In a feat of physics worthy of Star Trek's Romulans, researchers have for the first time used a form-fitting cloak to render an object invisible from any direction. Sorry, Harry Potter, this is not magic.
The experiment, conducted using microwave radiation, eventually could help pave the way for more-effective ways to hide military aircraft from radar, the researchers say. If extended to visible light, the approach could lead to novel biomedical applications, as well as tiny switches for optical computing.
The feat is the latest in a decade-long effort to develop an ability to hide objects from view. Other researchers have been able to hide objects at microwave, infrared, and even visible-light wavelengths, and in two and three dimensions. But the cloaking materials have been bulky.
Indeed, it's a desire that traces its roots to H.G. Wells' "Invisible Man," notes Andrea Alu, an assistant professor of engineering at the University of Texas at Austin and a member of the research team reporting the results this week in the New Journal of Physics.
"We see objects by collecting whatever they radiate," he says, referring to the light that materials reflect or scatter.
One approach to cloaking is to change the behavior of electromagnetic radiation – radio or light, for instance – in ways that send the radiation around the object, rather than scattering some of it back at the detector trying to "see" the object.
Cloaks to accomplish this generally have been made from so-called metamaterials – materials engineered to display traits that aren't found in nature.
The team led by Jason Soric, in the department of Electrical and Computer Engineering at the University of Texas at Austin, designed its cloaking system using metamaterials as well. But instead of trying to bend radiation around the object so that none is scattered back to an observer, the researchers opted to use a cloak to change the properties of the radiation itself in ways that would cancel out the radiation scattered from an object.
"The overall effect is transparency," says Dr. Alu.
To pull off the feat, the team relied on the wave-like properties of electromagnetic radiation. The team surrounded a seven-inch-long cylinder with an ultra-thin cloak made from a polycarbonate film. The film was criss-crossed with a fishnet-like mesh made from copper tape.
The mesh was designed to scatter the same amount of microwave radiation as the cylinder. But the wavelike peaks and valleys in the microwave radiation from the two sources were offset, so that the peaks in the cylinder's scattered radiation overlapped with the valleys in the mesh's scattered radiation, canceling each other out.
The combined effect rendered the cylinder invisible to microwaves from any direction. Any microwave shadow the object might have cast vanished as well, as though the microwave radiation went right through the cylinder unimpeded.
The loss of a shadow could have some useful applications in telecommunications, where large antennas are placed close together. Such "antenna farms" often sprout on the tops of tall buildings, where one antenna can block signals from another in a specific direction. By cloaking the offending blocker, other signals would pass on by, eliminating the dead zone that was once the shadow.
Because the cloak is thin and pliable, it may be possible to cloak a variety of odd shapes, the researchers say.
A key reason the researcher chose to use microwaves for their experiment, rather than visible light, is that cloaking with light works best when the object you're trying to hide is on the size scale roughly comparable to the wavelength of light – several hundred billionths of a meter. In principle the approach could work in visible light with tiny objects on scales of mere millionths of a meter, the team suggests.
Working with microwaves involves the same physics. But its longer wavelength allows experimenters to work with easy-to-handle objects.