Coating makes 3-D objects look flat

U. MICHIGAN (US) — Engineers have developed a carbon nanotube coating that can conceal an object’s 3-D geometry, absorbing 99.9 percent of its light to make it look like a flat black sheet.

The 70-micron coating that is about half the thickness of a sheet of paper has potential for use in display screens for ultra-high contrast, holds promise as a solar heating device, and may inspire a new type of camouflage paint for stealth aircraft. Currently, stealth planes use shape to scatter electromagnetic waves and avoid detection. The new technology could possibly absorb the waves.

“You could use it to completely hide any 3-D attributes of an object,” says Jay Guo, professor of electrical engineering and computer science at the University of Michigan and principal investigator. “It’s not cloaking, as the object can still cast a shadow. But if you put an object on a black background, then with this coating, it could really become invisible.”

To demonstrate the concept—reported online in Applied Physical Letters—researchers made a raised, microscopic tank shape on a piece of silicon and then grew the carbon nanotube carpet on top of the entire silicon chip. In photos taken through an optical microscope, the tank is imperceptible. As a control, they did the process again, carving out a rectangle that was not coated with carbon nanotubes. The rectangle is visible on this chip, but the tank remains hidden.

Here’s how the new coating works: Human eyes perceive an object based on how it reflects or scatters light. The “refractive index” of the new coating is similar to that of air, meaning light traveling through air doesn’t scatter or reflect when it hits the coating. It’s well known that carbon nanotubes are capable of absorbing light, but the researchers were able to push it to such a high percentage by spacing them just right.

“The carbon nanotube forest can absorb very wide range of electromagnetic wave from ultraviolet up to terahertz,” Guo says, “and in principle it can be applied to an arbitrary sized object.” Just how large an object? Guo suggests an intriguing possibility—perhaps entire planets or even stars.

“Since deep space itself is a perfect dark background, if a planet or star were surrounded by a thick, sooty atmosphere of light-absorbing carbon nanomaterial gases, it would become invisible due to the same principle,” Guo says. “It would become totally dark to our instruments that rely on the detection of electromagnetic waves. Could this explain some of the missing matter in the universe?”

X-rays or gamma rays would be able to penetrate through the hypothetical “dark veil” Guo proposes. Or, objects behind such veils would cast a shadow by distant stars behind them.

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