9

Energy

INTRODUCTION

Materials are playing an increasingly important role in the technological evolution of photonic and optical applications. Whether the applications are related to imaging of cellular functions, the development of new types of sensors and solar cells, or the integration of materials for optoelectronics, the study of techniques to alter how light interacts with materials has become an important element in the advancement of various applications. Moreover, defense applications require an assured and secure manufacturing source of key materials. While materials were not called out in the National Research Council’s (NRC’s) 1998 report Harnessing Light: Optical Science and Engineering for the 21st Century,1 the role of materials in optics and photonics has become much greater since its publication. Engineered materials, including photonic crystals, have come of age. We have seen key optoelectronic materials play an important role in negotiations between countries. Material development has always been a slow and expensive process, but with the advent of engineered materials and the rise of awareness of the importance of certain materials, materials have taken on a strong enough strategic importance to warrant their own chapter in the present report.

One specific challenge that optics faces in most industries where it is being used is that optics remains an enabling technology, supporting another area such

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1 National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press.



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9 Strategic Materials for Optics INTRODUCTION Materials are playing an increasingly important role in the technological evolu- tion of photonic and optical applications. Whether the applications are related to imaging of cellular functions, the development of new types of sensors and solar cells, or the integration of materials for optoelectronics, the study of techniques to alter how light interacts with materials has become an important element in the advancement of various applications. Moreover, defense applications require an assured and secure manufacturing source of key materials. While materials were not called out in the National Research Council’s (NRC’s) 1998 report Harnessing Light: Optical Science and Engineering for the 21st Century,1 the role of materials in optics and photonics has become much greater since its publication. Engineered materials, including photonic crystals, have come of age. We have seen key op- toelectronic materials play an important role in negotiations between countries. Material development has always been a slow and expensive process, but with the advent of engineered materials and the rise of awareness of the importance of certain materials, materials have taken on a strong enough strategic importance to warrant their own chapter in the present report. One specific challenge that optics faces in most industries where it is being used is that optics remains an enabling technology, supporting another area such 1  National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press. 248

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S t r at e g i c M at e r i a l s for Optics 249 as cancer treatment or welding. The principal value of a product is not always at- tributed to the optical technologies associated with the product’s applications. For example, developing new biological materials, such as fluorescent proteins,2 that have new optical characteristics is associated more with new advances in biotech- nology than with optical imaging. This chapter outlines the role that strategic materials play in the development of new optical phenomena in specific categories of applications, outlines a few key technological problems that need to be solved to enhance the impact of these materials on the evolution of those applications, and, finally, identifies a set of chal- lenges that need to be addressed by policy makers to support the research needed to solve the technological problems.3 ENERGY APPLICATIONS The Sun has been identified as one of the primary sources of alternative energy as the United States transitions from a fossil-fuel-driven energy infrastructure in the next 20 years. However, for solar energy to become a viable and cost-effective source, its price needs to drop significantly. As discussed in Chapter 5, energy criti- cal elements (ECEs) were identified as extremely important for the development of thin-film photovoltaics (TFPV), which will be instrumental in meeting cost targets. The ECEs are critical because of their limited supply in the United States. Gallium, germanium, indium, selenium, silver, and tellurium are all critical elements for de- velopment of TFPV. A challenge for the United States in connection with the ECEs is to determine whether there is a need to produce them in the United States as opposed to relying on a foreign supply.4 For example, most of the easily extractable lithium in the world is localized in South American countries. Lithium is present in most of the promising battery technologies being produced and developed, and access to this material could be restricted by government intervention. Several of the exotic elements required by emerging solar technologies rely on joint production methods, which pose another potential risk to scale-up. These 2  The 2008 Nobel Prize in chemistry was awarded jointly to Martin Chalfie, Osamu Shimomura, and Roger Y. Tsien “for the discovery and development of the green fluorescent protein, gfp.” More information is available at http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2008/. Ac- cessed May 31, 2012. 3  For further discussion of strategic materials, such as erbium and other rare earths, see National Research Council. 2012. The Role of the Chemical Sciences in Finding Alternatives to Critical Resources: A Workshop Summary. Washington, D.C.: The National Academies Press. 4  American Physical Society (APS) and Materials Research Society (MRS). 2011. Energy Critical Elements: Securing Materials for Emerging Technologies. A report by the APS panel on public affairs and the MRS. Available at http://www.aps.org/policy/reports/popa-reports/loader.cfm?csModule= security/getfile&PageID=236337. Accessed February 10, 2011.

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250 Optics and Photonics: Essential Technologies for O u r N at i o n materials are currently produced as by-products of refinement or extraction of more common materials and are bought from suppliers as waste products. In- creased demand for these materials may surpass the amount extracted with current production methods, thus placing a risk on the scaling up of systems reliant on these elements. Elements that are known to pose joint-production risks are gallium, tellurium, and selenium. Gallium, an essential component of many high-efficiency thin-film and multi-junction photovoltaic cells, is obtained as a by-product of aluminum refining, and tellurium and selenium are most often obtained as by- products of copper refining. The locations of these common production metals will determine production of these ECEs and are shown in Figure 9.1. Cadmium is usually produced from zinc processing but is refined out because of its toxic- ity and thus is commercially available at relatively low and stable prices and not qualified as an ECE.5 The use of light-emitting diodes (LEDs) for lighting is another example of how optics is affecting energy conservation in several industrial nations. Several coun- tries around the world have initiatives to phase out the use of incandescent light- bulbs by 2020. LEDs have been identified as the leading replacement technology for this mass market. One promising approach to obtain white light is to use gallium- nitride (GaN)-based blue LEDs in conjunction with fluorescent phosphors, which convert part of the blue light into yellow and red. Many of the efficient phosphors contain rare earth elements (REEs). The two key REEs that provide color LED light- ing are europium and terbium. Currently, the majority of the REEs are produced in China, and the Chinese government has imposed export restrictions.6 The United States will have to develop a novel strategy either to develop new materials for LED applications or to encourage research toward cost-effective and environmentally friendly purification, mining, and production. Finally, the United States should consider reclamation from waste to extract rare materials inasmuch as U.S. waste management derives most of its profit from processing waste to remove precious and rare metals and other substances. NOVEL STRUCTURES: SUB-WAVELENGTH OPTICS, METAMATERIALS, AND PHOTONIC CRYSTALS In addition to new core materials, there is much promise in tailoring existing materials in novel ways to produce innovative results. These new materials, known as metamaterials or nanophotonic materials, are materials that can be developed 5  APS and MRS. 2011. Energy Critical Elements. 6  Bradsher, K. 2010. “China Said to Widen Its Embargo of Minerals.” New York Times. October 19. Available at http://www.nytimes.com/2010/10/20/business/global/20rare.html?pagewanted=all. Accessed August 29, 2011.

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S t r at e g i c M at e r i a l s for Optics 251 FIGURE 9.1  Location of current copper and aluminum-ore production. Energy critical elements required for several emerging solar technologies are obtained as by-products of refinement.

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252 Optics and Photonics: Essential Technologies for O u r N at i o n to exhibit new optical properties that the original materials themselves would not naturally possess. Structuring materials with features less than or close to one wavelength of light can lead to these novel properties, with the optical behavior coming more from the nanopatterning or nanostructuring than from the specific underlying materials. Such sub-wavelength structuring can be used with metals, semiconductors, or dielectrics, including combinations of these. The resulting effective materials or complex structures with sub-wavelength patterning allow control of such properties as spectral and polarization dependence in transmission, refraction, reflection, absorption, and emission of light. If a sub-wavelength struc- ture is periodic in two or three dimensions, it is called a photonic crystal (PhC)7,8,9 because the structure shows effects on the motions of an optical wave similar to the effects of a semiconductor crystal on an electron wave.10,11 Since its inception, rapid progress has been made on this topic. Complex PhC structures with varied dimensions have been reported; these structures allow devices to be made that can control properties such as spectral and polarization dependence in transmission, refraction, reflection, absorption, and emission of light.12,13,14 Interesting metal patterns have been added to such periodic structures to produce negative refractive index and superlens effects;15,16 these patterns can focus light below the conven- tional diffraction limit. PhC devices are also being actively deployed in biosensors, semiconductor lasers, and hollow-core fibers.17,18,19,20 A new class of dielectric sub-wavelength grating exhibits very different prop- 7  Yablonovitch, E. 1987. Inhibited spontaneous emission in solid-state physics and electronics. Physical Review Letters 58:2059-2062. 8  John, S. 1987. Strong localization of photons in certain disordered dielectric superlattices. Physical Review Letters 58:2486-2489. 9  Joannopoulos, J.D., P.R. Villeneuve, and S. Fan. 1997. Photonic crystals: Putting a new twist on light. Nature 386:143-149. 10  Yablonovitch, E. 1987. Inhibited spontaneous emission in solid-state physics and electronics. 11  John, S. 1987. Strong localization of photons in certain disordered dielectric superlattices. 12  Joannopoulos et al. 1997. Photonic crystals: Putting a new twist on light. 13  Busch, K., G. von Freymann, S. Linden, S.F. Mingaleev, L. Tkeshelashvili, and M. Wegener. 2007. Periodic nanostructures for photonics. Physical Review Letters 444:101-202. 14  Painter, O., R.K. Lee, A. Scherer, A. Yariv, J.D. O’Brien, P.D. Dapkus, and I. Kim. 1999. Two- dimensional photonic band-gap defect mode laser. Science 284(5421):1819-1821. 15  Pendry, J.B. 2000. Negative refraction makes a perfect lens. Physical Review Letters 85:3966-3969. 16  Zhang, X., and Z. Liu. 2008. Superlenses to overcome the diffraction limit. Nature Materials 7:435-441. 17  Skivesen, N., A. Têtu, M. Kristensen, J. Kjems, L.H. Frandsen, and P.I. Borel. 2007. Photonic- crystal waveguide biosensor. Optics Express 15:3169-3176. 18  Noda, S., M. Yokoyama, M. Imada, A. Chutinan, and M. Mochizuki. 2001. Polarization mode control of two-dimensional photonic crystal laser by unit cell structure design. Science 293:1123-1125. 19  Knight, J.C. 2003. Photonic crystal fibres. Nature 424:847-851. 20  Russell, P. 2003. Photonic crystal fibres. Science 299:358-362.

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S t r at e g i c M at e r i a l s for Optics 253 erties from PhCs or traditional gratings. This grating leverages a high contrast in refractive indexes for the grating medium and its surroundings and sub-wavelength period to lead to extraordinary properties; hence the name high-contrast grating.21,22 They can be easily designed to exhibit super-broadband, high-reflectivity mirrors for light incident in a surface-normal direction and at a glancing angle, ultra- high-Q (over 106) resonators with surface-normal output, planar high-focusing power reflectors and lenses (numerical aperture, over 0.9), ultra-low-loss hollow- core waveguides, slow-light waveguides,23 and high-efficiency vertical to in-plane waveguide couplers.24 The United States has several research groups working on these types of ma- terials that can potentially lead to solar cells that more efficiently absorb and trap sunlight and convert it to electrical energy. Researchers have designed materials that can bend visible light at unusual but precise angles, regardless of its polarization,25 as a step toward perfectly transparent solar cell coatings that would direct all the Sun’s rays into the active area to improve solar power output. In addition, research- ers are working on novel antireflective solar cell coatings in the hope of getting more light into the cells.26,27 Finally, metastructured materials can be used to sepa- rate the solar spectrum efficiently to optimize solar energy conversion.28 The versatility of metamaterials and photonic nanostructures in tailoring optical properties could lead to radical improvements and completely new types of devices. Metamaterials have been proposed for superlens applications29 where they can to some extent go beyond the conventional diffraction limit of focusing in some special situations. Moreover, PhC devices are being deployed in biosensors, 21  Vlasov, Y.A., M. O’Boyle, H.F. Hamann, and S.J. McNab. 2005. Active control of slow light on a chip with photonic crystal waveguides. Nature 438:65-69. 22  Mateus, C.F.R., M.C.Y. Huang, Y. Deng, A.R. Neureuther, and C.J. Chang-Hasnain. 2004. U ­ ltrabroadband mirror using low-index cladded subwavelength grating. IEEE Photonics Technology Letters 16:518-520. 23  Chang-Hasnain, C.J. 2011. High-contrast gratings as a new platform for integrated optoelectron- ics. Semiconductor Science and Technology 26:014043. 24  Stöferle, T., N. Moll, T. Wahlbrink, J. Bolten, T. Mollenhauer, U. Scherf, and R. Mahrt. 2010. An ultracompact silicon/polymer laser with an absorption-insensitive nanophotonic resonator. Nano Letters 10:3675-3678. 25  Burgos, S.P., R. de Waele, A. Polman, and H.A. Atwater. 2010. A single-layer wide-angle negative- index metamaterial at visible frequencies. Nature Materials 9:407-412. 26  Xi, J.Q., M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.Y. Lin, W. Liu, and J.A. Smart. 2007. Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection. Nature Photonics 1:176-179. 27  Diedenhofen, S.L., G. Vecchi, R.E. Algra, A. Hartsuiker, O.L. Muskens, G. Immink, E.P.A.M. B ­ akkers, W.L. Vos, and J.G. Rivas. 2009. Broad-band and omnidirectional antireflection coatings based on semiconductor nanorods. Advanced Materials 21:973-978. 28  Burgos et al. 2010. A single-layer wide-angle negative-index metamaterial at visible frequencies. 29  Zhang and Liu. 2008. Superlenses to overcome the diffraction limit.

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254 Optics and Photonics: Essential Technologies for O u r N at i o n semiconductor lasers, and hollow-core fibers.30,31,32,33 Higher-dimensional meta- materials or custom nanophotonic structures provide extra design “knobs” to turn to modify optical characteristics and design devices with no precedent in conven- tional optics. Whereas one-dimensional photonic nanostructures have long been used in commercial products in the form of Bragg reflectors, dielectric interference filters, or antireflection coatings, higher-dimensional photonic nanostructures are not easy to fabricate and require more research on process technology. TECHNOLOGY CHALLENGES OF NANOSTRUCTURED MATERIALS Nanostructured materials became a subject of great interest because of the promise to “tailor” materials’ innate physical properties when they are made small enough for the wave functions of electrons, phonons, or photons to be signifi- cantly confined by the structured boundaries. Control of matter on nanometer scales would allow manipulation of absorption, emission, transmission, refraction, transport, and energy conversion and storage in innovative ways that could have profound implications for many applications. The applications of nanostructures for optoelectronic devices started when one-dimensional confinement structures, quantum wells, were demonstrated in 1974.34 Double heterostructured material with a thickness under 20 nm was shown to provide quantum confinement of electrons to alter the material’s effective band- gap energy. Today, most semiconductor diode lasers and integrated optoelectronics use quantum wells as a means to control wavelength, reduce threshold, and provide modulation. In the near future, it will be possible to extend this quantum confine- ment to two-dimensional (quantum wires) and three-dimensional (quantum dots, QDs) to achieve temperature-independent, ultra-low-threshold laser diodes and ultra-broadband semiconductor optical amplifiers. In addition to changing the effective bandgap due to the quantization effect, nanostructures may enable monolithic integration of lattice-mismatched materi- 30  Block, I.D., L.L. Chan, and B.T. Cunningham. 2006. Photonic crystal optical biosensor incorpo- rating structured low-index porous dielectric. Sensors and Actuators B: Chemical 120:187-193. 31  Park, H.G., S.H. Kim, S.H. Kwon, Y.G. Ju, J.K. Yang, J.H. Baek, S.B. Kim, and Y.H. Lee. 2004. Electrically driven single-cell photonic crystal laser. Science 305:1444-1447. 32  Allan, D.C., N.F. Borrelli, J.C. Fajardo, D.W. Hawtof, and J.A. West. 2001. Photonic Crystal Fiber. U.S. Patent 6 243 522. 33  Temelkuran, B., S.D. Hart, G. Benoit, J.D. Joannopoulos, and Y. Fink. 2002. Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission. Nature 420:650-653. 34  Dingle, R., W. Wiegmann, and C.H. Henry. 1974. Quantum states of confined carriers in very thin Al{x}Ga(1-x}As-GaAs-Al{x}Ga(1-x}As heterostructures. Physical Review Letters 33:827-829.

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S t r at e g i c M at e r i a l s for Optics 255 als on a single substrate by drastic increases in their critical thicknesses.35,36,37 For materials grown on lattice-mismatched substrates, there are thickness limits below which single-crystal material can be grown on substrates with mismatch. The larger the mismatch is, the lower the critical thickness. Typically, this layer is very thin; for 2 percent lattice mismatch, the thickness is about 8 nm, at which the quanti- zation energy can be easily observed. This is therefore called a strained quantum well. For typical III-V semiconductors, such as gallium arsenide (GaAs), indium phosphide (InP), or indium arsenide (InAs), on silicon (Si), the lattice mismatches are 4 percent, 8 percent, and 12 percent, respectively. Such mismatches are so large that the critical thicknesses are too small for device applications. This is not an issue for nanowires or nanopillars. For about 80-nm-diameter GaAs nanowires on Si, the height can be infinite without mismatches. Moreover, the core/shell layers grown later can accommodate much thicker lattice-mismatched materials. Various devices have already been demonstrated on such III-V nanopillars grown on silicon, including room-temperature operation of LEDs, avalanche photodi- odes, and optically pumped lasers.38,39 Such materials are grown at low enough temperature to be compatible with wafers with fabricated complementary metal- oxide-semiconductor (CMOS) circuits. That makes such nanostructured growth promising for integration of various III-V materials device structures on silicon. Besides III-V on silicon, the recent advancement of germanium (Ge) or ger- manium tin (GeSn) grown on Si has enabled optoelectronics devices operating in the telecommunication region, 1.3 to about 1.6 μm,40,41,42,43 to be fabricated in a 35  Chuang, L.C., M. Moewe, C. Chase, N. Kobayashi, S. Crankshaw, and C. Chang-Hasnain. 2007. Critical diameter for III-V nanowires grown on lattice-mismatched substrates. Applied Physics Letters ­ 90:043115. 36  Glas, F. 2006. Critical dimensions for the plastic relaxation of strained axial heterostructures in free-standing nanowires. Physical Review B 74:121302. 37  Chuang, L.C., M. Moewe, K.W. Ng, T. Tran, S. Crankshaw, R. Chen, W.S. Ko, and C. Chang- Hasnain. 2011. GaAs nanoneedles grown on sapphire. Applied Physics Letters 98:123101. 38  Chuang, L.C., F.G. Sedgwick, R. Chen, W.S. Ko, M. Moewe, K.W. Ng, T.D. Tran, and C. Chang- Hasnain. 2011. GaAs-based nanoneedle light emitting diode and avalanche photodiode monolithi- cally integrated on a silicon substrate. Nano Letters 11(2):385-390. 39  Chen, R., T.-T.D. Tran, K.W. Ng, W.S. Ko, L.C. Chuang, F.G. Sedgwick, and C. Chang-Hasnain. 2011. Nanolasers grown on silicon. Nature Photonics 5:170-175. 40  Fidaner, O., A.K. Okyay, J.E. Roth, R.K. Schaevitz, Y. Kuo, K.C. Saraswat, J.S. Harris, and D.A.B. Miller. 2007. Ge-SiGe quantum-well waveguide photodetectors on silicon for the near-infrared. IEEE Photonics Technology Letters 19:1631-1633. 41  Tang, L., S.E. Kocabas, S. Latif, A.K. Okyay, D. Ly-Gagnon, K.C. Saraswat, and D.A.B. Miller. 2008. Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna. Nature Photonics 2:226-229. 42  Tsybeskov, L., and D.J. Lockwood. 2009. Silicon-germanium nanostructures for light emitters and on-chip optical interconnects. Proceedings of the IEEE 97:1284-1303. 43  Michel, J., J. Liu, and L.C. Kimerling. 2010. High-performance Ge-on-Si photodetectors. Nature Photonics 4:527-534.

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256 Optics and Photonics: Essential Technologies for O u r N at i o n way that is compatible with CMOS electronic device fabrication, opening many new possibilities for the intimate integration of optoelectronics and electronics in manufacturable platforms, Ge-nanostructured emitters44 with high luminescence quantum efficiency, and Ge photodetectors with large 3-dB bandwidth and high responsivity have been demonstrated all on an Si-compatible platform. Strong optical modulation mechanisms previously observed practically only in III-V materials have been demonstrated in Ge quantum well layers grown on silicon.45 Such Ge-based materials and structures on silicon are thus very strong candidates for dense integration of optoelectronics and electronics in such applications as optical data interconnections. Another special feature of nanostructured materials is their large surface- to-volume ratio. For optoelectronic devices, that is detrimental inasmuch as the surface recombination represents dark current46 and non-radiative recombination. However, for sensors and applications requiring large surface-to-volume ratio, it is very desirable.47 The full potential for nanostructured materials is still limited to some extent by non-uniformities and large numbers of defects in currently available materials. In addition, the smaller structures have an associated larger quantization energy; hence the greater the impact of non-uniformity of size. There are two main approaches to synthesizing nanostructured materials: bottom-up and top-down. The former refers to various self-assembly methods, in- cluding molecular-beam epitaxy, metal-organic chemical vapor deposition (CVD), catalyst CVD, electrodeposition, pulsed-laser synthesis, and solution-based syn- thesis. Small and nearly uniform (for example, a few percent to 10 percent non- uniformity) particle-like structures can be created by using chemical solutions.48 Such QDs have seen wide applications in biosensing and bioimaging applica- tions, where the uniformity requirement is not very stringent.49,50 They face ma- jor challenges if one is to make electrical contacts to them or form p-n junctions 44  Tang et al. 2008. Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna. 45  Kuo, Y.-H., Y.-K. Lee, Y. Ge, S. Ren, J.E. Roth, T.I. Kamins, D.A.B. Miller, and J.S. Harris. 2005. Strong quantum-confined Stark effect in germanium quantum-well structures on silicon. Nature 437:1334-1336. 46  Dark current is current present even in the absence of a direct source. 47  Medintz, I.L., A.R. Clapp, H. Mattoussi, E.R. Goldman, B. Fisher, and J.M. Mauro. 2003. Self- assembled nanoscale biosensors based on quantum dot FRET donors. Nature 2:630-638. 48  Alivisatos, P. 2000. Colloidal quantum dots: From scaling laws to biological applications. Pure and Applied Chemistry 72:3-9. 49  Michalet, X., F.F. Pinaud, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, and S. Weiss. 2005. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 307:538-544. 50  Medintz, I.L., H.T. Uyeda, E.R. Goldman, and H. Mattoussi. 2005. Quantum dot bioconjugates for imaging, labelling and sensing. Nature Materials 4:435-446.

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S t r at e g i c M at e r i a l s for Optics 257 with them, however. Hence, they have limited device or integrated optics appli- cations. Self-assembled InAs QDs can be grown on GaAs substrates through the Stranski-Krastanov growth mode.51,52 This type of QD has about 5 to 10 percent non-uniformity. However, recent advances have been made in diode lasers and semiconductor optical amplifiers53,54 to exploit QDs. These are very promising for deployment in the near future. The top-down approach involves various lithography techniques to define the structures, using e-beam lithography, nanoimprint,55,56 or dip pen technologies57 and subsequent etching or growth of materials. These methods can lead to higher uniformity, but they still suffer from surface defects.58,59 OPTICAL MATERIALS IN THE LIFE SCIENCES AND SYNTHETIC BIOLOGY Synthetic biology is a new field of biological research and technology develop- ment that combines science and engineering with the goal of designing and con- structing novel and useful biological systems not found in nature. Synthetic biology has provided the means of both genetically engineering specific optical properties 51  Stranski, Ivan N., and Lubomir Krastanov. 1938. Abhandlungen der Mathematisch- Naturwissenschaftlichen Klasse IIb. Akademie der Wissenschaften Wien 146:797-810. 52  Eaglesham, D.J., and M. Cerullo. 1990. Dislocation-free Stranski-Krastanow growth of Ge on Si(100). Physical Review Letters 64:1943-1946. 53  Zhukov, A.E., A.R. Kovsh, V.M. Ustinov, Y.M. Shernyakov, S.S. Mikhrin, N.A. Maleev, E.Y. Kondrat’eva, D.A. Livshits, M.V. Maximov, B.V. Volovik, D.A. Bedarev, Y.G. Musikhin, N.N. Ledentsov, P.S. Kop’ev, Z.I. Alferov, and D. Bimberg. 1999. Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate. IEEE Photonics Technology Letters 11:1345-1347. 54  Akiyama, T., M. Ekawa, M. Sugawara, K. Kawaguchi, Hisao Sudo, A. Kuramata, H. Ebe, and Y. Arakawa. 2005. An ultrawide-band semiconductor optical amplifier having an extremely high penalty-free output power of 23 dBm achieved with quantum dots. IEEE Photonics Technology Let- ters 17:1614-1616. 55  Vieu, C., F. Carcenac, A. Pepin, Y. Chen, M. Mejias, A. Lebib, L. Manin-Ferlazzo, L. Couraud, and H. Launois. 2000. Electron beam lithography: Resolution limits and applications. Applied Surface Science 164:111-117. 56  Colburn, M., S.C. Johnson, M.D. Stewart, S. Damle, T.C. Bailey, B. Choi, M. Wedlake, T.B. M ­ ichaelson, S.V. Sreenivasan, J.G. Ekerdt, and C.G. Willson. 1999. Step and flash imprint lithography: A new approach to high-resolution patterning. Proceedings of the SPIE 3676:379. 57  Lee, K.B., S.J. Park, C.A. Mirkin, J.C. Smith, and M. Mrksich. 2002. Protein nanoarrays generated by dip-pen nanolithography. Science 295:1702-1705. 58  Cao, X.A., H. Cho, S.J. Pearton, G.T. Dang, A.P. Zhang, F. Ren, R.J. Shul, L. Zhang, R. Hickman, and J.M. Van Hove. 1999. Depth and thermal stability of dry etch damage in GaN Schottky diodes. Applied Physics Letters 75:232-234. 59  Tanaka, S., Y. Kawaguchi, N. Sawaki, M. Hibino, and K. Hiramatsu. 2000. Defect structure in selective area growth GaN pyramid on (111)Si substrate. Applied Physics Letters 76:2701.

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258 Optics and Photonics: Essential Technologies for O u r N at i o n into living organisms and manufacturing optically active materials, both of which are now routinely used in life science research. One of the major advances of the last two decades in the life sciences (the subject of a Nobel Prize in 2008) was the development of genetic engineering techniques that allow the programming of individual cells to produce protein molecules that fluoresce in response to specific stimuli. The approach is used for optical detection of the turning on and turning off of specific genes in cells in response to drugs or environmental conditions. Similarly, neurons engineered to produce fluorescent dyes based on proteins that report the active and inactive states of neurons in living animals are being used to map out the neuronal wiring of living brains and to track the flow of information through neural circuits in live animals. New, highly efficient inorganic dyes have been developed that can be chemically linked to nucleic acids and provide an optical readout of nucleic acid sequences in high-throughput DNA sequencing instruments. The optical materials are a critical element in the technology that will ultimately enable the $1,000 genome, which will help to make possible a new era of personalized medicine.60 New nanostructured materials have also demonstrated new methods for label- ing cells and intracellular organelles with biocompatible optical materials by us- ing nano-scale semiconductor structures (QDs) and nanometer gold spheres and rods. The new materials provide several advantages, including greatly improved resistance to photo-bleaching, tunable and very narrow spectral features, and the ability to functionalize the nanoparticle surface with antibodies to allow it to bind specifically to a wide variety of biological surfaces. FINDINGS This chapter illustrates the strategic role that materials and nanostructuring can play in the development of new optical systems. Below are the findings of the committee regarding strategic materials for optics. Finding:  There is much promise in tailoring existing materials in novel ways to produce innovative results. The new metamaterials and photonic nanostructures enable original optical properties that can be developed for innovative functions that could not be exhibited in traditional materials. Realizing the full potential of nanostructured materials is still hampered by non-uniformities and many defects. The smaller the structure, the more issues with non-uniformity. Finding:  Gallium, germanium, indium, selenium, silver, and tellurium are all criti- cal elements for development of thin-film photovoltaics (TFPV). 60  For more information, see discussions in Chapter 6 and Appendix C of this report.

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S t r at e g i c M at e r i a l s for Optics 259 Finding:  Most current white-light LEDs require rare materials to fluoresce in the proper wavelengths. Many of the efficient phosphors contain rare earth elements. The two key rare earth materials that provide color LED lighting are europium and terbium. RECOMMENDATIONS The committee presents the following recommendations with respect to stra- tegic materials for optics. Recommendation:  The U.S. R&D community should increase its leadership role in the development of nanostructured materials with designable and tailorable optical material properties, as well as process control for uniformity of production of these materials. Recommendation:  The United States should develop a plan to ensure the avail- ability of critical energy-related materials, including solar cells for energy genera- tion and fluorescent materials to support future LED development.