Nanophotonics Accessibility and Applicability (2008) / Chapter Skim
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3 Enabling Technologies
3 Enabling Technologies
Pages 83-130
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From page 83... ...
The development of these enabling technologies in a country is often an important indicator of the state of maturity of nanophotonics in that country. Realizing Hierarchical Synthesis, Growth, and Fabrication Structures at the Nanoscale Introduction Traditionally, synthesis, growth, and fabrication have been separately identified stages in the development of functional devices.
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From page 84... ...
Growth as used here is specifically limited to the epitaxial growth of semiconductor materials; it also includes self-assembled arrays via nanoparticle formation, such as Stranski-Krastanov formation of quantum dots by the interplay between strain and surface tension. Fabrication refers to the creation of ensembles of nanostructures -- for example, photonic crystals composed of nanoparticles.
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From page 85... ...
An example is shown in Figure 3-2, where an overcoating of zinc sulfide (ZnS) onto CdSe quantum dots (QDs)
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From page 86... ...
ZnS outer shell passivates CdSe quantum dots. The silica core particles are typically synthesized using the Stöber process (Stöber et al., 1968)
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From page 87... ...
The discovery that various ligands and co-ligands can promote anisotropic growth has led researchers to create a number of nanorods that afford uniform size and shape and good electrooptical properties, controlled by the molar ratio of the components to the coordinating solvent as well as annealing time. Nanowires are typically grown using chemical vapor deposition (CVD)
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From page 88... ...
Organic Materials Organic nanophotonic materials are more difficult to control with the structural precision of inorganic materials, but chemists can create intricate molecules and supramolecular assemblies with extremely highly specific interactions with chemicals and biological agents. Also, organics have an enormous range of electrooptical properties and are flexible hosts for metallic and inorganic materials to create multifunctional devices.
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From page 89... ...
. Reproduced with permission from Journal of Physics D: Applied Physics.
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From page 90... ...
particles can modify the dielectric properties of the assembly to provide, for example, increased dielectric contrast for photonic crystals. Liquid Crystals Liquid crystals spontaneously form modulated phases that are photonic band-gap materials and hence are interesting and useful for nanophotonics applications.
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From page 91... ...
ENABLING TECHNOLOGIES 91 a b FIGURE 3-4 Optically pumped mirrorless lasing by a dye-doped cholesteric liquid- crystal elastomer. The lasing wavelength is tuned by stretching the rubber laser (which results in compression of the one-dimensional Bragg reflector in the direction of the film thickness)
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From page 92... ...
Reflection band Laser emission λem = 668 nm FIGURE 3-5 Self-assembled liquid crystals can form one-dimensional reflectors and exhibit band-edge lasing.
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From page 93... ...
Recent research shows that materials, whose building blocks are not molecules, but nanoparticles, also show liquid-crystal phases. Examples are clays and mineral suspensions, such as gibbsite platelets and goethite nanorods, as well as synthesized semiconductor nanorods and viruses such as tobacco mosaic virus.
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From page 94... ...
Colloidal Synthesis The large effort at creating self-assembled photonic crystals to create a band gap in the visible spectrum employing colloids demands the synthesis of highly monodisperse spheres at targeted diameters (on the order of the wavelength)
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From page 95... ...
, quantum wires, and quantum dots) are not broadened beyond fairly specific requirements, typically determined by the particular application.
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From page 96... ...
Thus, development of these capabilities requires long lead times, and new activities of this type should be very apparent; it is unlikely that new participants in such activities would appear abruptly. Fabrication Planar Processing Approaches The integrated circuit industry has developed a broad suite of manufacturing tools that allow fabrication on scales that are immediately relevant to nanophotonics.
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From page 97... ...
Extensions to three-dimensional patterning such as multibeam interference are being explored specifically for photonic crystal applications. Optical Lithography Optical lithography is well established as the manufacturing technology of choice.
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From page 98... ...
Laser writing (etching) and focused ion-beam etching and/or material deposition can be used to place defects in twodimensional photonic crystals.
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From page 99... ...
ENABLING TECHNOLOGIES 99 FIGURE 3-8 A novel method for producing large-area three-dimensional nanostructured quasi-crystalline materials uses two-dimensional multiple-exposure lithography to produce an octagonal quasi-periodic surface-relief template 3-8 (background image)
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From page 100... ...
. Lin and colleagues developed a means of forming three-dimensional photonic crystals by planar processing using a repetitive succession of formation of silicon (Si)
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From page 101... ...
Recently, the stack-and-draw technique has been successfully used to fabricate photonic crystal fibers from sulfur hexafluoride glass (Wolchover et al., 2007) , a soft glass with high optical nonlinearity.
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From page 102... ...
They are then sleeved 10 about twice this diameter and drawn down to their final size, typically 100 microns to 400 microns in diameter. The first successful photonic crystal fibers were demonstrated using silica in 1995 (Knight et al., 1996)
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From page 103... ...
ENABLING TECHNOLOGIES 103 FIGURE 3-11 Example of top-down and bottom-up directed assembly. Templating of spherical block polymer domains within one-dimensional templates of varying width and scanning electron microscope micrographs of ordered arrays of spherical domains with N = 2 to 12 rows.
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From page 104... ...
Photonic crystals made of or incorporating organic materials are likely to provide sensitivity and specificity for the detection of chemical and biological agents. Modeling and Simulation in Nanophotonics One major reason for the rapid progress in the field of nanophotonics globally is the increasing availability of powerful computational methods for the design and simulation of nanophotonic structures, devices, and systems.
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From page 105... ...
This tool, rapidly becoming a world standard in both academia and industry, provides FEM solvers for electromagnetics as well as all major engineering disciplines, including heat transport and fluid flow, in a user-friendly environment that allows for the simultaneous solution of multiple coupled problems, such as electromagnetics and heat dissipation in a device structure. COMSOL is currently used by scores of nanophotonics research groups worldwide.
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From page 106... ...
This is highly useful for simulations for which the resonant frequencies are not known precisely (such as in the modes of a photonic crystal waveguide or resonator)
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From page 107... ...
. An emerging new approach is fast multipole methods (FMMs)
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From page 108... ...
This is seen most clearly in complex nanostructures, where plasmons on neighboring structures or surfaces interact, for then the plasmons mix and hybridize just like the electron wave functions of simple atomic and molecular orbitals. This property, termed plasmon hybridization, governs the optical properties of metallic nanostructures of increasingly complex geometries, providing the scientist with a powerful and general design principle that can be applied to guide the design of metallic nanostructures and to predict their resonant properties.
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From page 109... ...
Other characterization techniques for nanophotonics based on linear and nonlinear optical spectroscopies, both in the time and the frequency domains, are described in Chapter 2 of this report, in the section entitled "Techniques for Imaging and Spectroscopy of Plasmonic Structures." Other nonlinear optical microscopies can be used to provide even more information. For example, nonlinear optical techniques such as multiphoton excitation and Raman spectroscopy are now being used to image specific chemicals or nanostructures, with three-dimensional submicron resolution, and to study time-dependent processes involving these species.
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From page 110... ...
: The NSOM scans a very small light source very close to the sample. Detection of this light energy forms the image.
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From page 111... ...
ENABLING TECHNOLOGIES 111 143.30 nm 5 µm 5 µm 2.5 µm 2.5 µm 0.00 nm 0 µm 0 µm 5 µm 0 µm 2.5 µm 5 µm 0 µm 2.5 µm FIGURE 3-14 Topographic image (left) and near-field scanning optical microscope (NSOM)
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From page 112... ...
Nanophotonics Devices Wavelength-Scale Devices Wavelength-scale devices include photonic crystal-type devices that have a periodicity on the order of a vacuum wavelength, but they do incorporate precision at the nanometer scale, justifying the nanophotonic appellation. Of particular note is the success of fabrication methods for photonic crystal fibers.
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From page 113... ...
These lithographic grating couplers have now demonstrated mode-matching and insertion loss of less than 1 decibel (dB) and are part of the normal process flow in integrated circuit manufacturing.
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From page 114... ...
Flip-chip bonded lasers Silicon Optical Filters - DWDM Silicon 10G Modulators wavelength 1550nm electrically tunable driven with on-chip circuitry highest quality signal passive alignment integrated w/ control circuitry low loss, low power consumption non-modulated = low cost/reliable enables >100Gb in single mode fiber Complete 10G Receive Path Ge photodetectors trans-impedance amplifiers output driver circuitry The Toolkit is Complete • 10Gb modulators and receivers Fiber cable plugs here • Integration with CMOS electronics • Cost effective, reliable light source • Standard packaging technology Ceramic Package 4 FIGURE 3-18 Luxtera complementary metal oxide semiconductor�������������������������������������� ����������������������������������������������������������������������������� (CMOS) photonics technology.
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From page 115... ...
The optical antenna structures can be in the form of dipole antenna, monopole antenna, slot antennas based on Babinet's principle, grating couplers that resemble Yagi rooftop television antennas, and so on. Among the recent insights in this field is the recognition that well-designed focusing structures can focus light down to dimensions of a few- nanometers with reasonable efficiency. Some examples of these optical nano-antennas are shown in Figures 3-19 and 3-20.
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From page 116... ...
c E(ω) Ag Ag Complementary C-slot antenna Bow-tie slot antenna FIGURE 3-20 Some slot antenna options that are being promoted for capturing free-space lightwaves and focusing them to produce a high optical electric field E(ω)
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From page 117... ...
Technology Environment Nanophotonics -- at least in its current form -- relies heavily on semiconductor and CMOS fabrication capabilities and their associated package and integration technologies. In fact, nanophotonics depends -- at least to some extent -- on the continued progression of semiconductor manufacturing and integration capabilities as projected by the International Technology Roadmap for Semiconductors (ITRS)
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From page 118... ...
While major technology innovations such as high-k dielectrics will play a key role in the mitigation of the power crisis and novel device structures will be critical to reducing the variability impact, it is generally believed that the semiconductor industry has to find additional ways of enhancing performance besides traditional device scaling (Chen et al., 2006)
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From page 119... ...
It is expected that these more modular design strategies will more readily enable outsourcing and offshoring of circuit design and possibly eventually fabrication work, which could enhance the accessibility of nanophotonics in foreign countries. FIGURE 3-22 Evolvement of the role of optical communication in server computing systems.
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From page 120... ...
NOTE: CMOS, complementary metal oxide semiconductor; CPU, central processing unit; RF, radio frequency; HV, high voltage. 3-23 SOURCE: Semiconductor Industry Association (2005b)
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From page 121... ...
Because nanophotonics is still an infant technology, it is arguably a very difficult task to understand which packaging and integration technology is most relevant for its enablement. The complicated interplay between limitations of existing technologies, the potential enablement of new computing a rchitectures by optics and nanophotonics, the appropriate level of integration, and the advances of the different nanophotonic technology platforms is still an ongoing research topic.
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From page 122... ...
integration, but it realizes that this boundary is somewhat blurred with the introduction of new integration and packaging tech nologies such as three-dimensional silicon. Monolithic Integration: Silicon Photonics Monolithic integration using silicon photonics drives toward the seamless fabrication of CMOS and photonic circuits on the same circuit layer, creating an SoC-like chip with an on-chip communication layer (Reed and Knights, 2004)
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From page 123... ...
At this stage it remains a research topic to balance bandwidth, power consumption, losses, extinction ratio, temperature stability, and footprint requirements for these types of modulators. The committee notes that recently Si-based electrooptical modulators have also been demonstrated using quantum wells, although still with a low extinction ratio (Kuo et al., 2005)
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From page 124... ...
Heterogeneous Integration: Silicon Carrier, Three-Dimensional Silicon The monolithic integration of Si photonics with high-performance CMOS circuitry could be eventually the most cost-effective integration way; it may turn out, however, that the optimization of electrical and optical components leads to such different competing requirements that only heterogeneous packaging approaches can rectify this. As discussed, these heterogeneous approaches will provide openings for more diverse technology platforms.
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From page 125... ...
2004. Three-dimensional lithography of photonic crystals.
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From page 126... ...
2000. Fabrication of photonic crystals for the visible spectrum by holographic lithography.
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From page 127... ...
2001. Polymer-based photonic crystals.
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From page 128... ...
2003. Photonic band gap effect in layer-by-layer metallic photonic crystals.
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From page 129... ...
2002. Optical properties of inverse opal photonic crystals.
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From page 130... ...
2003. Three-dimensional photonic crystals fabricated by visible light holographic lithography.
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