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Researchers Create First Working Invisibility Cloak
By Imperial College, London and Duke University, USA
Oct 20, 2006, 23:03

Scientists have demonstrated the first ever working 'invisibility cloak', reports Science Express today. The team of researchers from Duke University, USA, worked with Professor Sir John Pendry New Window of Imperial College London to create a prototype 'cloak' based on a new design theory proposed by the same team earlier this year. Using a new design theory, researchers at Duke University's Pratt School of Engineering and Imperial College London have developed the blueprint for invisibility cloak. Once devised, the cloak could have numerous uses, from defense applications to wireless communications, the researchers said.
(Image courtesy: Imperial College London)

The cloak deflects microwave beams as they flow around an object hidden inside it, in the same way that water in a river flows around a stick. The Duke scientists have proven that the object inside the cloak is rendered invisible to microwaves, which could, in the long run, have a variety of applications for radar and communications technologies. The group's design methodology also may find a variety of uses other than cloaking, the scientists said. With appropriately fine-tuned metamaterials, electromagnetic radiation at frequencies ranging from visible light to electricity could be redirected at will for virtually any application. For example, the theory could lead to the development of metamaterials that focus light to provide a more perfect lens.

The researchers at Duke University used theoretical designs published in an earlier Science paper to build a small cloak, less than five inches across, which would provide invisibility in two dimensions. The cloak was built using specially-manufactured 'metamaterials' which 'grab' light heading towards the cloak and make it flow smoothly around the cloak instead of striking it.

To test the prototype cloak, the researchers aimed a microwave beam at it inside a test chamber between two metal plates, and then measured the electromagnetic fields both inside and outside the cloak. Their measurements showed that the electromagnetic waves separated and flowed around the centre of the cloak, as predicted by theory.

Sir John said: "Our first paper developed the concept of a cloak but there remained the huge challenge of making this a reality. We knew that no naturally occurring materials would do the job, but the new class of metamaterials which owe their properties to their internal structure rather than their chemistry, have proved yet again that they can meet some of the most extreme challenges."

Sir John and his collaborators at Duke are concentrating on perfecting their cloaking technology for microwaves, and are hoping to develop a three-dimensional cloak for invisibility to microwaves.

The metamaterials used to build the cloak are fashioned into concentric two-dimensional rings, which interact with magnetic waves in a very specific way. The precise variations in the shape of the copper element used determine the cloak’s electromagnetic properties. The cloak is thought to be one of the most complex metamaterial structures ever made, as it has a unique circular geometry and it’s electromagnetic properties vary across its surface.

Such a cloak could hide any object so well that observers would be totally unaware of its presence, according to the researchers. In principle, their invisibility cloak could be realized with exotic artificial composite materials called "metamaterials," they said.

"The cloak would act like you've opened up a hole in space," said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke's Pratt School. "All light or other electromagnetic waves are swept around the area, guided by the metamaterial to emerge on the other side as if they had passed through an empty volume of space."

Electromagnetic waves would flow around an object hidden inside the metamaterial cloak just as water in a river flows virtually undisturbed around a smooth rock, Smith said.
The research team, which also includes David Schurig of Duke's Pratt School and John Pendry of Imperial College London, reported its findings on May 25, 2006, in Science Express, the online advance publication of the journal Science. The work was supported by the Defense Advanced Research Projects Agency.

First demonstrated by Smith and his colleagues in 2000, metamaterials can be made to interact with light or other electromagnetic waves in very precise ways. Although the theoretical cloak now reported has yet to be created, the Duke researchers are on their way to producing metamaterials with suitable properties, Smith said.
"There are several possible goals one may have for cloaking an object,” said Schurig, a research associate in electrical and computer engineering. "One goal would be to conceal an object from discovery by agents using probing or environmental radiation."

"Another would be to allow electromagnetic fields to essentially pass through a potentially obstructing object," he said. "For example, you may wish to put a cloak over the refinery that is blocking your view of the bay."

By eliminating the effects of obstructions, such cloaking also could improve wireless communications, Schurig said. Along the same principles, an acoustic cloak could serve as a protective shield, preventing the penetration of vibrations, sound or seismic waves.

"To exploit electromagnetism, engineers use materials to control and direct the field: a glass lens in a camera, a metal cage to screen sensitive equipment, 'black bodies' of various forms to prevent unwanted reflections," the researchers said in their article. "Using the previous generation of materials, design is largely a matter of choosing the interface between two materials." In the case of a camera, for example, this means optimizing the shape of the lens.

The recent advent of metamaterials opens up a new range of possibilities by providing electromagnetic properties that are "impossible to find in nature," the researchers said.

Their design theory provides the precise mathematical function describing a metamaterial with structural details that would allow its interaction with electromagnetic radiation in the manner desired. That function could then guide the fabrication of metamaterials with those precise characteristics, Smith explained.

The theory itself is simple, Smith said. "It's nothing that couldn't have been done 50 or even 100 years ago," he said.

"However, natural materials display only a limited palette of possible electromagnetic properties," he added. "The theory has only now become relevant because we can make metamaterials with the properties we are looking for."

"This new design paradigm, which can provide a recipe to fit virtually any electromagnetic application, leads to material specifications that could be implemented only with metamaterials," Schurig added.

The team's next major goal is an experimental verification of invisibility to electromagnetic waves at microwave frequencies, the scientists said. Such a cloak, they said, would have utility for wireless communications, among other applications.

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