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Intense laser blast yields true black metal

PERFECT PITCH BLACK—Chunlei Guo, assistant professor of optics, and his team have discovered a way to transform any metal into the ultimate true black by firing it with an intense burst of laser light. The new process could have applications ranging from fuel cells to space telescopes, perhaps even the auto industry for those who think a matte-black Mercedes is a must-have.

Chunlei Guo, assistant professor of optics, and his research team wanted to see what would happen if they fired a super-intense laser beam at a piece of metal. The results have surprised them and many others: The laser turned the metal completely black.

Guo says this newly discovered process, which actually alters the reflective properties of metal, could have an amazing number of applications, from space telescopes to stealth bombers.

The key is an ultra-brief, ultra-intense beam of light called a femtosecond laser pulse. The 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 unleashes as much power as the entire grid of North America onto a spot the size of a needle point. That 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 small percentage of visible light before Guo hits them with the laser.

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.

The technique has worked on every metal Guo has tried. The added benefit: Since the process changes the very property of the metal itself, there’s no worry of the black wearing off.

There is a downside. It’s slow. To alter a strip of metal the size of a person’s little finger easily takes 30 minutes or more, but Guo is continuing to look at how different burst lengths, wavelengths, and intensities affect metal’s properties.

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

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