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.
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.
"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.