Intelligent Machines

Cloaking Breakthrough

New materials are on the U.S. Department of Defense’s radar.

Ever since H. G. Wells published The Invisible Man more than a century ago, the prospect of invisibility – or cloaking – has been a mainstay of science fiction. But now physicists say they have finally figured out how to make objects invisible, and what’s more, they are just months away from putting this theory into practice.

Blue cloaking “metamaterial” is able to bend light around an object (orange circle), ensuring it casts no shadow or reflection. (Credit: D. Schurig, Duke University.)

The trick is to find a way to guide light and other types of electromagnetic radiation around an object so that it casts no shadow and produces no reflection. Normally, this kind of manipulation would be a tall order, says John Pendry of Imperial College London, England. But, he adds, the recent development of a new class of materials called “metamaterials” makes it tantalizingly feasible.

Metamaterials are engineered materials whose properties are determined by their physical structure rather than their chemistry, says Pendry. Such properties include the ability to bend light, he says.

Now working with David Smith and David Schurig of Duke University, Pendry has formulated a way to design metamaterials that can bend light around an object no matter what direction the light is coming from. “You can apply it to any shape,” says Smith. This means that in theory, anything could be cloaked, he says.

Building on Pendry’s work, which is described in the current issue of Science, Smith and Schurig are developing a proof-of-principle device, with funding from the U.S. Department of Defense’s research arm, the Defense Advanced Research Projects Agency. “It’s fair to say that this year there will be a demonstration on the basic physics of cloaking,” says Schurig.

The cloaking effect depends on a material’s “refractive index,” or its ability to influence the direction of light that passes through it. Light tends to prefer the quickest route between two points, which is normally a straight line. With metamaterials, however, the quickest path can be one that bends around an object.

But bending light is just one of the requirements for cloaking. “You have to return the light to the same path it was pursuing before it hit the cloak; otherwise it casts a shadow,” says Pendry. Similarly, when light enters the cloak, it must not be reflected. “One way to think about it is that this material gives the appearance of being like space,” says Smith, in that space can bend light and also has no reflection.

“It’s a breakthrough,” says George Eleftheriades, an expert in metamaterials at the University of Toronto. However, he says, there is a limitation: “It won’t work for every frequency.”

Indeed current materials are capable of redirecting only microwaves, which means the cloaking device Smith and Schurig are developing will work only against radar or other microwave emitters. While this is likely to prove useful for future stealth planes, we are still at least a decade away from cloaking objects from visible light.

The reason is that, to produce the cloaking effect, the substructures of the metamaterials must be smaller than the wavelength of light being redirected. That’s currently feasible for microwaves, which have a wavelength of about three centimeters. But redirecting visible light, which has a wavelength of around half a micrometer, or half a millionth of a meter, would require metamaterials with structures engineered at the molecular level. “We would like to do it on a molecular scale, but nanoengineering is not yet up to it,” says Pendry. Recent developments in nano metamaterials, however, could speed the development process up.

For now, then, the prototype cloak consists of arrays of millimeter-sized copper rods and C-shaped rings embedded in a composite fiber board, much like the kind of printed circuit boards that normally house computer chips. Both the rods and the C-rings are capable of passively creating electromagnetic fields when exposed to microwave radiation. When oriented just right, these components can specify the path that the radiation will follow.

There is also another application for cloaking, says Schurig: it can be used as a kind of shield. “Sometimes you want to protect or isolate things from the electromagnetic spectrum,” he says. For example, cloaking could be used on space probes to protect sensitive equipment from cosmic radiation.

But there is a catch. While any cloaked object would be invisible, it would also be blind within the cloaked frequency range, since any light directed toward it would be rerouted around it. In the case of a radar-cloaked plane, this should not be a major problem, says Schurig. The pilot would be unable to use radar, but she could still navigate visually.

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