Invisibility-Cloak Breakthrough

New software has enabled metamaterials to work with a broad band of frequencies.

Metamaterials interact with light in ways that appear to violate the laws of physics. They can bend light around an object as if it weren't there, or narrow the resolution of light microscopes down to a few nanometers. But metamaterials must be painstakingly structured at the nano- and microscales in order to achieve these exotic effects. Now the Duke University researcher who built the first invisibility cloak in 2006 has created software that speeds up the design of metamaterials. He and his colleagues have used the program to build a complex light cloak that's invisible to a broad band of microwave light--and they did it in only about 10 days.

Now you see it: A new device that can reroute microwave radiation is made up of about 600 I-shaped copper structures and works over a broad spectrum. Credit: David R. Smith

David R. Smith of Duke and Tai Jun Cui of Southeast University, in Nanjing, China, led the work, which is a landmark in the field of metamaterials. The cloak that the researchers built works with wavelengths of light ranging from about 1 to 18 gigahertz--a swath as broad as the visible spectrum. No one has yet made a cloaking device that works in the visible spectrum, and those metamaterials that have been fabricated tend to work only with narrow bands of light. But a cloak that made an object invisible to light of only one color would not be of much use. Similarly, a cloaking device can't afford to be lossy: if it lets just a little bit of light reflect off the object it's supposed to cloak, it's no longer effective. The cloak that Smith built is very low loss, successfully rerouting almost all the light that hits it.

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"Their cloak . . . answers the naysayers who predicted that cloaks would always be narrowband and lossy," says John Pendry, chair in theoretical solid-state physics at Imperial College London. Pendry did the theoretical work upon which both the first invisibility cloak and its new successor are based. "Needless to say, I am delighted with this development," says Pendry. He and his Imperial College colleague Jensen Li proposed a theoretical version of a broadband cloak just last year, and at that time, he says, he "did not expect such rapid experimental progress."

The broadband cloak is a rectangular structure measuring about 50 by 10 centimeters, with a height of about 1 centimeter. It's made up of roughly 600 I-shaped copper structures. Making each structure is a simple matter, says Smith. "They're copper patterns on a circuit board, cut up and arranged. It's a well-known, inexpensive technology." The hard part is determining the dimensions of each of these 600 structures and how to arrange them. With the first light cloak, which had only 10 such pieces, "we had to design each element by numerical simulations," Smith says. Applying the same approach to the more complicated cloak would have eaten up months.