British scientists have found a way of changing the color of metals including gold, silver and aluminium.
The breakthrough, described in a paper in the Journal of Optics (nanophotonics.org.uk’s version), opens up the prospect of coloring metals without having to coat or chemically treat them.
“This is the first time the visible color of metal has been changed in this way,” said Prof Nikolay Zheludev of the University of Southampton’s Optoelectronics Research Center, senior author of the study. “The colors 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.”
“This is the fundamental principle we have exploited in this project,” he said. “By embossing metals with patterns only around 100 nanometers across, we’ve found that we can control which wavelengths of light the metal absorbs and which it reflects.”
The precise shape and height or depth of the patterns determine exactly how light behaves when it strikes the metal and therefore what color is created. The technique can be used to produce a wide range of colors on a given metal.
“A silver ring, for example, could be decorated with a number of different patterns, making one part of it appear red, another part green and so on; metal features with sophisticated optical properties that would be almost impossible to imitate could be incorporated into documents as security features.”
The nano-patterning is carried out at the research level using well-established techniques such as ion beam milling, which may be envisaged as sand-blasting on the atomic scale.
Bibliographic information: J Zhang et al. 2012. Optical response of plasmonic relief meta-surfaces. Journal of Optics, vol. 14, no. 11; doi: 10.1088/2040-8978/14/11/114002