[Paleopsych] SW: Light, Matter, and Metamaterials
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Materials Science: Light, Matter, and Metamaterials
http://scienceweek.com/2004/sa041001-2.htm
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:
OPTICS: ON NEGATIVE REFRACTION
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
overlap.(2-5)
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
(2003)
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:
MATERIALS SCIENCE: ON TERAHERTZ MAGNETIC RESPONSE
The following points are made by T.J. Yen et al (Science 2004
303:1494):
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
applications.
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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