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Duke Scientists Lay Claim to ‘Invisibility Cloak’

-October 20, 2006

Sci-fi concepts have moved closer to reality in a Duke University lab.

A team led by scientists at Duke's Pratt School of Engineering have laid claim to demonstrating 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.

Although it sounds like the fodder of fantasy books, the scientists claimed that the technology could very well have serious, real-life utility. Cloaks that render objects essentially invisible to microwaves could have a variety of wireless communications or radar applications, according to the researchers.

The team reported its findings on Thursday, in Science Express, the advance online publication of the journal Science. The researchers manufactured the cloak using "metamaterials" precisely arranged in a series of concentric circles that confer specific electromagnetic properties. Metamaterials are artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce.

The cloak represents "one of the most elaborate metamaterial structures yet designed and produced," the scientists said in a statement. It also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property, they claimed.

Earlier scientific approaches to achieving invisibility often relied on limiting the reflection of electromagnetic waves. In other schemes, scientists attempted to create cloaks with electromagnetic properties that, in effect, cancel those of the object meant to be hidden. In the latter case, a given cloak would be 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," David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke, said in the statement. "The cloak reduces both an object's reflection and its shadow, either of which would enable its detection."

While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend instead on their physical structure. In the case of the new cloak, that structure consists of copper rings and wires patterned onto sheets of fiberglass composite that are traditionally used in computer circuit boards.

To simplify design and fabrication in the current study, the team said it set out to develop a small cloak, less than five inches across, that would provide invisibility in two dimensions, rather than three. The cloak design is unique among metamaterials in its circular geometry and internal structural variation, the researchers said. All other metamaterials have been based on a cubic, or grid-like, design, and most of them have electromagnetic properties that are uniform throughout.

"Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position," Smith added. "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."

To assess the cloak's performance, the researchers aimed a microwave beam at a cloak situated between two metal plates inside a test chamber, and used a specialized detecting apparatus to measure the electromagnetic fields that developed both inside and outside the cloak. By examining an animated representation of the data, they found that the wave fronts of the beam separate and flow around the center of the cloak.

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, the researchers said. The researchers said they plan to work toward developing a three-dimensional cloak and further perfecting the cloaking effect.

Although the same principles applied to the new microwave cloak might ultimately lead to the production of cloaks that confer invisibility within the visible frequency range, that eventuality remains uncertain, the researchers said.

"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 concluded in the statement.

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. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise.

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