University of Rochester

Skin deep.
Ultrafast laser pulses change the colors (center circles) of various metals.

Alchemy in the Age of the Laser

It might never be able to compete with those dirt-cheap auto paint jobs, but a new laser technique could revolutionize the way metals are colored for varied applications such as jewelry, home appliances, military camouflage, and even optical filters in telescopes. It could also eliminate the need for environmentally unfriendly paints and other coatings.

Among the seemingly miraculous properties of lasers is the ability to alter the optical characteristics of materials. The intense beam of laser light can change a material's color without coating it with paint or pigment. But there's a problem: Lasers powerful enough to tint materials also tend to damage them with heat.

So researchers at the University of Rochester in New York sought to develop a laser coloring technique that could avoid the destructive heat effect. Optical physicists Chunlei Guo and A. Y. Vorobyev used laser pulses lasting only a femtosecond, or a quadrillionth (a billionth of a millionth) of a second. The technique works by melting extremely small areas of the metals, which then congeal in ways that reflect different wavelengths of light. The duo reported online 31 January in Applied Physics Letters that by fine-tuning their ultrafast laser light output and by modifying the strength of the beam, the number of treatment pulses, and the processing duration, they were able to change the hues of a number of metal samples. They could give small sheets of aluminum a gold cast, for instance, and turn gold black.

The process directly transforms the colors of metals, meaning no paints are necessary. And Guo notes that laser-treated surfaces tend to resist wear and tear, especially at high temperatures, making them more durable than painted surfaces. Because the technique is still new, relatively slow, and energy-intensive, Guo says that it's at least several years from commercial applications. Next up will be to improve its speed and efficiency. But as the technique matures, he says, "I think we should be able to apply it to things in our everyday life."

Maybe, but it will likely take a while, says physicist Joseph Shinar of the Department of Energy's Ames Laboratory and Iowa State University in Ames. The results are "very interesting and appealing, and the physics appear solid," he says, but there are already many ways to color metals and materials using proven coatings. The laser technique appears to affect only the topmost layers of the metal, so scratches or abrasions might change or remove the coloring--just as with coatings. Therefore, "it may be some time before we find out what sort of application needs this fulfills," Shinar says.

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