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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.
