Metal chameleon: Red, gold and green, by George!

Golden traffic lights

 

Researchers in the UK have demonstrated how embossing or indenting tiny patterned features on the surface of a piece of gold can change the way it reflects light and so allow them to make gold appear red and green instead of its usually lustrous yellow colour.

As far as precious metals go, you don't get any more iconic than gold. Its seeming inertness, its lustre, and its colour have made it the focus of human wonder and greed for millennia. But, would it have been held in such high esteem by humanity for so long if gold had been red or even green. Perhaps one character from 1980s BBC comedy show, Blackadder, who made "precious green" should now feel vindicated. The latest research into metamaterials shows how the surfaces of gold, silver and aluminium (revered as a rare, precious commodity during the Victorian era) can be modified in such a way that the natural colour is masked by an iridescent effect. The modified surface features are at a scale below the shortest wavelength of visible light and as with the scales of a butterfly's wing and other natural colourations not involving pigment, such as feathers and mother of pearl, reveal themselves as modified colour to the human eye.

The research now opens up the possibility of changing the colour of metals without the need to use additives or to coat them chemically or electrolytically. The process might have wide-ranging applications not only in exploiting the modified aesthetics of precious metals for jewellery and other adornments but in engineering applications where, for instance, thin films might be created that have the specific properties of the metal being used but different characteristics in the field of optics. One obvious example would be to use such embossed metals to print bank notes and security documents make them almost impossible to forge.

"This is the first time the visible colour of metal has been changed in this way," says project leader Nikolay Zheludev who is Deputy Director of Southampton University's Optoelectronics Research Centre. "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," he adds. "This is the fundamental principle we have exploited in this project."

Photonic features

The features embossed on the metal surface consist of patterns with details approximately 100 nanometres across, and the technology used to generate the patterns allows the team to fine tune the wavelengths of light reflected. The team adds that they can produce a wide range of colours on a given piece of metal by varying the shape and height or depth of the patterns created by ion beam milling, which is essentially akin to sand-blasting but on the nanoscale.

Scaling up the process might be possible by creating a master template akin to the masters used to "print" CDs and DVDs. "We've filed a patent application to cover our work," Zheludev adds, "and we're currently talking to a number of organisations about taking our breakthrough towards commercialization."

The team's Kevin MacDonald of the University of Southampton told SpectroscopyNOW about the researchers' future plans. "This result is just one part of a very broad-ranging research programme dedicated to exploring the many different ways in which materials can be structured and engineered on the nanometre scale to provide new and extraordinary electromagnetic (visible, infrared, THz, mm- and micro-wave) properties beyond those available in naturally occurring media," he explains. "Our society today depends fundamentally photonic technologies (fibre-optic data networks, optical data storage, laser-assisted manufacturing, laser-based medical diagnostics and treatments...) and the future advancement of these will rely on the development of radically new nanotechnology-enabled materials." MacDonald suggests that, "Our research programme is at the core of this global  movement. We aim to develop a new generation of revolutionary switchable and active nanostructured photonic media to provide ground-breaking solutions for telecoms, energy and light generation, imaging, lithography, data storage, sensing, and security and defence applications."