Invisibility Cloak for Almost-Visible Light
An invisibility cloak that works for visible light might soon be in sight, now that a team has made one that works for the slightly longer wavelengths of near-infrared light. The cloak is only about a micrometer in size--a little snug for Harry Potter--but other researchers say it's a major advance.
The ages-old fantasy of invisibility became scientifically acceptable in 2006, after work by theoretical physicist John Pendry and colleagues at Imperial College London and, working independently, Ulf Leonhardt of the University of St. Andrews in the United Kingdom. The theorists imagined stretching space so that light rays would flow around an object and rejoin seamlessly beyond it, rendering the object imperceptible (Science, 26 May 2006, p. 1120).
Of course, researchers can't stretch space. But Pendry and Leonhardt showed how to fill a region with a "metamaterial," an assemblage of metallic rods, rings, and other bits that together interact with light to manipulate it in novel ways, to mimic such stretching. Five months later, David Smith and colleagues at Duke University in Durham, North Carolina, built a cloak for microwaves with a fixed wavelength roughly 46,000 times longer than visible light waves (Science, 20 October 2006, p. 403).
Now, Xiang Zhang, an applied physicist at the University of California, Berkeley, and colleagues have fashioned a cloak that works for near-infrared light with wavelengths 1.8 to 2.4 times longer than visible light. Whereas the first cloak was a circular shield that ferried microwaves around something inside it, Zhang's cloak is a so-called carpet cloak. The scheme, first introduced last year by Pendry and Jensen Li, now a member of Zhang's team, consists of a layer of metamaterial covering a flat mirror. The layer has a small space beneath it, like a hump in a carpet, but is tailored in such a way that light continues to reflect as usual, so that something can be concealed in the void.
You can't even hide your pocket change in Zhang's cloak, however. The whole experiment takes place within a slab of silicon 250 nanometers thick, to which the light is confined. The mirror consists of the metal-coated edge of the slab, which reflects light back into the material. A notch in the edge serves as the hiding place. The researchers sculpted the optical properties of the silicon by drilling closely spaced 110-nanometer-wide holes through it. Regions with more holes would have a lower "index of refraction," which means that light travels through them at a higher speed. Light reflected neatly off the flat edge but scattered wildly off an uncloaked notch. When the researchers tailored the pattern of holes above the notch to make a cloak, the light once again reflected as if it were hitting a mirror, the researchers report online this week in Nature Materials. Michal Lipson and colleagues at Cornell University report similar work in a paper posted to the arXiv preprint server (www.arXiv.org) on 22 April.
Experts praise the work. "I think it's incredible," says Pendry, who has laid much of the conceptual groundwork for cloaking in the past few years. "It amazes me how quickly the experimental groups can react to theoretical suggestions." "It's a huge step," agrees Duke's Smith. "I think it's very possible that this will be pushed into the visible." The carpet cloak mimics the bowing of plane into a hump, which is much less radical than the stretching of a line into a cylinder in the original cloak, he notes. As a result, it's possible to make one using only insulating "dielectric" materials such as silicon or glass and no metals, which tend to strongly absorb visible light. That, in turn, should make it easier to design carpet cloaks and mathematically related devices that work at shorter wavelengths, Smith says: "This should open people's eyes" to the possibilities.