[Paleopsych] SW: Light, Matter, and Metamaterials

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Materials Science: Light, Matter, and Metamaterials

    The following points are made by W. Barnes and R. Sambles (Science
    2004 305:785):
    1) In recent years, there has been an explosion of interest in
    controlling the interaction between light and matter by introducing
    structure on length scales equal to or smaller than the wavelength of
    the light involved. Functionality is thus as much a property of
    geometry as of material parameters -- a concept sometimes referred to
    as "metamaterials". In the 1980s, Yablonovitch (1) and John (2) showed
    that by introducing three-dimensional periodicity on the scale of the
    wavelength of light, one can alter how light interacts with the
    material, specifically by blocking the propagation of light to make
    photonic band gap (PBG) materials. More recently, by introducing
    structure smaller than the wavelength of light involved, synthetic
    "left-handed" materials have been created that have the fascinating
    property of negative refraction (3). Pendry et al (4) have recently
    demonstrated how in theory we may be able to exploit another aspect of
    structure on the subwavelength scale, this time to create a new family
    of surface electromagnetic modes.
    2) Wavelength-scale periodic structures with even a very small
    refractive index contrast may lead to the strong selective reflection
    of light and photonic stop-bands. However, producing a truly
    three-dimensional PBG material that reflects over a range of
    wavelengths for all directions is very challenging. Indeed, practical
    applications of the PBG idea have been most fruitfully pursued in
    restricted dimensions, notably in the photonic crystal fiber and in
    two-dimensional planar slabs. An interesting variant is to apply the
    same idea to surface waves. In 1996 Kitson etal (5) demonstrated that
    a full PBG for surface plasmon-polariton (SPP) modes could be made by
    introducing periodic texture into the metallic surface that supports
    SPPs. In the latest development by Pendry et al (4), surface structure
    is used not just to control surface modes but also to create them.
    3) Surface plasmon-polaritons (SPPs) are surface modes that propagate
    at metal-dielectric interfaces and constitute an electromagnetic field
    coupled to oscillations of the conduction electrons at the metal
    surface. The fields associated with the SPP are enhanced at the
    surface and decay exponentially into the media on either side of the
    interface. In the visible domain, there is a very short penetration
    depth of the field into the metal and a relatively short penetration
    depth into the dielectric, thus allowing one to concentrate light on a
    scale much smaller than the wavelength involved.
    4) In the microwave regime, metals have a very large complex
    refractive index (n+ik), where n and k are the real and imaginary
    parts, respectively, and -n ~ k ~ 10^(3). The SPP mode is then very
    nearly a plane wave that extends huge distances into the dielectric
    but only very short distances into the metal. Metals in the microwave
    domain are therefore frequently treated as ideal conductors, thus
    reflecting microwaves perfectly, which limits the usefulness of
    applying near-field concepts being developed in the visible domain to
    the microwave regime. What Pendry et al (4) have demonstrated is that
    by puncturing the metal surface with subwavelength holes, some of the
    field may penetrate the (effective) surface. This changes the
    field-matching situation at the bounding surface and leads to a new
    effective surface plasmon resonance frequency.
    References (abridged):
    1. E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987)
    2. S. John, Phys. Rev. Lett. 58, 2486 (1987)
    3. R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 77 (2001)
    4. J. B. Pendry, L. Martin-Moreno, F. J. Garcia-Vidal, Science 305,
    847 (2004)
    5. S. C. Kitson, W. L. Barnes, J. R. Sambles, Phys. Rev. Lett. 77,
    2670 (1996)
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by J.B. Pendry and D.R. Smith (Physics
    Today 2004 June):
    1) Victor Veselago, in a paper(1) published in 1968, pondered the
    consequences for electromagnetic waves interacting with a hypothetical
    material for which both the electric permittivity (e) and the magnetic
    permeability (m) were simultaneously negative. Because no naturally
    occurring material or compound has ever been demonstrated with
    negative (e) and (m), Veselago wondered whether this apparent
    asymmetry in material properties was just happenstance or perhaps had
    a more fundamental origin. He concluded that not only should such
    materials be possible, but if ever found, they would exhibit
    remarkable properties unlike those of any known materials and would
    give a twist to virtually all electromagnetic phenomena. Foremost
    among these properties is a negative index of refraction.
    2) Veselago always referred to the materials as "left handed", because
    the wave vector is antiparallel to the usual right-handed cross
    product of the electric and magnetic fields. The authors prefer the
    negative-index description. The names mean the same thing, but the
    authors suggest their description appeals more to everyday intuition
    and is less likely to be confused with chirality, an entirely
    different phenomenon.
    3) Why are there no materials with negative (e) and (m)? One first
    needs to understand what it means to have a negative (e) or (m) and
    how negative values occur in materials. The Drude-Lorentz model of a
    material is a good starting point, because it conceptually replaces
    the atoms and molecules of a real material by a set of harmonically
    bound electron oscillators resonant at some frequency (F). At
    frequencies far below (F), an applied electric field displaces the
    electrons from the positive cores and induces a polarization in the
    same direction as the applied field. At frequencies near resonance,
    the induced polarization becomes very large, as is typical in
    resonance phenomena; the large response represents accumulation of
    energy over many cycles, such that a considerable amount of energy is
    stored in the resonator (in this case, the medium) relative to the
    driving field.
    4) So large is this stored energy that even changing the sign of the
    applied electric field has little effect on the polarization near
    resonance. That is, as the frequency of the driving electric field is
    swept through the resonance, the polarization flips from in-phase to
    out-of-phase with the driving field, and the material exhibits a
    negative response. If instead of electrons the material response were
    due to harmonically bound magnetic moments, then a negative magnetic
    response would exist.
    5) Although somewhat less common than positive materials, negative
    materials are nevertheless easy to find. Materials with negative (e)
    include metals (such as silver, gold, and aluminum) at optical
    frequencies; materials with negative (m) include resonant
    ferromagnetic or antiferromagnetic systems.
    6) That negative material parameters occur near a resonance has two
    important consequences. First, negative material parameters will
    exhibit frequency dispersion: They will vary as a function of
    frequency. Second, the usable bandwidth of negative materials will be
    relatively narrow compared with positive materials. These consequences
    can help answer our initial question as to why materials with (e) and
    (m) both negative are not readily found. In existing materials, the
    resonances that give rise to electric polarizations typically occur at
    very high frequencies -- in the optical for metals, and at least in
    the terahertz-to-IR region for semiconductors and insulators. On the
    other hand, resonances in magnetic systems typically occur at much
    lower frequencies and usually tail off toward the THz and IR region.
    In short, the fundamental electronic and magnetic processes that give
    rise to resonant phenomena in materials simply do not occur at the
    same frequencies, although no physical law would preclude such
    References (abridged):
    1. V. G. Veselago, Sov. Phys. Usp. 10, 509 (1968)
    2. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, IEEE
    Trans. Microwave Theory Tech. 47, 2075 (1999)
    3. R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 77 (2001)
    4. A. A. Houck, J. B. Brock, I. L. Chuang, Phys. Rev. Lett. 90, 137401
    5. C. G. Parazzoli, R. B. Greegor, K. Li, B. E. C. Koltenbah, M.
    Tanielian, Phys. Rev. Lett. 90, 107401 (2003)
    Physics Today http://www.physicstoday.org
    Related Material:
    The following points are made by T.J. Yen et al (Science 2004
    1) The range of electromagnetic material response found in nature
    represents only a small subset of that which is theoretically
    possible. This limited range can be extended by the use of
    artificially structured materials, or metamaterials, that exhibit
    electromagnetic properties not available in naturally occurring
    materials. For example, artificial electric response has been
    introduced in metallic wire grids or cell meshes, with the spacing on
    the order of wavelength (1); a diversity of these meshes are now used
    in THz optical systems (2).
    2) More recently, metamaterials with subwavelength scattering elements
    have shown negative refraction at microwave frequencies (3), for which
    both the electric permittivity and the magnetic permeability are
    simultaneously negative. The negative-index metamaterial relied on an
    earlier theoretical prediction that an array of nonmagnetic conductive
    elements could exhibit a strong, resonant response to the magnetic
    component of an electromagnetic field (4).
    3) Conventional materials that exhibit magnetic response are far less
    common in nature than materials that exhibit electric response, and
    they are particularly rare at THz and optical frequencies. The reason
    for this imbalance is fundamental in origin: Magnetic polarization in
    materials follows indirectly either from the flow of orbital currents
    or from unpaired electron spins. In magnetic systems, resonant
    phenomena, analogous to the phonons or collective modes that lead to
    an enhanced electric response at infrared or higher frequencies, tend
    to occur at far lower frequencies, resulting in relatively little
    magnetic material response at THz and higher frequencies.
    4) Magnetic response of materials at THz and optical frequencies is
    particularly important for the implementation of devices such as
    compact cavities, adaptive lenses, tunable mirrors, isolators, and
    converters. A few natural magnetic materials that respond above
    microwave frequencies have been reported. For example, certain
    ferromagnetic and antiferromagnetic systems exhibit a magnetic
    response over a frequency range of several hundred gigahertz (5) and
    even higher. However, the magnetic effects in these materials are
    typically weak and often exhibit narrow bands, which limits the scope
    of possible THz devices. The realization of magnetism at THz and
    higher frequencies will substantially affect THz optics and their
    5) In summary: The authors demonstrate that magnetic response at
    terahertz frequencies can be achieved in a planar structure composed
    of nonmagnetic conductive resonant elements. The effect is realized
    over a large bandwidth and can be tuned throughout the terahertz
    frequency regime by scaling the dimensions of the structure. The
    authors suggest that artificial magnetic structures, or hybrid
    structures that combine natural and artificial magnetic materials, can
    play a key role in terahertz devices.
    References (abridged):
    1. R. Ulrich, Infrared Phys. 7, 37 (1967)
    2. S. T. Chase, R. D. Joseph, Appl. Opt. 22, 1775(1983)
    3. R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 79 (2001)
    4. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, IEEE
    Trans. Microw. Theory Tech. 47, 2075(1999)
    5. P. Grunberg, F. Metawe, Phys. Rev. Lett. 39, 1561 (1977)
    Science http://www.sciencemag.org

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