Advanced Engineering 2018

Composites Enable a Working Invisibility Cloak

29 October 2006

A team led by scientists at Duke University's Pratt School of Engineering has demonstrated 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.

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, Oct. 19, in Science Express, the advance online publication of the journal Science. The research was funded by the Intelligence Community Postdoctoral Fellowship Program.

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. 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.


""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,"" Schurig said. ""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.""


""You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response,"" Schurig continued. ""The required materials are quite complex, but can be implemented using metamaterial technology.""

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 fibreglass composite that are traditionally used in computer circuit boards.



""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.""

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. ""The waves' movement is similar to river water flowing around a smooth rock,"" Schurig said. Moreover, the observed physical behaviour of the cloak proved to be in ""remarkable agreement"" with that expected based on a simulated cloak, the researchers 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 team member Steven Cummer, a professor of electrical and computer engineering at Duke. 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 said.

