In recent years optics researchers have come up with numerous concepts for invisibility cloaks—camouflaging that would effectively reroute light around an object to be concealed. Most of these approaches have relied on so-called metamaterials, which are carefully engineered structures that have bizarre optical properties. A much simpler cloaking apparatus could do away with the need for metamaterials entirely.
Researchers at BAE Systems in Washington, D.C., Towson University and Purdue University have devised a cloaking device based on two gold surfaces, one coated on a curved lens and one on a flat piece of glass. Stacked together, they can conceal an area in between by forming what is known as a tapered waveguide. The trick lies in the gradient of the material’s refractive index, which allows light shining parallel into the stack to bend around a central area “like water flowing around a stone,” says study co-author Vladimir M. Shalaev, a professor of electrical and computer engineering at Purdue.
Shalaev was part of a group that in 2007 designed a visible-light cloak using metamaterials. But that cloak worked only at a predefined wavelength of light and concealed a very small area. In contrast, the waveguide appears to work for multiple wavelengths of visible light and can hide a bigger area. “From the very beginning we realized the huge challenge” of making such a cloak, Shalaev says. “It’s not fundamentally impossible, but it’s really, really hard.”
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John Pendry, a physicist at Imperial College London, says that the tapered waveguide strategy is “a very clever idea.” Physicist Ulf Leonhardt of the University of Saint Andrews in Scotland agrees, calling the paper, in the May 29 Physical Review Letters, “a brilliant piece of work, a wonderfully simple idea.” Both researchers point out, however, that the new approach conceals a two-dimensional area rather than a three-dimensional one. “The things you might want to cloak are probably not confined to two dimensions,” Pendry remarks. Still, the system could find use in optical communications.