The first "invisibility cloak" has been designed by a team at Duke University's Pratt School of Engineering, North Carolina. The cloak deviates microwave rays to go around a "hidden" object inside with little distortion, making it almost "disappear". These cloaks could have wireless communications or radar applications, hiding jets from waves.
Stealth technology which protects bombers from radar deflects the radar, disguising the jets, but do not turn them invisible.

The five-inch cloak (12.7 centimeters) is made from "metamaterials" - complex artificial products with electromagnetic properties which natural materials do not possess, made of copper rings and wires patterned onto fiberglass composite sheets that are traditionally used in computer circuit boards.

Natural materials' properties are due to their chemistry, but the properties of metamaterials depend on their physical structure. The material is designed to distort space so that microwaves are not reflected back, but are instead bent around the cloaked object, whatever its shape, and allowed to flow on as if it didn't exist.

The result is that the beams are deflected like water flowing around a rock in a river, without noticeably interrupting the main current. The cloak represents "one of the most elaborate metamaterial structures yet designed and produced," said the research team. "It also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property," they added.

Previous attempts to produce "invisibility" tried to limit electromagnetic waves' reflection or to cancel the electromagnetic waves of the object meant to be hidden or were suitable for hiding only objects with very specific properties. "By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally," said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke.

"The cloak reduces both an object's reflection and its shadow, either of which would enable its detection."

"One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads," said cloak designer David Schurig, a research associate in Duke's electrical and computer engineering department

"In such a space, light or other electromagnetic waves would be confined to the warped 'threads' and therefore could not interact with, or 'see,' objects placed inside the resulting hole."

"The researchers used a mathematical description of that concept to develop a blueprint for a cloak that mimics the properties of the imagined, warped space," he said.

"You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response," Schurig continued.

The small cloak provides invisibility in two dimensions, rather than three. It incorporates strips of metamaterial positioned into concentric two-dimensional rings that distort a narrow array of microwave radiation. This cloak uses a different metamaterials' geometry. Other metamaterials previously designed have used cubic, or gridlike, design, conferring them uniform electromagnetic properties. "Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position," Smith said.

"Rather than its material properties being the same everywhere, the cloak's material properties vary from point to point and vary in a very specific way. Achieving that gradient in material properties was a fairly significant design effort."
The scientists testes the cloak in the lab, aiming a microwave ray at a cloak positioned between two metal plates, meanwhile measuring electromagnetic fields both inside and outside the cloak.

The team discovered that the wave fronts of the beam split and went around the center of the cloak. "The waves' movement is similar to river water flowing around a smooth rock," Schurig said.

"Although the new cloak demonstrates the feasibility of the researchers' design, the findings nevertheless represent a "baby step" on the road to actual applications for invisibility," said Steven Cummer, a professor of electrical and computer engineering at Duke.

The team wants to design a three-dimensional cloak and to improve the cloak's effect. And to produce invisibility, the cloak must deviate the whole spectrum of visible wavelengths, while the current cloak material only does that with microwaves.

This requires greater advance in engineering metamaterials, fact that eventuality remains uncertain. "It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device," Smith said.

"To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colors, that make up light," he said.