Scientists at the University of Rochester use an incredibly intense burst of laser light to change the properties of almost any metal to render it pitch black. The process holds the promise of making everything from fuel cells to space telescope detectors more efficient.
"We've been surprised by the number of possible applications," says Chunlei Guo, assistant professor of optics at the University of Rochester. "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."
The team uses an ultrabrief, ultraintense beam of light called a femtosecond laser pulse to turn the metal surface literally pitch black, i.e., it can absorb virtually all the light that falls on its surface.
The laser burst lasts only a few quadrillionths of a second. To get a grasp of that kind of speed — a femtosecond is to a second what a second is to about 32 million years.
During its brief burst, Guo's laser focuses as much power as the entire grid of North America onto a spot the size of a needle point. The intense blast forces the surface of the metal to form nanostructures — pits, globules, and strands — that dramatically increase the surface area and its ability to capture radiation. Larger structures also form in subsequent blasts.
Similar processes using gas have chemically etched microstructures in silicon. But silicon already absorbs most of the visible light that falls on it, so etching only improves absorption by about 30%. In contrast, untreated metals absorb only a few percent of visible light.
Efficient light absorption comes in handy anytime radiation gathering is needed. For example, detectors of all kinds, from space probes to light meters, could benefit from the new technique.
And the ability to turn a metal black without paint, scoring, or burning could easily lead to such everyday uses as replacing black paint on automobile trim. Guo also says metals surfaced with the new nanostructures could help derive more energy from fuel-cell reactions.
Currently, the process is slow. It easily takes 30 min or more to alter a strip of metal the size of your little finger, says Guo. So the team is looking at how burst lengths, wavelengths, and intensities affect metal properties. Fortunately, despite the incredible intensity involved, femtosecond lasers can get power from ordinary wall outlets. Thus, if refined, the process would be relatively easy to implement. For more information visit Guo's research group Web site at
www.optics.rochester.edu/workgroups/guo/index.htm.