Any colour you like

Using a tabletop laser, University of Rochester optical scientists have turned pure aluminium, gold. And blue. And grey. And many other colours. And it works for every metal tested, including platinum, titanium, tungsten, silver, and gold.

Chunlei Guo, the researcher who a year ago used intense laser light to alter the properties of a variety of metals to render them pitch black, has pushed the same process. He now believes it is possible to alter the properties of any metal to turn it any colour - even multi-coloured iridescence like a butterfly's wings.

Since the process changes the intrinsic surface properties of the metal itself and is not just a coating, the colour won't fade or peel, said Guo, associate Prof of optics at the Institute of Optics at the University of Rochester.

He suggests the possibilities are endless - a cycle factory using a single laser to produce bicycles of different colours; etching a full-colour photograph of a family into the refrigerator door; or proposing with a gold engagement ring that matches your fiancée's blue eyes.

Guo and his assistant, Anatoliy Vorobeyv, use an incredibly brief but incredibly intense laser burst that changes the surface of a metal, forming nanoscale and microscale structures that selectively reflect a certain colour to give the appearance of a specific colour or combinations of colours.

The metal-colouring research follows up on Guo's breakthrough in late 2006, when his research team was able to create nanostructures on metal surfaces that absorbed virtually all light, making something as simple as regular aluminium into one of the darkest materials ever created.

Guo's black metal, with its very high absorption properties, is ideal for any application where capturing light is desirable. The potential applications range from making better solar energy collectors, to more advanced stealth technology, he says. The ultra-brief/ultra-intense light Guo uses is produced by a femtosecond laser, which produces pulses lasting only a few quadrillionths of a second.

The intense blast forces the surface of the metal to form nanostructures - pits, globules, and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures.

Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, said Guo. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what colour the metal reflects.

Guo and Vorobyev achieve the iridescent colouring by creating microscale lines covered with nanostructures. The lines, arranged in regular rows, cause reflected light of different wavelengths to interfere differently in different directions. The result is a piece of metal that can appear solid purple from one direction, and grey from another, or multiple colours all at once.

To alter an area of metal the size of a five pence coin currently takes 30 minutes or more, but the researchers are working on refining the technique. Fortunately, despite the incredible intensity involved, the femtosecond laser can be powered by a simple wall outlet, meaning that when the process is refined, implementing it should be relatively simple.