Metamaterial

An engineered 3-D metamaterial that can reverse the natural direction of visible and near-infrared light, a development that could help form the basis for higher resolution optical imaging, nanocircuits for high-powered computers.

Metamaterials are artificial materials engineered to provide properties which "may not be readily available in nature". These materials usually gain their properties from structure rather than composition, using the inclusion of small inhomogeneities to enact effective macroscopic behavior.

The primary research in metamaterials investigates materials with negative refractive index Negative refractive index materials appear to permit the creation of 'superlenses' which can have a spatial resolution below that of the wavelength, and a form of 'invisibility' has been demonstrated at least over a narrow wave band. Although the first metamaterials were electromagnetic, acoustic and seismic metamaterials are also areas of active research.

Potential applications of metamaterials are diverse and include remote aerospace applications, sensor detection and infrastructure monitoring, smart solar power management, public safety, radomes, high-frequency battlefield communication and lenses for high-gain antennas, improving ultrasonic sensors and even shielding structures from earthquakes.

The research in metamaterials is interdisciplinary and involves such fields as electrical engineering, electromagnetics, solid state physics, microwave and antennae engineering, optoelectronics, classic optics, material sciences, semiconductor engineering, nanoscience and others.

Electromagnetic metamaterials

See also: Photonic crystal

Metamaterials are of particular importance in electromagnetism (especially optics and photonics). They show promise for optical and microwave applications such as new brands of beam steerers, modulators, band-pass filters, lenses, microwave couplers, and antenna radomes. Metamaterials usually consist of periodic structures, and thus have many similarities with photonic crystals and frequency-selective surfaces such as diffraction gratings, dielectric mirrors, and optical coatings. However, these are usually considered distinct from metamaterials, as their features are of similar size to the wavelength at which they function, and thus cannot be approximated as a homogeneous material.

A metamaterial affects electromagnetic waves by having structural features smaller than the wavelength of electromagnetic radiation it interacts with. For instance, if a metamaterial is to behave as a homogeneous material accurately described by an effective refractive index, its features must be much smaller than the wavelength. For visible light, where the middle of the spectrum has a wavelength of approximately 560 nm (for sunlight), the metamaterial structures are generally half this size or smaller, that is <280 nm. For microwave radiation, the structures need only be on the order of few centimeters. Microwave frequency metamaterials are usually synthetic, constructed as arrays of electrically conductive elements (such as loops of wire) which have suitable inductive and capacitive characteristics.

Artificial materials and "handedness"

Chirality with hands and two enantiomers of a generic amino acid

The attempt to use artificial materials to control electromagnetic properties dates back to Jagadish Chandra Bose in 1898 who researched substances with chiral properties and to studies by Karl Ferdinand Lindman on wave interaction with metallic helices as artificial chiral media in the early twentieth century. In the 1950s and 1960s, artificial dielectrics were studied for lightweight microwave antennas. Microwave radar absorbers moved into the research arena in the 1980s and 1990s as applications for artificial chiral media.

The term chiral (pronounced /ˈkaɪrəl/) describes an object, especially a molecule, which has or produces a non-superimposeable mirror image of it itself. In chemistry, such a molecule is called an enantiomer or is said to exhibit chirality or enantiomerism. The term "chiral" comes from the Greek word for the human hand, which itself exhibits such non-superimposeability of the left hand precisely over the right. Due to the opposition of the fingers and thumbs, no matter how the two hands are oriented, it is impossible for both hands to exactly coincide. It is a mathematical approach to the concept of 'handedness'. Helices, chiral characteristics (properties), chiral media, order, and symmetry all relate to the concept of left- and right-handedness.

The chirality (or handedness) is an important characteristic in metamaterial design and fabrication as it relates to the direction of wave propagation. Metamaterials as left-handed media occur when both permittivity ε and permeability µ are negative. Furthermore, left handedness occurs mathematically from the handedness of the vector triplet E, H and k.

Wave propagation as handedness is wave polarization and described in terms of helicity (occurs as a helix). Electrical polarization occurs perpendicular to the direction of propagation. If during the wave propagation through material the electric vector rotates clockwise then the material is right-handed, and if rotation occurs counterclockwise then material is left-handed. (See Federal Standard 1037C) Historically, the orientation of a polarized electromagnetic wave has been defined for light by the orientation of the electric vector, and for radio waves, by the orientation of the magnetic vector.

Mirror inversion should convert all coordinate axes to equal, but opposite values. This process should, and often does demonstrate parity transformation, known as parity symmetry. Although parity symmetry is ordinary in our macroscopic world, parity is broken at the subatomic level. This is a basic tenet of physics. Furthermore, although the visual evidence of our everyday lives contradicts this, broken parity actually occurs at several different levels in nature. Left-handed amino acids are the building blocks of essentially all life on earth. Furthermore, from DNA molecules to bacteria, winding plants, and right-handed human beings, to spiral galaxies, one of the handedness dominates over the other. Chirality is also evident in structural objects, transmission media, and electromagnetic effects.

In natural occurring transmission media right handedness dominates, i.e., permittivity and permeability are both positive resulting in an ordinary positive index of refraction. However, metamaterials have the capability to exhibit a state where both permittivity and permeability are negative, resulting in an extraordinary index of negative refraction, i.e. a left-handed material. The term left-handed material (LHM), is interchangeable with the term double negative metamaterial (DNG).

Incident wave

sRGB rendering of the spectrum of visible light
Color	Wavelength	Frequency
violet	380–450 nm	668–789 THz
indigo	420–450 nm	668–714 THz
blue	450–495 nm	606–668 THz
green	495–570 nm	526–606 THz
yellow	570–590 nm	508–526 THz
orange	590–620 nm	484–508 THz
red	620–750 nm	400–484 THz

The scientific literature published about metamaterials uses the term incident wave or incident wave radiation or incident radiation, which is not the same concept as incident light. An incident wave is, in essence, light of a certain, well-defined frequency (color) traveling in a given direction and having a defined wavefront shape. For simplicity, the wavefront shape is usually taken as flat, spherical or cylindrical. The former two shapes correspond to the plane and spherical waves, respectively.

In pratical experiments, and simulations which mimic practical experiments, the incident wave is the light which strikes, or makes contact with, the surface of a chosen material. With most all of the experiments and simulations cited in this article, the material is the subject, and in this case it is a metamaterial. The researchers measure, observe, compute, and then interpret the effect the incident wave has when it strikes the subject material.

Furthermore, Light, when seen through a prism, is "refracted" into a visible rainbow of colors. Each visible color has its own frequency. For example the color red is actually a light wave at 400–484 THz, and the color green is actually a light wave at 526–606 THz. Red and green light are frequency components of a light wave. See the chart to the right.

Light waves also occur at frequencies that cannot be seen with the human eye. These types of frequencies would include, microwave frequencies from 300MHz (0.3 GHz) and 300 GHz, and infrared from around 300 GHz to 400 THz. In sceintific metamaterial literature the infrared frequencies are often referred to as "optical" frequencies. Anything in the visible "color" range are usually referred to as visible frequencies. Microwave frequencies are referred to as microwave frequencies.

Regarding metamaterials, an incident wave could be any of these frequencies mentioned above.

Negative refractive index

Main article: Negative index metamaterials

A comparison of refraction in a left-handed metamaterial to that in a normal material

The greatest potential of metamaterials is the possibility to create a structure with a negative refractive index, since this property is not found in any non-synthetic material. Almost all materials encountered in optics, such as glass or water, have positive values for both permittivity ε and permeability µ. However, many metals (such as silver and gold) have negative ε at visible wavelengths. A material having either (but not both) ε or µ negative is opaque to electromagnetic radiation (see surface plasmon for more details).

Although the optical properties of a transparent material are fully specified by the parameters ε and µ, refractive index n is often used in practice, which can be determined from $\scriptstyle n=\pm {\sqrt {\epsilon \mu }}$. All known non-metamaterial transparent materials possess positive ε and µ. By convention the positive square root is used for n.

However, some engineered metamaterials have ε < 0 and µ < 0. Because the product εµ is positive, n is real. Under such circumstances, it is necessary to take the negative square root for n. Physicist Victor Veselago proved that such substances can transmit light.

The foregoing considerations are simplistic for actual materials, which must have complex-valued ε and µ. The real parts of both ε and µ do not have to be negative for a passive material to display negative refraction. Metamaterials with negative n have numerous interesting properties:

• Snell's law (n₁sinθ₁ = n₂sinθ₂), but as n₂ is negative, the rays will be refracted on the same side of the normal on entering the material.
• The Doppler shift is reversed: that is, a light source moving toward an observer appears to reduce its frequency.
• Cherenkov radiation points the other way.
• The time-averaged Poynting vector is antiparallel to phase velocity. This means that unlike a normal right-handed material, the wave fronts are moving in the opposite direction to the flow of energy.

Electromagnetic, acoustic and seismically metamaterials have been proposed and built.

For plane waves propagating in electromagnetic metamaterials, the electric field, magnetic field and wave vector follow a left-hand rule, thus giving rise to the name left-handed (meta)materials. It should be noted that the terms left-handed and right-handed can also arise in the study of chiral media, but their use in that context is unrelated to this effect. The effect of negative refraction is analogous to wave propagation in a left-handed transmission line, and such structures have been used to verify some of the effects described here.

Different classes of electromagnetic metamaterials

Electromagnetic metamaterials have the potential of an enormous impact, because with the capability to direct wave propagation at the electromagnetic level, whole systems can be refined. For example, low density of materials means that components, devices, and systems can be extremely lightweight and increasingly small, while at the same time enhancing system and component performance.

Because physicists can now probe deeper into elementary particles, the border between synthetic materials and metamaterials is vague and novel properties are being discovered in natural materials. Unusual properties are also produced in conventional materials by processing them at nanoscales. However, a distinguishing feature of metamaterials is that they can be specifically fabricated to fulfill a certain objective and to fit the desired application. The size and spacing of elements in the material are created smaller than the radiated wavelength. This incident radiation, therefore distinguishes the metamaterial as homogenous.

Electromagnetic metamaterials have been synthesized by embedding various constituents/inclusions with novel geometric shapes and forms in some host media. Various types of composite material, both electromagnetic and other types have been and are being studied by various research groups worldwide (see all sections and references below).

Often the behavior, designed structure, and designed parameters of electromagnetic metamaterials are described in by certain terms without reference to their frequency dependence. However, in this type of composite media electromagnetic waves interact with the designed inclusions, inducing electric and magnetic moments, which in turn affect the macroscopic effective permittivity and permeability of this, bulk composite "medium".

Since electromagnetic metamaterials can be synthesized by embedding artificially fabricated inclusions (as large-scale artificial atoms) in a specified host medium, or on a host surface, this provides the designer with a large set of available, independent parameters. Those parameters define how the metamaterial is to be engineered. They include the properties of the host materials, and the size shape and composition of the inclusions. Other parameters to consider are the density, arrangement, and alignment of these inclusions. By defining all these parameters during fabricaion, a metamaterial is engineered for specific electromagnetic response functions. Additionally, these response functions are not found in the individual constituents. All these design parameters can play a key role in the final outcome of the synthesis process. Among these the geometry (or shape) of the inclusions is one parameter that can provide the new possibilities for processed metamaterials.

In light of these developments, electromagnetic metamaterials are represented by different classes, as follows:

Double negative metamaterials

See also: Negative index metamaterials and Negative refraction

In double negative metamaterials (DNG), both permittivity and permeability are negative resulting in a negative index of refraction. DNGs are also referred to as negative index metamaterials (NIM). Other terminologies for DNGs are "left-handed media", "media with a negative refractive index", and "backward-wave media", along with other nomenclatures.

In 1968 Victor Veselago published a paper theorizing plane wave propagation in a material whose permittivity and permeability were assumed to be simultaneously negative. In such a material, he showed that the Poynting vector would be antiparallel to the direction of phase velocity. This is contrary to wave propagation in natural occurring materials. In the years 2000 and 2001, papers were published about the first demonstrations of an artificial material that produced a negative index of refraction. By 2007, research experiments which involved negative refractive index had been conducted by many groups.

In peer reviewed journal articles (see References), there are several (mathematical) material models which describe frequency response in DNGs. One of these is the Lorentz model. This describes electron motion in terms of a driven-damped, harmonic oscillator. When the acceleration component of the Lorentz mathematical model is small compared to the other components of the equation, then the Debye model is applied. When the restoring force component is negligible, and the coupling coefficient is generally the plasma frequency, then the Drude model is applied. There are other component distinctions that call for the use of one of these models, depending on its polarity, or purpose.

Studies have elucidated applications for negative refractive index materials. These applications are phase compensation with electrically small resonators, negative angles of refraction, subwavelength waveguides, backward wave antenna, Cherenkov radiation, photon tunneling, and enhanced electrically small antenna. The concept of continuous wave excitation is a key component of these studies to obtain the negative index refraction using DNG media, and then to introduce the results of research into these applications. DNG metamaterials are innately dispersive, so their permittivity ε, permeability µ, and refraction index n, will alter with changes in frequency. To date DNGs have only been demonstrated as artificially constructed materials.

Electromagnetic bandgap structured metamaterials

Electromagnetic bandgap materials (known as photonic crystals (PCs) or photonic bandgap (PBG) materials) are a novel class of artificially fabricated structures, which have the ability to control and manipulate the propagation of electromagnetic waves. PBGs (or EBGs) have emerged as a new class of materials providing capabilities along new dimensions for the control and manipulation of light. Properly designed photonic crystals can prohibit the propagation of light, or allow it along only certain directions, or localize light in specified areas. They can be constructed in one, two, and three dimensions (1D, 2D,and 3D) with either dielectric or/ and metallic materials.

Intended material fabrication of PBGs has the goal of creating periodic, dielectric structures, with low loss, and that are of high quality. A PBG affects the properties of the photon in the same way semiconductor materials affect the properties of the electron. So, it happens that the PBG is the perfect bandgap material, becuae it allows no propagation of light. However, the manufactured defects in the crystal allows light for certain prescribed bandwidths. Each unit of the prescribed periodic structure acts like large scale atoms.

Electromagnetic bandgap structured (EBG) metamaterials are designed to prevent the propagation of an allocated bandwidth of frequencies, for certain arrival angles and polarizations. With EBG materials new methods utilize the properties of various dielectrics to achieve better performance. A variety of geometries and structures have been proposed to fabricate the special EBG metamaterial properties. However, in practice it is impossible to build a flawless EBG device. Factors such as advances in ideas, research, testing and development, along with the prospects of significant technological solutions, have driven the development of EBG applied science. Commercial production of dielectric EBG devices has lagged, because commercial rewards are not readily apparent. However, start-up companies are cropping up solely focused on exploiting EBG metamaterials. These metamaterials have been manufactured for frequencies ranging from a few gigahertz (GHz) up to several terahertz (THz). In other words, applications have achieved fabricated media for radio frequency, microwave and mid-infrared regions. "It now appears that EBG concepts can, in many cases act as improved replacements for conventional solutions to electromagnetic problems." Applicable developments include an EBG transmission line fabricated utilizing the special properties of metamaterials, EBG woodpiles made of square dielectric bars, and several different types of low gain antennae.

Further information: Electronic band structure and Coupled oscillation

Single negative metamaterials

In single negative (SNG) metamaterials either permittivity or permeability are negative, but not both. These are ENG metamaterials and MNG metamaterials discussed below. Interesting experiments have been conducted by combining two SNG layers into one metamaterial. These effectively create another form of DNG metamaterial. A slab of ENG material and slab of MNG material have been joined to conduct wave reflection experiments. This resulted in the exhibition of properties such as resonances, anomalous tunneling, transparency, and zero reflection. Like DNG metamaterials, SNGs are innately dispersive, so their permittivity ε, permeability µ, and refraction index n, will alter with changes in frequency.

• Epsilon negative media (ENG) – permittivity ε is negative while permeability µ is positive. Many plasmas exhibit this characteristic. For example noble metals such as gold or silver will exhibit this characteristic in the infrared and visible spectrums.
• Mu-negative media (MNG) – permittivity ε is positive while permeability µ is negative. A material, which called gyrotropic or gyromagnetic exhibits this characteristic. A gyrotropic material is a medium that has been altered by the presence of a quasistatic magnetic field. This results in the magneto-optic effect. A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field. In such a material, left- and right-rotating elliptical polarizations can propagate at different speeds, leading to a number of important phenomena. When light is transmitted through a layer of magneto-optic material, the result is called the Faraday effect: the plane of polarization can be rotated, forming a Faraday rotator. The results of reflection from a magneto-optic material are known as the magneto-optic Kerr effect (not to be confused with the nonlinear Kerr effect). Two gyrotropic materials with reversed rotation directions of the two principal polarizations are called optical isomers.

Double positive medium

Double positive mediums (DPS) do occur in nature such as naturally occurring dielectrics. Permittivity and permeability are both positive and wave propagation is in the forward direction. Artificial materials have been fabricated which have DPS, ENG, and MNG properties combined.

Bi-isotropic media

Isotropic electromagnetic metamaterials, can have interesting electromagnetic interactions, such as the demonstration of negative index of refraction with a metamaterial prism. In electromagnetic metamaterials, the electric field causes magnetic polarization, and the magnetic field induces an electrical polarization, i.e., magnetoelectric coupling. Intrinsic to the structure of electromagnetic metamaterials is net handedness of the medium. The net handedness brings with it magnetoelectric coupling

See also: Gauss's flux theorem

Intrinsic to magnetoelectric coupling of bi-isotropic media, are four material parameters interacting with the electric (E) and magnetic( H) field strengths, and electric (D) and magnetic (B) flux densities. These four material parameters are ε, µ, k and χ or permittivity, permeability, strength of chirality, and the Tellegen parameter respectively. Furthermore, in this type of media, the material parameters do not vary with changes along a rotated coordinate system of measurements. In this way they are also defined as invariant or scalar.

The intrinsic magnetoelectric parameters, k and χ, affect the phase of the wave. Furthermore, the effect of the chirality parameter is to split the refractive index. In isotropic media this results in wave propagation only if ε and µ have the same sign. If both ε and µ are positive this results in propagation in the forward direction. If both ε and µ are negative, a backward wave is produced. If ε and µ have different polarities, then this does not result in wave propagation. 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.

In bi-isotropic media with χ assumed to be zero, and k a non-zero value, different results are shown. Both a backward wave and a forward wave can occur. Alternatively, two forward waves or two backward waves can occur, depending on the strength of the chirality parameter. Furthermore, a negative index metamaterial can be created if k > √εµ. In this case, it is not necessary that either or both ε and µ be negative for backward wave propagation.

Bi-anisotropic media

The bianisotropy is related to the existence of magnetoelectric coupling in the artificial constituents (artificial, large-scale, atoms) of the medium.

Split-ring resonators

Main article: Split-ring resonator

Left-handed metamaterial array configuration, which was constructed of copper split-ring resonators and wires mounted on interlocking sheets of fiberglass circuit board. The total array consists of 3 by 20×20 unit cells with overall dimensions of 10×100×100 mm.

A split-ring resonator (SRR) is a component part of a negative index metamaterial (NIM), also known as Double Negative metamaterials (DNG). They are also component parts of other types of metamaterial such as Single Negative metamaterial (SNG). SRR's are also used for research in Terahertz metamaterials, Acoustic metamaterials, and Metamaterial antennas. SRRs are a pair of concentric annular rings with splits in them at opposite ends. The rings are made of nonmagnetic metal like copper and have small gap between them.

A magnetic flux penetrating the metal rings will induce rotating currents in the rings, which produce their own flux to enhance or oppose the incident field (depending on the SRR's resonant properties). This field pattern is dipolar. Due to splits in the rings the structure can support resonant wavelengths much larger than the diameter of the rings. This would not happen in closed rings. The small gaps between the rings produces large capacitance values which lower the resonating frequency, as the time constant is large. The dimensions of the structure are small compared to the resonant wavelength. This results in low radiative losses, and very high quality factors.

At frequencies below the resonant frequency, the real part of the magnetic permeability of the SRR becomes large (positive), and at frequencies higher than resonance it will become negative. This negative permeability can be used with the negative dielectric constant of another structure to produce negative refractive index materials.

Terahertz metamaterials

Main article: Terahertz metamaterials

Terahertz radiation lies at the far end of the infrared band, just before the start of the microwave band.

Terahertz metamaterials are metamaterials which interact at terahertz frequencies. For research or applications of the terahertz range for metmaterials and other materials, the frequency range is usually defined as 0.1 to 10 THz. This corresponds to the submillimeter wavelengths between 1 mm (EHF band) and 0.1 mm (long-wavelength edge of far-infrared light.

Photonic metamaterials

Main article: Photonic metamaterials

Photonic metamaterials are artificial, periodic materials, hence a metamaterial. The periodic structure is derived from nanostructures which work well at optical frequencies.

In a conventional material, the response to electric and magnetic fields, and hence to EM radiations such as light, is determined by the atoms.. In a metamaterial, the response is engineered through many identical macroscopic materials sometimes known as structural units , or meta-atoms, but with dimensions much less than the wavelength at the operating frequency. In effect, the engineered units behave like macroscopic atomic dipoles when radiated from an external source.

Metamaterial antennas

Main article: Metamaterial antennas

Metamaterial antennas are a class of antennas which use metamaterials to improve the performance of the antenna systems. Applying metamaterials to increase performance of antennas has garnered much interest. Demonstrations have shown that metamaterials could enhance the radiated power of an antenna. Materials which can attain negative permeability could possibly allow for properties such as an electrically small antenna size, high directivity, and tunable operational frequency.

Tunable metamaterials

Main article: Tunable metamaterials

A tunable metamaterial is a metamaterial which has the capability to arbitrarily adjust frequency changes in the refractive index at will. A tunable metamaterial encompasses the development of expanding beyond the bandwidth limitations in left-handed materials by constructing various types of metamaterials.

Frequency selective surface (FSS) based metamaterials

Main article: Tunable metamaterials

FSS based metamaterials have become an alternative to the fixed frequency metamaterial. The former allow for optional changes of frequencies in a single medium (metamaterial), rather than the restrictive limitations of a fixed frequency response.

Development and applications

The first metamaterials were developed by W. E. Kock in the late 1940s. and metallic delay lenses.

The unique properties of metamaterials were verified by full-wave analysis. However, the left-handed structures devised up to 2002 were impractical for microwave applications because their applicable bandwidth was too narrow and their coefficients of transmission were low. A method was provided in 2002 to realize left-handed metamaterials using artificial lumped-element loaded transmission lines in microstrip technology.

Superlens

Main article: Superlens

A superlens uses metamaterials to go beyond the diffraction limit. The diffraction limit is inherent in conventional optical devices or lenses.

It was first postulated by John Pendry and colleagues in Physical Review Letters that a negative refractive material would enable a superlens because of two properties:

1. A wave propagating in a negative-refractive medium exhibits a phase advance instead of a phase delay in conventional materials;
2. Evanescent waves in a negative-refractive medium increase in amplitude as they move away from their origin.

However, it was demonstrated via simple geometrical arguments that in order to enable property #1 above, negative time must be enforced. Furthermore, if property #2 is actually possible, this would lead to infinite energy creation at infinite distances. Both properties thus appear to yield non-causal behaviors.

The first superlens with a negative refractive index provided resolution three times better than the diffraction limit and was demonstrated at microwave frequencies. Subsequently, the first optical superlens (an optical lens which exceeds the diffraction limit) was created and demonstrated, but the lens did not rely on negative refraction. Instead, a thin silver film was used to enhance the evanescent modes through surface plasmon coupling.

Two developments in superlens research were reported in 2008. In the first case, alternate layers of silver and magnesium fluoride were deposited on a substrate. Then nanoscale grids were cut into the layers, which resulted in a 3-dimensional composite structure with a negative refractive index in the near-infrared region. In the second case, a metamaterial was formed from silver nanowires which were electrochemically deposited in porous aluminium oxide. The resulting material exhibited negative refraction down to 660 nm.. In early 2007, a metamaterial with a negative index of refraction for a visible light wavelength was announced. The material had an index of −0.6 at 780 nm.

Cloaking devices

Main article: Negative index metamaterials

Metamaterials are a basis for attempting to build a practical cloaking device. The possibility of a working invisibility cloak was demonstrated on October 19, 2006. According to the article, a team led by scientists at Pratt School of Engineering, Duke University has demonstrated the first working "invisibility cloak." The cloak deflects microwave beams so they flow around a "hidden" object inside with little distortion, making it appear almost as if nothing were there at all. The associated report was published in the journal Science.

Such a device typically involves surrounding the object to be cloaked with a shell which affects the passage of light near it. It was claimed that plasmons could be used to cancel out visible light or radiation coming from an object. This "plasmonic cover" would work by suppressing light scattering by resonating with illuminated light, which could render objects "nearly invisible to an observer." The plasmonic screen would have to be tuned to the object being hidden, and would only suppress a specific wavelength—an object made invisible in red light would still be visible in multicolored daylight.

In October 2006, a US-British team of scientists created a metamaterial which rendered an object invisible to microwave radiation. As light is one of the bands of electromagnetic radiation, this was considered the first step toward a cloaking device for visible light, although more advanced nanoengineering techniques would be needed due to light's short wavelengths.

On 2 April 2007, Vladimir Shalaev at Purdue University announced a theoretical design for an optical cloaking device based on the 2006 British concept. The design deploys an array of tiny needles projecting from a central spoke that would render an object within the cloak invisible for red light (wavelength of 632.8 nanometers).

In 2009 at Duke University the latest advance—a series of algorithms were developed, to guide the design and fabrication of new metamaterials. David Smith of the Duke Engineering department, comparing the 2006 device, says, "The difference between the original device and the latest model is like night and day. The new device can cloak a much wider spectrum of waves—nearly limitless—and will scale far more easily to infrared and visible light. The approach we used should help us expand and improve our abilities to cloak different types of waves." The article also noted that "once the algorithm was developed, the latest cloaking device was completed from conception to fabrication in nine days, compared to the four months required to create the original, and more rudimentary, device."

Acoustic metamaterials

Main article: Acoustic metamaterials

Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound in the form of sonic, infrasonic, or ultrasonic waves, as these might occur in gases, liquids, and solids. The hereditary line into acoustic metamaterials follows from theory and research in electromagnetic metamaterials. Furthermore, with acoustic metamaterials, sonic waves can now be extended to the negative refraction domain.

Seismic metamaterials

Main article: Seismic metamaterials

Seismic metamaterials, are metamaterials which are designed to counteract the adverse effects of seismic waves on man-made structures, which exist on or near the surface of the earth.

Non-linear metamaterials

EM field shielding by non-linear metamaterials

It is well known that over certain frequencies, typical metals can reflect electromagnetic (EM) fields and can thus be used as electromagnetic shielding materials. However, conventional linear LHMs cannot be used to shield electromagnetic fields. This is drastically modified when nonlinearity of the magnetic response is taken into account, creating a controllable shielding effect in LHMs, accompanied by a parametric reflection.

Sub-diffraction limit for non-linear MM lens

By covering a thin flat nonlinear lens on the sources, the sub-diffraction-limit observation can be achieved by measuring either the near-field distribution or the far-field radiation of the sources at the harmonic frequencies and calculating the IFT to obtain the sub-wavelength imaging. The higher order of harmonics are used, the higher resolution is obtained.

Non-linear electric metatmaterial

A new type of nonlinear metamaterial is designed, and analyzed with a dominant negative electric response. Introducing nonlinearity into the electric response makes it tunable while leaving the magnetic response unchanged. It is expect that our results would constitute the building blocks of a complete nonlinear negative index metamaterial containing both nonlinear or tunable electric and magnetic elements, which can be engineered independently.

Negative refraction at near visible and visible frequencies

Negative refraction at 813 nm and 772 nm

813 nm and 772 nm nearly reaches into the visible spectrum. A double negative refraction occurs at 813 nm and single negative refraction occurs at 772 nm.

Negative refraction for the blue-green light

Plasmonic waveguide device for demonstration of negative refraction at visible frequencies

This is a planar metal-insulator-metal two-dimensional NIM in a waveguide configuration. Negative effective permeability and negative effective permittivity characterize the NIM, whcn averaged over the thickness of the waveguide.

Negative refraction for the red light

An effective negative index of refraction of -0.6 is demonstrated at around 780 nm wavelegnth. This wavelength can easily be seen with the naked eye in our laser experiments.

Optical negative-index metamaterials

Describing the recent progress (in 2006) made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

Experimental demonstration of near-infrared NIMs

Received 7 March 2005 and published in September of that year was the first fabrication and experimental verification of a transversely structured metal-dielectric-metal multilayer exhibiting a negative refractive index around 2 μm. Both the amplitude and the phase of the transmission and reflection were measured experimentally, and are in good agreement with a rigorous coupled wave analysis.

Three-dimensional optical metamaterial

Here is a 3D optical metamaterial having negative refractive index with a very high figure of meritref of 3.5. This metamaterial is made of cascaded ‘fishnet’ structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. Constructed as a 21-layer fishnet structure with a unit cell of p5860 nm, a5565nm and b5265 nm.

Controllable magnetic response at optical frequencies

Negative permeability material for red light

Desired permeability achieved in one wavelength of the visible spectrum at 780 nm.

Controllable permeability across visible spectrum

Coupled nanostrips demonstrate control of magnetic responses across the visible spectrum.

Magnetic response at telecommunication and visible frequencies

At telecommunication or visible frequencies the desired magnetic response, negative permeability (μ_eff < 0) does not occur in natural materials. One stated technological challenge in November 2005 was to achieve μ_eff < 0 and this would be accomplished with an adapted SRR/wire metamterial. Prior research had established that the SRR/wire metamaterials were used in the first demonstrations of μ_eff < 0 at microwave frequencies. By November 2004 magnetic resonance was demonstrated at 100 THz (3 μm wavelength), which is an increase of more than 4 orders of magnitude within four years.

Other uses

Metamaterials have been proposed for designing agile antennas. Research at the National Institute of Standards and Technology has demonstrated that thin metamaterial films can greatly reduce the size of resonating circuits that generate microwaves, potentially enabling even smaller cell phones and other microwave devices. It has been theorized that metamaterials could be built to bend matter around them because of the subatomic properties of matter. Such a matter cloak could for example bend a bullet around a person rather than absorb the impact as traditional bulletproof vests do.

Theoretical models

Left-handed materials were first described theoretically by Victor Veselago in 1967.

John Pendry was the first to theorize a practical way to make a left-handed metamaterial. Left-handed in this context means a material in which the right-hand rule is not followed, allowing an electromagnetic wave to convey energy (have a group velocity) in the lode against its phase velocity. Pendry's initial idea was that metallic wires aligned along the direction of propagation could provide a metamaterial with negative permittivity (ε < 0). Note however that natural materials (such as ferroelectrics) were already known to exist with negative permittivity; the challenge was to construct a material which also showed 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 provide a negative permeability. In the same paper, he showed that a periodic array of wires and ring could give rise to a negative refractive index. A related negative-permeability particle, which was also proposed by Pendry, is the Swiss roll.

The analogy is as follows: All materials are made of atoms, which are dipoles. These dipoles modify the light velocity by a factor n (the refractive index). The ring and wire units play the role of atomic dipoles: the wire acts as a ferroelectric atom, while the ring acts as an inductor L and the open section as a capacitor C. The ring as a whole therefore acts as an LC circuit. When the electromagnetic field passes through the ring, an induced current is created and the generated field is perpendicular to the magnetic field of the light. The magnetic resonance results in a negative permeability; the index is negative as well. (The lens is not truly flat, since the capacitance of the structure imposes a slope for the electric induction.)

Groups engaged in metamaterial research

The number of groups studying metamaterials is continuously increasing. For example, Duke University has initiated an umbrella organization researching metamaterials under the banner "Novel Electromagnetic Materials" and became a leading metamaterials research center. The center is a part of an international team, which also includes California Institute of Technology, Harvard University, UCLA, Max Planck Institute of Germany, and the FOM Institute of the Netherlands. In addition, there are currently six groups connected to this umbrella organization, which are conducting intense metamaterial research:

See also

• List of emerging technologies
• METATOY, an acronym for metamaterial for rays
• Fresnel lens
• Zone plate
• Litz wire is a 3D structure designed to reduce skin effect
• Phonon is a quantized mode of vibration occurring in a rigid crystal lattice.
• Surface phonon
• Negative refraction
• Scattering Matrix Method

External links

Research groups (in alphabetical order) having educational pages on metamaterials

1. Min Qiu's Nanophotonics group, Royal Institute of Technology (KTH), Sweden
2. Christophe Caloz' research group — Canada
3. George Eleftheriades's research group — Canada
4. Nader Engheta – US
5. FGAN-FHR — Germany
6. M. Saif Islam's Research Group, University of California at Davis – US
7. Yang Hao's Group, Queen Mary, University of London – UK
8. Sir John Pendry's group — free-download papers — Imperial College — UK
9. Viktor Podolskiy's group — Oregon State University — US
10. Shvets Research Group, University of Texas at Austin – US
11. David Smith's research group — Duke University — US
12. Costas Soukoulis at IESL, Greece — Photonic, Phononic & MetaMaterials Group
13. Srinivas Sridhar's Group, Northeastern University — US
14. Irina Veretennicoff's research group, Vrije Universiteit Brussel — Belgium
15. Martin Wegener's Metamaterials group, Universität Karlsruhe (TH) — Germany
16. Georgios Zouganelis's Metamaterials Group, NIT — Japan
17. Xiang Zhang's group, Berkeley US
18. Sergei Tretyakov's group, Helsinki University of Technology, Finland

Internet portals

1. Journal "Metamaterials" published by Elsevier (homepage)
2. Online articles: "Metamaterials" in ScienceDirect
3. RSS feed for Metamaterials articles published in Physical Review Journals
4. MetaMaterials.net Web Group
5. Virtual Institute for Artificial Electromagnetic Materials and Metamaterials ("METAMORPHOSE VI AISBL")
6. European Network of Excellence "METAMORPHOSE" on Metamaterials
7. SensorMetrix Formed with a specific directive to exploit the recent advances in electromagnetic metamaterials

More articles and presentations (most recent first)

1. Dr. Sebastien Guenneau (Seismic MMs) Research on Metamaterials and Photonic Crystal Fibres
2. UWB Tunable Delay System, Prof Christophe Caloz, Ecole Polytechnique de Montreal)
3. Metaphotonics.de, Information about Photonic Metamaterials in Karlsruhe (HHNG Dr. Stefan Linden and Prof. Dr. Martin Wegener)
4. Realistic raytraced images, videos and interactive web-based demonstrations of materials with negative index of refraction.
5. Cloaking devices, nihility bandgap, LF magnetic enhancement, perfect radome NIT Japan
6. Left-Handed Flat Lens HFSS Tutorial Electromagnetism Tutorial
7. Journal of Optics A, February 2005 Special issue on Metamaterials
8. Experimental Verification of a Negative Index of Refraction
9. How To Make an Object Invisible
10. Metamaterials hold key to cloak of invisibility