All metals can turn black

Summary:Researchers at the University of Rochester have found a way to change the properties of almost any metal by using a femtosecond laser pulse which forms nanostructures at the surface of the metal. As the process needs surprisingly low power, it could soon be used for a variety of applications, such as stealth planes, black jewels or car paintings.

Researchers at the University of Rochester have found a way to change the properties of almost any metal by using a femtosecond laser pulse. This ultra-intense laser blast creates true 'black metal' from copper, gold or zinc by forming nanostructures at the surface of the metal. As these nanostructures capture radiation, the metals turn black. And as the process needs surprisingly low power, it could soon be used for a variety of applications, such as stealth planes, black jewels or car paintings. But read more...

This discovery has been made by Chunlei Guo, an assistant professor of optics at the University of Rochester (UR), and his team at the Femtosecond Laser Laboratory.

But before looking at this discovery, here are two paragraphs from the UR press release that are worth reading -- for the comparisons made by the writer.

The key to creating black metal is an ultra-brief, ultra-intense beam of light called a femtosecond laser pulse. 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 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.

Despite of these dramatic words, the femtosecond laser can be powered by a simple wall outlet. So please don't think you'll suffer from a power outage when Guo switches his laser on.

But as you can guess, the results of this work are not really photogenic: a black image wouldn't help you to understand anything. Instead, please find below a picture of the UR femtosecond laser system which "consists of a femtosecond laser oscillator, a regenerative, a two-pass external amplifier, and an optical parametric amplifier." (Credit: Chunlei Guo's Femtosecond Laser Laboratory).

The UR femtosecond laser system

In "Ultra-short laser pulses turn metals pitch black," New Scientist gives additional details.

Chunlei Guo and colleagues at Rochester University in New York, US, used a titanium-sapphire laser, which requires only a normal power supply, to repeatedly blast samples of polished metal with pulses lasting 65 femtoseconds each.
After just a few pulses, "we found femtosecond pulses can reshape the metal's surface into a range of different nanostructures," says Guo. The resulting nanoscale pattern of cavities and protuberances traps light so efficiently that a shiny surface turns jet black.

Right now, the blackening process is slow, because it needs to be repeated at various spots on the piece of metal. But Guo is working at how different burst lengths, different wavelengths, and different intensities affect metal's properties. And he thinks that there are many possible applications. Here is what he said to New Scientist about possible military applications.

"Metals blasted using the laser could absorb radar and infrared very efficiently, making boats or planes hard to detect." And the effect should last longer than conventional coatings as the laser blasts completely reshape the surface of a metal.

The UR press release describes other possible uses for this process: "Turning a metal black without paint, scoring, or burning could easily lead to everyday uses such as replacing black paint on automobile trim, or presenting your spouse with a jet-black engagement ring." Imagine this: "Honey, here is the 'femtoblasted' ring I've created for you." Will it be well received?

Anyway, this latest research work has not been published yet, but a previous paper on the same subject has been accepted earlier this year by Optics Express under the name "Femtosecond laser nanostructuring of metals" (Volume 14, Issue 6, Pages 2164-2169, March 2006). Here are two links to the abstract and to the full paper (PDF format, 6 pages, 283 KB).

Sources: University of Rochester Press Release, November 21, 2006; Tom Simonite, New Scientist, November 22, 2006; and various websites

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