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Dukies unveil invisibility cloak

So, if you're not magician Harry Houdini, how hard is it to make something disappear? Not as hard as Duke researchers thought.

Science Express published a report Thursday recording a successful test of the first invisibility cloak-a year before it was supposed to work.

The experimental data were gathered by a team of scientists, led by David Smith, associate professor of electrical and computer engineering, and research associate David Schurig.

The data demonstrates that the cloak can redirect microwaves around itself and have the waves appear behind it relatively undisturbed-a technique that could eventually be applied to visible light, researchers said.

"One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads," Schurig explained. "In such a space, light or other electromagnetic waves would be confined to the warped 'threads' and therefore could not interact with-or 'see'-objects placed inside the resulting hole."

Schurig said it was not easy to "warp space" but the scientists solved the problem by using "metamaterials."

These metamaterials, which have unique electro-magnetic properties not found in nature, are what make it possible for waves to be deflected around the object.

"Ordinary materials like glass have an index of refraction. This material does not have a uniform refraction index, so its properties vary as a function of its position," said Anthony Starr, president of SensorMetrix in San Diego, Calif., who helped construct the metamaterials.

Simply put, Starr explained, this means that scientists can control the way the waves bend around a cloak in a manner that would not be possible with materials occurring in nature.

A cloak is perhaps a misleading term for the device Duke scientists have constructed, said Bryan Justice, Pratt '05, who aided in the experimental verification of the project.

"I would call it a microwave shield," he said.

The cloak, a small device no more than five inches across, is really a series of concentric circles-called split-ring resonators-constructed out of metamaterials, said Schurig, who designed the device.

The team produced the cloak according to electromagnetic specifications determined by a new design theory proposed by Sir John Pendry, chair in Theoretical Solid State Physics at Imperial College London, in collaboration with the Duke scientists. Liheng Guo, a junior, wrote the control software that allowed for automated data acquisition in the 2-D mapping system used to test the cloak.

The cloak's success has created a recent stir in the news, but team member Jack Mock cautioned it may be a while before a cloak for visible light is created.

"When the media gets a hold of it they start talking about Predator, Romulan spaceships and Harry Potter-and it's not quite what we've made here," said Mock. "However, it is an invisibility cloak. The theory that came out in the paper in late May works for all wavelengths; it's really very flexible, and you could potentially make a cloak out of it."

The team was mainly concerned with getting the data out quickly, so they constructed the experiment using imperfect microwave cloaking in two dimensions, which was easier to test, Mock said.

The cloak's imperfections meant the waves experienced some losses due to absorption by the metamaterials and did not appear perfectly on the other side of the cloak, he said. Applied to a theoretical cloak for visible light, this would mean that there would still be some visible shadow of the concealed object.

Although a perfect cloak could feasibly be constructed, the more complicated the cloak gets, the more problems inevitably arise, Justice said. Building a bigger cloak means there are potentially more absorption losses and, therefore, more of a shadow.

In addition, testing the cloak's success is a feat in itself, Mock said.

The cloak is placed in the middle of a black foam circle which prevents the microwaves from escaping and being bounced back into the sensor equipment. The whole apparatus is then sandwiched in between two aluminum plates, while microwaves are constantly beamed at the cloak, and a sensor travels around to different points on the device, pausing at each point and taking measurements.

"We want to know what the microwaves are doing at many different points in this space that we want to map out, so we have to measure, essentially, at over 60,000 data points," Mock said.


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