Monday, November 12, 2012

Invisibility cloak 'inevitable'

Duke University creates perfect, centimeter-scale invisibility cloak

Scientists at Duke University have created the first invisibility cloak that perfectly hides centimeter-scale objects. While invisibility cloaks have been created before, they have all reflected some of the incident light, ruining the illusion. In this case, the incident light is perfectly channeled around the object, creating perfect invisibility.

There are some caveats, of course. You didn’t really think you’d soon be sneaking away with Ron or Hermione (delete as applicable) into the woods, did you? For now, the Duke invisibility cloak only works with microwave radiation — and perhaps more importantly, the cloak is unidirectional (it only provides invisibility from one very specific direction). The big news here, though, is that it is even possible to create an invisibility cloak of any description. It is now just a matter of time before visible-light, omnidirectional invisibility cloaks are created.
Duke University's invisibility cloak

So far, all invisibility cloaks — including Duke’s (pictured right) — are constructed out of metamaterials. In general, a metamaterial is an artificial, man-made material that has unnatural properties. In practice, metamaterials nearly always refer to materials that have a negative refractive index — a property that never occurs in nature, where every material has a positive refractive index. A negative refractive index allows for the creation of some crazy things, such as superlenses that go beyond the diffraction limit, or invisibility cloaks.

In theory, invisibility cloaks operate by bending electromagnetic waves around objects — so that instead of seeing the object, you see what’s behind the object. Negative index metamaterials get you most of the way, but they still need to be arranged in such a way that the illusion is perfect. To get around a 3D object, you have to turn a corner at some point — and previous cloaks have struggled to fashion metamaterials that bend waves around corners without causing reflections, which ruin the illusion. In this case, Nathan Landy, a graduate student at Duke University, tweaked the metamaterial itself, and then fashioned it into a diamond, which is apparently the best shape for minimizing reflections.

The end result is an invisibility cloak that can perfectly hide a 3×0.4-inch (7.5x1cm) cylinder from microwave radiation. This obviously isn’t quite as awesome as an invisibility cloak that operates in the visible light spectra, but microwaves are still very important in the realms of telecommunications and defense (think radar, and the ultimate stealth vehicles). A very basic application might be in fiber optic networks, where invisibility cloaks could be used to bend light around corners without attenuating the signal.

The next step for the Duke University team is to create an omnidirectional, 3D microwave invisibility cloak. Then, eventually, when we work out how to create metamaterials that can bend visible light in interesting ways, we might get an actual invisibility cloak. That’s a few years away yet, though.