Scientists Move Forward in Quest for ‘Invisibility Cloak’
The paper, by theoretical physicists Ulf Leonhardt and Tomas Tyc, of the University of St. Andrews, outlines a plan for building a structure that would guide many different wavelengths of light around an object, hiding the object from view.
Such a structure hasn’t yet been built and there are several technical obstacles to overcome before one can be, according to Leonhardt.
“But the point is that there is no fundamental obstacle anymore,” he said. “It’s feasible in principle.”
Researchers first began to move invisibility from the realm of Harry Potter to the realm of reality two years ago. British physicist John Pendry, working with American colleagues David Smith and David Schurig at Duke University, developed a “cloak” that could hide an object from a microwave beam. Microwaves, which range from one millimeter to one meter long, are much longer than visible light waves, which range from about 400 to 700 nanometers.
Smith and Schurig built the cloak with a type of laboratory-made material called a metamaterial, in their case made of copper wires patterned onto sheets of fiberglass. Metamaterials take advantage of the fact that all materials refract, or bend, light — for example, water bends light so that underwater objects appear closer than they really are.
The metamaterial that Smith and Schurig built bent the microwaves in such a way that it guided them around the object, like water flowing around a rock in a stream.
In August, scientists at the University of California, Berkeley announced that they had developed a metamaterial that could negatively refract visible wavelengths of light — a necessary step toward building a visible-light cloaking device.
All of these studies, however, shared an important limitation — the metamaterials would only work for one particular wavelength, be it in the microwave or visible end of the spectrum. But humans see across the visible spectrum, from 400 to 700 nanometers, so to truly hide something from the human eye a material would have to work for all those wavelengths.
In their new paper, Leonhardt and Tyc outline a plan for building such a material, based on working in the geometry of curved, or non-Euclidean, space.
David Schurig, who was not involved in the new research, says that it appears that Leonhardt and Tyc have succeeded in designing a theoretical cloak that would work over a broad range of wavelengths — but that the new design would come with a tradeoff. Unlike the microwave cloak, which made an object perfectly invisible to the microwave beam, the new design would create a “phase distortion,” slightly distorting light the way a thin glass lens does.
“The earlier work you could argue in principle you could make work perfectly at one frequency,” Schurig explained. “The new work will work over a wider range, but it won’t work perfectly at any frequency.”
There’s also an engineering challenge to overcome. Researchers are not yet able to build visible light metamaterials that work as well as metamaterials for the microwave range, according to Leonhardt.
“To be honest, it’s still not possible for people to do this right away,” he said. “It would require work on the materials side.”
One of the problems is that many of the metamaterials absorb light, so that even if they are able to bend the light correctly, not enough light gets through to the other side.
But Schurig predicts that researchers will be able to design better visible light metamaterials within the next few decades: “It’s hard to imagine it will be more than 20 or 30 years, but it could be 10 years,” he said.