility can only be realized through a resonant response. Metallic split-ring resonators and similar structures, which function like LC circuits, can give rise to large negative magnetic susceptibility, but only near resonance and only when the magnetic field is perpendicular to the plane of the ring or ring- like planar structure. Both types of elements are inherently anisotropic; that is, their susceptibility depends on the orien- tation of the applied fields relative to the elements. Metamaterials consisting of regular lattices of such elements tend to be anisotropic. Anisotropy, which implies polarization dependence, is not desirable, but may be acceptable for some applications. It may be eliminated by incorporating elements with different orientation in the metamaterial, but the orientationally averaged susceptibilities of the elements may be far from ideal, and significant loss in performance may result. An alternate strategy for high-definition imaging has been proposed; it relies on anisotropy and may overcome the problem of losses (Jacob et al., 2006; Liu et al., 2007). The basic idea is to abandon nega-

28 Nanophotonics tive magnetic permeability, with its requirement of operating very near resonance and the high attendant losses. Instead, it is noted that evanescent waves occur when the magnitude of the wave vector carrying image information is greater than 2pn / lO. If the refractive index n could be made sufficiently large, then arbitrarily high resolution image information could be carried by the wave without the wave vector exceeding the limit of 2pn / lO and thus without evanescent decay. In uniaxial anisotropic media, there are two modes of propagation, with the dispersion relation for the extraordinary mode being k⊥ k||2 ω 2 2 + = ε || ε ⊥ c 2 where k┴ and k║ are components of the wave vector perpendicular and parallel to the optic axis. If one of the principal values of the dielectric tensor is negative, then the magnitude of k, that is, the refractive index n, may be arbitrarily large. Thus, anisotropic metamaterials, consisting of positive and negative dielectric components, such as oriented metallic nanowires in a dielectric host, should be capable of subwavelength imaging with modest losses. Issues To date, experimental metamaterials rely predominantly on metallic structures and current flow to produce the negative permeability, and the associated losses are too large to allow many applications. A figure of merit, –Re(n)/Im(n), has been introduced to capture the loss information. Table 2-1 presents the reported results. Fabrication is another major issue. To date, the demonstrations have all been in thin-film materials with a total thickness (for all three layers) of much less than a wavelength. Recently, a theoretical predic- tion suggested that a thicker stack of material (up to 10 layers) would have a lower loss and a dramatically improved figure of merit (Zhang et al., 2005c). No experiments have yet been reported. This is still a thin film, and it does not seem likely that the current approach will yield bulk materials, both because of the excessive losses and because of the difficulty of extending thin-film approaches to macroscopic scales. Two different fabrication techniques have been used to date: electronic-beam direct write and interferometric lithography (Dolling et al., 2006; Shalaev et al., 2005; Zhang et al., 2005a; 2005b). Direct write is a serial technology that is not scalable to large volumes of material. Interferometric lithography, as a simpler version of traditional optical lithography, is a large-area technique that is directly scalable to manufacturing volumes. Additional discussion of fabrication approaches is presented in Chapter 3. TABLE 2-1  Reported Metamaterials Experiments in the Near Infrared Spectral Region Figure of Merit Material Structure λ(µm) [–Re(n)/Im(n)] Reference Au/Al2O3/Au Two-dimensional perforated films 2.0 0.5 Zhang et al. (2005b) (symmetric) Au Metal line pairs 1.5 0.1 Shalaev et al. (2005) Au/Al2O3/Au Two-dimensional perforated films 2.0 1.0 Zhang et al. (2006a) (asymmetric) Ag/MgF2/Ag Two-dimensional perforated films 1.4 3.0 Dolling et al. (2006) (asymmetric fishnet)

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 29 An exciting new direction is the introduction of active materials (gain) and the integration of these negative-index materials with semiconductor and other gain media. The challenges are large as a result of the short range of the interactions and the nonradiative losses introduced by the close proximity of the gain media to the metal films. Impact New and improved optical materials have always led to advances in optical systems. Currently, the first tentative steps at realizing these materials are under way. As always, the materials are too difficult to work with and too lossy to realize the benefits. However, these are very early days in this process, and it is clear on the basis of analogies with other major advances in optical characteristics that there will be many new capabilities associated with these hitherto-unavailable characteristics. Some promising directions include nonlinear optics, subwavelength cavities and field concentration for both sources and detectors, imaging at scales much less than a wavelength, negative dispersion and dispersion compensa- tion, and many others. These are discussed at length in later chapters in this report. To date, most of the work on metamaterials has focused on the fabrication and demonstration of homogeneous materials. Recently, the Duke group demonstrated an inhomogenous metamaterial lens by systematically varying the structure of the metamaterial elements (Driscoll et al., 2006). Because the lens is fabricated with only few metamaterial layers, it is much more lightweight than traditional approaches. In another set of experiments, the same group has demonstrated the “cloaking” of electromagnetic radia- tion by arranging an inhomogenous array of metamaterial elements in concentric rings around an object (Schurig et al., 2006). These experiments point to exciting new directions for metamaterials and confirm the hypothesis stated above—new materials lead to new functionality and to new applications. The enhancement associated with subwavelength apertures will be of particular importance in mid- and long-wave infrared applications such as focal plane arrays. Room-temperature IR detectors are either very noisy as a result of large thermal dark currents in narrow band-gap semiconductor materials or very slow as in microelectromechanical systems (MEMS)-based microbolometers because of the thermal response of the isolated materials. In both cases, plasmonic antenna concepts offer revolutionary new capabilities. The dark current scales with the detector area and the noise scales as the square root of the area; thus, the figure of merit is the relative signal for a small detector versus a large-area detector divided by the square root of the area ratio. For microbolometers, the speed scales directly as the area (capacitance and thermal time constant) of the small elements. Box 2-1 and Box 2-2 provide examples of optical system advances made possible by improved optical materials. Plasmonics Plasmonics is a subfield of nanophotonics concerned primarily with the manipulation of light at the nanoscale, based on the properties of surface plasmons. Plasmons are the collective oscillations of the electron gas in a metal or a semiconductor. Rigorously, the plasmon is the quasi-particle resulting from the quantization of plasma oscillations, a hybrid of the electron plasma and the photon. Although p ­ lasmons are quantum mechanical in nature, their properties, most specifically with respect to the c ­ oupling of light to plasmon oscillations, can be described rigorously by classical electrodynamics. Surface plasmons (SPs) are the electromagnetic waves that propagate along metallic/dielectric interfaces; they can exist at any interface, and for any frequency region, where the complex dielectric constants of the media constituting the interface are of opposite sign and the sum of the dielectric constants are negative. SPs are supported by structures at all length scales. They largely determine the optical proper-

30 Nanophotonics BOX 2-1 Prospects of Far-Field Imaging with the Superlens One of the most compelling aspects of nanophotonics is the possibility of high-resolution imaging, as illustrated in Figure 2-1-1. FIGURE 2-1-1  The superlens. SOURCE: Reproduced with permission of Melissa Thomas. The superlens, proposed by Pendry, relies on the negative refractive index that can be ­realized using metamaterials. The possibility of subwavelength resolution has captured the popular imagi- nation, as well as the attention of device developers. There are two fundamental challenges, how- ties of metal-based nanostructures. In fact, the first quantitative theoretical success of electromagnetic theory was the explanation of the preferential absorbance of green light by gold nanoparticles, impart- ing an intense red color to the material (glass) in which they are embedded, known historically as ruby glass. The field of plasmonics is based on utilizing SPs for a large variety of tasks by designing and manipulating the geometry of metallic structures and consequently their specific plasmon-resonant or plasmon-propagating properties. While many metals support SPs, gold and silver have thus far dominated experimental work in this area. Within this rapidly developing and highly multidisciplinary field, several key research directions have been established. These range from the propagation of signals and information on metal-based waveguides, to enhanced sensing and spectroscopies for the chemical identification and detection of

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 31 ever, to realizing superlenses capable of high-resolution far-field imaging: loss and ­impedance matching. These challenges pose critical obstacles to practical device ­development. For a metamaterial slab, imaging as shown in Figure 2-1-1, the image distance + object dis- tance are equal to the slab thickness (L). The use of a macroscopic object in Figure 2-1-1, the coffee cup, suggests that this structure will work for L >> λ; as it turns out, this is a mis­perception. Losses in metamaterials are characterized by the figure of merit, FOM ≡ –Re(n)/Im(n), and the attenuation factor for light traversing the slab is e–4pL/lFOM. The highest-performance optical metamaterials today are those with fishnet structure (Dolling et al., 2006) with FOM ~3; hence, even for a slab with a thickness equal to the wavelength, more than 98 percent of the light is absorbed. For imaging objects more than a few wavelengths away from the slab, the attenuation is absolutely prohibitive. Another formidable problem is that of impedance matching. Any mismatch between the ­mpedance of the host (usually air) and the impedance of the metamaterial (e.g., η and η + δ) i gives a limit on the spatial bandwidth (Smith et al., 2003). The maximum resolution enhance- λ  δ ment R = l / lmin of the lens is R = – ln   , where L is the thickness of the slab. For a 2pL  η  slab of thickness equal to the wavelength, a resolution enhancement of R = 2 requires that δ < 3.5 × 10–6!—and since the scaling is exponential, for a slab of thickness 2λ, the requirement is that δ < 10–11!! Since negative-index materials are inherently dispersive, the range of ­frequencies for which this condition can be satisfied becomes vanishingly small, along with the prospects for transmitted information content. In contrast, for a slab of thickness λ/10, the constraint is only δ < 0.28. The same constraints hold in the case of losses; that is, where d is imaginary. This indicates that, with materials comparable to the best-performing materials achieved to date, subwavelength resolution is only possible for thin (L < l) slabs and in the near field, at distances from the slab comparable to l. For far-field imaging, which requires large phase shifts and large slab thicknesses of L >> l, the constraints on impedance matching with practical finite bandwidth sources such as typical lasers are unachievable. Finding 2-1. The committee finds that, in spite of their enormous appeal, beyond the diffraction limit, near-“perfect” slab lenses, which image in the far field, do not appear to be feasible, barring some unforeseen breakthrough. biomolecules or biological agents, to near-field optics and scanning microscopies employing ­metallic probe tips, to enhanced absorption and fluorescence processes in solid-state detectors and devices, to molecular systems, active plasmonic devices, and biomedical applications. Fabrication methods in p ­ lasmonics include both top-down and bottom-up strategies using clean-room and chemical techniques, also spawning novel hybrid fabrication approaches that combine both wet and dry fabrication tech- niques. Quantitative and computationally intensive electromagnetic modeling has assumed a dominant and increasingly important role in this field, employing fully numerical methods such as finite element, boundary element, and finite difference-time domain approaches; analytical or semianalytical methods such as plasmon hybridization and discrete dipole approximation; and lumped circuit concepts for the design of complex plasmonic systems.

32 Nanophotonics BOX 2-2 A Cloak of Invisibility Metamaterials provide new ranges of optical properties, permittivity (ε) and permeability (µ), that can be combined in almost arbitrary configurations using emerging fabrication capabilities. Much attention has been paid to the negative refractive index (n = ± εµ ) that arises when both ε and µ are negative, and to the consequences of this negative n for traditional optical elements such as prisms and lenses (see Box 2-1). Metamaterials offer degrees of freedom in optical design that are not possible with naturally occurring materials. Many of these are combined in the concept of optical cloaking, shown by a ray-tracing analysis in Figure 2-2-1, that was first introduced by Pendry et al. (2006). FIGURE 2-2-1  (A) Ray-tracing of a cloak extending from radius R1 to radius R2 around a spherical object, (B) showing the light bending around the cloaked region (radius <R1) without penetration of the object and exiting from the cloak as if both the object and the cloak did not exist. SOURCE: Pendry et al. (2006). Reprinted with permission from AAAS. Localized Surface Plasmon Resonance Sensing The excitation of conduction electrons by light is denoted as a surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures. The plasmon-resonant frequency is determined by the dielectric properties of the metal, and specifically for nanoscale metallic structures by the size, shape, and local environment of the nanostructure. In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Localized Surface Plasmon Resonance Sensing” for brief descriptions of the work of researchers in this field who have developed most of the concepts discussed in this section.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 33 The cloak is based on a systematic variation of the optical properties (ε and µ) of the meta- material in order to deflect the radiation around the object while at the same time eliminating reflections from the outer surface of the cloak. That is, at the outer edge of the cloak, the meta- material impedance is matched to air; internal to the cloak, the electromagnetic waves are bent around the object. The spatial variation of the optical properties that gives rise to this functionality is (Pendry et al., 2006). R1 < r < R 2 R 2 (r − R1 ) 2 εr = µ r = R 2 − R1 r R2 εθ = µ θ = εφ = µ φ = R 2 − R1 r > R2 ε = µ =1 Note that since ε and µ are equal everywhere, the impedance is unity everywhere, and there are no reflections at any of the boundaries. At the inner boundary of the cloak, both εr and µr go to zero. This requirement of having components with the optical properties <1 is characteristic of the cloak, since the path length around the object is physically longer than the path length through the space occupied by the object, requiring a phase velocity greater than that in the space. Causality then requires that the cloak be dispersive, setting a bandwidth constraint on the cloaking that may pose an issue for practical applications. In a first experiment, Schurig et al. (2006) demonstrated a cylindrical cloak using split-ring resonators with a somewhat modified variation of the optical parameters (0.003 < µr < 0.28) and showed good agreement with the model. Since cloaking does not depend as critically on the resonant conditions as the perfect lens does, it appears to be more tolerant of small deviations from the ideal conditions (Cummer et al., 2006). Metamaterials offer a much wider range of optical properties than those available from natural materials. Scientists have always been able to exploit new materials to provide new functional- ity. It would seem that the present case is no exception and that many exciting applications of metamaterials have yet to emerge. Plasmon-resonance-based chemical sensing can be accomplished in a variety of ways. Historically, surface plasmon propagation (SPP) on functionalized continuous metal films was exploited. Modifica- tions in the chemical environment due to the binding of molecules to the functionalized film can be moni- tored as changes in the angle of incidence required for SP excitation in an evanescent coupling geometry. More recently, metal nanostructures and nanopatterned surfaces have also been used as nanoscale SPR sensors, both in solution and immobilized on surfaces. In SPR spectroscopy, the wavelength shift of the plasmon resonance is monitored while the refractive index of the medium surrounding the metal is changed. These LSPR shifts, usually reported in eV/RIU (shift in photon energy per change in refrac- tive index unit), help quantify their applicability as biological and chemical LSPR sensors. Another

34 Nanophotonics I II RICE 15.0 kV X100,000 100 nm WD 10.1 mm III III IV V V FIGURE 2-6  Various nanoparticles used in localized surface plasmon resonance sensing applications: (I) Gold nanostars (Hafner group, Rice University), Copyright 2006 American Chemical Society; (II) Silica-gold nanoshells (Halas group, Rice University) Permission from Rice University; (III) Atomic force microscopy image of silver 2-6 triangles using nanosphere lithography (Van Duyne group, Northwestern University)reprinted with permission from the Annual Review of physical Chemistry, Copyright 2007; (IV) Nanocube particles (Xia group, University of Washington, and Van Duyne group, Northwestern University) Copyright 2006 American Chemical Society; (V) Nanorice nanoparticles (Halas group, Rice University) Copyright 2006 American Chemical Society. In A ­ ppendix D, see the section “Localized Surface Plasmon Resonance” for information on the research groups named. SOURCES: Reprinted with permission from Nehl et al. (2006); Willets and Van Duyne (2007); Sherry et al. (2005); Wang et al. (2006b). dimensionless figure-of-merit criterion for this sensing application is the ratio of the LSPR shift to the linewidth of the plasmon resonance (see Figure 2-6). The recent interest in the field can be attributed to large LSPR shifts reported for a variety of nano- structures such as silver and gold colloid, silver triangles deposited using nanosphere lithography (pat- terning using self-assembled arrays of microparticles as a shadow mask for metal deposition), (Hulteen and Van Duyne, 1995) gold nanoshells, (Tam et al., 2004) nanostars, (Nehl et al., 2006) nanorice, (Wang et al., 2006b), and so on. The high sensitivity has also led to real-time sensitive detection of binding Ibid.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 35 TABLE 2-2  Various Nanoparticles Used for Localized Surface Plasmon Resonance Sensing SOURCE: Reproduced, with permission from Future Medicine Ltd., from Liao et al. (2006). events studied using LSPR spectroscopy. LSPR spectroscopy is used for biological and chemical sens- ing by transducing changes in the local refractive index via a wavelength-shift measurement. Table 2-2 (Liao et al., 2006) lists various nanoparticles used for LSPR sensing. Surface-Enhanced Spectroscopy When an electromagnetic wave interacts with a roughened metallic surface or a metallic nanoparticle film, the electromagnetic (EM) fields in the vicinity of the surface or nanoparticle are greatly enhanced as compared to the incident EM field. This phenomenon has been attributed to the excitation of SPs at the metallic interface. This enhanced field has been exploited to enhance various vibrational and electronic spectroscopic signatures of molecules adsorbed onto the metallic surfaces or that are in close proximity to the surface, and is collectively known as surface-enhanced spectroscopy (SES). The best studied of these is surface-enhanced Raman spectroscopy (SERS). Surface-enhanced spectroscopy also includes surface-enhanced fluorescence (SEF), surface-enhanced infrared absorption spectroscopy (SEIRA), and other surface-enhanced nonlinear optical spectroscopies such as surface-enhanced second harmonic generation and surface-enhanced sum frequency generation (see Figure 2-7). In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Surface-Enhanced Spectroscopy.”

36 Nanophotonics (a) FIGURE 2-7  (a) Schematic representation of tip-enhanced Raman spectroscopy (TERS). (b) A silicon cantilever is coated with a 40 nm layer of silver and used for TERS. SOURCE: Adapted with permission from Hayazawa et al. (2003). Copyright Elsevier, Chemical Physics Letters, 2003. 2-7 Surface-Enhanced Raman Spectroscopy (SERS) The field of surface-enhanced Raman spectroscopy was pioneered by the Van Duyne group at North- western University (Jeanmaire and Duyne, 1977) very low res image the University of Kent (Albrecht and this is a and Creighton at Creighton, 1977) when they discovered so maybeRaman signal from molecules attached to roughened that the it should remain at this size silver electrodes demonstrates a large enhancement of the Raman scattered intensity. Enhancement ­ actors of 106-fold intensity have been observed over normal Raman scattering. This large enhancement f is understood to be the product of two major contributions: (1) an EM enhancement mechanism and (2) a chemical enhancement mechanism (CHEM). Since the original discovery, aggregated gold and silver nanoparticles have been used as efficient nanoantennas to focus the incident light and enhance the EM fields in the vicinity of the nanoparticles. Enhancement factors of up to 1014 in SERS signal of dye molecules, attained using aggregated metal nanoparticles, have led to single-molecule detection using SERS (Kneipp et al., 1997; Nie and Emory, 1997). For the observation of SERS, a strong correlation has been observed between the SERS excitation and the SPR maximum (Jackson and Halas, 2004; McFarland et al., 2005). Individual silver and gold colloid particles exhibit strong plasmon resonances in the visible parts of the spectrum. Aggregated colloid particles have been shown to have plasmon resonances that are redshifted and lead to localized areas of intense local fields, or “hot spots.” These localized hot spots give rise to the high enhancement factors that make single-molecule detection possible. In addition to aggregated colloid, many groups have developed robust substrates for SERS. A variety of shapes and geometries have been explored as SERS substrates, including metal island films (Jennings et al., 1984), large silver and gold colloid (Michaels et al., 2000), silver triangle arrays (Haynes and Van Duyne, 2003), silver and gold nanoshells (Jackson and Halas, 2004), and fractal silver films (Drachev et al., 2004). In addition to the experi­mental develop- ment of optimal SERS substrate, there has been a parallel effort to understand the physical mechanism behind SERS.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 37 The SERS substrates made with gold and silver colloid produced large enhancement factors in small localized areas, but the heterogeneity of the hot spots makes quantitative measurements unreliable. For the past few years there has been great emphasis on rationally designing substrate geometries to achieve large enhancement factors. In many applications, where single-molecule detection is not required, substrates are being designed to optimize between the enhancement factors and are achieving a dense coverage of the adsorbate molecules to be detected. Simultaneously, the development of scanning spectroscopic techniques with subwavelength resolution allows a very small area to be imaged on any substrate. SERS is rapidly maturing as a spectroscopic tool. This has led to application of SERS in many direc- tions, most notably for the detection of chemical and biological molecules. Some of these applications are discussed in greater detail in later sections in this chapter. Surface-Enhanced Infrared Absorption (SEIRA) Molecules adsorbed on metal island films or metallic nanoparticles exhibit 10 to 1,000 times more intense infrared absorption than would be expected from conventional measurements without the metal. This effect is referred to as surface-enhanced infrared absorption. The enhanced field due to the SPRs supported by the metal plays predominant roles in enhancing the absorption of light by the attached molecules (Chang et al., 2006). The chemical interactions of the molecules with the surface can give additional enhancement (Huo et al., 2005). SEIRA is a complementary spectroscopic technique to SERS. SEIRA is being used as a molecule-specific technique for qualitative and quantitative chemical sensing and catalysis research (Ayato et al., 2006). Surface-Enhanced Fluorescence (SEF) Fluorescence is the emission of photons as a molecule relaxes from an excited electronic state to the ground state. The presence of a vicinal metallic nanostructure to a fluorophore strongly influences both the radiative and nonradiative decay of the fluorophore and its lifetime. The influence on the radiative rate, nonradiative decay rate, and lifetime of the fluorophore also depends on the distance between the metallic surface and the fluorophore. This discovery has led to the radiative decay engineering of SEF (Lakowicz et al. 2002), which offers new ways to increase the intensity of low-quantum-yield fluoro- phores and to improve the stability of fluorophores that can easily photobleach. Most of the applications of SEF have been in improving the fluorescence intensity of amino acids, oligonucleotides, and dye molecules used in imaging biological samples. Techniques for Imaging and Spectroscopy of Plasmonic Structures The development of advanced plasmonic devices is inseparable from the development of techniques to probe the properties of such devices. Applications of plasmonics range from macroscopic applications in thermal signature management and SPR sensing of biological and chemical agents; to microscopic studies of SERS, surface-enhanced fluorescence, and far-field microspectroscopy of isolated single nano- structures; to nanoscale probing of the detailed propagation of plasmons on micro- and nanostructures using near-field optical microscopy. Major techniques for the imaging and spectroscopy of plasmonic structures, in order from macro to nano, include macroscopic absorbance spectroscopy, dark-field In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Techniques for Imaging and ­Spectroscopy of Plasmonic Structures.”

38 Nanophotonics microcroscopy and microspectroscopy, confocal microscopy, photothermal microspectroscopy, near- field optical microscopy, two-photon induced photoemission microscopy, and cathodoluminescence, as shown in Figures 2-8 through 2-10. The development of imaging and spectroscopy techniques is also critical for the development of sensors based on SPs, as some method of reading out the information from the sensor is required. Macroscopic absorbance spectroscopy employs a grating monochromator and an incoherent light source (quartz-tungsten-halogen, deuterium, and arc lamps are typically used) to measure the transmis- sion of structures containing plasmon-resonant nanostructures. This technique is applied to studying the properties of nanoparticles produced in bulk (i.e., with wet chemistry), large-area substrates for surface- enhanced spectroscopy, and as readout in LSPR sensing (see Figure 2-11). FIGURE 2-8  Dark-field microscopic image of a mixture of silver particles with a variety of shapes. NOTE: The particles appear to be different colors due to the dependence of the plasmon resonance wavelength on particle g ­ eometry. SOURCE: Orendorff et al. (2006). Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA. Repro- duced with permission.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 39 FIGURE 2-9  Illustration of the scattered light from gold nanoparticles using different Gaussian modes for excita- tion to determine particle orientation. Images on the left show scattering from a spherical Au nanoparticle and on the right, an Au nanorod. Three different types of excitation beam are employed: (a) and (b) use a linearly polarized Gaussian beam; (c) and (d) use an azimuthally polarized doughnut mode; (e) and (f) employ a radially polarized doughnut mode. The nonspherical nature of the nanorod can clearly be seen in (d) and (f). SOURCE: R ­ eprinted with permission from Failla et al. (2006). Copyright 2006 American Chemical Society. 2-9 FIGURE 2-10  Photon scanning tunneling microscope (PSTM) image showing the propagation of light along a metal nanowire. The wire acts as a resonant cavity, and the resulting nodes are visible in the PSTM ­image. (a) Illustration of the scheme employed to excite the wire, (b) far-field microscopic image of the wire—the right spot on the left is the excitation light and the ­emitted light from the end of the wire is indicated with an ­arrow; (c) PSTM image of the end portion of the wire; (d) the standing wave pattern seen along the length of the wire. SOURCE: Reprinted with permission from Ditlbacher et al. (2005). Copyright 2005 by the American Physical Society. 2-10

40 Nanophotonics FIGURE 2-11  Plasmon propagation on a silver film measured using time-resolved photoelectron emission m ­ icroscopy (PEEM). Upper portion of (a): Topography of the sample where the groove used to excite sur- face plasmons is clearly visible; (a) through (f): Images of the plasmon propagation at increasing delay times: (g): Experi­mental propagation of the plasmon wave measured from the images in (a) through (f); (h): Results from a simulation of the plasmon propagation. SOURCE: Reprinted with permission from Kubo et al. (2007). Copyright 2007 American Chemical Society. 2-11 Dark-field microscopy allows the background-free observation of scattering from plasmon-­resonant nanoparticles or nanoholes. The sample is illuminated with light at high numerical aperture (large angles), and only the light scattered at smaller angles is collected. Direct reflection off the surface will occur at equal angles to the incident light and therefore will not be collected. Only light that is scattered by the particles into the objective will be captured. The scattered light can then be coupled into a spec- trometer, allowing the scattering spectra of single nanostructures to be measured. The downside to this technique is that, unlike the case with near-field approaches, the diffraction limit applies, and therefore objects must be spaced at least on the order of a micron apart. Determining the geometry of the par- ticles measured on subwavelength scale can still be achieved by using an indexed substrate and another m ­ icroscopy technique such as scanning electron micrographs (SEMs) to image the nano­structure. Despite its downside, this technique has been quite successfully employed to study the scattering properties of the plasmon resonance of small Au and silver (Ag) nanospheres, nanorods, holes in thin metallic films, and Au nanoshells. Further, this technique has been applied to study SPR shifts of single nanostructures and may ultimately allow the observation of single-molecule binding events (Ditlbacher et al., 2005; Nehl et al., 2004; Sönnichsen et al., 2002).

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 41 Confocal microscopy has recently been demonstrated in the Meixner group at Eberhard-Karls- Universitat Tubingen, Germany, to be usable for determining the orientation of nonspherical metallic nanoparticles. This is of importance, as previously either electron microscopy or topographic imaging with an atomic force microscope or near-field scanning optical microscope (NSOM) tip was required to determine the orientation (Failla et al., 2006). In a related technique called spatial modulation s ­ pectroscopy (SMS), by dithering the sample position and employing a coherent supercontinuum source for illumination, it is also possible to obtain directly the absorbance spectrum of small nanoparticles (Muskens et al., 2006). Confocal microscopy is also frequently employed for surface-enhanced Raman spectroscopy (see the subsection above entitled “Surface-Enhanced Spectroscopy”). Photothermal heterodyne imaging allows for the measurement of the absorbance spectrum of very small particles using far-field excitation. In this technique, a laser resonant with the plasmon resonance of the nanoparticle is used to heat the particle, which in turn heats up the surrounding medium. Due to the temperature dependence of the index of refraction of the medium, a process called thermal lensing, a second laser beam is scattered off of the larger “particle” formed by the changed refractive index. Essen- tially, this process results in converting the problem of measuring the absorbance of a very tiny particle to one of measuring the scattering of a much larger particle. This process has been used to measure the size-dependent plasmon resonance of Au nanospheres as small as 1.5 nm in diameter (Berciaud et al., 2004, 2005). Using this technique, the plasmon resonance of these tiny particles can be clearly seen in biological samples, enabling their use as a contrast agent free of blinking of photobleaching effects that plague fluorescence from molecules or quantum dots (Lasne et al., 2006). SPs on smooth films cannot be excited directly with light, owing to the mismatch in momentum. One classic solution to this problem is to use a prism on the bottom side of the film to excite a plasmon on the top surface of a film (the Kretschmann geometry). In this way the photon wave vector can be increased by a factor of the index of refraction of the prism material and thereby matched to a propa- gating SP polariton mode of the metal film-air interface. This same effect occurs in reverse when metal films on a dielectric substrate are used as a SP waveguide. While usually this would be a detrimental effect to the performance of the waveguide, it can also be used to observe the propagation of SPs in a technique called surface leakage radiation microscopy (Drezet et al., 2007). Due to its recent commercialization, NSOM is rapidly becoming the tool of choice for the routine study of the optical properties of nanoplasmonic structures and devices such as plasmonic waveguides. Currently the two main vendors of near-field optical microscopes are Nanonics Imaging, Ltd., in Israel (bent tapered fiber tip) and the German firm WITec (microfabricated cantilever probes). The basic point of near-field imaging is to create or capture light with larger wave vectors than are allowed to propagate in free space. The resolution that can be obtained in optical imaging is essentially limited by the range of wave vectors that can be employed in the measurement of an image. This mea- surement can be accomplished by using a small aperture or scattering particles either to confine light to a small spatial volume or to collect light from a small region. NSOM can be broadly separated into two main categories: apertured and apertureless techniques. The latter technique relies on scanning a nanoscale scattering object, usually a metalized atomic force microscope tip or small metal particle, over a surface illuminated from the far field with a highly focused laser beam. The scattered light is then collected and analyzed. This technique is used for tip-enhanced spectroscopy, such as TERS, because the plasmon excited in the tip creates a large, highly localized near field at the end of the tip. Such a tip can also be used to scatter evanescent waves, such as those from propagating plasmons on surfaces or waveguides, into the far field to be detected (Huber et al., 2005). Apertured NSOM relies on the confinement of light by a small hole in a metalized tip. Tips can either be made by tapering a standard optical fiber to a fine point and metallizing the outside to define

42 Nanophotonics an aperture or by microfabrication of a cantilever with a triangular point that is metallized on the outside to form an aperture. While the resolution obtained with apertured NSOM can approach 10 nm, typi- cally 100 nm apertures are employed as a reasonable trade-off between decreased spatial resolution and increased signal relative to smaller tips (Hecht et al., 2000). Owing to relative experimental ease, the illumination mode is the more common variation of apertured NSOM. For experiments involving wave- guides or active structures, collection-mode NSOM or photon scanning tunneling microscopy (PSTM) is the technique of choice, as either of these allows the field extending from the plasmon propagating along a waveguide or device to be imaged directly. Two-photon photoelectron emission microscopy (PEEM) imaging can be used for ultrafast-phase resolved imaging of surface plasmon propagation. This recently developed technique is very promising for the study of SPP, as it can directly resolve the propagation of plasmons in time and space to extract not only the decay length, but also the relative phase of the plasmon mode. By using phase-locked pairs of 10 femtosecond (fs) ultrafast pulses in a pump-probe experiment with +/– 25 as delay resolution, and collecting the resulting two-photon photoemission, the propagation of surface plasmon polariton wave packets can be measured with 60 nm spatial and 10 fs temporal resolution (Kubo et al., 2007). The experimental complexity of such an apparatus means that this will remain a specialized technique; it is nonetheless important, as it is currently the ultimate measuring tool for SPP. In cathodoluminescence spectroscopy, an electron beam is used to directly excite plasmons in a metallic nanostructure. While light can only efficiently excite dipole-active plasmon modes, electrons are far less restricted. For example, an electron beam can directly excite plasmons in a metal film. Very high resolution can be obtained using this technique, because the electron beam creates a point source of SPs on the order of the beam diameter, potentially allowing structures to be studied on sub-10 nm scales. This technique was recently demonstrated by measuring the length of SPP on Au and Ag films (van Wijngaarden et al., 2006); it can potentially can be used to study plasmonic waveguides directly where direct excitation from the far field is cumbersome owing to the low resolution obtainable with such a technique. Extraordinary Transmission, Subwavelength Holes In 1998 Thomas W. Ebbesen and his colleagues published some remarkable results concerning how much light is transmitted through an array of holes having a diameter smaller than the wavelength of light (Ebbesen et al., 1998). They found that at certain wavelengths of incident light, such an array of holes is quite transparent, yet it had long been predicted by Hans Bethe that the light transmitted through a single subwavelength hole should be negligible (Bethe, 1944). However, not only was the transmission greater than the theoretical predictions, but in fact the amount of light transmitted was about twice as much as was incident on the total surface area occupied by the holes. Ebbesen et al. (1998) suggested that SP polaritons, or charge-density waves propagating on a metal surface, were responsible for the extraordinary transmission. See Figure 2-12, which shows various NSOM images. A laser illuminates the bottom side of the Au film while the NSOM tip is scanned over the top side. Interference between the light transmitted through the film and the surface plasmons generated on the top surface by the presence of the hole creates fringe patterns characteristic of the propagating surface plasmon wave- length. This experiment concentrated on the demonstration of the coupling between a normal-incident optical beam and surface plasmons provided by a subwavelength hole. Although the interhole spacing was deliberately too large for efficient coupling into a resonant surface plasmon mode, an eight-fold enhanced transmission compared to a bare film was observed, and the images provide direct evidence of the excitation of SPs in a nanohole array system (Gao et al., 2006). These images confirm the pres-

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 43 FIGURE 2-12  Scanning electron micrographs of a single nanometric hole in a gold film (a) and of a nanohole array (d), and near-field scanning optical micrographs of a single nanohole (b, c) and of nanohole arrays (e, f). SOURCE: Reprinted with permission from Gao et al. (2006). Copyright 2006 American Chemical Society. 2-12 ence of SPs in the nanohole array system and provide direct evidence for their role in the extraordinary transmission phenomenon. Some controversy has accompanied the assertion that surface plasmons are responsible for the extraordinary transmission effect. Papers published subsequent to Ebbesen (1998) presented results confirming the SP model (Ghaemi et al., 1998; Martín-Moreno et al., 2001). Some studies, however, countered this view. For example, Treacy (1999) suggested that ordinary diffraction effects might play a major role in the extraordinary transmission. Phillippe Lalanne’s group then argued that their models indicated that SPs should actually suppress the extraordinary transmission effect and thus could not be the cause of it (Cao and Lalanne, 2002). Despite these differences, the majority opinion was that SPs indeed were the dominant effect in the extraordinary transmission model. However, in a 2004 publication (Lezec and Thio, 2004), two of the authors of the original 1998 paper in Nature changed their stance on this issue. They showed experimental results of extraordinary transmission through nonmetallic films, and since only metals can support SPs, they argued that the SPs were not responsible for the extraordinary transmission. They further proposed a new In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Extraordinary Transmission, Subwave- length Holes.”

44 Nanophotonics model based on what they called composite diffracted evanescent waves (CDEWs), and they showed that their model agrees with experimental data. This model has been received with skepticism, however, and Philippe Lalanne’s group has offered results showing that the CDEW model may not be sufficient to explain extraordinary transmission, and that SPs are still the likely candidate (Lalanne and Hugonin, 2006). Figure 2-13 shows a scanning electron micrograph of two regions of small dimples in a silver film arranged in square arrays. Some of the dimples were milled completely through the silver film to make holes in a pattern forming the letters “hν.” Due to the extraordinary transmission phenomenon, the holes permit light of a color determined by the array’s period (center-to-center hole or dimple spacing) to be transmitted through the silver film when white light is shone on the back of the film. The array contain- ing the “h” has a period of 550 nm, allowing red light to be transmitted, while the array containing the “ν” has a spacing of 450 nm, allowing only green light to be transmitted (pictured in the inset). This demonstrates the ability to tune the wavelength of the transmitted light and suggests great potential for optical filtering applications. Regardless of which theory best explains the extraordinary optical transmission phenomenon, sub- wavelength holes and subwavelength-hole arrays have many practical applications. First, because the wavelength of the transmitted light in a hole array is dependent on the period (or interhole spacing), hole arrays can serve as optical filters in which the allowed transmission can be tuned by changing the hole spacing (Genet and Ebbesen, 2007). Arrays of holes also offer potential for display devices. In 1999, Tineke Thio’s group (Arinna, LLC) showed that the wavelength and intensity of transmission through hole arrays could be changed elec- tooptically by immersing the hole array in a liquid crystal matrix and varying the applied voltage (Kim et al., 1999). This structure simultaneously combines the functionality of the crossed polarizers, the FIGURE 2-13  Scanning electron micrograph of two regions of small dimples in a silver film arranged in square arrays. SOURCE: Reprinted by permission from Genet and Ebbesen (2007). Copyright 2007 by Macmillan Pub- lishers Ltd. 2-13

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 45 liquid-crystal modulator, and the color filters in traditional liquid-crystal displays, offering the potential of enhanced efficiency and simpler manufacturing. Hole arrays also offer promise in the realization of all-optical switching components. Thomas W. Ebbesen’s group (Institut de Science d’Ingeniere Supramoleculaire, Universite Louis Pasteur, Strasbourg, France) has recently performed experiments which show that putting molecules inside of the hole arrays allows for terahertz-speed all-optical switching of the refractive index (Dintinger et al., 2006). Optical components such as these are essential for the realization of useful optical circuits. Recently, attention has turned to alternatives to simple hole arrays. C-apertures have been investi- gated by the Stanford University group of Hesselink, and annular (coaxial) apertures have been shown to provide significantly improved transmission, particularly on high-index substrates (Fan et al., 2005). Another exciting direction is the incorporation of nonlinear materials inside the aperture. Coupled with the large fields associated with the plasmonics, very large second-harmonic signals, comparable to those generated in much longer conventional nonlinear media such as lithium niobate, have been observed (Fan et al., 2006). Nonlinear plasmonics holds potential for dramatically extending the domain of optical nonlinearities and possibly eliminating the need for extended phase matching, making the equivalent of radio-frequency mixers available throughout the optical spectrum. Plasmonic Waveguides and Other Electromagnetic Transport Geometries On-chip optical data transfer could greatly enhance computation. However, in order for this technol- ogy to be feasible, light must be confined and routed in dimensions smaller than its own wavelength to allow for sufficient miniaturization of chips employing this technology. To date, SP waveguides exhibit relatively high losses, and these losses become worse as the wavelength is decreased to the near infrared (NIR) and visible—the important wavelengths for the application to on-chip communications. Nonethe- less, this is a very active area of research because of the importance of the application, and innovations are being introduced at a rapid pace. Several geometries of plasmon waveguides exist, as discussed in the following subsections. Metal Stripe Waveguides One possible geometry of plasmon waveguides is a long, thin metal stripe. In 2000, Pierre Berini’s group (School of Information Technology and Engineering, University of Ottawa, Canada) first dem- onstrated long-distance plasmon propagation in such a geometry (Charbonneau et al., 2000). While a very large propagation length (3.5 mm) was achieved at a wavelength of 1.55 μm, the width of the metal stripe was 8 μm wide, which is not subwavelength confinement. However, early experiments indicated that subwavelength confinement and propagation on such structures may be possible. For example, Franz Aussenegg’s group was able to get propagation over several micrometers in a wire that was 200 nm wide (Krenn et al., 2002). This group also showed that the propagation lengths depended heavily on the width of the metal stripes (Lamprecht et al., 2001), as demonstrated in Figure 2-14. Mark Brongersma’s group (Geballe Laboratory for Advanced Materials, Stanford University) con- structed a model explaining the dependence of the propagation length on the width of the metal stripe (Zia et al., 2005) and pointing out the differences between leaky SP modes and bound guided modes as well as the difficulty involved in distinguishing between the two. In Zia et al. (2005), the group predicted In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Plasmonic Waveguides and Other Electromagnetic Transport Geometrics.”

46 Nanophotonics FIGURE 2-14  Near-field micrograph of plasmon propagation down a metallic stripe waveguide. (Left) Width of stripe: 1.25 µm. (Right) Width of stripe: 3.5 µm. Light is launched down the waveguide from an illumination source below the structure and photon scanning tunneling microscopy images the plasmon propagating down the stripe. These images demonstrate that the distance which the light propagates down the stripe is dependent on the 2-14 stripe’s width. For a wider stripe, a longer propagation length can be achieved. SOURCE: Reprinted with permis- sion from Zia et al. (2006). Copyright 2006 by the American Physical Society. that at a certain width of metal stripe, no guided wave modes can exist. In a subsequent, detailed experi- mental study (Zia et al., 2006), the group was able to show that indeed, as the metal stripe waveguide becomes narrower, one can predict the finite number of guided modes and the stripe width at which each mode will be cut off. Zia et al. (2006) found that at a certain stripe width, no more guided modes exist, and that propagation on the stripe is thus very limited. These findings suggest that while the metal stripe waveguide can support long propagation lengths for large stripe widths, it is not useful for the purpose of subwavelength confinement. Metal Nanowire Waveguides In 2000, Robert Dickson’s group at the Georgia Institute of Technology reported observing plasmon propagation down very long, chemically prepared silver and gold nanorods where the transverse dimen- sion of the rods is less than 100 nm and the longitudinal dimension is ~4 μm. The group was able to couple light in with a diffraction-limited laser spot on one end of the rod and to observe the other end of the rod light up, proving that plasmons were in fact propagated along the rod (Dickson and Lyon, 2000).

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 47 FIGURE 2-15  Micrographs showing propagation of light down silver nanowires. (A) A laser is focused to a dif- fraction-limited spot and positioned at the bottom of the end of a silver nanowire. Light is then propagated down the wire via the SP and coupled out of the nanowire at the top end. This demonstrates that a metal nanowire is an effective plasmonic waveguide. (B) The nanowire is the same as in A, but the light is coupled into the top end of the wire and out of the bottom end, illustrating that light can be propagated in either direction. (C) A silver wire is used; light is coupled into the left end. (D) The wire is the same as in (C), but now the laser is focused onto the 2-15 center of the wire. Here, a plasmon is not excited, and thus light is not guided down the wire. SOURCE: Reprinted with permission from Sanders et al. (2006). Copyright 2006 American Chemical Society. Graff et al. (2005) also saw this in 2005. This group had actual silver wires that were 25 nm wide and ~50 μm long. They used a similar method of coupling the light into the wire, but embedded the wire in a fluorescent matrix. Light propagated down the wire excited the fluorophore, allowing the researchers to observe that the plasmon was propagating for about 15 μm down the wire (see Figure 2-15). Recently, Mark Reed’s group also performed experiments showing the propagation of SPs down silver nanowires (Sanders et al., 2006). However, this group was also able to observe that plasmons are coupled back into free space at sharp bends in the wire and at places where the wire branches into multiple paths. They also observed that at such bends in the wire, the plasmon continues to propagate down both paths of the wire, suggesting an optical splitting component. Nanoparticle Chain Waveguides The idea of using chains of nanoparticles as plasmonic waveguides for optics was first proposed by Franz Aussenegg’s group in 1998 (Quinten et al., 1998). Since then, Harry Atwater’s research group

48 Nanophotonics (Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology) has extensively studied this system. This group has found that metal nanoparticles arranged in linear chains coupled to each another via the optical near field when light was incident on them (Maier et al., 2002a) and proceeded to characterize the utility of these structures as waveguides (Maier et al., 2002b). Because each of these studies relied on far-field techniques, the entire nanoparticle chain had to be illuminated at the same time, which disallowed the direct observation of optical energy transport from one end of the chain to the other (see Figure 2-16). However, in 2003, Atwater’s group presented results that employed a near-field scanning optical microscope to excite plasmons on one end of the nanoparticle chain and observe that plasmons propagated down the chain transporting energy, thereby proving the waveguiding abilities of such structures (Maier et al., 2003). Stephen Mann’s group has demonstrated the capability of making highly anisotropic branched chains of nanoparticles via a wet chemistry technique and has observed plasmonic mode coupling in the chains (Lin et al., 2005). This technique of fabricating nanoparticle chains could be far surperior to conventional top-down approaches such as electron-beam lithography. The group of Marcus Dantus has done a similar study in which waveguiding via plasmons was observed in a dendritic silver nanoparticle mesh. The group was able to control the direction of the propagation using polarization and was able to observe propagation distances of up to 100 microns (Gunn et al., 2006). FIGURE 2-16  Schematic representation of a nano­ particle chain plasmonic waveguide. The ­schematic shows a tip of a near-field scanning ­optical micro- scope coupling light into a nano­particle at the far end of a linear chain of nano­particles. Light is then propagated down the nanoparticle waveguide via near-field coupling between ­ plasmon resonances in each successive nanoparticle. Light is then coupled out of the waveguide by the last particle in the chain. SOURCE: Stefan Maier, 2008, with p ­ ermission.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 49 Metal-Insulator-Metal Waveguides Recently, the waveguide geometry consisting of an insulator with a metal cladding has gained increased interest. Theoretical studies first suggested that these metal-insulator-metal (MIM) structures offer superior subwavelength confinement for plasmon waveguides. Mark Brongersma’s group theoreti- cally compared insulator-metal-insulator (IMI) and MIM waveguiding structures. The group concluded that by the MIM structures had far superior ability to confine the plasmon modes into subwavelength sizes (Zia et al., 2004). Harry Atwater’s group then extended the study of MIMs by theoretically studying them using realistic optical properties of metals. This group focused specifically on planar-­multilayered MIM systems and found that such structures could support both plasmonic modes and photonic modes (normal propagating electromagnetic modes) (Dionne et al., 2006a). The group then fabricated and studied these structures experimentally (Dionne et al., 2006b). In this study the researchers fabricated Ag/Si3N4/Ag layered MIM structures and characterized their waveguiding properties. They were able to couple light into and out of these structures using slit openings in the metal. They were able to observe broadband propagation of electromagnetic energy over distances of several micrometers. They were also able to observe that these structures support both the plasmonic and photonic modes as they had previously predicted. Channel Plasmon Polaritons The history of channel plasmon polaritons (CPP) is briefly reviewed by Francisco J. García-Vidal (2006) in a Nature news brief. In 1990, the group of Alexei. A. Maradudin theoretically predicted that guided electrostatic modes would exist in a V-shaped groove in a metal film (Lu and Maradudin, 1990). In 2002, the group revisited the problem, extending Maradudin’s theory past the electrostatic limit to include propagating electromagnetic modes bound inside such a groove, dubbing them “channel polari- tons” (Novikov and Maradudin, 2002). Figure 2-17 demonstrates that channel plasmons can propagate effectively over long distances and can propagate around sharp bends. The waveguide-ring (WR) resona- tor structure can be used to control the wavelength that is allowed to propagate along the waveguide. Soon after this work, D.K. Gramotnev and D.F.P. Pile began to study channel plasmon polaritons using the Finite-Difference-Time-Domain (FDTD) numerical method. In 2004, they modeled the propa- gating modes and discussed the properties of CPP and also suggested that CPP seems to experience less dissipation over long propagation distances than other plasmonic waveguide structures do (Pile and Gramotnev, 2004). Next, they predicted that CPP should undergo nearly no dissipation at sharp bends in the V-groove (Pile and Gramotnev, 2005). In 2005, Sergey I. Bozhevolnyi’s group (Aalborg University, Denmark) first demonstrated experi- mentally that the earlier theoretical studies were correct (Bozhevolnyi et al., 2005). This work involved measuring the propagation distance that could be achieved in such a waveguide structure by using an NSOM to image the electromagnetic near-field as it propagates down the channel groove. They observed propagation lengths of ~100 μm at wavelengths of 1,425 nm to 1,620 nm, similar to typical telecom- munications wavelengths. While this study only included straight-line propagation, in 2006 this same group demonstrated that the V-groove waveguide could, as predicted, support remarkably low loss at sharp bends in the channel’s path (Bozhevolnyi et al., 2006). The group also reported the achievement of making several on-chip optical components from this structure, such as a Y-splitter, an interferometer, and a ring resonator. The further characterization and optimization of this waveguiding system remain an active area of study.

50 Nanophotonics t In K Out α exp(iθ) FIGURE 2-17  A plasmonic waveguide ring (WR) resonator. (a) Scanning electron micrograph of WR resonator structure. (b) Topographical image of the same structure. (c) Near-field scanning electron micrograph of the WR resonator showing light of wavelength 1,525 nm propagating through the structure. SOURCE: Reprinted by per- mission from Bozhevolnyi et al. (2006). Copyright 2-17by Macmillan Publishers Ltd. 2006 Plasmon-Based Active Devices Going beyond the passive on-chip information-transportation applications discussed above, active generation, amplification, and switching of plasmons will allow on-chip routing and integrated sensors.  In conjunction with the plasmonic detectors discussed in the next subsection and the passive on-chip transportation and filtering discussed above, active devices may allow the creation of highly integrated single-chip sensors for biological or chemical detection (see Figure 2-18). Directly switching, routing, and modulating optical fields is an area of intense technological impor- tance for optical networking and on-chip routing applications. Removing the need to convert optical s ­ ignals into electronic signals to route information would allow significant improvements in band- width. The electrical modulation of surface plasmons is also important for interfacing electronics with p ­ lasmonics in on-chip applications. In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Plasmon-Based Active Devices.”

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 51 a b c FIGURE 2-18  (a) Schematic of a device to electrically modulate the plasmon resonance of gold nanorods. The nanorods are deposited on one surface of a cell containing aligned liquid crystals. Applying a voltage across the cell causes the liquid crystals to change orientation, which in turns changes the dielectric environment to which the plasmon resonance in the nanorods responds. This change results in modulation of the spectral position of the 2-18 plasmon resonance. (b) One technique employed to amplify surface plasmons propagating on a metal film. A laser at 580 nm excites a population inversion in the dye solution, which coherently delivers energy to the ­plasmon on the surface of the film by using a 633 nm laser by stimulated emission. The energy-level diagram for this process is illustrated in the bottom-left part of (b). (c) Integrated plasmon detector. Surface plasmons, launched on the film using a slit, propagate into the device where they generate carriers in the semiconductor layer giving rise to a detectable electric current. ������������������������������������������������������������������������������� SOURCES: (a) Reprinted with permission from Chu et al. (2006), ���������������� Copyright������� 2006, American Institute of Physics; (b) reprinted with permission from Seidel et al. (2005), Copyright 2005 by the 2005), American Physical Society; and (c) reprinted with permission from Ditlbacher et al. (2006), ���������������� Copyright������� 2006, American Institute of Physics. In the same fashion as for LSPR sensing, changing the refractive index of the medium surrounding a plasmonic nanostructure can be used to shift the plasmon resonance. This effect can be exploited to bring about modulation by using a material that changes the refractive index under either electrical or optical stimulation. One such technique is to electrically modulate the plasmon resonance of a nano- structure by exploiting the change in refractive index that a nematic liquid crystal undergoes when an electrostatic field is applied (Chu et al., 2006; Muller et al., 2002). Another proposed system that may allow for higher-speed modulation is the electrooptic effect in ferroelectric films (Liu and Xiao, 2006). Thermal modulation of the medium surrounding metal strip waveguides can also be exploited to modu- late plasmons, in which case the metal strips supporting the plasmon propagation can themselves be used as ohmic heating elements to spatially localize the effect (Nikolajsen et al., 2004). Rather than changing the dielectric constant of the surrounding medium, it is also possible to con- struct waveguides in which the optical properties of the waveguide material itself can be changed. Just as for gold or silver films, thin films of gallium (Ga) on dielectric substrates support surface plasmons. Unlike other materials used for plasmonics, the phase of Ga can be changed between metastable ­metallic phases (m-Ga) and a polymorphic phase (α-Ga) which exhibits strong absorption across the visible and NIR due to the presence of covalently bound Ga2 molecules. In thin films, this phase change can be

52 Nanophotonics triggered either optically or thermally. Remarkably, this effect can happen on timescales as short as 4 picoseconds (ps), allowing for high-speed modulation of SPs (Krasavin et al., 2005). In the visible spectrum, optical modulation of a 633 nm light by a continuous wave (CW) laser at 488 nm using χ(3) nonlinearity of poly-3-butoxy-carbonyl-methyl-urethane deposited in nanohole arrays in a thin Au film has been demonstrated (Smolyaninov et al., 2002). This switching mechanism in principle can allow for very high speed switching. Due to the strong field enhancement arising from the surface modes of the hole array, this effect is strong enough to be observed with a CW control beam rather than requiring pulsed excitation. Semiconductors exhibit a Drude response in the terahertz range similar to that of metals at optical frequencies. Unlike metals, in which the carrier concentration is essentially fixed, the permittivity of semiconductors can be easily modified by changing the free carrier concentration by, for example, chang- ing the temperature or optically generating electron-hole pairs. This effect has been exploited to change the transmission of surface plasmons propagating on grating structures fabricated on the surface of an InSb wafer by optically creating electron-hole pairs. While this technique has only been demonstrated thus far with a CW modulation laser, potentially this switch mechanism can result in a transmission rise time on the picosecond timescale, enabling ultrafast all-optical switching of terahertz plasmons with low optical fluences on the order of µJ/cm2 (Gomez-Rivas et al., 2006). As early as 1989, the use of amplification by creating a population inversion in a medium adjacent to a metallic film to increase the SPP propagation length was proposed. More recently, Mark I. Stockman has introduced the idea of SPASER (surface-plasmon amplification by stimulated emission of radiation) (Bergman and Stockman 2003), and several groups have experimentally demonstrated the amplifica- tion and generation of surface plasmons. Owing to the momentum mismatch between light and surface p ­ lasmons, efficient coupling between far-field excitation and propagating surface plasmons in waveguides is difficult to achieve. For on-chip transport applications or integrated sensors, an ideal solution to this problem is to create plasmons using, for example, a SPASER. In routing and long-distance applications, amplification of signals for off-chip communications without an optical-electronic-optical conversion is necessary in order to maintain signal levels. In addition, the high loss frequently encountered in SPP waveguides may be counteracted by the use of amplification. Surface plasmon lasers, or SPASERs, based on the coupling of a gain medium directly with surface plasmons, have been demonstrated in the mid-infrared (IR) spectral region using metallic structures on top of quantum cascade lasers (Bahriz et al., 2006; Moreau et al., 2006). The mid-IR is of particular interest, as plasmons propagate with far lower losses in this spectral region than in the visible. In addition, on-chip sensing applications can be imagined as well as traditional routing. In the visible, plasmon amplification by coupling to organic laser dyes has been demonstrated (Seidel et al., 2005). In solution, the effective negative imaginary part of the dielectric constant of excited rhodamine 6G dye was demonstrated to increase the Rayleigh scattering efficiency of aggregates of Ag nanoparticles by a factor of 6 due to partial cancellation of the imaginary part of the Ag dielectric function at the plasmon resonance (Noginov et al., 2006). The final component necessary for active plasmonic devices to be used for on-chip communica- tions or integrated sensor application is a surface plasmon detector. The next subsection discusses the concentration of light into small photodiodes, which is one way that surface plasmons could be detected on a small scale. Recently, however, a detector based on an organic photodiode constructed with an integrated surface plasmon waveguide to detect light propagating into the device from the waveguide directly has been demonstrated (Ditlbacher et al., 2006).

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 53 a b FIGURE 2-19  (a) Schematic illustration of a plasmonic focuser used to collect light from a large area and ­focus it into a high-speed nanophotodiode. (b) Silicon (Si) light-emitting diode employing the plasmon resonance of a silver island film to couple light trapped in the waveguide mode of the device to the far field. SOURCES: (a) Reprinted with permission from Ishi et al. (2005), Copyright 2005, Institute of Pure and Applied Physics; (b) reprinted with permission from Pillai et al. (2006). Copyright 2006, American Institute of Physics. 2-19 Plasmon-Enhanced Devices Plasmon-enhanced devices generally fall into two categories: enhanced emission devices and enhanced detection devices. In the former case, plasmons are used to increase the emission efficiency of LEDs and other light emitters by either improving the coupling of light out of the structure or changing the decay rates of the emitting material directly to enhance the emission of light. Enhanced detectors can fall into two categories as well: in one category plasmons are used to concentrate the light to smaller areas than could be achieved using conventional optics for ultrahigh speed photodiodes, and in the other category the coupling of light into a conventional device is enhanced by scattering off plasmonic nanoparticles into the device (see Figure 2-19). Further, because plasmonic enhancement is generally performed with metallic structures, one could also employ these structures as electrical contacts, thereby creating contacts that enhance the performance of a device rather than just having the detrimental effect of reducing the overall efficiency of a device by shadowing part of the active area. Shrinking high-speed photodiodes enables higher speeds because the response time is limited by the junction capacitance and transit time of photogenerated carriers. Shrinking the devices reduces both effects. However, coupling light into nanoscale photodetectors with conventional optics is limited by the diffraction limit to devices with active areas of at least several hundred nanometers in diameter. Plasmonics allows the concentration of light into nanometer-scale volumes, thereby enabling ultrahigh- speed nanophotodiodes to be fabricated. Two structures have recently been demonstrated to collect and concentrate light into nanoscale volumes at the surface of photodiodes using surface plasmons: C-­apertures in a metal film to enhance a germanium (Ge) photodiode operating at 1310 nm (Tang et al., 2006) and concentric metallic rings in a metallic film (Ishi et al., 2005) to enhance the performance of an Si nanophotodiode (see Figure 2-20). A similar idea can also be employed in on-chip interconnect In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Plasmon-Enhanced Devices.”

54 Nanophotonics a b FIGURE 2-20  (a) Sample geometry for enhancing the radiative decay rate of indium gallium nitride (InGaN) quantum well (QW) emission. The metal film on the top surface supports a plasmon resonance that enhances the radiative decay rate of the InGaN QW. (b) Emission spectrum of the samples with metal films showing the enhancement in photoemission. The plasmon resonance of the Ag film is closest to the emission wavelength of the device and therefore causes maximum enhancement of the output. These devices have potential for improving white-light-emitting diodes. SOURCE: Reprinted by2-20 permission from Okamoto et al. (2004). Copyright 2004 by Macmillan Publishers Ltd. systems to collect light from a larger waveguide structure and couple it to nanophotodiodes on size scales commensurate with the transistors making up the electronic part of the device. For mid-IR detectors, reducing the volume of the semiconductor sensing element also has the impor- tant effect of reducing the thermal noise in the detector. Using the intensely concentrated near field of a plasmon, it is possible to concentrate light from a large area to enhance absorption by a small volume of material. It has recently been proposed that the enhanced near field caused by surface plasmons on metallo-dielectric diffraction gratings can be used to significantly increase the absorption of light by nanoscale mercury cadmium telluride (HgCdTe) (MCT) detectors. In this device, an Au film with 50 nm wide strips of MCT at regular spacing is shown to increase the amount of light absorbed in the MCT, owing to the intense near-field enhancement in the gaps, by as much as a factor of 250 at 9.8 µm compared to the same volume of MCT material as part of a thick slab with a conventional antireflection (AR) coating. In addition, the Au stripes could potentially be used for electrodes (Yu et al., 2006). The performance of larger devices can also be enhanced in two ways: by increasing the coupling of light into devices through (1) far-field scattering of surface plasmons or (2) near-field enhanced absorption. Of particular importance is the development of inexpensive methods for enhancing the per- formance of solar cells. Recent theoretical work has developed techniques for determining the optimal nanoparticles for the collection of sunlight in such applications (Cole and Halas, 2006). One technique frequently used to enhance the performance of silicon devices, such as solar cells, is to texture the surface by etching depressions into the surface to trap light, allowing more efferent absorption of light

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 55 by the device. Unfortunately, such texturing of the surface frequently results in degraded electrical per- formance, particularly in thin-film devices. The plasmon resonance of nanoparticles, metallo-dielectric diffraction gratings, or holes in a metal film could potentially all be used to scatter light into the device, enhancing the overall efficiency in a manner similar to that of conventional texturing techniques, without modifying the underlying device structure and thereby maintaining the electrical characteristics (Pillai et al., 2006). In addition, because metal structures are used for visible plasmonics, one could potentially devise structures in which the plasmonic structure acted as a front contact, eliminating the problems of “shadowing” part of the active area of the device by the front electrical contacts. Another technique is to use the enhanced near field directly to enhance the absorption of light by using the near field to concentrate light in a shallow active layer (Stuart and Hall, 1998). The focusing effect of plasmons can also be used in the other direction; by depositing a plasmonic antenna on the surface of a conventional semiconductor diode, the light can be concentrated into a nanoscale volume of space directly above the surface of the laser. This is applicable as an active near-field optical microscope probe and to dramatically increase the density achievable with optical data-storage devices by replacing the conventional lens assembly in optical disk drives (Cubukcu et al., 2006). Enhancing the performance of new solid-state lighting sources is of great current importance due to concerns over environmental conservation and energy security. In addition to the pressing need for improved illumination devices, other important applications include enhancing silicon light emission for integrated on-chip photonic devices and enhancing the performance of organic LEDs for display applications. For enhancing the light emission from devices composed of materials such as InGaN quantum wells (Okamoto et al., 2004), light-emitting polymers for organic LEDs (Neal et al., 2006), and Si nanocrystals (Biteen et al., 2006), enhancing the radiative decay rate of the light emission material allows a significant overall improve- ment in the device quantum efficiency. In the case of thin-film Si band-edge emission devices, coupling light out of the natural waveguide modes of the Si thin-film structure has traditionally been accomplished by surface texturing in the same fashion as for the solar cell applications discussed above (Pillai et al., 2006). As with detectors, nanoparticles on the surface can couple to these waveguide modes directly and efficiently, scattering light out to the far field, enhancing the overall external device efficiency. Quantum cascade lasers (QCLs) for terahertz emission into the far field can also be enhanced by using surface plasmons. A plasmon waveguide constructed by applying gold films to both sides of the device, essentially a MIM waveguide, as discussed above, can confine light within the active region. Etch- ing grooves in a gold film on the top of a QCL allows the creation of a second-order diffraction grating. The result is a plasmon band gap allowing feedback at only one laser mode. In addition, a second-order diffraction grating can be used to couple light out of the waveguide by scattering light into the far field in a direction normal to the top of the laser, enabling the creation of vertical emitting terahertz QCLs. Plasmonic waveguides are particularly advantageous for use with QCLs because the optical modes generated in the QCL are transverse-magnetic (TM) polarized, matching the TM polarization of surface plasmon waveguides. In addition, the gold used for the waveguides in these lasers also serves the dual purpose of acting as electrical contacts to the device (Fan et al., 2006; Tredicucci et al., 2000). Plasmonics in Biotechnology and Biomedicine As the field of plasmonics matures, there are many applications of plasmonic properties of nano­ particles in the field of biotechnology and medicine. These applications can be broadly divided into the InAppendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Plasmonics in Biotechnology and Biomedicine.”

56 Nanophotonics subfields of chemical and biological sensing, plasmonic heating of nanoparticles, and optical imaging using nanoparticles. The sensing applications include LSPR sensing and SERS of biomarkers of disease. Most notable in this regard is the optical biosensor for Alzheimer’s disease that uses LSPR sensing to detect beta amyloid aggregation (Haes et al., 2005) and SERS sensors for in vivo blood glucose monitoring (Lyandres et al., 2005; Stuart et al., 2006b). Other SERS-based sensors for bioterrorist agents such as anthrax (Zhang et al., 2005c; Zhang et al., 2006b) and half-mustard gas (Stuart et al., 2006a) also demonstrate high sensitiv- ity. SERS and surface-enhanced Raman optical activity (SEROA) (a vibrational spectroscopic technique that relies on the difference in the intensity of Raman-scattered right and left circularly polarized light due to molecular chirality) are also being investigated as highly molecular-specific techniques. LSPR sensing is also used to detect nanomolar quantities of antibodies in whole blood (Hirsch et al., 2003a). Plasmonic heating of nanoparticles is being used to enhance laser tissue welding (Gobin et al., 2005) and photothermal cancer ablation therapies (Hirsch et al., 2003b; O’Neal et al., 2004). These have been demonstrated successfully in vivo in mice. Figure 2-21 shows a comparison of conventional sutures versus nanoshell-enhanced laser welding of incisions after surgery. Plasmonic nanoparticles are being extensively used as optical contrast agents for imaging biological tissues (Stone et al., 2007). There are many groups using a variety of nanoparticles, from simple solid gold nanospheres to nanorods and antibody-targeted nanoshells for imaging applications, as shown in Figures 2-22 and 2-23. Figure 2-22 shows therapy of SKBr3 breast cancer cells using anti-HER2 nanoshells. In Figure 2-23, as the collagen network is deformed by cell traction forces, the pattern of scattered light from the embedded nanorods also shifts and deforms. Digital image analysis can then be used to track the movement and deformation of the light pattern and to calculate local material deformations. Simultaneous fluorescence imaging can also be used to identify cell locations, in order to associate strain fields with the relevant cell spatial positions, morphologies, and orientations. FIGURE 2-21  Comparison of conventional sutures (upper row of pictures) versus nanoshell-enhanced laser w ­ elding (lower row of pictures) of incisions after surgery. Photographs of an individual following surgery, day 0 to day 32. After 10 to 14 days, the scab on the soldered incisions fell off, leaving a fine scar where the animal is healing. The soldered row leaves a more defined scar compared to the sutured side but diminishes over time. SOURCE: Gobin et al. (2005). Copyright 2005. Reprinted with permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. 2-21

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 57 FIGURE 2-22  Therapy of SKBr3 breast cancer cells using anti-HER2 nanoshells. Cell viability assessed via c ­ alcein staining (top row), and silver stain assessment of nanoshell binding (bottom row). Cytotoxicity was observed in cells treated with a near-infrared (NIR) emitting laser following the exposure of cells targeted with anti-HER2 nanoshells only. Note the cytotoxicity (dark spot) in cells treated with an NIR emitting laser follow- ing nanoshell exposure (top row, right column) compared to the controls (left and middle columns). SOURCE: Reprinted with permission from Loo et al. (2005). Copyright 2005 American Chemical Society. 100 µm FIGURE 2-23  (Left) Dark-field optical micrograph of light scattered from gold nanorods embedded in cell- p ­ opulated collagen layers. (Right) Simultaneous fluorescence image of cardiac fibroblast cells present on the c ­ ollagen. The two images superimposed with some transparency (center panel). Scale bar = 100 μm. Inset: Transmission electron micrograph of the gold nanorods. Scale bar (inset) = 100 nm. SOURCE: Reprinted with permission from Stone et al. (2007). Copyright 2007 American Chemical Society 2-23 new text superimposed

58 Nanophotonics Emerging Topics of Phonon Polaritons and TerAhertz Waveguides Phonon Polaritons In 2002, Rainer Hillenbrand’s group (Nano-Photonics Group, Max-Planck-Institute fur Biochemie and Center for Nanoscience, Germany) proposed that many of the subwavelength optics applications provided by surface plasmons in the visible/near-infrared spectral range had direct analogs in the mid- infrared range arising from surface phonon polaritons on polar crystalline dielectrics (Hillenbrand et al., 2002).10 These phonon polaritons are propagating waves of lattice deformations of the polar crystal that can couple resonantly to electromagnetic radiation. The group studied this phenomenon at a sili- con carbonide (SiC) surface using scattering-type apertureless scanning near-field optical microscopy (s-SNOM). The group demonstrated the ability to resonantly couple a subwavelength metal tip to a surface in the mid-infrared spectral region, which has opened the door to many potential applications. For example, this group demonstrated that because the phonon resonance depends greatly on the crystal- line structure of the material, applications related to sensing the local crystal structure can be realized (Ocelic and Hillenbrand, 2004). The researchers demonstrated that they see a resonance for crystalline SiC but not for amorphous SiC. Using focused ion beam implantation to cause lattice defects in the SiC crystal, they wrote a subwavelength checkerboard pattern of intact crystalline SiC and damaged SiC (see Figure 2-24). Then, using their s-SNOM, they scanned the surface and found resonant coupling at the crystalline sites and little or no coupling at the damaged sites. They were able to resolve 200 nm (λ/50 resolution)-wide squares in the checker pattern and 100 nm (λ/100 resolution)-wide stripes in a stripe pattern. This work demonstrates a promising potentional for optical data-storage applications at subwavelength dimensions. The Hillenbrand group also has studied surface phonon polariton propagation (Huber et al., 2005) and has demonstrated that they can detect and image slight variations of structure in a crystal lattice (Huber et al., 2006). Surface phonon polaritons on SiC have also been proposed to be useful for employment in particle acceleration. Gennady Shvets’s group (Department of Physics and Institute for Fusion Studies, Univer- sity of Texas at Austin) has proposed this application and is actively working toward its realization. The group has designed the structure, performed numerical simulations, and begun fabricating this structure (Shvets et al., 2004). The structure consists of an Si grating on one side of an Si wafer with an SiC layer grown on the other side. Two such structures are positioned together such that the SiC layers face each other and have a small vacuum gap between them. Carbon dioxide (CO2) laser light (on resonance with SiC) is shone onto the Si grating and excites propagating phonons on the SiC surfaces, which produces an enhanced electric field directed down the vacuum cavity where particles can be accelerated. This group has recently performed additional tests to confirm and characterize the surface waves excited at the SiC surface (Kalmykov et al., 2006). The realization of such a device would offer an inexpensive desktop accelerator, making particle physics experiments much more accessible. Terahertz Plasmonic Waveguides Waveguides in the terahertz frequency range of the electromagnetic spectrum have proven to be a difficult technology to realize owing to high losses and group velocity dispersion. 11 In 2004, Daniel M. Mittleman’s group (Rice University) reported the realization of a practical terahertz waveguide arising from surface plasmon polariton modes supported on bare metal wires (Wang and Mittleman, 2004). 10In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Phonon Polaritons.” 11In Appendix D, “Selected Research Groups in Plasmonics,” see the section entitled “Emerging Topics in Plasmonics.”

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 59 FIGURE 2-24  Near-field micrograph of a silicon carbonide (SiC) surface. A checkerboard pattern has been made on the SiC surface by using focused ion-beam implantation to effectively destroy the crystalline structure of SiC in the pattern. The near-field image was obtained using a scattering-type scanning near-field optical microscope (s-SNOM) with 11.22 μm wavelength light. Because only the crystalline sections of the SiC surface support p ­ honon-polaritons, only those regions are bright in the s-SNOM image. The inset shows a near-field image of a 2-24 checker pattern in which the squares are 200 nm wide, demonstrating a very fine spatial resolution, which suggests the potential for optical data storage applications. SOURCE: Reprinted by permission from Ocelic and Hillenbrand (2004). Copyright 2004 by Macmillan Publishers Ltd. The group was able to couple terahertz radiation into the wire by placing another wire oriented per- pendicular and proximal to the waveguide wire and focusing terahertz radiation at the place where the two wires cross. The researchers observed virtually dispersionless propagation along such a wire 24 cm long with a loss coefficient of 0.03 cm–1 carried by radially polarized SPP mode. This work represented the first viable waveguide for the terahertz frequencies. Daniel Grischkowsky’s group (Oklahoma State University) then further characterized these wire waveguides and identified the propagating modes as Sommerfeld waves (Jeon et al., 2005). Mittleman’s group then characterized the SPP dispersion, which, although small, was found to be nonlinear with wire thickness (Wang and Mittleman, 2006). Paul C.M. Plancken’s group (Delft University of Technology, The Netherlands) showed that these wire waveguides become highly dispersive with even a thin coating of a dielectric material, suggesting the possibility of a sensor based on this change (van der Valk and Plancken, 2005). Despite the promise of the bare metal wire waveguides, Stefan Maier (University of Bath, United Kingdom) has recently suggested that this geometry is not the best terahertz waveguide, because at tera- hertz frequencies, SPPs are not highly localized but instead extend out several wavelengths radially and suffer significant losses near bends and proximal objects. Thus, he has suggested alternative geometries for terahertz plasmon waveguiding based on structures containing periodic holes or grooves in a metal surface which support SPP-like modes that are highly confined similar to SPPs in the visible spectral

60 Nanophotonics region (Maier and Andrews, 2006). Subsequently, Maier has also proposed terahertz waveguides using cylindrical metal wires with periodic radial grooves. Such a structure is predicted to support highly confined SPP-like modes with dispersion relations similar in trend to SPPs in the visible spectral range (Maier et al., 2006). Reduced Dimensionality and Quantum Confinement in Nanophotonics Introduction and Background This section addresses the ongoing and anticipated advances in photonics technology that are occurring as a result of emerging new ways of incorporating reduced-dimensionality structures, and the resulting quantum confinement of charge carriers, into optoelectronic devices. While quantum confine- ment has been used in optoelectronic devices for a few decades now, new approaches and new device concepts continue to be developed, producing dramatic new levels of capability and performance, over broad ranges of wavelength. It is anticipated that these advances will continue to occur at a rapid rate into the foreseeable future. These advances will benefit from and occur in parallel with ongoing advances in the sophistication and control of materials growth techniques and nanoscale fabrication and process- ing methods. For the purposes of this report, the Committee on Nanophotonics Accessibility and Applicability takes its topic to include optoelectronic devices and device components that employ reduced dimen- sionality and quantum confinement to manipulate energy levels and tailor charge-carrier transport for improved performance. The committee provides a brief description of how some of these techniques have been used in the past. The majority of this section then describes emerging approaches in which quantum confinement is being used to develop entirely new approaches to photonic devices. Finally, a short speculative section discusses new types of device approaches, which at this stage are speculative and very long term, that might ultimately be developed. Definition of Reduced Dimensionality and Quantum Confinement Reduced dimensionality and quantum confinement occur when the structure extent in one or more dimensions becomes comparable to the Fermi wavelength of the charge carrier (i.e., the electron or hole). 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. How- ever, the committee takes the definition to be somewhat broader than the basic one provided above. The definition should include such techniques as the placement of thin barriers to manipulate the energy- dependent tunneling of carriers from one region to another, and the use of nanostructures to influence phonon transport independently from carrier transport—in which case the relevant length scale becomes the phonon mean free path rather than the electron Fermi wavelength. The Double Heterostructure Laser: Earliest Use of Quantum Confinement in Photonics The double heterostructure enabled the charge carriers to be concentrated into a thin layer of material having smaller band gap than the material surrounding it. This (1) suppressed electron-hole recombina- tion outside the active region, (2) prohibited carriers from “overshooting” the active region by providing “blocking” layers of wider band gap, and (3) increased optical confinement owing to the higher index of refraction of the lower band gap material. Further increases in recombination efficiency and mode

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 61 confinement were achieved by making the double heterostructure layer ever thinner, until eventually the electron energy levels became quantized and the structure became known as a quantum well. At present, the highest-efficiency semiconductor light emitters are QW-based lasers. The ability to epitaxially grow heterostructure layers with low-defect densities was enabled by several atom-by-atom growth methods developed at around the same time, including molecular beam ­epitaxy (MBE), developed by A.Y. Cho and others, and metal organic chemical vapor deposition (MOCVD), with H. Manasevit, R. Dupuis, and P.D. Dapkus as major inventors. Several groups contributed to the development of this technology, including Alferov’s group in Russia at the Ioffe Institute and groups at Bell Laboratories, Rockwell, IBM, and RCA in the United States. These groups rapidly advanced through a number of critical breakthroughs, including growth of lattice-matched structures, the use of band diagrams, and single heterostructure lasers. The first continuous-wave laser operation at room temperature was achieved in a double heterostructure independently by Alferov’s group and M. Panish’s group at Bell Laboratories in 1970. During this period, a group under M.G. Craford at Monsanto applied heterostructure concepts to new compound semiconductor alloys, achieving the first yellow LED and a 10-fold brightness increase in red and orange LEDs in the early 1970s. In the 1990s, efficient and durable blue LEDs and laser diodes were demonstrated by S. Nakamura at Nichia Chemical Industries, Ltd., using gallium nitride (GaN)-based heterostructures. Thus, the fundamental solid-state physics concepts of quantum confinement and the use of quantum wells to localize excitons, increase oscillator strengths, enhance radiative recombination efficiencies, and control charge-carrier transport were largely developed in parallel with optoelectronics device research. This extremely close cycle between theory and experiment and between science and technology was unusually successful. (The basic science portion of this effort was recognized by a Nobel Prize in ­physics awarded to Alferov and Kroemer in 2000.) In the following subsections, the committee discusses how these concepts, and new ones as well, are being extended and combined in new configurations to create optoelectronic devices with novel properties and enhanced performance. New Devices: Emitters Following is a discussion of a number of optoelectronic emitters being developed that employ extended concepts of reduced dimensionality and/or quantum confinement. Quantum Dot (QD) Lasers Quantum dot lasers are conventional semiconductor lasers in which a layer of quantum dots is embedded within the active quantum wells of the laser structure. The quantum dots are typically formed by a spontaneous self-assembly process driven by strain. The canonical example is the formation of indium arsenide (InAs) QDs within a gallium arsenide (GaAs) quantum well. The QDs can be created by either MBE or MOCVD growth methods. It has been known for a while that the incorporation of quantum dots in the active region of opto- electronic devices would drastically improve device performance, primarily owing to the enhanced Dirac-delta-like density of states function (Arakawa and Sakaki, 1982; Asada et al., 1986). The density of states, which is a measure of the total number of quantum mechanically allowed energy states per unit volume, is a very important parameter in solid-state physics, and a large density of states is highly desirable for optoelectronic devices such as lasers and detectors. As one goes from a bulk material in which there is no quantum confinement to a system in which the carriers are allowed to move in two dimensions (quantum well), one dimension (quantum wire), or in quasi-zero dimensions (quantum dots),

62 Nanophotonics the density-of-states function increases in magnitude and becomes discretized. The density of states in a semiconductor for various levels of confinement is shown in Figure 2-25. Some of the major advantages of lasers with quantum dots in the active region are as follows: • A significant decrease in the threshold current and in the temperature dependence of the threshold current (Bimberg et al., 1997; Lester et al., 1999); • A large increase in the differential gain and modulation bandwidth (Kamath et al., 1997); and • A vastly reduced chirp and low linewidth-enhancement factor (Newell et al., 1999; Saito et al., 2000). But the fabrication of these ultra-small (~10Å to 100Å) objects in the laboratory has been a major challenge. Even when these objects were formed, by using direct patterning or otherwise, they lacked not only the large areal density that was required to make them feasible for devices but also the high optical quality that was an essential prerequisite for optoelectronic devices such as lasers, modulators, and detectors (Prins et al., 1993). It was only a decade ago that a novel technique, called self-organization or self-assembly (Stranski- Krastanov process), enabled the realization of coherently strained three-dimensional islands or “quantum dots” (Berger et al., 1988; Leonard et al., 1993; Tabuchi et al., 1991; Xie et al., 1995). It was observed that under certain conditions, during the heteroepitaxy of lattice-mismatched systems, either by MBE or by MOCVD, these dots were formed. Soon after, various electronic and optoelectronic devices such FIGURE 2-25  Plot of the electronic density of states g(E) as a function of energy E for structures of different dimensionality. SOURCE: Image provided by Paul Alivisatos, University of California at Berkeley. Reprinted with permission. 2-25

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 63 as lasers, mid-infrared detectors, emitters, single-electron transistors, and electrooptic devices were reported using quantum dot active regions (Grundmann et al., 2000; Kamath et al., 1996; Kastner, 1992; Kirstaedter et al., 1994; Krishna et al., 2000a; 2000b; 2001; 2002; Pan et al., 1998; Phillips et al., 1998a, 1998b; Raghavan et al., 2002; Ye et al., 2002). By embedding InAs dots in a conventional 980 nm quantum well pump laser structure, 1.3 micron lasing is achievable. Similarly, 1.48 micron to 1.6 micron emission is accessible by embedding the dots in a typical AlGaInAs quantum well laser design manufactured on an InP substrate. Threshold current densities in QD lasers have been demonstrated as low as 16 A/cm2 (Liu et al., 1999). In particular, mid-infrared detectors based on QDs have demonstrated very good performance. Recently, the first two-color camera was demonstrated using quantum dots in the active region (Krishna, 2005). Such cameras would be invaluable for a variety of applications ranging from night-vision equip- ment to chemical agent and pollutant monitoring. The main advantage envisioned by using QDs in the laser active region is that (1) quantum con- finement is greatly increased owing to the QDs’ dimensionality being restricted in all three dimensions rather than only one, as is the case for a quantum well. This leads to the benefits of exciton localization, enhanced oscillator strength, and increased radiative recombination efficiency, but in greater degree than can be achieved in a single quantum well. In addition, because of the reduced electronic densities of states occurring in a QD, it is in principle easer to achieve population inversion. As a result, QD lasers can be engineered to be highly temperature-insensitive and have a very low linewidth-enhancement factor. For instance, the GaAs-based QD laser offers higher, more stable, better-uncooled performance over broader temperatures compared to QW designs. QD distributed feedback lasers (DFBLs) show a less than +/– 5 percent slope efficiency variation over a temperature range of 10ºC to 85ºC, which is approximately five times better than typical InGaAsP/InP QW DFBLs. For fixed-wavelength applications, the QD laser eliminates the need for a thermoelectric cooler and thermistor. In the case of Fabry-Perot lasers, T 0’s, or characteristic temperatures, range from 50 K to 80 K for undoped dots and 160 K to infinity for p-type doped QD (Shchekin and Deppe, 2002). The trade-off with doped dots is an increase in internal loss from a typical 1.5-2 cm–1 to 4-5 cm–1, which compromises the slope efficiency of the laser somewhat. For both fixed-wavelength and external cavity laser applications, QD lasers offer extremely small linewidth enhancement factor, α (<0.1), not achievable with QW lasers. This result leads to the additional benefits of low chirp under direct modulation, insensitivity of the laser operation to optical feedback or back-reflections, and filamentation suppression. III-N Lasers and LEDs Lasers and LEDs fabricated from the aluminum indium gallium nitride ((Al)(In)(Ga)N) system (often called the III-N system, referring to column III of the periodic table) are relatively recent arrivals on the optoelectronics scene, having only been demonstrated in the early 1990s. (Prior to this, there were no bright LEDs available in the ultraniolet (UV), blue, and green regions of the spectrum.) However, the advances that have occurred since then have been astonishing: bright, high efficiency III nitride (III-N) LEDs are now seeing wide commercial use in signaling applications, and a nascent solid-state lighting industry is emerging. This development promises highly efficient, long-lived, environmentally benign, and mechanically robust lighting sources for residential and commercial lighting, aviation, and military applications. III-N lasers in the UV have applications to the sensing of chemical, biological, and nuclear agents. In the visible spectral region, III-N lasers have important military uses for image projection. Blue lasers are already being developed for information storage (the widely heralded “Blue Ray” and High Density Digital Versatile Disc [HD-DVD] standards). Because of the immense market potential

64 Nanophotonics of III-N emitters, many of the innovative nanophotonics approaches being explored today are using this system as a platform—including such approaches as using QDs as wavelength conversion materials, photonic lattices for achieving improved light extraction, directionality of emission, and internal quantum efficiency. Coupling of internal excitonic states to surface plasmons is also being explored as a means of improving quantum efficiency. While the use of photonic lattices and surface plasmons is discussed in further detail in Chapter 3 of this report, the role of quantum confinement in III-N emitters is discussed here. Two major chal- lenges currently face the further development of III-N emitters. First, because no native substrate exists in GaN or AlN (beyond small experimental efforts producing pieces on the order of 1 cm 2 in area), epitaxially grown material suffers from defect densities as high as 1011 cm–2. These defects provide sites at which electrons and holes can recombine nonradiatively—that is, without producing a photon but instead producing waste heat. It is thus highly desirable to reduce the density of dislocations in order to improve efficiency and performance. The use of reduced-dimensionality nucleation layers—that is, initial growth layers composed of dense arrays of one-dimensional wires—offers a pathway toward lowering the defect densities. The one-dimensional wires act as a kind of suspension network, support- ing a subsequent planar growth layer with a greatly reduced density of dislocations. The dislocations are effectively turned sideways during this growth process, annihilating each other. Initial research in this area is being sponsored by the Department of Energy.12 For earlier related work, albeit not in the nanoscale regime, see Follstaedt et al. (2002). Role of V-Defects Interacting with Dislocations The second challenge concerns the difficulty in achieving high-efficiency green LEDs. In order to achieve green, the semiconductor quantum wells need to have high-In-content InGaN. However, as increasing amounts of In are added to the quantum well, the efficiency of the LED drops dramatically, tailing off around 550 nm, which is right at the center of the eye response curve. The origin of this effect is the subject of some controversy. However, one belief is that the In may spontaneously segregate at the nanoscale, forming QD-like regions of high-In content within the quantum wells. Initially these QDs may help to capture excitons and increase oscillator strengths. However, because most GaN-based emitters are grown on polar crystal faces, as the QDs become larger—as would be expected with increasing In content—they may actually lower the oscillator strengths. This is because the polar electric field will cause the electron and hole to become localized at opposite sides of the QD. It is believed that a deeper understanding of how In-rich QDs form and how they interact with dislocations may lead to methods of controlling their size and position so as to form ordered arrays with narrow size distributions. This could lead to the engineering of the nanoscale atomic structure of the active layers so as to achieve great improvements in efficiency and could ultimately lead to the development of III-N lasers spanning the range from pure InN (700 nm) to pure AlN (~200 nm). Type II “W” Lasers for Infrared (InAs/GaSb) Mid-infrared detectors based on InAs/indium (In), gallium (Ga) antimony (Sb) strained layer superlattices (SLSs) have been investigated for the past 15 years, ever since they were first proposed by Smith and Mailhiot (1987). The main advantage of this system lies in the fact that the band gap of 12See,for example, the Nanowire Templated Lateral Epitaxial Growth of Low Dislocation Density GaN, described at http:// www.netl.doe.gov/ssl/portfolio-07/current-light/NanowireTemplatedLateral.htm. Accessed July 18, 2007.

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 65 the s­uperlattice can be tailored over a wide range of wavelengths (2 µm < λc < 30 µm) by varying the thickness of the constituent materials. Thus, using two “mid” band gap semiconductors, devices can be fabricated with an operating wavelength spanning the entire regime—mid-wave infrared (MWIR), 3 µm < λ < 5 µm; long-wave infrared (LWIR), 8 µm < λ < 14 µm; and very long wave infrared (VLWIR), λ > 14 µm (Aifer et al., 2003; Fuchs et al., 1997; Johnson et al., 1996; Miles et al., 1990; Plis et al., 2006; Wei et al., 2005). Kaspi et al. (2002) have demonstrated high-power mid-infrared lasers using this technology. Quantum Wire Heterojunctions and Carbon Nanotube Emitters One-dimensional nanostructures, including semiconductor nanowires and carbon nanotubes (CNTs), have recently been investigated by a number of groups as potential nanoscale optoelectronic and photonic building blocks. In particular, GaN-based nanowires have shown promise as nanoscale light emitters and lasers in the UV to visible light range. Compared to conventional, planar GaN technologies that form the basis for current solid-state light emitters (LEDs) and lasers in the UV-to-green spectrum, GaN-based nanowires offer several potential advantages. One inherent advantage of nanowires, which can be con- sidered one-dimensional systems, over planar systems is the ability to relieve strain energy by means of lateral relaxation. This quality has enabled the growth of high-quality, dislocation-free nanowires directly on lattice mismatched substrates, and it raises interesting possibilities for achieving highly efficient LEDs based on nanowire heterostructures. Because mismatch strain inhibits indium incorporation in InGaN on GaN, which is necessary for green emission, InGaN/GaN heterostructure nanowires may provide a route for achieving higher indium mole fractions due to strain relaxation. Reduced strain may also result in reduced piezoelectric fields in the active region, which may result in higher radiative recombination efficiency due to the increased hole and electron wavefunction overlap. The ability to grow nanowires free of dislocations is a further advantage that would result in additional improvements in the efficiency and lifetimes of nanowire-based emitters versus planar-based devices. Another factor limiting the performance of conventional, planar LEDs is poor light-extraction efficiency due to internal reflection, which effectively traps the light within the device. Light-emitting nanowires may provide a solution to this issue, as they may have inherently high extraction efficien- cies owing to their high number of facets, high aspect ratios, and subwavelength dimensions. Enhanced extraction efficiency has been observed in LEDs that have undergone “chip-shaping” to create benefi- cially angled facets (Krames et al., 1999), as well as in LEDs with reduced die areas, and so it is antici- pated that quantum wires, with their large number of facets and high surface-to-volume ratio, will exhibit very high extraction efficiencies. Further, quantum wires of subwavelength dimension are expected to exhibit little internal reflection, if any. Recently, Lieber and coworkers have demonstrated intense, color-tunable, and efficient light emis- sion from individual core-multishell GaN/InGaN nanowires, as described in Figure 2-26. Recent advances have also demonstrated the growth of dense, vertical arrays of dislocation-free GaN nanowires (Wang et al., 2006a). The absence of dislocations arises due to the efficient strain relaxation enabled by the nanoscale sizes of the wires, and it is speculated that their absence could result in drastically reduced nonradiative exciton recombination rates. Dense, ordered arrays of heterostructure GaN-based n ­ anowires could form the basis for highly efficient LEDs and lasers that address many of the issues afflicting current, planar-based technologies. In addition, electroluminescence in CNT devices has been demonstrated, by taking advantage of the presence of Schottky barriers at the contacts. This allows the nanotube transistor to be biased such that holes are injected at one contact while electrons are injected at the other contact. The recombination of

66 Nanophotonics FIGURE 2-26  Core/shell nanowires allow the formation of color-tunable light emitters by changing the shell composition. (Top left) Sketch of the cross-section of a core/shell nanowire and (top right) a scanning electron microscope image of a single nanowire light-emitting diode and its current-voltage behavior. (Bottom) Light spots from nanowires of different shell composition. SOURCE: Reprinted with permission from Qian et al. (2005). 2-26 Copyright 2005 American Chemical Society. the electrons and the holes in the nanotube leads to light emission, with a wavelength that can be tuned by changing the diameter of the nanotube (Misewich et al., 2003). A variation of this approach uses very sharp band bending along the length of the nanotube to give very bright light emission from nanometer scale regions (Chen et al., 2005). New Devices: Detectors and Modulators Plasmonic Detectors As discussed in earlier sections of this chapter, plasmons are the collective oscillations of the electron gas in a metal or a semiconductor. Plasmons naturally oscillate at very high frequencies, typically in the infrared region for metallic nanoparticles. However, the previous sections of this chapter have primarily dealt with plasmons in metallic nanostructures. In metallic nanostructures, the electron densities of the metals are essentially fixed, and the primary method of changing plasmon resonances is by changing the dielectric constant of the surrounding matrix material. Recently, however, it has been shown that

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 67 the plasmon oscillations occurring in laterally confined quantum well structures can be used as resonant detectors of electromagnetic radiation. This opens up the possibility of an entire new class of active plasmonic semiconductor devices. These devices may enable revolutionary new capabilities based on two distinct advantages that they have over metallic plasmonic structures: 1. Because the charge layer is in a semiconductor QW, the charge-carrier density can easily be tuned with an electrostatic gate, as in a typical metal-oxide semiconductor field-effect transistor ( ­ MOSFET). This enables tuning of the plasmon oscillation frequency over an extremely wide range. 2. The QW in which the plasmon oscillation occurs can simultaneously be used as an electrical sensing channel in a MOSFET configuration. The plasmon frequency easily extends into the terahertz in standard semiconductor materials, which opens up the possibility of creating a variety of devices, with new capabilities that are unattainable using conventional electronics and device concepts. It also turns out that the wavelength of plasmons is typically hundreds of times smaller than the wavelength of light, opening up the possibility for sub- wavelength detectors and nanoscale high-speed devices. Tunable narrowband detectors of terahertz radiation, called plasmonic grating-gate detectors, which take advantage of the unique properties of plasmons, have recently been demonstrated. Essentially, these devices are field-effect transistors (FETs), except that the gate is fashioned into a grating to define distinct plasmon oscillations in the FET channel. The grating serves the additional function of coupling radia- tion into the plasmon channel. The detector sensitivity does not yet compete with broadband Schottky diode detectors, but using tunable plasmon excitations for the absorption mechanism provides unique “spectrometer-on-a-chip” functionality. The frequency response of the detectors is determined by the period of the grating the voltage bias on the gate (variable). To date, detection of radiation has been demonstrated from 100 gigahertz (GHz) (16 micron grating period) to 1 THz (4 micron grating period), although higher frequencies are easily attainable (see Figure 2-27). Since the grating-gate detectors operate at normal incidence, they naturally lend themselves to focal plane arrays (FPAs) for terahertz imaging. Additionally, the subwavelength nature of semiconductor plasmons opens up the exciting possibility of multispectral FPAs. The useful pixel size, or system spot- size, of an imaging system is limited by the wavelength that one is looking at. Grating-gate detectors can easily operate at sizes significantly below these limits; this enables the placement of detectors within what would normally be considered the size of a single pixel (see Figure 2-27). Since these detectors have spectral analysis capability, each pixel could look at a different frequency range, or each pixel could look at the same frequency, but have a different plasmon harmonic in order to increase noise immunity. Nanoschottky Diodes The rectifying electrical contact between a metal and a semiconductor is called a Schottky diode and is a well-established and widely used radio-frequency and microwave signal detector. Schottky diodes are particularly valued as heterodyne mixer detectors, which generate a difference frequency between an unknown signal to be detected and a known frequency from a local oscillator. The ability to determine an unknown signal’s frequency relative to the known local oscillator and the very high sensitivity pos- sible using heterodyne mixing detection make Schottky diode-based receivers the technology of choice in many radio and microwave communications and radar applications.

68 Nanophotonics FIGURE 2-27  (a) Photoresponse of a plasmonic grating-gate detector as a function of gate bias for many dif- ferent radiation frequencies. Clearly, each frequency produces a peak at a different gate voltage, demonstrating that the device can function as a “spectrometer-on-a-chip.” Inset shows the normal incidence detection geometry. (b) Response of a single detector to two vastly different frequencies, 584 GHz and 993 GHz, covering 400 GHz bandwidth. Higher harmonic plasmon oscillations are clearly present. (c) Demonstration of true spectrometer- 2-27 on-a-chip functionality with a 12.5 ms duration full spectral sweep. The multiple plasmon peaks in the detector response, while understood and predictable, tune in slightly different ways. The gate bias has been calibrated in units of frequency, with the lower axis corresponding to peak A, and the upper axis to peak B. Inset: illustration of how the subwavelength nature of the detectors enables multifunctional single pixels that operate within the d ­ iffraction-limited spot-size of an imaging system. SOURCE: Provided by committee member, Jerry Simmons. Extending heterodyne mixing methods to higher frequencies, namely, as high as the near-infrared, presents a significant technological challenge. To function at higher frequencies, the parasitic capacitance of a diode mixer must be minimized so that high- frequency signals are not inadvertently shunted around the detector. For a Schottky diode, reducing capacitance requires making the contact area between metal and semiconductor extremely small. Using sophisticated nanofabrication methods, Schottky diodes with contact diameters of about 250 nm have been made with electron-beam lithography and carefully controlled etching. Such a contact area allows Schottky diodes to function as mixers up to far-infrared frequencies around 3 THz (Siegel et al., 1999). Such electron-beam nanofabrication methods are very expensive, slow, and may not be able to pro- duce Schottky mixers that work above 10 THz. However, recent research in semiconductor nanowires and carbon nanotubes may provide a “bottom-up” alternative route to fabricating Schottky mixers en masse at much lower cost and usable to much higher frequency, possibly to the mid- or near-infrared region. This possibility derives from the fact that such nanowires and nanotubes can be easily, control- lably, and reproducibly synthesized with intrinsic diameters of 5 nm to 100 nm, eliminating the need for lithographic definition and precision etching down to nanometer length scales. Such nanomaterials are also amenable to inexpensive and fast self-assembly methods, such as dielectrophoresis, to place nano-

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 69 materials at specific points in a circuit. Most importantly, it has been shown that the contact between a semiconductor nanowire or semiconducting CNT and a good metal, such as gold, forms a Schottky diode contact with good electrical characteristics. If the nanotube/wire-to-metal contact area can be reliably limited to the diameter of the nanomaterial, area scaling arguments imply that the parasitic capacitance can potentially be reduced enough to enable mixer operation as high as near-infrared frequencies around 100 THz (Manohara et al., 2005). Such nano-Schottkys would bring to the near-infrared the benefits of heterodyne detection that are used so extensively in the microwave (Highstrete et al., 2006). Detectors Based on Quantum Wires and Carbon Nanotubes Devices that convert photons into electrical signals typically fall in two categories: those used ­simply to detect an optical signal (photodetectors) and those used to convert optical energy into electrical energy (photovoltaic devices or “solar cells”). In the photodetector area, one of the challenges is to detect low light intensities. Nanowires and carbon nanotubes offer potential because of their small size and ­ballistic electron transport. For example, a recent approach has used carbon nanotubes functionalized with chromophores to optically switch transistors with low-intensity UV light (Simmons et al., 2007). In the photovoltaic area, current approaches use either inorganic or organic materials. For maximum efficiency, inorganic solar cells require a semiconductor with a direct band gap such as GaAs. However, the cost per watt of such solar cells exceeds economic practicality because of the high cost of fabrication with such materials. On the other hand, silicon solar cells can be fabricated at much lower cost, but because Si has an indirect band gap, the efficiency is not as high. A similar situation arises with organic solar cells, which are inexpensive but have low efficiencies. Nanowires and nanotubes may provide solutions to these problems. For example, not only are all the band gaps in all carbon nanotubes direct, but each carbon nanotube possesses a series of direct band gaps that span the infrared, the visible, and the ultraviolet. Furthermore, because the transport in nanowires and carbon nanotubes may be made ballistic, energy loss to scattering is reduced, giving additional improvements in efficiency. Recent calculations for carbon nanotube p-n junction devices have indicated that relatively high energy conversion efficiencies can be achieved (Stewart and Leonard, 2005; Stewart and Léonard, 2004); experimental realizations (Lee, 2005) of such devices show that they behave as near-ideal diodes with high conversion efficiencies (Lee, 2005; Stewart and Leonard, 2005; Stewart and Léonard, 2004). Quantum-Confined Stark Effect Electro-Absorption Modulators Early in the development of single- and multi-quantum well lasers, workers observed that the e ­ xcitonic absorption edge of quantum wells red-shifts (moves to longer wavelength or lower energy) under the influence of an applied field (Yuh and Wang, 1988). This characteristic was quickly exploited to create external optical modulators for high-speed telecommunications over optical fiber. By placing several quantum wells in the center of a reverse-biased p-n junction and engineering the band gap to sweep through the transmitted photon energy under the applied voltage, quantum-confined stark effect (QCSE) electro-absorption modulators (EAM) can be made to reduce optical transmission by 35 deci- bels (dB) or more (Kawano et al., 1997; Takeuchi et al., 1997). These EAMs quickly replaced direct modulation of laser diodes for long-haul telecommunications because of their superior modulation speed and low residual wavelength modulation, or chirp. Today, QCSE EAMs have shown direct electrical modulation bandwidths in excess of 60 GHz, with some highly specialized structures exceeding 150 GHz (Kodama et al., 2004).

70 Nanophotonics With the development of quantum well intermixing (QWI), these QCSE EAMs can be monolithically integrated with QW diode lasers and other optical devices such as photodiodes. QWI is an extremely powerful post-growth nanoengineering technique by which the width of a QW can be reduced with sub- nanometer precision causing the zero-field absorption edge to blue-shift as much as 120 nm (Skogen et al., 2002). Intermixing of quantum wells is achieved by artificially lowering the thermodynamic barrier to diffusion of individual atoms within the crystal lattice, having the effect of locally grading the as-grown abrupt heterojunction. Grading the band gap of the heterojunction alters the shape of the quantum well potential, giving the desired blue-shift. One method used to initiate intermixing is to implant phosphorus (P) atoms into InP. Under thermal treatment these excess P atoms and In vacancies diffuse through the crystal causing local intermixing. Together the development of QW lasers and QCSE EAMs with QWI have played a major role in the development of modern photonic integrated circuits (PICs). New Class of Optoelectronic Devices Based on Intraband Transitions Band Structure Engineering: Overthrowing Nature’s Band Gap Tyranny Up until now, this section of the report has discussed a number of new types of optoelectronic devices enabled by the introduction of nanophotonic structures. Thus, for instance, the QD laser promises great enhancements in performance owing to the confined density of states of the QD. The basic operating principle, however, remains similar to that of conventional semiconductor lasers. At this point, however, let us turn to a set of completely novel nanophotonic device operating princi- ples enabled by quantum confined structures: those based on intraband transitions. These devices employ energy-level control and population through band structure engineering, modulation doping, and electron tunneling. The canonical examples include quantum well infrared photodetectors (QWIPs) (Levine et al., 1987) and quantum cascade lasers (Faist et al., 1996). Unlike their conventional ­predecessors, these devices depend on using the physical concepts of tunneling, superlattice energy bands, and quantum confinement to create new optical devices whose properties depend only secondarily on the intrinsic characteristics of the semiconductor materials from which they are made. Rather, their properties are primarily determined by the thickness of the few-nanometer-thick layers that compose them—that is, the quantum confinement effects of the quantum wells and the tunneling characteristics of the ­barriers between them. Indeed, these devices are enabling a revolutionary new generation of optoelectronic devices operating in wavelength regions previously inaccessible to solid-state technology. Their revolu- tionary character arises in their use of engineered electron transitions solely in a single-electron band. Indeed, many of these devices are actually unipolar optoelectronics, since they involve carriers of only one charge polarity, either electrons or holes. By contrast, most optoelectronic devices heretofore employed electronic transitions between the conduction and valence bands, involving both electrons and holes. This meant that device characteristics were almost entirely determined by the band gap of the material. Hence, by incorporating new physical concepts and approaches, scientists have been able to overthrow nature’s band gap tyranny and access new spectral regions. QWIP Detectors One of the first engineered unipolar optoelectronic devices was the quantum well infrared photo­ detector, or QWIP (Levine, 1993). The QWIP uses a series of quantum wells with typically n-type doping to populate the ground state of the QW. Under bias in the dark, the structure draws little current since all the electrons are trapped in the ground-state level of the QWs. However, when exposed to

NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 71 light, the ­electrons are excited out of the QWs and into continuum energy levels, where they are free to carry ­ current to the device contacts. Conceptually, the QWIP can be thought of as an engineered photoconductor, where instead of electrons being excited out of defect levels determined by nature, they come from QW subband energy levels engineered by the magnitude of the QW potential and the width of the QW. The QWIP is one of the most successful optoelectronic devices based on quantum confinement reduced-dimensionality effects. Large-imaging focal plane arrays (FPAs), 2,000 x 2,000 pixels, are now manufactured. At operating temperatures above 70 K, the detectivity of the QWIP is considerably lower than that of HgCdTe photodiodes; however, they have the advantage of not being subject to the high degree of defect problems that HgCdTe exhibit. Additional innovations being explored in QWIP-type structures include, for instance, voltage-tunable two-color QWIPs, which could allow increased levels of target discrimination (Majumdar et al., 2003). One extension of QWIPs that holds promise for matching and exceeding the performance of HgCdTe photodiodes is QDIPs, or quantum dot infrared photodetectors. The quantum dots are formed through the strain-energy-driven two-dimensional to three-dimensional morphology transition when compressively strained films (InAs or InGaAs) are grown on larger band gap matrices (GaAs, AlGaAs, or InGaP). The QD ground state is populated with electrons by intentional n-type doping. Historically, efforts on QDIPs looked at quantum dot inclusion in a GaAs matrix (Chakrabarti et al., 2004; Pal et al., 2003). This approach requires tuning of the QD electronic levels to change the wavelength response and makes any device extremely sensitive to process changes. Also the wavefunction overlap between the occupied QD ground state and the continuum of states in the GaAs matrix is predicted to be low. An improved approach is to utilize a dot-in-a-well (DWELL) structure that couples the ground state of the QD to the unoccupied state that is determined by the QW width (Raghavan et al., 2004). Changing the QW width can vary the transition energy between the QD to QW. This design also results in a larger wavefunction overlap between bound states in the QD and the QW. QDIP devices have been the subject of intense ­interest as a possible replacement for HgCdTe and as a possible route to VLWIR radiation sensors (Krishna et al., 2003). The development of QDIP sensors could then leverage the developed FPA technology for QWIP FPAs to provide a higher-yield, lower-cost MWIR and LWIR imaging solution. QDIPs have several potential advantages over QWIPs: sensitivity to normal incidence radiation, increased responsivity due to increased excited carrier lifetime, and higher-temperature operation due to reduced overlap of the QD density of states with the three-dimensional carrier distribution. The three- dimensional structure of the quantum dot allows it to couple to normal incidence radiation. This elimi- nates the need for surface patterning, such as gratings to couple only a fraction of the normal incidence radiation into the QWIP. The increased carrier lifetimes in quantum dots, tens of nanoseconds compared with tens of picoseconds for quantum wells, is attributed to reduced carrier-phonon coupling and results in higher detectivity. Higher-temperature operation is a result of reduced overlap of the quantum dot density of states with the distribution of states in the matrix. If these advantages can be realized, QDIPs should represent a superior alternative to QWIPs for infrared sensing. Displacing HgCdTe photodiodes is more difficult for middle-wavelength-infrared (3 µm to 7 µm) imaging. QDIPs represent at best an alternative technology in this part of the spectrum. At longer wavelengths, QDIPs should be competitive with HgCdTe photodiodes, and for VLWIR (>15 µm) QDIPs represent an opportunity to obtain radia- tion sensors operating at 70 K. State-of-the-art QDIPs are at best equal to QWIPs in performance. Most of this development has targeted the LWIR atmospheric window (8 µm to 12 µm). The development of QDIP sensors could easily leverage the development of QWIP FPA technology to provide a higher-yield, lower-cost LWIR imaging solution.

72 Nanophotonics Quantum Cascade Lasers Quantum cascade lasers are one of the most dramatic examples of a completely new device operat- ing principle emerging from the ability to confine electron levels in reduced-dimensionality structures. The first QCL was demonstrated in 1994, at a wavelength of 4.2 µm (Faist et al., 1994). QCLs operate by engineering the electron energy levels and tunneling coefficients in a multilayer structure. The device is an all-electron device, with the only the conduction band having any influence on its operation, and it can be fabricated in many material systems but is typically made with InGaAs/InAlAs or AlGaAs/GaAs heterostructures. While many design variations exist, a single cell or “unit period” of the QCL can be thought of as a three-level structure where the energy separations between the levels are completely engineered, allowing the designer to choose the operating wavelength. ­Electrons are injected electrically into the upper energy level and transition to the middle level by emitting a desired photon. The electron transition to the lowest level from the middle level is engineered to keep the middle energy level at a very low population, enabling population inversion between the top two levels. Since the electrons are still in the conduction band, by clever use of band structure engineering and tunneling, the electrons are transported from the lowest level into the highest level of the next cell, allowing the process to be repeated. By stacking many cells together—up to 200 or more for the longest wavelengths—sufficient gain can be achieved that the structure will produce lasing. Thus, a single electron will emit a photon for each cell it traverses, resulting in a cascade-like motion as it moves through the QCL structure, which is of course the origin of the name for the device. QCLs are a triumph of band structure engineering, simulation and modeling, and high-precision epitaxial growth techniques. The energy levels, tunneling probabilities, wavefunction distributions, and decay rates must all be extremely well designed and controlled. A typical design is to space the middle and lowest levels apart by the optical phonon energy, ensuring that the middle level is rapidly depopu- lated so that population inversion can occur. QCL emission wavelengths are limited to less than the conduction band offset in the host ternary compound semiconductor system—for example, to less than the conduction band offset energy in the material system (and in actuality about half the offset). In practice, QCLs have been demonstrated with wavelengths between 3 to 24 microns and between 60 and 200 microns, covering large portions of the electromagnetic spectrum from the mid-infrared out to the terahertz. Their advantages, in addition to a broad range of wavelength ranges, include high power and high-temperature operation. (Of course, achievable powers and operating temperatures are a strong function of wavelength. Record powers are near 10 watts (W) in the mid-IR and 250 milliwatts (mW) in the terahertz, while record operating tem- peratures are >300 K in the mid-IR and up to 164 K in the terahertz. Terahertz Quantum Cascade Lasers The first quantum cascade laser to operate in the terahertz frequency range was reported in 2001 (Kohler et al., 2002). Now terahertz QCLs have been demonstrated to operate with frequencies from 1.6 THz to 4.9 THz and record powers of 250 mW (pulsed) and 140 mW (continuous wave) ­(Williams et al., 2006). Prior to this, the standard for continuous-wave terahertz lasers was molecular gas lasers. These bulky and expensive systems consist of a meter-long gas tube, pumped by a CO2 laser in another meter- long tube, and weigh on the order of 100 kilograms (kg). The development of terahertz QCLs enables highly compact (less than 1 mm long), low-weight, and inexpensive laser sources in this frequency regime. By integrating terahertz QCLs with coherent detectors, it may be possible to build compact terahertz transceivers with far greater sensitivity and frequency resolution than that of direct detection

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The Committee on Technology Insight-Gauge, Evaluate & Review set up by the NRC at the request of the Defense Intelligence Agency, has selected a number of emerging technologies to investigate for their potential threats to and opportunities for national security. This first study focused on emerging applications of nanophotonics, which is about the interaction of matter and light at the scale of the wavelength of the light. Manipulation of matter at that scale allows tailoring the optical properties to permit a wide-range of commercial and defense applications. This book presents a review of the nanoscale phenomena underpinning nanophotonics, an assessment of enabling technologies for developing new applications, an examination of potential military applications, and an assessment of foreign investment capabilities

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