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How black is black?

21 February, 2008

invisible.jpgResearchers in New York reported this month that they have created a paper-thin material that absorbs 99.955 percent of the light that hits it, making it by far the darkest substance ever made — about 30 times as dark as previously thought possible.

The material, made of hollow fibers, is a death trap for photons — light checks in, but it never checks out. By voraciously sucking up all surrounding illumination, it can give those who gaze on it a dizzying sensation of nothingness (sounds like one of my students).

But scientists are not satisfied. Using other new materials, some are trying to manufacture rudimentary Harry Potter-like cloaks that make objects inside of them literally invisible under the right conditions — the pinnacle of stealthy technology.

Both advances reflect researchers’ growing ability to manipulate light, the fleetest and most evanescent of nature’s offerings. The nascent invisibility cloak now being tested, for example, is made of a material that bends light rays “backward,” a weird phenomenon thought to be impossible just a few years ago.

Known as transformation optics, the phenomenon compels some wavelengths of light to flow around an object like water around a stone. As a result, things behind the object become visible while the object itself disappears from view.

“Cloaking is just the tip of the iceberg,” said Vladimir Shalaev, a professor of electrical and computer engineering at Purdue University and an expert in the fledgling field. “With transformation optics you can do many other tricks,” perhaps including making things appear to be located where they are not and focusing massive amounts of energy on microscopic spots.

U.S. military and intelligence agencies have funded the cloaking research “for obvious reasons,” said David Schurig, a physicist and electrical engineer at North Carolina State University who recently designed and helped test a cloaking device. In that experiment, a shielded object a little smaller than a hockey puck was made invisible to a detector that uses microwaves to “see.”

The first working cloaks will be limited that way, he said — able to steer just a limited part of the light spectrum around objects — and it could be years before scientists make cloaks that work for all wavelengths, including the visible spectrum used by the human eye.

But even cloaks that work on just a few key wavelengths could offer huge benefits, making objects invisible to laser beams used for weapons targeting, for example, or rendering an enemy’s night goggles useless because objects would be invisible to the infrared rays those devices use.

The newest black — which when held next to something conventionally black, such as a tuxedo jacket, is noticeably blacker — reflects just 0.045 percent of visible light.

It is made of carbon nanotubes: microscopic, hollow fibers whose walls are just one atom thick. Importantly, the fibers are widely spaced, providing plenty of space to allow light in and almost no surfaces to bounce it back out.

(Source: Washington Post 20/02/2008)

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