CLOAKING DEVICE depicted in this diagram would make a bulge in its inner surface (white) appear flat thanks to variations in the speed of light through the material (color). Image: Courtesy of Jensen Li
Researchers say they have come up with a new concept for a two-dimensional cloak of invisibility that would be much easier to make than the three-dimensional version demonstrated last year in rudimentary form. This, however, is not the kind of cloaking device the military might be interested in: When viewed under red light, it would make a lump in an otherwise flat surface appear smooth and shiny like the image in a mirage.
Scientifically, the idea represents a simpler version of a three-dimensional cloaking idea proposed two years ago. In that concept, a spherical shell of metamaterial—specially shaped metal pieces that bend light in ways other materials cannot—would cause light to skirt the inner edge of the shell like wind around an airplane wing, making anything inside the shell invisible. But in practice it is hard to make a metamaterial powerful enough to prevent distortions in the invisibility, says physicist John Pendry of Imperial College London, one of those who came up with the concept.
Making a flat cloak would do away with the need for those extreme properties by relaxing the amount of light-bending necessary, Pendry and his colleague Jensen Li, now at the University of California, Berkeley, report in a preprint published online. Instead of metamaterial, they recommend drilling holes in a layer of silicon and placing it over a metal surface that bulges up in the spot where an object is to be hidden. They say that if the sizes of the holes were graded in the right way, light shining into the silicon would reflect from the metal lump as if it was flat, rendering it invisible.
The dream of cloaking research is to hide an object illuminated by visible light. In 2007 researchers partially cloaked an object from radio waves. But current metamaterials absorb too much visible light to make a convincing cloak. Pendry says that using silicon would potentially solve the problem, because near-infrared light shines right through it. To keep costs realistic, however, he says the cloaking region would have to be no larger than a centimeter (about 0.4 inch).
There's also the question of why anyone would need something disguised as a mirror. He and Li suggest that the design might, however, be a useful element in proposed integrated circuits that work at high speeds by flicking light beams back and forth.
That's a bit of a stretch at this point, says Aref Chowdhury, a senior manager at Alcatel–Lucent in Murray Hill, N.J., who notes that a plain old reflector would do the same trick as the cloak but without the fuss. "I just don't see how [it's] going to move the light," he says. What is nice, he adds, is the way the authors used the original cloaking concept to come up with a design that gets around many of the sticking points of three-dimensional invisibility.