Invisibility Cloak Sees Light of Day


Mere months after making headlines for proposing a technologically feasible way of rendering objects invisible, a research team has demonstrated a rudimentary example of an invisibility cloak. Concentric rings of so-called metamaterial caused microwaves to partially bend around an enclosed object like a water flows around a stone, the group reports.

A metamaterial is a composite structure, built of metal rings and wires embedded in fiberglass, that makes light behave in weird ways. Metamaterials can be used, for example, to bend light sharply or to focus it to a higher resolution than is normally possible. More recently, researchers pointed out that the technology should make it possible to construct spheres or cylinders capable of cloaking an object almost perfectly from detection by a single wavelength of light. When light strikes a metamaterial it causes the electrons in the metal pieces to vibrate; these vibrations in turn affect the speed of the light. A metamaterial shell with the right gradient of metal elements should cause light of a particular wavelength to wrap around the shell's interior.

Engineers David Schurig and David Smith of Duke University say they were concealing something themselves last May when they and their colleagues reported their proposal: "We had a cloak we liked pretty well in May, and it got better from there," Schurig reveals. In the group's current version a central copper ring--the object to be cloaked--is surrounded by concentric rings of metamaterial standing one centimeter tall and spanning 12 centimeters. The rings are sandwiched between two plates so that microwaves can only travel through the cloak in the plane of the rings, as described in a paper published online October 19 by Science.

When the microwaves strike the shell they interact with its C-shaped copper wires and, theoretically, should be absorbed and reflected less by the enclosed object than if the shell wasn't there. The researchers sampled the electric field component of the microwaves at many points in the apparatus to see how the radiation was affected, and the results match well with their simulations, they report. "We don't say anything quantitatively about how well this is cloaking, but we've reduced both the reflection and the shadow generated by the object, and those are the two essential features of the invisibility cloaking," Schurig says.

"Although this prototype is not an ideal invisibility device, it's definitely a very important breakthrough," says optics theorist Ulf Leonhardt of the University of St. Andrews College in Scotland, who was not part of the research. "It worked surprisingly well." Getting the technology up and running was easier than they anticipated, Schurig and Smith say, but add that creating a spherical shell or cloaking shorter wavelengths of light, such as the visible spectrum, are likely to be very challenging tasks.

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