Nanotechnology researchers have taken a step toward creating an "optical cloaking" device that could render objects invisible by guiding light around anything placed inside the "cloak".
Purdue University engineers, following mathematical guidelines devised in 2006 by physicists in the UK, have created a theoretical design that 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, according to Vladimir Shalaev, Purdue's Robert and Anne Burnett Professor of Electrical & Computer Engineering.
A limitiation of the design is that it works only for any single wavelength, and not for the entire frequency range of the visible spectrum, says Shalaev.
"But this is a first design step toward creating an optical cloaking device that might work for all wavelengths of visible light," he adds.
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, says Shalaev.
"How to create a design that works for all colors of visible light at the same time will be a big technical challenge, but we believe it's possible. In principle, this cloak could be arbitrarily large, as large as a person or an aircraft."
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 says. "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." Meta in Greek means beyond, so the term metamaterial means to create something that doesn't exist in nature. Unlike designs for invisibility in the microwave range, the new design has 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 the cloaked object.
"But this could, in principle, be offset by other means; for example, with antireflective coatings," Shalaev says. "The big challenge is how to make rays bend around the object, which we have described how to do in this paper."