Ultraintense Laser Blast Creates True 'Black Metal'

A process involving an incredibly intense burst of laser light that can change the properties of almost any metal to render it black could potentially make everything from fuel cells to a space telescope's detectors more efficient, said the scientists who inadvertently discovered it.

"We wanted to see what would happen to a metal's properties under different laser conditions, and we stumbled on this way to completely alter the reflective properties of metals," said Chunlei Guo, assistant professor of optics at the University of Rochester. "We've been surprised by the number of possible applications for this."

Chunlei Guo, assistant professor of optics at the University of Rochester, and students in his laser lab. (Photo: University of Rochester)

The key to creating black metal is an ultrabrief, ultraintense beam of light called a femtosecond laser pulse. The laser burst lasts only a few quadrillionths of a second. A femtosecond is to a second what a second is to about 32 million years.

During its brief burst, Guo's laser unleashes as much power as the entire grid of North America onto a spot the size of a needle point. That intense blast forces the surface of the metal to form nanostructures -- pits, globules and strands -- that both dramatically increase the area of the surface and capture radiation. Some larger structures also form in subsequent blasts.

Guo's research team has tested the absorption capabilities for the black metal and confirmed that it can absorb virtually all the light that falls on it, making it pitch black. Other similar attempts have turned silicon black, but those use a gas to produce chemically etched microstructures. Regular silicon already absorbs most of the visible light that falls on it, so the etching technique only offers about a 30 percent improvement, whereas regular metals absorb only a few percent of visible light before Guo hits them with the laser.

A four-inch strip of blackened aluminum. (Photo: University of Rochester)

The huge increase in light absorption enabled by Guo's femtosecond laser processing means nearly any metal becomes extremely useful anytime radiation gathering is needed. For instance, detectors of all kinds, from space probes to light meters, could capture far more data than an ordinary metal-based detector could. And turning a metal black without paint, scoring, or burning could easily lead to everyday uses, even potentially replacing black paint on automobile trim.

Guo said this remarkable increase in a metal's surface area is also a perfect way to catalyze chemical reactions. Along with one of his research group members, postdoctoral student Anatoliy Vorobyev, he hopes to learn how the metal can help derive more energy from fuel cell reactions.

The new process has worked on every metal Guo has tried, and since it's a property of the metal itself, the black doesn't wear off.

At this point, the process is slow. To alter a strip of metal the size of a little finger takes easily 30 minutes or more, but Guo is looking at how different burst lengths, wavelengths and intensities affect metal properties. Fortunately, despite the incredible intensity involved, the femtosecond laser can be powered by a simple wall outlet. So when the process is refined, it should be relatively simple to implement.

But don't expect to see "home-blackening" kits anytime soon.

"If you got your hand in the way of the focused laser beam, even though it's only firing for a few femtoseconds, it would drill a hole through your skin," said Guo. "I wouldn't recommend trying that."

For more information, visit: www.optics.rochester.edu/workgroups/guo/index.htm