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How to Make an Object Invisible

A new theoretical design using nanowires provides a way to hide devices from visible light.

A hairbrush-shaped device has been theoretically designed that would use bristles made out of nanowires to bend light around it, rendering the object invisible. The researchers who came up with the design say that it’s the first practical design for an “optical cloak” to work in the visible spectrum. They are now working on building an actual device based on their calculations.

Cloak on, cloak off: Simulations show how light interacts with the cross section of the cloaking device. When it’s uncloaked (top), light is reflected off the object. But when it’s cloaked (bottom), light is guided around the object and anything within it.

Although still only a theoretical design, it is the first to show how a recently discovered cloaking effect could be made to work for all wavelengths of visible light, says Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University, in West Lafayette, IN, who led the research effort.

“It sets out a road map for building these sorts of structures,” says John Pendry, a professor of theoretical physics at Imperial College London, U.K. Besides making it possible to turn things invisible, the work could lead to ways to create heat shields by bending infrared light around objects, he says. Pendry’s initial research led to last year’s creation of the first working cloaking device, which operated in the microwave range. (See “Cloaking Breakthrough.”) This latest work now shows a way to extend this into the visible-light range, says Pendry.

To become invisible, an object must do two things: it has to be able to bend light around itself, so that it casts no shadow, and it must produce no reflection. While naturally occurring materials are unable to do this, a new class of materials called metamaterials is now making it possible. (See “TR10: Invisible Revolution.”)

Bending light around an object requires a material to have a negative refractive index. The refractive index is a property that dictates how light passes through a medium; it’s the reason a stick will look bent when placed in water. If water had a negative refractive index, it would make the stick look as though it were bending back on itself.

Last year, Pendry demonstrated that it is theoretically possible to design structures of very thin conducting wires that could have an effect on the electric and magnetic fields of microwaves, causing them to bend in unnatural ways such as this. This theory was later backed up by experiments carried out by David Smith and David Schurig at Duke University, in Durham, NC.

But repeating the success for visual light seemed to present problems. For one thing, making the design used by Smith and Schurig work for visible light would require components just 40 nanometers in size.

The solution was to design a device with tightly spaced needles of nanowires, 10 nanometers in diameter and 60 nanometers long, emanating from a cylindrical central spoke. In the current issue of the journal Nature Photonics, the researchers show how–in theory at least–this would cloak the object from red light of wavelength 632.8 nanometers long.

There are limitations to this approach, however. A very small percentage of light would still be reflected, so the object would not be entirely invisible. Also, while the design can be adapted to work for other frequencies in the visible range, the design will still only work for a very narrow band of light.

“This is a real problem,” says Ulf Leonhardt, a professor of theoretical physics at St. Andrews University, in Scotland, and an expert in this field. “It would look completely odd, and you would definitely see something.” But he says that this is not an indictment of the Purdue research; rather, it’s a general problem with research into cloaking so far.

“It’s still an important step to go into the visible range,” says Leonhardt. “And it’s a definite step forwards.” But to make things truly disappear before our eyes, a way will need to be found to make devices work across a broad range of frequencies, he says.

Even so, using nanowires is a very practical way forward, says Pendry. “It’s very useful because what we really want now is to see how well people can build them,” he says. Indeed, this is what the group is working on now. “The next step is to fabricate and test an actual sample,” says Alexander Kildishev, a research scientist at Purdue. This work will be carried out in collaboration with Purdue’s Birck Nanotechnology Center.

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