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Metamaterials (from the Greek word "meta", μετά meaning "to go
beyond") are materials engineered to have properties that have not
yet been found in nature.
[3] They are made from assemblies of
multiple elements fashioned from composite materials such as
metals or plastics. The materials are usually arranged in repeating
patterns, at scales that are smaller than the wavelengths of the
phenomena they influence. Metamaterials derive their properties
not from the properties of the base materials, but from their newly
designed structures. Their precise shape, geometry, size, orientation
and arrangement gives them their smart properties capable of
manipulating electromagnetic waves: by blocking, absorbing,
enhancing, bending waves, to achieve benefits that go beyond what
is possible with conventional materials.
Appropriately designed metamaterials can af ect waves of
electromagnetic radiation or sound in a manner not observed in
bulk materials.
[4][5][6] Those that exhibit a negative index of
refraction for particular wavelengths have attracted significant
research.
[7][8][9] These materials are known as negative index
metamaterials.
Potential applications of metamaterials are diverse and include optical filters, medical devices, remote
aerospace applications, sensor detection and infrastructure monitoring, smart solar power management,
crowd control, radomes, highfrequency battlefield communication and lenses for highgain antennas,
improving ultrasonic sensors, and even shielding structures from earthquakes.
[10][11][12][13] Metamaterials
of er the potential to create superlenses. Such a lens could allow imaging below the dif raction limit that is
the minimum resolution that can be achieved by a given wavelength. A form of 'invisibility' was
demonstrated using gradientindex materials. Acoustic and seismic metamaterials are also research
areas.
[10][14]
Metamaterial research is interdisciplinary and involves such fields as electrical engineering,
electromagnetics, classical optics, solid state physics, microwave and antennae engineering,
optoelectronics, material sciences, nanoscience and semiconductor engineering.
[5]
History
Explorations of artificial materials for manipulating electromagnetic waves began at the end of the 19th
century. Some of the earliest structures that may be considered metamaterials were studied by Jagadish
Chandra Bose, who in 1898 researched substances with chiral properties. Karl Ferdinand Lindman studied
wave interaction with metallic helices as artificial chiral media in the early twentieth century.
Winston E. Kock developed materials that had similar characteristics to metamaterials in the late 1940s. In
the 1950s and 1960s, artificial dielectrics were studied for lightweight microwave antennas. Microwave
radar absorbers were researched in the 1980s and 1990s as applications for artificial chiral media.
[5]
Negative index materials were first described theoretically by Victor Veselago in 1967. He proved that such
materials could transmit light. He showed that the phase velocity could be made antiparallel to the
direction of Poynting vector. This is contrary to wave propagation in naturallyoccurring materials.
[15]
John Pendry was the first to identify a practical way to make a lefthanded metamaterial, a material in
which the righthand rule is not followed. Such a material allows an electromagnetic wave to convey
energy (have a group velocity) against its phase velocity. Pendry's idea was that metallic wires aligned
along the direction of a wave could provide negative permittivity (dielectric function ε < 0). Natural
materials (such as ferroelectrics) display negative permittivity; the challenge was achieving negative
permeability (µ < 0). In 1999 Pendry demonstrated that a split ring (C shape) with its axis placed along the
direction of wave propagation could do so. In the same paper, he showed that a periodic array of wires and
rings could give rise to a negative refractive index. Pendry also proposed a related negativepermeability
design, the Swiss roll.
In 2000, Smith et al. reported the experimental demonstration of functioning electromagnetic metamaterials
by horizontally stacking, periodically, splitring resonators and thin wire structures. A method was
provided in 2002 to realize negative index metamaterials using artificial lumpedelement loaded
transmission lines in microstrip technology. In 2003, complex (both real and imaginary parts of) negative
refractive index
[16] and imaging by flat lens
[17] using left handed metamaterials were demonstrated. By
2007, experiments that involved negative refractive index had been conducted by many groups.
[4][13] At
microwave frequencies, the first, imperfect invisibility cloak was realized in 2006.
[18][19][20][21][22]
Electromagnetic metamaterials
An electromagnetic metamaterial af ects electromagnetic waves incident on it via structural features that
are smaller than the wavelength. To behave as a homogeneous material accurately described by an
ef ective refractive index, its features must be much smaller than the wavelength.
For microwave radiation, the features are on the order of millimeters. Microwave frequency metamaterials
are usually constructed as arrays of electrically conductive elements (such as loops of wire) that have
suitable inductive and capacitive characteristics. One microwave metamaterial is a suitably scaled splitring
resonator.
Photonic metamaterials, nanometer scale, manipulate light at optical frequencies. To date, subwavelength
structures have shown only a few, questionable, results at visible wavelengths.
[6][7] Photonic crystals and
frequencyselective surfaces such as dif raction gratings, dielectric mirrors and optical coatings exhibit
similarities to subwavelength structured metamaterials. However, these are usually considered distinct
from subwavelength structures, as their features are structured for the wavelength at which they function
and thus cannot be approximated as a homogeneous material. However, material structures such as
photonic crystals are ef ective in the visible light spectrum. The middle of the visible spectrum has a
wavelength of approximately 560 nm (for sunlight). Photonic crystal structures are generally half this size
or smaller, that is <280 nm.
Plasmonic metamaterials utilize surface plasmons, which are packets of electrical charge that collectively
oscillate at the surfaces of metals at optical frequencies.
Frequency selective surfaces (FSS) can exhibit subwavelength characteristics and are known variously as
artificial magnetic conductors (AMC) or High Impedance Surfaces (HIS). FSS display inductive and
capacitive characteristics that are directly related to their subwavelength structure.
[23]
Negative refractive index
Almost all materials encountered in optics, such as glass or water,
have positive values for both permittivity ε and permeability µ.
However, metals such as silver and gold have negative permittivity
at shorter wavelengths. A material such as a surface plasmon that
has either (but not both) ε or µ negative is often opaque to
electromagnetic radiation. However, anisotropic materials with
only negative permittivity can produce negative refraction due to
chirality.
Although the optical properties of a transparent material are fully
specified by the parameters εr and µr, refractive index n is often
used in practice, which can be determined from . All
known nonmetamaterial transparent materials possess positive εr
and µr. By convention the positive square root is used for n.
However, some engineered metamaterials have εr < 0 and µr < 0.
Because the product εrµr is positive, n is real. Under such circumstances, it is necessary to take the
negative square root for n.
The foregoing considerations are simplistic for actual materials, which must have complexvalued εr and
µr. The real parts of both εr and µr do not have to be negative for a passive material to display negative
refraction.
[24][25] Metamaterials with negative n have numerous interesting properties:
[5][26]
Snell's law (n1
sinθ1 = n2
sinθ2
), but as n2
is negative, the rays are refracted on the same side of the
normal on entering the material.
Cherenkov radiation points the other way.
The timeaveraged Poynting vector is antiparallel to phase velocity. However, for waves (energy) to
propagate, a –µ must be paired with a –ε in order to satisfy the wave number dependence on the
material parameters .
Negative index of refraction derives mathematically from the
vector triplet E, H and k.
[5]
For plane waves propagating in electromagnetic
metamaterials, the electric field, magnetic field and wave
vector follow a lefthand rule, the reverse of the behavior of
conventional optical materials.
Classification
Electromagnetic metamaterials are divided into dif erent
classes, as follows:
[4][5][27]
Negative index
In negative index metamaterials (NIM), both permittivity and
permeability are negative, resulting in a negative index of
refraction. These are also known as double negative metamaterials or double negative materials (DNG).
Other terms for NIMs include "lefthanded media", "media with a negative refractive index", and
"backwardwave media".
[4]
In optical materials, if both permittivity ε and permeability µ are positive, wave propagation travels in the
forward direction. If both ε and µ are negative, a backward wave is produced. If ε and µ have dif erent
polarities, waves do not propagate.
Mathematically, quadrant II and quadrant IV have coordinates (0,0) in a coordinate plane where ε is the
horizontal axis, and µ is the vertical axis.
[5]
To date, only metamaterials exhibit a negative index of refraction.
[4][26][28]
Single negative
Single negative (SNG) metamaterials have either negative relative permittivity (εr) or negative relative
permeability (µr), but not both. They act as metamaterials when combined with a dif erent, complementary
SNG, jointly acting as a DNG.
Epsilon negative media (ENG) display a negative εr while µr is positive.
[4][26] Many plasmas exhibit this
characteristic. For example, noble metals such as gold or silver are ENG in the infrared and visible
spectrums.
Munegative media (MNG) display a positive εr and negative µr.
[4][26] Gyrotropic or gyromagnetic
materials exhibit this characteristic. A gyrotropic material is one that has been altered by the presence of a
quasistatic magnetic field, enabling a magnetooptic ef ect. A magnetooptic ef ect is a phenomenon in
which an electromagnetic wave propagates through such a medium. In such a material, left and rightrotating
elliptical polarizations can propagate at dif erent speeds. When light is transmitted through a layer
of magnetooptic material, the result is called the Faraday ef ect: the polarization plane can be rotated,
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forming a Faraday rotator. The results of such a reflection are known as the magnetooptic Kerr ef ect (not
to be confused with the nonlinear Kerr ef ect). Two gyrotropic materials with reversed rotation directions
of the two principal polarizations are called optical isomers.
Joining a slab of ENG material and slab of MNG material resulted in properties such as resonances,
anomalous tunneling, transparency and zero reflection. Like negative index materials, SNGs are innately
dispersive, so their εr, µr and refraction index n, are a function of frequency.
[26]
Bandgap
Electromagnetic bandgap metamaterials (EBM) control light propagation. This is accomplished either with
photonic crystals (PC) or lefthanded materials (LHM). PCs can prohibit light propagation altogether.
Both classes can allow light to propagate in specific, designed directions and both can be designed with
bandgaps at desired frequencies.
[29][30] The period size of EBGs is an appreciable fraction of the
wavelength, creating constructive and destructive interference.
PC are distinguished from subwavelength structures, such as tunable metamaterials, because the PC
derives its properties from its bandgap characteristics. PCs are sized to match the wavelength of light,
versus other metamaterials that expose subwavelength structure. Furthermore, PCs function by dif racting
light. In contrast, metamaterial does not use dif raction.
[31]
PCs have periodic inclusions that inhibit wave propagation due to the inclusions' destructive interference
from scattering. The photonic bandgap property of PCs makes them the electromagnetic analog of
electronic semiconductor crystals.
[32]
EBGs have the goal of creating high quality, low loss, periodic, dielectric structures. An EBG af ects
photons in the same way semiconductor materials af ect electrons. PCs are the perfect bandgap material,
because they allow no light propagation.
[33] Each unit of the prescribed periodic structure acts like one
atom, albeit of a much larger size.
[4][33]
EBGs are designed to prevent the propagation of an allocated bandwidth of frequencies, for certain arrival
angles and polarizations. Various geometries and structures have been proposed to fabricate EBG's special
properties. In practice it is impossible to build a flawless EBG device.
[4][5]
EBGs have been manufactured for frequencies ranging from a few gigahertz (GHz) to a few terahertz
(THz), radio, microwave and midinfrared frequency regions. EBG application developments include a
transmission line, woodpiles made of square dielectric bars and several dif erent types of low gain
antennas.
[4][5]
Double positive medium
Double positive mediums (DPS) do occur in nature, such as naturally occurring dielectrics. Permittivity
and magnetic permeability are both positive and wave propagation is in the forward direction. Artificial
materials have been fabricated which combine DPS, ENG and MNG properties.
Institutional networks
MURI
The Multidisciplinary University Research Initiative (MURI) encompasses dozens of Universities and a few
government organizations. Participating universities include UC Berkeley, UC Los Angeles, UC San
Diego, Massachusetts Institute of Technology, and Imperial College in London, UK. The sponsors are
Of ice of Naval Research and the Defense Advanced Research Project Agency.
[73]
MURI supports research that intersects more than one traditional science and engineering discipline to
accelerate both research and translation to applications. As of 2009, 69 academic institutions were
expected to participate in 41 research ef orts.
[74]
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Metamorphose
The Virtual Institute for Artificial Electromagnetic Materials and Metamaterials ”Metamorphose VI
AISBL” is an international association to promote artificial electromagnetic materials and metamaterials.
It organizes scientific conferences, supports specialized journals, creates and manages research programs,
provides training programs (including PhD and training programs for industrial partners); and technology
transfer to European Industry.