The tiny patterns are formed on the gold surface using a process called ion beam milling (the focused ion beam system is used to create nanoscale intaglio metamaterial patterns on the metal surface. Here we can see the gold substrate being loaded. (c) University of Southampton

No, this isn’t some kind of reinvented alchemy or optical illusion. Scientists at University of Southampton have changed the colour of gold, silver and other metals without coating, by using a nanotechnology patterning technique.  Applications may include harder to forge currency or encryption of valuable documents, among other.

The team of researchers embossed the surface of metals with tiny raised or indented patterns only around 100 nanometres across or 400 times thinner than the human hair. This way, they managed to control which wavelengths of light the metal absorbs and which it reflects. Since our eyes do not perceive objects themselves but the light which bounces off them, this change in the way light is being absorbed and reflected allowed for interesting effects. For instance gold’s appearance was changed to red gold or green gold.

“This is the first time the visible colour of metal has been changed in this way,” says Professor Nikolay Zheludev, Deputy Director of Southampton’s Optoelectronics Research Centre, who led the project.

“The colours of the objects we see all around us are determined by the way light interacts with those objects. For instance, an object that reflects red light but absorbs other wavelengths will appear red to the human eye.

The key to this capability is the fact that each individual feature contained within the pattern is smaller than the wavelength of the incident light. Technically speaking, the nano-patterned metal is therefore a metamaterial, engineered to provide properties not found in nature.

This technique could allow for a number of applications. The jewelry industry could benefit especially from this novel research. A silver ring  could be decorated with a number of different patterns, making one part of it appear red, another part green and so on. Also, metals with certain optical properties, extremely difficult to replicate without the proper resources, could be used a safe key for important classified documents.

The findings were documented in three separate papers in the journals Optics Express, Journal of Optics and an US patent paper.

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