Metamaterial

From Wikipedia, the free encyclopedia

Metamaterials are exotic composite materials that display properties beyond those available in naturally occurring materials. Instead of constructing materials at the chemical level, as is ordinarily done, these are constructed with two or more materials at the macroscopic level. One of their defining characteristics is that the electromagnetic response results from combining two or more distinct materials in a specified way which extends the range of electromagnetic patterns because of the fact that they are not found in nature.^[1]

The term was coined in 1999 by Rodger M. Walser of the University of Texas at Austin. He defined metamaterials as^[2]

macroscopic composites having a manmade, three-dimensional, periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation.

How a metamaterial, used as a cloaking device, currently works.

Cloaking device deactivated: Light is reflected and absorbed by the object, causing it to be visible.

Cloaking device active: Light is deflected around, in front of, and behind the object, making it appear as if nothing were there at all.

In other words, these are artificial composite materials of enhanced abilities over natural materials, and are three dimensional for which they can take shape in the world outside the drawing board. According to The Research Group of David R. Smith, "Over the last several years there has been a surge of interest in artificial materials because of their potential to expand the range of electromagnetic properties in materials. In a paper published in 2001, Rodger Walser from the University of Texas, Austin, coined the term metamaterial to refer to artificial composites that "...achieve material performance beyond the limitations of conventional composites." The definition was subsequently expanded by Valerie Browning and Stu Wolf of DARPA (Defense Advanced Research Projects Agency) in the context of the DARPA Metamaterials program that started also in 2001. Their basic definition: Metamaterials are a new class of ordered composites that exhibit exceptional properties not readily observed in nature. While the original metamaterials definition encompassed many more material properties, most of the subsequent scientific activity has centered on the electromagnetic properties of metamaterials gains its properties from its structure rather than directly from its composition."^[1]

In other words, electromagnetics researchers often use the term quite narrowly for materials which exhibit negative refraction. W. E. Kock developed the first metamaterials in the late 1940s with metal-lens antennæ^[3] and metallic delay lenses.^[4]

A negative refractive index is important to researchers in the field of metamaterials. With a negative refractive index researchers have been able to create a device known as a cloaking device, or an invisibility cloak,^[5] which is not possible with natural materials. Refraction is the bending of light as it moves through some transparent medium, such as the lenses of eyeglasses, or a glass of water. Something such as a finger through the glass may look greater or smaller. A pencil stuck in a glass of water seems to sharply bend at an angle. At each bend the light through the glass brakes inward, and the index of refraction in natural materials has a positive value. A negative refractive index is when light brakes outward, and bends outward in a thicker medium. Dr. Michio Kaku wrote^[6], "every optics textbook says this is impossible." In 1967, when metamaterials were first theorized by Soviet Victor Veselago, they were thought to be bizarre and preposterous.^[7]

Usually when a beam of light is bent entering a glass of water it keeps faring in a straight line at the angle that it entered, and the index of refraction is constant. Suppose one could shape the index over the medium's span: With metamaterials it can be controlled so that the object becomes invisible—a negative refraction index. Ames Laboratory in Iowa created a metamaterial of index of −0.6 for red light (780 nanometers).^[8] Previously, physicists were only successful in bending infrared light with a metamaterial at 1,400 nm, which is outside the visible light range.^[9]

[edit] Electromagnetic metamaterials

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 wavespan of the 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 wave. For visible light, whose wave is less than one micrometre typically (560 nm for sunlight), the structures are generally half this size or smaller, q.e. < 280 nm. For microwave radiation, the structures need only be on the order of 10 cm. 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.

[edit] Negative refractive index

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 naturally occurring 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, in practice the refractive index N is often used. N may be determined from . All known transparent materials possess positive values for 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.^[10]

Metamaterials with negative N have numerous startling properties:

• Snell's law (N₁sinθ₁ = N₂sinθ₂) still applies, 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.

For plane waves propagating in such 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.

[edit] Development and applications

The first metamaterials were developed by W.E. Kock in the late 1940s.^[3][4] The unique properties of metamaterials were verified by full-wave analysis.^[11] However, the LH 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.^[12][13]

[edit] Superlens

It was first postulated by John Pendry^[14] of Imperial College, London and colleagues in Physical Review Letters that a negative refractive material would enable a perfect lens 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. ^[15]

The first superlens with a negative refractive index provided resolution three times better than the diffraction limit and was demonstrated at microwave frequencies.^[16] Subsequently, the first optical superlens (an optical lens which exceeds the diffraction limit) was created and demonstrated,^[17] 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.^[18] In the first case, alternant 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.^[19] In the second case, a metamaterial was formed from silver nanowires which were electrochemically deposited in porous aluminum oxide. The resulting material exhibited negative refraction down to 660 nm.^[20]. In early 2007, a metamaterial with a negative index of refraction for visible light wavelengths was announced. The material had an index of −0.6 at 780 nm.^[21]

[edit] Cloaking devices

Metamaterials are currently a basis for building a cloaking device. A possibility of a working invisibility cloak was demonstrated on October 19, 2006. According to the article, "A team led by scientists at Duke University's Pratt School of Engineering 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."^[22] The associated report was published in the journal Science.^[23]

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.^[24]

In October 2006, a US-British team of scientists created a metamaterial which rendered an object invisible to microwave radiation.^[25] As light is one of the bands of electromagnetic radiation, this was considered the first step toward a cloaking device for witsom light, although more advanced nanoengineering techniques would be needed due to light's short wavelengths.

On 2 April 2007, two Purdue University engineers 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 in a wavelength of 632.8 nanometres.^[26]

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."^[27]

[edit] Other uses

This section requires expansion with: more information from the source provided.

Metamaterials have been proposed for designing agile antennas ^[28]. 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.^[29] 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. ^[30]

[edit] Theoretical models

Left-handed (LH) materials were first described theoretically by Victor Veselago in 1967.^[31]

John Pendry was the first to theorize a practical way to make a left-handed metamaterial (LHM). 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 an open 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 Professor Pendry, is the Swiss roll.

The analogy is as follows: Natural 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.)

[edit] 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

[edit] References

[edit] External links

[edit] 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

[edit] 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

[edit] More articles and presentations (most recent first)

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