Tuesday, 3 April 2007

Hello, Hogwarts: invisibility cloak created

Researchers using nanotechnology have taken a step toward creating an ‘optical cloaking’ device that could render objects invisible by guiding light around anything placed inside the cloaked area.

The theoretical design uses an array of tiny needles radiating outward from a central spoke. The design, which resembles a round hairbrush, would bend light around the object being cloaked.

Background objects would be visible but not the object surrounded by the cylindrical array of nano-needles, said Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University and one of the researchers developing the cloak.

The design does, however, have a major limitation: It works only for any single wavelength, and not for the entire frequency range of the visible spectrum, Shalaev said.

"But this is a first design step toward creating an optical cloaking device that might work for all wavelengths of visible light," he said.

Calculations indicate the device would make an object invisible in a wavelength of 632.8 nanometers, which corresponds to the colour red. The same design, however, could be used to create a cloak for any other single wavelength in the visible spectrum, Shalaev said, and could potentially cloak an area as large as a person or an aircraft.

"What we propose is the cloaking of objects of any shape and size," Shalaev said.

Images depicting scientific simulations (Cloak off, top. Cloak on, bottom) to show how objects might be "cloaked" to render them invisible - Birck Nanotechnology Center, Purdue University
Two requirements are needed to render an object invisible: Light must not reflect off of the object, and the light must bend around the object so that people would see only the background and not the cloaked object itself.

"If you satisfied only the first requirement of preventing light from reflecting off of the object, you would still see the dark shadowlike shape of the object, so you would know something was there," Shalaev said. "The most difficult requirement is to bend light around the cloaked object so that the background is visible but not the object being cloaked. The viewer would, in effect, be seeing around, or through, the object."

The device would be made of so-called ‘non-magnetic metamaterials’, which are synthetic materials that have no magnetic properties. Having no magnetic properties makes it much easier to cloak objects in the visible range but also causes a small amount of light to reflect off of the cloaked object, researchers say.

A key factor in the design is the ability to reduce the amount by which the material refracts light, so that its index of refraction is less than one. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another.

Natural materials typically have refractive indices greater than one. The new design reduces a refractive index to values gradually varying from zero at the inner surface of the cloak, to one at the outer surface of the cloak, which is required to guide light around the cloaked object.

Creating the tiny needles would require the same sort of equipment already used to fabricate nanotech devices. The needles in the theoretical design are about as wide as ten nanometers, or billionths of a meter, and as long as hundreds of nanometers. They would be arranged in layers emanating from a central spoke in a cylindrical shape.

Although the design would work only for one frequency, it still might have applications, such as producing a cloaking system to make soldiers invisible to night-vision goggles.

"Because night-imaging systems detect only a specific wavelength, you could, in theory, design something that cloaks in that narrow band of light," Shalaev said. Another possible application is to cloak objects from "laser designators" used by the military to illuminate a target, he said.

Researchers say that creating a cloak for rendering total invisibility in the entire visible spectrum would be a bigger challenge, requiring further advances in optical metamaterials and new combinations of nanotechnology with highly abstract ideas.

More information is available from Purdue University's press release.

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