Nanophotonics Accessibility and Applicability (2008) / Chapter Skim
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2 Nanoscale Phenomena Underpinning Nanophotonics
2 Nanoscale Phenomena Underpinning Nanophotonics
Pages 19-82
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From page 19... ...
. These papers formed the foundations of the tremendously 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.
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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)
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. Various photonic crystal waveguides have since been fabricated with much smaller lattice constants (<0.4 micrometer [µm]
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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)
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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 material 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)
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From page 24... ...
Figure 2-5 illustrates some of the general trends in research as measured by publications. 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.
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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)
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. Materials, of course, consist of atoms and molecules with a spatial scale much less than the optical wavelength, so these continuum approximations are appropriate.
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NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 27 then to the IR (Linden et al., 2004; Yen et al., 2004; Yen et al., 2005)
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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.
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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.
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SOURCE: Reproduced with permission of Melissa Thomas. The superlens, proposed by Pendry, relies on the negative refractive index that can be realized using metamaterials.
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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.
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From page 32... ...
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.
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Plasmon-resonance-based chemical sensing can be accomplished in a variety of ways. Historically, surface plasmon propagation (SPP)
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Copyright 2006 American Chemical Society. In A ppendix D, see the section "Localized Surface Plasmon Resonance" for information on the research groups named.
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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)
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. Copyright Elsevier, Chemical Physics Letters, 2003.
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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.
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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.
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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.
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: Topography of the sample where the groove used to excite surface plasmons is clearly visible; (a) through (f)
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. 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)
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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 wavelength. This experiment concentrated on the demonstration of the coupling between a normal-incident optical beam and surface plasmons provided by a subwavelength hole.
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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)
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Regardless of which theory best explains the extraordinary optical transmission phenomenon, subwavelength 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)
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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)
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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 dimension of the rods is less than 100 nm and the longitudinal dimension is ~4 μm.
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NANOSCALE PHENOMENA UNDERPINNING NANOPHOTONICS 47 FIGURE 2-15 Micrographs showing propagation of light down silver nanowires.
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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.
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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)
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Removing the need to convert optical s ignals into electronic signals to route information would allow significant improvements in bandwidth. The electrical modulation of surface plasmons is also important for interfacing electronics with p lasmonics in on-chip applications.
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(b) One technique employed to amplify surface plasmons propagating on a metal film.
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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)
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From page 53... ...
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)
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From page 54... ...
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)
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Quantum cascade lasers (QCLs) for terahertz emission into the far field can also be enhanced by using surface plasmons.
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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)
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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)
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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)
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has recently suggested that this geometry is not the best terahertz waveguide, because at terahertz 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
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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.
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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.
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, 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)
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Recently, the first two-color camera was demonstrated using quantum dots in the active region (Krishna, 2005)
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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 challenges currently face the further development of III-N emitters.
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From page 65... ...
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.
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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
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From page 67... ...
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.
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From page 68... ...
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, controllably, 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.
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From page 69... ...
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.
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. 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.
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From page 71... ...
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)
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From page 72... ...
in the terahertz, while record operating temperatures 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)
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Applied Physics Letters 82(25)
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Applied Physics Letters 88(6)
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quantum cascade lasers operating above room t emperature. Applied Physics Letters 68(26)
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Applied Physics Letters 70(22)
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InAs/InGaAs quantum-dots-in-a-well detector. Applied Physics Letters 83(14)
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2004. Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals.
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As/GaAs quantum dot infrared photodetectors. Applied Physics Letters 73(14)
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2005. Integrated four-channel GaAs based quantum dot laser module with photonic crystals.
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From page 81... ...
Applied Physics Letters 87(7)
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From page 82... ...
Applied Physics Letters 89(15)
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