The research team's far-field bistatic measurement setup with the cloaked cylinder in the centre (CREDIT: New Journal of Physics)

'Invisibility cloak' hides 3D objects from microwaves

An ultra-thin “invisibility cloak” developed by US researchers can hide three-dimensional objects from microwaves.

The team from the University of Texas at Austin have used a new, ultrathin layer called a "metascreen", just micrometres thick that can hide objects in their natural environment, in all directions and from all of the observers' positions.

Presenting their study today in the Institute of Physics and German Physical Society's New Journal of Physics, the researchers explain how the metascreen cloak was made by attaching strips of 66 micrometre-thick copper tape to a 100 micrometre -thick, flexible polycarbonate film in a fishnet design.

It was used to cloak an 18cm cylindrical rod from microwaves and showed optimal functionality when the microwaves were at a frequency of 3.6 GHz and over a moderately broad bandwidth.

While previous cloaking studies have used metamaterials to divert or bend incoming waves – whether light, x-rays or microwaves – around an object, this new method, which the researchers dub "mantle cloaking", uses the ultra-thin metascreen to cancel out the waves as they are scattered off the cloaked object.

"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," says co-author of the study Professor Andrea Alu.

Moving forward, one of the key challenges for the researchers will be to use "mantle cloaking" to hide an object from visible light.

"In principle this technique could also be used to cloak light," says Alu.  “However, the size of the objects that can be efficiently cloaked with this method scales with the wavelength of operation, so when applied to optical frequencies we may be able to efficiently stop the scattering of micrometre-sized objects.

"Still, we have envisioned other exciting applications using the mantle cloak and visible light, such as realizing optical nanotags and nanoswitches, and noninvasive sensing devices, which may provide several benefits for biomedical and optical instrumentation."

The researchers have predicted that due to the inherent conformability of the metascreen and the robustness of the proposed cloaking technique, oddly shaped and asymmetrical objects can be cloaked with the same principles.

Last year, the same group of researchers were the first to successfully cloak a 3D object in another paper published in New Journal of Physics, using a method called "plasmonic cloaking", which used more bulky materials to cancel out the scattering of waves.

“Metascreens are easier to realize at visible frequencies than bulk metamaterials and this concept could put us closer to a practical realization,” says Alu.

"The advantages of the mantle cloaking over existing techniques are its conformability, ease of manufacturing and improved bandwidth.

“We have shown that you don't need a bulk metamaterial to cancel the scattering from an object; a simple patterned surface that is conformal to the object may be sufficient and, in many regards, even better than a bulk metamaterial."

Visit the New Journal of Physics website to read the team’s paper.

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