Metallic Mesh Becomes Invisible to Antenna Signals

Most of modern science’s attempts to recreate the invisibility cloaks found in TV’s Star Trek and the wizarding world of Harry Potter have focused on bending light waves around the object meant to be hidden. A team of U.S. and Chinese researchers have taken a very different direction by creating the first practical “invisible” material that allows certain electromagnetic signals to pass unimpeded as they would through air.

This is not going to lead to a fantastical invisibility cloak that can hide large starships or boy wizards from sight. But it represents a huge leap for real science. Previously, researchers could only make a single tiny sphere or cylinder invisible to certain electromagnetic wavelengths by taking advantage of a phenomenon called “dark state.”

Typically when electromagnetic signals hit a material, some of the signal may get scattered by reflecting off at a certain angle or being refracted as they pass through. Past research has shown that a single tiny object coated by a dielectric material—capable of holding an electrostatic field without conducting electricity—can mostly minimize the scattering signals by having them cancel each other out in one channel through destructive interference.

That destructive interference creates the “dark state” phenomenon. As a result, the material only allows certain wavelengths to pass directly through as though the material was invisible.

The researchers, from MIT and Zhejiang University, in Hangzhou, China, have taken such “dark state” invisibility to the next level by creating an entire sheet of metallic mesh that becomes effectively invisible to certain electromagnetic signals coming from any direction. The paper detailing their work appeared in the 15 Feb 2016 online issue of the journal Proceedings of the National Academy of Sciences.

“To me, this is the first time that people are able to do invisible materials,” said Ling Lu, an applied physicist at MIT in Boston, Mass. “Although the research for cloaking has been widely pursued previously, having a uniform material that is omnidirectionally invisible is quite exciting for us.”

So what good is an “invisible” material that can still be seen by the naked eye? First of all, it could provide a perfect version of the spherical “radomes” that protect antennas or radar dishes from outside conditions such as bad weather. Radomes also protect antennas or radar installations at government facilities and aboard military warships from public view. A radome made from the “invisible” mesh could even selectively allow the antenna or radar signals to pass through while screening out other electromagnetic signals.

There’s even a science fiction reference for this particular application. Fans of the classic or new Star Wars films may recall seeing sci-fi radomes in the form of two gray spheres perched on the bridge towers of the imposing Star Destroyers used by the Galactic Empire and First Order. Clearly, both Darth Vader and Kylo “Vader-fanboy” Ren would approve.

The new material could also enable new freedom and flexibility for antenna designs by providing support functions without concerns about affecting the antenna signal, Lu explained. He and his colleagues based the mesh on all-purpose copper, a cheap conductor material with many possible applications.

“You could improve the heat conduction, current conduction or mechanical stability—that’s very important—without altering the electrodynamic properties,” says Lu. “Or you can build an invisible wall blocking other wavelengths to just let this channel go through for communication.”

The international effort was primarily funded by the National Natural Science Foundation of China and the China Postdoctoral Science Foundation. But individual researchers also had a mix of U.S. military and civilian support from the Institute for Soldier Nanotechnologies under the U.S. Army Research Office, the National Science Foundation, and MIT. 

The research began with the computer simulations of Dexin Ye, an electrical engineer at Zhejiang University. His simulation work suggested that a material in the shape of a wire with alternating thicker and thinner segments could align the dark-state frequencies in different channels in such a way as to extend the dark state phenomenon beyond a single tiny object to any arbitrary size and shape.

That encouraged the Chinese-U.S. team to test the invisibility idea by making sheets of the material. Each sheet consists of arrays of tiny copper cubes—individually about 4 millimeters on each side—all connected together by thin, square-shaped copper rods. The copper array was sandwiched between two polysulfone covers that act as a dielectric material coating, creating a solid slab of material. Each slab was about the size of a sheet of paper.

Lab experiments with the slabs of material showed that the “invisibility” effect worked from any direction around an antenna. Researchers tested a 10.4-gigahertz signal frequency commonly used in radio astronomy and satellite communications.

In the future, Lu and his colleagues believe, it should be possible to adapt the material to become invisible to shorter wavelengths such as those in the terahertz range (often used in airport security screening devices). It might even work in the near infrared range that is closer to the visible light spectrum.