Imagine a scene from a 1940s crime movie. Backlit by a streetlamp, a shadowy figure lurks within a luminous fog. A new invisibility cloak could render that person even more mysterious by making him disappear. The new, simpler cloak overcomes many of its predecessors' limitations, researchers say. But it works only in foggy conditions, and some researchers question whether the technique should count as cloaking at all.
"It's a very ingenious idea," says Shuang Zhang, a physicist at the University of Birmingham in the United Kingdom who was not involved with the new work. "It solves a lot of the major problems" with traditional cloaks.
Cloaking jumped from science fiction to science in 2006, when theorists predicted that light could be funneled around an object to make it undetectable. They imagined placing the object in a cylindrical or spherical shell of “metamaterial”—an assemblage of tiny rods and rings with tunable electric and magnetic properties. The shell would gently guide incoming light waves around the object within its center, rather than allowing them to hit it. Instead of seeing the object, an observer would see what was behind it. Within months, researchers had made a cloak that worked for microwaves.
Since then, physicists have made a variety of cloaks, all of them limited in some way. The original cloak can hide an object from light coming from any direction, but it works only for light of a single wavelength. In contrast, a "carpet cloak" works over a wider range of wavelengths but can hide only an object sitting on a surface, so that it cannot be viewed from all angles. In 2011, a team of researchers developed a particularly simple carpet cloak made of two crystals that could hide centimeter-sized objects, but it worked only for light polarized in one direction. In general, physicists have struggled to develop a cloak that is broadband, omnidirectional, and big enough to hide a macroscopic object.
Now, Robert Schittny, Martin Wegener, and colleagues at Karlsruhe Institute of Technology in Germany have developed a cloak that has all of those properties—although it works only in foggy or "diffusive" conditions. Ordinarily, light zips along in straight rays. In a medium such as fog or milk, however, photons ping randomly off myriad objects within the medium—water droplets in fog, milk solids in milk. That scattering makes the light diffuse more slowly, just as heat diffuses through your cold fingers or a scent diffuses through air.
Under those conditions, designing a cloak becomes easier. In the original circular cloak, the light detouring around the object must speed up to catch the light that passes by the cloak altogether. To avoid violating the rule from relativity that neither energy nor information can travel faster than the speed of light in a vacuum, that hurrying along can work at only one frequency. In contrast, in a diffusive medium, light travels much more slowly to begin with. So the cloak need only be made of a material through which light diffuses faster than it does through the surroundings.
That's how the new cloak, described online today in Science, works. It consists of a metal tube 3.21 centimeters in diameter and painted white. Researchers coated the core with 3.85 millimeters of a polymer doped with light-scattering particles. They placed the cloak in a tank of water tainted with enough white paint to make light diffuse through it more slowly than through the polymer. Finally, they backlit the tank with white light. Without the rubbery polymer layer, the cylinder blocks some of the light diffusing through the liquid and casts a shadow. But with it, the shadow disappears, rendering the cylinder undetectable. The researchers created a similar spherical cloak, too.
"It's imaginative," says John Pendry, a theoretical physicist at Imperial College London and one of the inventors of the original cloaking idea. The diffusive approach could have various applications, perhaps to backlight displays, Pendry says. But it won't be much help for cops and crooks. "I can't imagine why you would want to cloak somebody already cloaked in fog," he says.
Indeed, Xiang Zhang, a physicist at the University of California, Berkeley, questions whether the technique really counts as cloaking, as it works only if the fog is already thick enough to obscure the details of an object and everything behind it. "Probably we shouldn't call it a cloak, or call it a poor man's cloak," he says. He notes that diffusive cloaking was demonstrated earlier this year by physicists in Singapore who developed cloaks for heat.
The new technique may underscore the fact that in cloaking, it's not possible to achieve everything at once, says Birmingham's Shuang Zhang: "It seems like there's a fundamental barrier there." And that reality is proving hard to hide.