14 DAY TRIAL // Researchers have employed nanotechnology on the way towards the creation of an invisibility device that could make objects invisible.
A Purdue University team has made a theoretical design that employs an array of tiny needles radiating outward from a central spoke, which would bend light around the cloaked object. "Background objects would be visible but not the object surrounded by the cylindrical array of nano-needles," said Vladimir Shalaev, Purdue's Professor of Electrical and Computer Engineering.
"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. 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 color red. The same design, however, could be used to create a cloak for any other single wavelength in the visible spectrum," Shalaev said. "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."
Ulf Leonhardt at the University of St. Andrews in Scotland, a professor of theoretical physics, sees the Purdue design similar to the Roman " first optical metamaterial", a type of glass incorporating nanometer-scale particles of gold.
A cup made of this glass looks green in daylight, but when glowed from inside it appears ruby. "The Purdue research represents theoretical simulations that show that a modified Roman cup based on modern nanofabrication technology will act as an invisibility device ... Any object you put inside will disappear as if dissolved in air, provided it is viewed through polarizing tinted glasses of precisely that color," said Leonhardt.
Other approaches developed methods for cloaking objects in the microwave range of the spectrum, over the wavelengths of visible light, but this is the first design for cloaking in the range of the visible light.
Invisibility requires two conditions: light must not reflect off of the object, and it must bend around the object, to see only the background and not the shadowlike shape of the cloaked object. "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", said Shalaev.
"Non-magnetic metamaterials" ease the cloaking of objects in the visible range but also determines a smaller light level to be reflected. "But this could, in principle, be offset by other means, for example, with antireflective coatings," Shalaev said. "The big challenge is how to make rays bend around the object, which we have described how to do in this paper."
A crucial factor is decreasing the "index of refraction" under 1. Refraction happens when electromagnetic waves, like light, change direction when passing from one material into another, like the bent-stick-in-water effect. The refraction index of a material assesses how much light will bend in that particular material.
Most natural materials present refractive indices above 1. The new design models a refractive index that varies gradually from zero at the inner surface of the cloak, to 1 at the outer surface of the cloak, guiding light around the cloaked object.
The needles of the device have 10 nanometers (1/1,000,000,000 m) width and hundreds of nanometers in length. They would be located in layers spreading from a central spoke in a cylindrical shape. Even the current design, that works just for one frequency, still could be employed for making cloaking devices that turn 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. Another possible application is to cloak objects from "laser designators" used by the military to illuminate a target," said Shalaev.