Two New Cloaking Devices Close In on True Invisibility

Richard A. Lovett
for National Geographic News
May 6, 2009
Cloaking devices, like the Star Trek technology that can make whole Romulan warships disappear, came a step closer to reality last week.

Two independent teams have developed silicon-based materials that can hide microscopic objects. The materials are the first to work in near-infrared light, wavelengths very close to visible light.

On a flat surface, an object covered with a piece of cloth would normally be detectable based on its telltale bump. But with the new materials, even the bump seems to vanish.

A team led by Xiang Zhang at the University of California, Berkeley, achieved this effect by drilling scores of tiny nano-holes into the cloaking material.

(See pictures of materials that use nanotechnology.)

These holes change the material's optical properties, allowing light to bend around the hidden object, as described in the April 29 issue of Nature Materials.

The other team, led by Michal Lipson at Cornell University, achieved the same effect by covering their material's surface with tiny pillars also designed to bend light. The Cornell group's paper is currently under review by Nature Photonics.

Jason Valentine, one of the Berkeley researchers, calls his team's material a "carpet cloak." In both cases the process is like hiding something beneath a rug.

"The object is effectively hidden on the ground," Valentine said.

Small But Effective

Both materials still have a long way to go before they're ready for stealth military operations. To start with, both cloaks work only in infrared light for now. The next step is to try to develop a version that works in visible wavelengths.

The cloaks are also capable of hiding only microscopic objects.

But even that has potential uses, Valentine said. In optical computing, for example, such cloaks could be used to allow light to move more efficiently, by hiding the parts of a computer chip that get in the way of the beams.

Also, expensive dielectric mirrors—special mirrors used to make printed circuits for electronics—can be ruined by tiny defects in their surfaces.

"You could cloak a defect and make it look like a perfect mirror again," Valentine said.

Scaling up to hide an object the size of a BB-gun pellet might be possible, but going larger will be difficult, he said.

In the Berkeley material, for instance, "the holes have to be smaller than the wavelength of light, and many are required."

From Theory to Practice

The development of these working invisibility cloaks is exciting, but it's not a huge theoretical breakthrough, Ulf Leonhardt, a theoretical physicist at Scotland's University of St. Andrews, said in an email.

"Such an idea already appeared in my very first paper on cloaking," published in the journal Science in 2006, he said.

(Read "Invisibility Cloaks Possible, Study Says.")

Carl Poitras, a researcher with the Cornell group, agrees.

"Like a lot of things we can do," he said, "the theory is predicted ahead of time, but the technology isn't there to show it."

Now, for cloaking devices at least, the theoretical models are starting to become real-life products.

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