New Material Makes Objects Appear Invisible

A very black carbon nanotube coating renders objects into silhouettes and could form the basis of a future stealth device. Like a cloak of invisibility, the material makes things disappear completely when viewed against a black background.

"The most exciting thing is that it makes any 3-D object look like a shadow," said researcher Haofei Shi at the University of Michigan. "You know something is there but you don't know what it is."

The effect works even under direct light and from multiple viewing angles, showcasing the first perfect light-absorbing coating, Shi said. Capable of swallowing a broad spectrum of light, the paper-like material has potential for roomy stealth airplanes and sun-gulping solar cells.

"You can wear a coat made of this material and you will look like a flat sheet of black paper," said co-researcher Jay Guo at the University of Michigan. "They cannot perceive any 3-D aspects of the person in the coat … This would be a very nice winter coat. All of the light waves from outside could be totally absorbed and turned into heat."

A forest of carbon nanotubes, each much smaller than a wavelength of light, manages to trap more than 99.99 percent of light, ranging from ultraviolet to infrared, because of the low density of the tree pattern, Shi said. At less than 10 nanometerswide, each tube is separated its neighbors by an average space of 100 nm.

"If it was a higher density it would be shiny, you would see the profile of the object," Shi said.

While engineers could also use nanotube forests of silicon and metal to construct very black materials, the team chose carbon for its relative ease of fabrication and its impressive abilities as a light absorber.

To create the cloaking material, the researchers grew 1-inch [2.54 cm]squares of the carbon nanotube forest and pasted them onto an object like a piece of paper.

The material's ability to absorb and totally turn light into heat makes it suitable for use in solar heaters and other highly energy-efficient solar-energy applications. "The device works very well in natural sunlight," Shi said.
"Also it could be a stealth technology, a stealth airplane that can fire undetected because it absorbs radar rather than directing back to the radar stations," Guo said.

Though the plane would clearly appear as a very black object in the sky during daytime, in total darkness it would escape detection completely, without having "to be made into this sharp, weird shape" as with current stealth aircraft such as the Blackbird, Guo said. Satellites, as well, could sit against the perfect black of deep space and remain invisible to current instruments.

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