New research brings invisibility cloaks closer to reality

New metamaterials may take engineers one step closer to building invisibility cloaks or even shields that can conceal military airplanes, scientists say.
Metamaterials are artificial materials engineered to bend electromagnetic, acoustic and other types of waves in ways not possible in nature.
Hao Xin, a professor of electrical and computer engineering at the University of Arizona, has made a discovery with these synthetic materials that may pave the way for microscopes with superlenses that see molecular-level details, or shields that conceal military airplanes and even people.
In the UA's Millimeter Wave Circuits and Antennas Laboratory, Xin uses a 3-D printer to make metamaterials from metals, plastics and other substances.

Resembling porous plastic bowling balls and tiny copper wire circuit boards, these objects are configured in precise geometrical patterns to bend waves of energy in unnatural ways.
In particular, they exhibit a property called negative refraction, meaning they can bend a wave backwards.

Through a prism with negative refraction, a straw leaning in a glass of water would appear inverted: the piece above the water's surface would appear below the water and leaning in the opposition direction.
In a more futuristic scenario, someone looking at a person wearing a cloak with artificially designed refraction properties would see part or none of the person, depending on the cloak's refractive index distribution and whether the light bouncing off it reached the viewer's eye, researchers said.

"One of the biggest problems with metamaterials is that they produce energy loss," Xin said.
"The waves decay as they pass through the artificial material. We have designed a metamaterial that retains negative refraction but does not diminish energy," Xin added.
In fact, the synthetic material not only prevented energy loss - it actually caused energy gain, with the microwave intensifying in strength as it passed through the material.
Xin achieved this by embedding simple battery-powered tunnel diodes (a type of semiconductor device) and micronanofabrication technologies into the new material.
"Many people did not think it was possible to achieve energy gain along with negative refraction," Xin said.

Xin first showed it was possible, with one-dimensional metamaterials, in a paper published in Physical Review Letters in 2011.
His new findings reported in the journal Nature Communications have broader implications, because they involve 3-D metamaterials.