University of Toronto researchers, Michael Selvanayagam and George V. Eleftheriades, have offered a popular summary of their work. from the popular summary (on the website where they’ve published their academic paper),
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.
A Nov. 12, 2013 news item on ScienceDaily, offers information augmenting the popular summary,
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. Their paper ‘Experimental demonstration of active electromagnetic cloaking’ appears today in the journal Physical Review X.
“We’ve taken an electrical engineering approach, but that’s what we are excited about,” says Eleftheriades. “It’s very practical.”
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.
The Nov. 13, 2013 University of Toronto news release, which originated the news item and was posted a day later, provides more specific details about the research,
“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.
For now, the antenna loops must be manually attuned to the electromagnetic frequency they need to cancel. But in future, researchers say, they could function both as sensors and active antennas, adjusting to different waves in real time, much like the technology behind noise-cancelling 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 that 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.
The cloaking technology holds possiblities that go beyond obvious applications such as hiding military vehicles or conducting surveillance operations. For example, structures that interrupt 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.
For those who feel inclined to explore this work further,
Experimental Demonstration of Active Electromagnetic Cloaking by Michael Selvanayagam and George V. Eleftheriades. Phys. Rev. X (Volume 3 Issue 4) or Phys. Rev. X 3, 041011 (2013) [13 pages] DOI: 10.1103/PhysRevX.3.041011
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
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