Two researchers at the University of Toronto have created and tested a new type of active invisibility cloak that can hide objects over a wide range of frequencies.

Prof George Eleftheriades and his co-author, PhD student Michael Selvanayagam, effectively cloaked an aluminum cylinder from radio waves using one layer of loop antennas. The system can be scaled up to cloak larger objects using more loops, and the loops could become printed and flat, like a blanket or skin.

“It’s very simple: instead of surrounding what you’re trying to cloak with a thick metamaterial shell, we surround it with one layer of tiny antennas, and this layer radiates back a field that cancels the reflections from the object,” Prof Eleftheriades explained.

Currently the antenna loops must be manually attuned to the electromagnetic frequency they need to cancel, but in future they could function both as sensors and active antennas, adjusting to different waves in real time, much like the technology behind noise-canceling headphones.

Work on developing a functional invisibility cloak began around 2006, but early systems were necessarily large and clunky – if you wanted to cloak a car, for example, in practice you would have to completely envelop the vehicle in many layers of metamaterials in order to effectively shield it from electromagnetic radiation.

The sheer size and inflexibility of the approach makes it impractical for real-world uses. Earlier attempts to make thin cloaks were not adaptive and active, and could work only for specific small objects.

Beyond obvious applications, such as hiding military vehicles or conducting surveillance operations, this cloaking technology could eliminate obstacles – for example, structures interrupting signals from cellular base stations could be cloaked to allow signals to pass by freely.

The system can also alter the signature of a cloaked object, making it appear bigger, smaller, or even shifting it in space.

And though their tests showed the cloaking system works with radio waves, re-tuning it to work with Terahertz or light waves could use the same principle as the necessary antenna technology matures.

“There are more applications for radio than for light. 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,” said r Eleftheriades, who is the senior author of the paper published in the journal Physical Review X.

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Bibliographic information: Michael Selvanayagam and George V. Eleftheriades. 2013. Experimental Demonstration of Active Electromagnetic Cloaking. Phys. Rev. X, vol. 3, no 4; doi: 10.1103/PhysRevX.3.041011