Researchers Perfect a Kind of 'Invisibility Cloak' for Microwaves
By Barry Levine / CIO Today. Updated November 12, 2012.
Move over, Harry Potter. Scientists at Duke University say they have created a perfect "invisibility cloak."
Of course, this advance comes with a few catches. The cloaking is currently invisible only from one direction of viewing, and it is designed to work with microwaves, for possible uses in telecommunications or radar.
The cloak causes microwaves to flow around the object. To a perceiver, such as radar, space where the object resides appears to be empty, filled by the scene behind the object. The cloaking has worked on a cylinder that is 7.5 centimeters in diameter and 1 centimeter tall.
Invisibility research kicked into higher gear in 2006, following the publication of a paper by Duke researchers and Sir John Pendry of Imperial College London that presented the theory of "transformation optics." A cloak was demonstrated in 2006, but the Duke researchers described that incarnation as "very poor," largely because of reflections.
The researchers use a cloak created from artificial metamaterials that are constructed in a series of concentric circles and that interact with electromagnetic waves in ways different from natural materials. Theoretically, the metamaterials could hide an object of any size or material, at least from microwaves, but their first incarnation caused reflections that hindered the illusion.
Researchers Professor David Smith and graduate student Nathan Landy told news media that the reflections occurred at the boundaries of the cloak, not unlike reflections seen on clear glass -- a viewer can see through the glass, but is also aware of the presence of the glass because of reflections from the surface. In the first experiments six years ago, the purpose was primarily to demonstrate a proof of concept, so the reflections issue was left unresolved.
To solve this problem, they eventually tried to rework how the edges of the microwave cloak lined up, so that there would be no reflection to give away the object's position.
That original cloak was composed of parallel and intersecting strips of fiberglass etched with copper. The current cloak uses a similar row-by-row design, but copper strips have been added to create better performance and fewer reflections.
Professor Smith told BBC News that "this, to our knowledg, is the first cloak that really addresses getting the transformation exactly right to get you that perfect invisibility."
He also compared the current limitation of the technology -- being invisible only if it is seen from one direction -- to "the card people in Alice in Wonderland." The card people, Smith noted, can be nearly invisible if they turn their sides toward you, "but they're obviously visible if you look from the other direction."
The researchers have indicated that this kind of invisibility would be difficult to achieve with visible light, but, so far, they've not ruled out that possibility. In addition to potential military uses, the current microwave-targeted incarnation could also find application in fiber optics, in order to bend light efficiently around physical corners in fiber cables.
The new research has been published by Duke researchers in the Nov. 11 online issue of Nature magazine.