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Negative index metamaterials are metamaterials that have the capability to direct and regulate waves due to their negative refractive index. Metamaterial broadly refers to any synthetic material with unusual refractive properties, in this context. The application of negative index metamaterials involves blending objects with the environment, and this application is being actively developed in the laboratory. This on-going research is funded by several US government agencies, including DARPA, the United States Air Force and the United States Navy.

The basic idea involves controlling a material such that it bends light or other electromagnetic waves around a region or object such that they emerge on their original path. It would appear to an observer as if light was propagating across empty space. An object inside the concealed region would not be visible. The theory of negative index metamaterials was originally outlined in 1968, and it was confirmed by experiment in 2001. As a practical demonstration of the associated capabilities, a small object was hidden from view, appearing to be empty space, using negative index metamaterials in 2006. Currently, "invisibility" has been achieved only with small objects, and only in several frequency ranges within the electromagnetic spectrum. In the future, concealment might be achieved across the entire electromagnetic spectrum, from the lowest radio frequencies, through microwaves, to the visible light.

Military applications include improved stealth aircraft. Besides military applications the technology has projected applications for everyday life.

Limitations of current stealth technology
The F-117 Nighthawk, was the first operational combat stealth aircraft. The F-117 Nighthawk was designed to fufill the need for an "aircraft capable of attacking high value targets without being detected by enemy radar". It fufilled expectations throughout its life of service between its first operational flight in 1981, until its retirement in 2008. For example, a group of F-117's flew over 1,270 sorties, over four nights in Operation Desert Storm. These flew low into combat past what was at the time the "world's most densely concentrated network of air defenses", which would direct and deliver thousands of anti-aircraft guns and surface-to-air missiles. Wing leaders expected losses of 50%. However, engineers gave positive assurances. On the fourth night, at the conclusion of their part in the campaign, "not one F-117 recieved battle damage." The F-117 demonstrated the effectiveness of today's stealth technology.

The F-117s effectively neutralized the Iraq Integrated Air Defense system within the first few minutes of the war, and gave control of the skies, to Coalition Forces. However, they were limited to night flying with clear skies. Several decades from now, with a fully developed electromagnetic cloaking capability, there would be no limitation to night flying. It could not be visually observed during daylight hours because the propagation of controlled electromagnetic waves would make it appear as if no aircraft were present. Several decades from now radar would not be a concern at both night and day hours because with electromagnetic cloaking capability, the fighter would potentially return no signal to enemy radar because the intersecting beam would be shifted around in the direction of the shielding rays. Furthermore, several decades from now, since cloaking would have been achieved through manipulation of the electromagnetic spectrum, development of any type of aircraft desired or required becomes possible.

When in service, the F-117 had a specialized coating on its outer skin known as radar absorbing material (RAM). This is effective in attenuating any incident radar beam thus avoiding detection. However, an air bubble in the coating is enough for radar detection. The physical design consisting of unconventional edges and angles also dampens the return signal. The windshield is coated with a special film to hide the pilot's helmet from radar. However, compared to most conventional military aircraft which are constructed to be aerodynamically stable the F-117's design makes the craft unstable. It is difficult to keep the F-117 straight and level during flight. A computer is needed to maintain stability. With an electromagnetic shielding capability, it would be possible to fly either aerodynamically stable or aerodynamically unstable military aircraft, as desired or required, because the cloak is in the control of electromagnetic waves.

The F-22 Raptor stealth fighter experiences similar problems with its specialized RAM coating. In 2003 it was observed that the coating process takes 15 days. Then the skin is meticulously inspected for flaws, which can cause the FA-22 Raptor to become visible on radar. In addition, any contamination on uncoated surfaces can affect the cohesion of the coat, and it then must be partially or fully reapplied. In July 2009, it has been reported that, while in service, the FA-22 requires more than 30 hours of maintenance for every hour in the skies, and the RAM coating is the principal cause. Similar to the B-2 stealth bomber, the coating is key to the craft's stealth capability, and, as with the B-2, the coating is vulnerable to rain and abrasions. The ratio of maintenance hours to flight hours means that FA-22 flies at a rate of $44,000 per hour.

Furthermore, each FA-22 requires hand-crafted components, which exemplifies a general flaw, or limitation, of the Stealth program. There are often multiple chains of supply lines for specialized components for the B-2, and F-22 stealth aircraft. For example, "the B-2 (stealth bomber)...used dozens of different materials, each with its own supply chain, and required its own maintenance, and handling training." Moreover, on the B-2, some of the materials can be restored only slowly, and laboriously. Low-observable materials have to be removed and replaced at the slightest sign of damage. These hurdles for the FA-22, and B-2 aircraft helps to create large expenditures when compared to conventionally built aircraft. For example the F-16 F-16C/D has a unit cost of US$ 18.8 million (1998 dollars). This compares to a unit cost of US$ 42.6 M for the F-117, and US$ 137.5 million unit cost for the F-22 Raptor. Conventional aircraft have of the availability of interchangeable parts, and do not require special RAM coating. With electromagnetic shielding, radar absorption would be in the electromagnetic fields, as the beam is directed around the secreted object, returned to their original trajectory, appearing as empty sky. . When mature, this cloaking capability could utilize conventionally produced aircraft or other types. This is because as early as May 2006 there was discussion about military applications, in the news. John Pendry, a pioneer in this science, stated that this technology could be available to the military within 18 months at radar frequencies. A Pendry and Smith research paper presents theoretical methods of cloaking for radar. This was posted on Science Express, in May 2006, in advance of print publication in the AAAS Science journal.

More research into this technology
Other accomplishments related to electromagnetic manipulation followed. In early 2007, a wave expansion method was analyzed which would create a scattering field to ideally conceal an object, or a person, by developing a 2D cylindrically shaped, electromagnetic shielding field. Later in 2007, a mathematical improvement in the cylindrical shielding to produce an electromagnetic "wormhole", analyzed in three dimensions.

Broadband ground-plane cloak
If a transformation to orthogonal coordinates is applied to Maxwell's equations in order to conceal a perturbation on a flat conducting plane, then an object can be hidden underneath the perturbation. An automated process, guided by a set of algorithms, was used to construct a metamaterial consisting of thousands of elements. each with its own geometry. "The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss." Developing the algorithm allowed the manufacturing process to be automated, which resulted in fabrication of the metamaterial in nine days. The previous device used in 2006 is rudimentary in comparison, and the manufacturing process required four months in order to create the device.

Proposed applications of negative index metamaterials

 * This technology can be used in other applications, such as wireless communications, defense and radar. When the technology is fully developed, it may be possible to prevent detection of objects by infrared, radar and radio waves. Structures that would otherwise disrupt wireless signals could route the signals around the structure, improving reception nearby. As the technology progresses, with materials that enable more and more precise control of the electromagnetic field, advanced lenses could be developed for cameras, and unwanted reflections from objects could be reduced. It could improve microwave antennae design, and optical components.


 * Collaborating researchers from University College London, Helsinki University of Technology, the University of Rochester and the University of Washington stated, in 2009, that:

"...some possible applications [with] electromagnetic wormholes include the creation of invisible fiber optic cables, for example for security devices, and scopes for MRI-assisted medical procedures for which metal tools would otherwise interfere with the magnetic resonance images. The invisible optical fibers could even make three-dimensional television screens possible in the distant future. The effectiveness and implementation of [this technology] in practice, however, are dependent on future developments in the design, investigation, and production of metamaterials."


 * "Configurations of geometrical optical designs are now possible that could not be realized by positive index materials." Any material that exhibits the property of negative refractive index, a property not observed in naturally occurring materials, will have a variety of practical applications, such as beam steerers, modulators, band-pass filters, and lenses permitting subwavelength point source focusing."


 * "Our work has relevance to exotic lens design and to the cloaking of objects from electromagnetic fields."


 * "With (optical conformal mapping) the use of modern metamaterials, practical demonstrations of such devices may be possible. The method developed here can also be applied to escape detection by other electromagnetic waves or sound."I In addition, "With these artificial dielectrics, invisibility could be reached for frequencies in the microwave-to-terahertz range. In contrast, stealth technology is designed to make objects of military interest as black as possible to radar where, using impedance matching, electromagnetic waves are absorbed without reflection, i.e., without any echo detectable by radar. Recently, nanofabricated metamaterials with custom-made plasmon resonances have been demonstrated that operate in the visible range of the spectrum and may be modified to reach invisibility."


 * "Acoustic 'superlens' could mean finer ultrasound scans - Negative refraction — Devices based on such acoustic metamaterials could produce ultra-sharp medical scans, more detailed seismic maps, and even earthquake-resistant buildings. Conventional lenses cannot focus on anything smaller than roughly half the wavelength of light or sound they use. That is because they cannot recover and focus weak scattered waves that are needed to "see" the really small features. Thanks to their odd refractive properties, though, metamaterial superlenses can — so the acoustic superlens designed by Guennea and colleagues could focus on details that are invisible to today's equipment. Such a superlens could be used at a variety of scales. "You can build one to see a foetus," says Guenneau, "or something underground". Imaging underground features requires long-wavelength sound that consequently gives poor resolution, an acoustic superlens could help.


 * "Earthquake protection — Guenneau's group also showed that a checkerboard array of their halved ring-shapes can stop sound waves passing. This pattern could be used to damp noise or vibrations on a wide range of scales, from tiny electronic or mechanical components to large buildings protected from the earthquake shear waves using columns with the metamaterial incorporated within. Although the work is so far mathematical, the team is confident its acoustic metamaterial can be built. The model is based on the known properties of silica and the voids cut into it are accompanied by stiff bars with properties similar to carbon nanotubes."


 * September 26th, 2008 - A team of physicists has shown that its possible to make a type of dam that acts as an invisibility cloak which hides off-shore platforms from water waves and tsunamis. The collaboration of physicists is from the Centre National de la Recherche Scientifique (CNRS) and Aix-Marseille Universite in France and the University of Liverpool in England. They have conducted laboratory experiments showing that it’’s possible to make type of dike that acts as an invisibility cloak that hides off-shore platforms from water waves. Laboratory experiments show that obstacles arranged in fluids in certain patterns can effectively make objects they surround invisible to waves. Tsunami invisibility cloaks wouldnt make structures disappear from sight, but they could manipulate ocean waves in ways that makes off-shore platforms, and possibly even coastlines and small islands, effectively invisible to tsunamis.


 * January 10, 2008- "Duke University engineers will reveal Friday (Jan. 11) details of an acoustic cloak fabricated from metamaterials that they claim can render objects invisible to sonar. If this works then we will have unprecedented control to hide from the effects or to enhance the effects of sound and other waves in all kinds of material. Submarines invisible to sonar would extend the security of nuclear weapons deployed on submarines against future sensing technology. Even if one side had nanotechnology it would take a lot to find invisible to sonar stealth submarines that were carrying nuclear weapons. Extending the deterrent of nuclear weapons makes for a more militarily stable future world. Further, the engineers claim that the technique proves that waves can be redirected around objects in different media, opening up the possibility of improving the acoustics in concert halls by cloaking structural beams from sound waves in air. It may even be possible to redirect seismic waves around buildings [making them earthquake resistant], or ocean waves around ships."


 * "July 07, 2009 - Earthquake cloak: Adapting Optical Invisibility Techniques for Earthquake Shockwave Resistant Buildings Correcting article: There are several papers on cloaking buildings from earthquake waves. The new theoretical cloak comprises a number of large, concentric rings made of plastic fixed to the Earth's surface. The stiffness and elasticity of the rings must be precisely controlled to ensure that any surface waves pass smoothly into the material, rather than reflecting or scattering at the material's surface." "When waves travel through the cloak they are compressed into tiny fluctuations in pressure and density that travel along the fastest path available. By tuning the cloak's properties, that path can be made to be an arc that directs surface waves away from an area inside the cloak. When the waves exit the cloak, they return to their previous, larger size." Unlike some of the optical invisibility cloaks that have been studied in physics labs in recent years, the new cloak is "broadband", meaning that it can divert waves across a range of frequencies."