‘Metascreen’ forms ultrathin invisibility cloak

Ashley N. Rice, ashley.rice@photonics.com

A new invisibility cloak can hide 3-D objects from microwaves while in their natural environment, in all directions and from all of the observers’ positions.

Most invisibility cloaks up to this point have been large, cumbersome contraptions. The University of Texas at Austin’s cloak, however, uses an ultrathin material called a “metascreen” that was fabricated by attaching strips of 66-µm-thick copper tape to a 100-µm-thick, flexible polycarbonate film in a fishnet design.

In tests, the material was used to cloak an 18-cm cylindrical rod from microwaves and showed optimal functionality with 3.6-GHz microwaves over a moderately broad bandwidth. Researchers predict that the metascreen’s inherent conformability and the proposed cloaking technique’s robustness will allow cloaking of oddly shaped and asymmetrical objects.

Previous cloaking studies have used metamaterials to divert, or bend, the incoming waves around an object. The new method, dubbed “mantle cloaking,” uses the ultrathin metallic metascreen to cancel out the waves as they are scattered off the cloaked object.

Experimental setup for the far-field measurement of the cloaked cylinder. As shown in the schematic, the transmitter (Φ_tx, θ_tx ) and the receiver (Φ_rx, θ_rx ) are both at distance R = 17.5λ0 from the objects under test.

“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,” said professor Andrea Alu, study co-author. “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.”

Last year, the same group successfully cloaked a 3-D object using bulkier materials in a plasmonic cloaking method to cancel out the wave scattering.

The investigators will now look at using mantle cloaking to hide objects from visible light.

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

Other possible applications include optical nanotags and nanoswitches, and noninvasive sensing devices for biomedical and optical instrumentation.

The findings were reported in the New Journal of Physics (doi:10.1088/1367-2630/15/3/033037).