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Scientists Create Cloak of Partial Invisibility

By Ker Than
LiveScience Staff Writer
posted: 19 October 2006 10:10 am ET

Scientists have created a cloaking device that can reroute certain wavelengths of light, forcing them around objects like water flowing around boulders in a stream. To creatures or machines that see only in microwave light, the cloaked object would appear nearly invisible.

"The microwaves come in and are swept around the cloak and reconstructed on the other side while avoiding the interior region," said study team member David Smith at Duke University's Pratt School of Engineering. "So it looks as if they just passed through free space."

The device only works in the microwave range of light, so cloaked objects are still visible to humans. Also, it only works in two dimensions and only for microwaves moving in a plane. A three-dimensional invisibility cloak would hide an object completely.The microwave cloak is also slightly reflective and casts a partial shadow.

Despite these shortcomings, however, the new device is "a very good achievement," said Ulf Leonhardt, a theorist at the University of St. Andrews in England who was not involved in the study.

"It's surprising that it's as simple as it is and that it works so well," Leonhardt said in a related news article about the work in the journal Science.

The achievement, reported online today by the journal, comes five months after the same team published a study detailing the precise mathematical specifications a needed to build such a cloaking device.

Metamaterials

The apparatus was made using "metamaterials," artificial materials engineered to have precisely patterned surfaces that interact with and manipulate light in novel ways.

Although called a cloak, the device is not something that can be worn. Rather, it consists of a series of concentric circles, made of copper rings and wires patterned onto sheets of fiberglass, and resembles a loosely coiled reel of film.

The patterns enable the manipulation of light, and the size of the patterns determines which wavelengths of light can be manipulated. Smaller patterns affect shorter the wavelengths. Microwaves have relatively long wavelengths and can be affected with metamaterials having relatively large patterns. Manipulating visible light, which has much shorter wavelengths, will require metamaterials with much finer patterns.

While making those finer patterns is possible with current nanomanufacturing technologies, the metals used to make the microwave cloak would behave differently with visible light, Smith said.

"They act very differently at optical wavelengths; they become very absorptive. A cloaked object would just become very opaque, rather than transparent," he told LiveScience.

But even if metamaterials are made that can deflect visible light, don't expect the kind of invisibility offered by Harry Potter's cloak or Star Trek cloaking devices any time soon.

Human eyes are sensitive to many different wavelengths of light, as evidenced by the rainbow of colors that we see, and it's still uncertain if metamaterials can deflect so many wavelengths simultaneously.

Still useful

But even imperfect cloaking devices might be useful, the researchers say. Cloaks that deflect radio waves could render an object invisible to radar or improve cell phone receptions by rerouting signals around obstructions. They might also be used to protect people from penetrating and harmful radiation.

"If you knew that you had radiation of a certain bandwidth frequency, you could have it skirt around some region that you wanted shielded," Smith said.

The team says the next step is to create a cloak that works in three dimensions and to perfect the cloaking effect.

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