US scientists can change almost any metal into gold—at least as far as appearances go: with the help of extremely short and energy-intensive laser pulses they change the surface of metals such as silver, tungsten, or titanium so that they take on a completely different color. Read what is possible with this technology.

US scientists can change almost any metal into gold—at least as far as appearances go: with the help of extremely short and energy-intensive laser pulses they change the surface of metals such as silver, tungsten, or titanium so that they take on a completely different color. For example, in this way they have been able to color normally silvery aluminum gold, blue, and gray. Even iridescent colors, such as those used in special paints for automobiles, are possible with this technology, say the scientists. As this is not a coating but a property of the material itself, the color effects do not wear off. However, at present red and green are still missing from the palette, report Anatoliy Vorobyev and Chunlei Guo from the University of Rochester.

Guo and his colleagues already laid the foundations for the colored metals back in 2006: they were able to blacken metals—also with the help of a laser—to the extent that they hardly reflected any light. The energy of the laser created minute hills, holes, spheres, and other uneven points on the surface that more or less hold the light, the scientists reported then. The pulses required were just a few femtoseconds, in other words, billionths of one millionth of a second long. "After we developed the black metals we decided that we wanted to gain full control of the process and get the metals to reflect only one specific color and absorb the rest," explains Guo.

And he and his colleague Vorobyev have now managed just that: by varying the intensity, length, and number of laser pulses they created different patterns from the millionth of a millimeter large uneven points on the surface of the metal. Depending on the arrangement of the bumps and cavities these patterns influence the light falling on the surface in different ways and also change which parts are reflected. If regular lines are also laser engraved into the surface, this causes an interference effect in which the different wavelengths of the light strengthen or extinguish each other. This produces iridescent colors that change depending on which angle they are viewed from.

So far the process has worked with every metal they have tested, says Guo; these include platinum, titanium, tungsten, silver, gold, and aluminum. This leads him to believe that the process would also work with every other metal. He is sure that there are many applications for metals colored in this manner; after all, they could be used everywhere where metal is painted at present. The process could also be used to decorate everyday objects—as an example, Guo mentions a color photo of the family in the stainless steel door of the refrigerator. However, for the present he and his colleagues are trying to create the colors that are still missing.


Sources
Guest article: www.wissenschaft.de
Ilka Lehnen-Beyel
Anatoliy Vorobyev and Chunlei Guo (University of Rochester): Applied Physics Letters, Volume 92, p. 041914