PORTLAND, Ore. -- A few years ago, a flurry of research results demonstrated that invisibility cloaks could be constructed from meta-materials -- assemblies of metals and insulators that channel signals around objects. However, despite continued development efforts none have made it out of the laboratory yet. Now an approach to invisibility cloaks that uses the same principle as noise-canceling headphones has been demonstrated at the University of Toronto, for which the researchers claim commercialization is more feasible.
"We tried to build invisibility cloaks using metamaterials, but the big problem was that they have to be very thick," professor George Eleftheriades told us. "So when you want to cloak a big object the invisibility cloak becomes very large and bulky -- metamaterials are just not very practical, even though its a very elegant concept to channel waves around an object."
Professor George Eleftheriades (left) and doctoral candidate Michael Selvanayagam showing their new approach to invisibility cloaking. (Credit: Marit Mitchell) SOURCE: University of Toronto
Instead of using metamaterials, Eleftheriades's approach surrounds the object to be cloaked with tiny antennas tuned to the frequency band in which the cloaking is to occur, such as radar. The antennas then send out a signal that cancels out the reflected signal -- effectively cloaking the object. In experiments, the researchers effectively cloaked an aluminum cylinder with an array of 12 magnetic-dipole loop antennas. By changing the weights controlling the current applied to each element of the array, the cylinder was effectively cloaked in the forward and backward directions. And by adjusting the weights in various configurations, they were able to demonstrate how the object could be disguised to be a different size or in a different location, feats that were never demonstrated by meta-material cloaks that merely channel signals around objects.
"With antennas, you don't have to channel waves around the object -- the antennas just adaptively sense the incoming signal and feedback a signal that cancels it, sort of like noise-canceling headphones," Eleftheriades told us. "This is the way to do it practically, and for electrical engineers this is a perfect way because we can hook up electronics and control them to make the object invisible, we change its scattering cross-section to make it look smaller or of a different material or even displaced in space -- all by adjusting the weights on these tiny antennas."
The research team, which included doctoral candidate Michael Selvanayagam, also claimed that its antenna-based solution to invisibility cloaks is scalable to large objects and can be made very thin -- by using flat loop antennas that could be printed on the object like a skin.
For the future, the researchers are experimenting with adaptive electronics that first uses the antenna array to sense the incoming signal, then adjusts its weights accordingly. This adaptive approach could also allow an objects profile to be changed in real time at the user's or system's discretion.
Besides hiding and disguising military vehicles and surveillance aircraft, the researchers hope their system can also be used in civilian applications, such as reducing interference caused by cellular base stations by cloaking them to allow signals to more freely pass nearby. Theoretically, the approach should also work at visible wavelengths, once the necessary nanoscale antenna technologies are perfected.
— R. Colin Johnson, Advanced Technology Editor, EE Times