Object Made Flawlessly Invisible
By Ron Grunsby, Editor
Duke University scientists have built an invisibility cloak that perfectly channels electromagnetic waves around a cylinder 7.5 cm in diameter and 1 cm tall. This is significant because it creates perfect invisibility — previous invisibility cloaks have reflected some, but not all, of the electromagnetic waves, such as the functional cloaking device reported in 2006 by Duke University electrical engineers. The current Duke University invisibility cloak, detailed in Nature Materials, hides the object from microwave radiation, and invisibility only occurs from one direction.
The cloak uses metamaterials, which are manmade objects that have properties, such as a negative refractive index, that often do not exist in natural objects. Structures that contain metamaterials can direct electromagnetic waves around an object, with the waves emerging on the other side as if they had passed through empty space — “cloaking” the object. This type of invisibility cloak could eventually be useful in fiber optic networks, where it could bend light around corners without attenuating the signal.
To overcome the loss of waves due to reflections at the boundaries of the 2006 cloaking device, Nathan Landy, a graduate student at Duke University, changed the fabrication strategy. While the original cloak had parallel and intersecting strips of fiberglass etched with copper, Landy's version added copper strips to create a better-performing material. The strips of the two-foot-square device form a diamond shape with an empty center, and the cloak is divided into four quadrants.
Landy said the reflections observed in the earlier cloak occurred along the edges and corners of the spaces within and around the metamaterial. "Each quadrant of the cloak tended to have voids, or blind spots, at their intersections and corners with each other," Landy said. "After many calculations, we thought we could correct this situation by shifting each strip so that it met its mirror image at each interface. We built the cloak, and it worked."