November 12, 2013

Thin, active invisibility cloak demonstrated for first time with antennas that cancel out radar

Researchers have demonstrated an effective invisibility cloak that is thin, scalable and adaptive to different types and sizes of objects.

Professor George Eleftheriades and PhD student Michael Selvanayagam have designed and tested a new approach to cloaking—by surrounding an object with small antennas that collectively radiate an electromagnetic field. The radiated field cancels out any waves scattering off the cloaked object.

Picture a mailbox sitting on the street. When light hits the mailbox and bounces back into your eyes, you see the mailbox. When radio waves hit the mailbox and bounce back to your radar detector, you detect the mailbox. Eleftheriades and Selvanyagam’s system wraps the mailbox in a layer of tiny antennas that radiate a field away from the box, cancelling out any waves that would bounce back. In this way, the mailbox becomes undetectable to radar.

“We’ve demonstrated a different way of doing it,” says Eleftheriades. “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.”

Their experimental demonstration effectively cloaked a metal cylinder from radio waves using one layer of loop antennas. The system can be scaled up to cloak larger objects using more loops, and Eleftheriades says the loops could become printed and flat, like a blanket or skin. 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-cancelling headphones.


This is the set up of the lab where U of T Professor George Eleftheriades and PhD student Michael Selvanayagam have designed and tested a new approach to cloaking -- by surrounding an object with small antennas that collectively radiate an electromagnetic field. The radiated field cancels out any waves scattering off the cloaked object. Credit: University of Toronto



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 (T-rays) or light waves could use the same principle as the necessary antenna technology matures.

“There are more applications for radio than for light,” says 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.”

Physical Review X - Experimental Demonstration of Active Electromagnetic Cloaking

We “see” a physical object by detecting electromagnetic waves scattered from the object. A device that can “correct” or cancel that scattering would take the notion of a magic invisibility cloak from the realm of science fiction to reality. In fact, such physical devices already exist, accomplishing their feat based on metamaterials that bend light around the object to be cloaked, “correcting” the scattering. Designing metamaterials with the right light-bending properties for this purpose is, however, quite challenging, and the designs often require a thick “cloak.” An alternative approach to this problem is “active cloaking”: surrounding the object to be cloaked with electromagnetic sources that are carefully tuned to cancel the electromagnetic field scattered by the object. In this work, we demonstrate the first experimental realization of such a thin active cloak for microwaves.

The sources we have used are specially designed antennas and phase shifters, which can be configured into thin layers with flexibility in shape. We have succeeded in cloaking a sizable metallic cylinder by properly tuning the phase of the radiation from the antennas so that the radiation cancels the field scattered by the cylinder. We have gone a step further than cloaking and have also demonstrated how the object can be disguised as another object by tuning the antennas in a controlled way. The catch with active cloaking, however, is that knowledge of the incident field is required to tune the antennas. To tackle this issue, we have discussed some potential solutions that also utilize the antennas as sensors to detect the incident field.

Future work along this line will aim to extend the bandwidth of the cloak (with respect to pulsed incident fields) as well as design active cloaks that can adaptively respond to an incident field.

The cloak consists of a 12-element magnetic-dipole array. By controlling the weights of the current on each element of the array, the scattering off of the cylinder is reduced in the backward and forward directions. The ability to disguise the aluminum cylinder as another object by varying the weights of the dipole array is also demonstrated. Finally, potential ways of overcoming the constraint of requiring a priori knowledge of the incident field leading to camouflaging-type behavior are discussed. From materials by the American Physical Society.

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