Optical alchemy turns base metals gold

Using a tabletop laser rather than the fabled philosopher’s stone, physicists at the University of Rochester in New York have turned aluminium and other metals gold, along with blue, grey and many other colours.

Gold aluminium, blue titanium and gold platinum, the colours of which are the result of nanoscale modification of the metals’ surfaces with intense laser light

The breakthrough achieved by Chunlei Guo and Anatoliy Vorobyev follows their previous discovery that randomly organised nanoscale pits burned into a metal surface by intense laser light can alter the properties of the surface so as to render it pitch black.

“The technique has many unique advantages over simply painting or enamelling processes,” says Guo. “First of all, the laser-treated surfaces are still part of the metal itself and will thus resist wear and tear, especially for applications at high-temperatures and under severe conditions.”

Since the surface structures created by the femtosecond laser are smaller than the wavelength of light, the way they reflect that light is highly dependent on their size and shape. By varying the laser intensity, pulse length and number of pulses, Guo and Vorobyev can control the configuration of the surface pitting and hence the wavelength of the reflected light.

As well as block colours, Guo and Vorobyev can achieve an iridescent effect by creating microscale lines covered with nanostructures. The interference caused by this modified optical grating can lead to a metal that appears purple when viewed in one direction, grey from another or multiple colours all at once.

“Aesthetically pleasing colours could be produced on metal surfaces, gadgets and appliances, thereby eliminating harmful paints,” says laser specialist Sacharia Albin at Old Dominion University in Norfolk, Virginia. “The nanoscale surface is as hard as the metal, and hence quite durable.”

The process is not quick, however. To process a piece of metal the size of a small coin currently takes half an hour, but the researchers are working on improving the efficiency of the technique.

Further reading: “Colorizing metals with femtosecond laser pulses”, Vorobyev and Guo, Appl. Phys. Lett. 92, 041914 (2008).

Figure: Gold aluminium, blue titanium and gold platinum, the colours of which are the result of nanoscale modification of the metals’ surfaces with intense laser light (source: Richard Baker/University of Rochester).

Article first published in Nanomaterials News.