University of Toronto researchers have demonstrated an invisibility cloak that hides objects within an electromagnetic field, rather than swaddling it in meta-materials as other approaches require.
Rather than covering an object completely in an opaque cloak that then mimics the appearance of empty air, the technique developed by university engineering Prof. George Eleftheriades and Ph.D. candidate Michael Selvanayagam makes objects invisible using the ability of electromagnetic fields to redirect or scatter waves of energy.
The approach is similar to that of “stealth” aircraft whose skin is made of material that absorbs of the energy from radar systems and deflects the rest away from the radar detectors that sent them.
Rather than scattering radio waves passively due to the shape of its exterior, however, the Toronto pair’s “cloak” deflects energy using an electromagnetic field projected by antennas that surround the object being hidden.
Most of the proposals in a long list of “invisibility cloaks” announced during the past few years actually conceal objects by covering them with an opaque blanket, which becomes “invisible” by displaying an image of what the space it occupies would look like if neither the cloak nor the object it concealed were present. An invisibility cloak concealing an adolescent wizard hiding in a corner, for example, would display an image of the walls behind it in an effort to fool observers into thinking there was no young wizard present to block their view of the empty corner.
“We’ve taken an electrical engineering approach, but that’s what we are excited about,” Eleftheriades said in a public announcement of the paper’s publication. (A summary is available here; the full text is available as a free PDF here.)
A similar approach was published in March by researchers at the University of Texas-Austin, who created an ultrathin metallic screen that scattered incoming microwaves using both a generated electromagnetic field and the metallic surface of the screen itself. “When the scattered fields from the cloak and the object interfere, they cancel each other out and the overall effect is transparency and invisibility at all angles of observation,” wrote Andrea Alu, a co-author of the study.
In the University of Toronto experiment, the two researchers used an array of 12 magnetic dipoles to create an electromagnetic field, then carefully modulated the current on each element to modify the field such that it deflected microwaves aimed at an aluminum cylinder in every direction except back toward the source of the microwaves, where the object could be detected.
With a little more deliberate manipulation of the electromagnetic field, the same technique was able to make the radar/microwave reflection look like something other than the aluminum cylinder it actually “saw.”
In this case, researchers were able to disguise the radar signature of an aluminum cylinder as… the radar signature of either of two other sizes of aluminum cylinder. But the same technique should allow objects to be disguised in more sophisticated ways, or have their radar signatures appear from somewhere other than the object’s actual location. Accomplishing that would require using 3D arrays of antennas to cover objects on every side rather than just one, improving control over the electromagnetic field the array produces, and by using the dipoles to actually broadcast false signals rather than simply scattering the originals, the authors wrote.
The same principle should also work on energy within the visible spectrum, potentially making objects invisible to the eye as well as the radar detector.
“There are more applications for radio than for light,” said Eleftheriades. “It’s just a matter of technology—you can use the same principle for light, and the corresponding antenna technology is a very hot area of research.”
Image: Univ. Toronto