A new take on the Midas touch -- changing the color of gold

October 24, 2012

Red gold, green gold - a ground-breaking initiative has found a way of changing the colour of the world's most iconic precious metal.

A University of Southampton team have discovered that by embossing tiny raised or indented patterns onto the metal's surface they can change the way it absorbs and reflects light - ensuring our eyes don't see it as 'golden' in colour at all.

The finding results from a major initiative funded by the Engineering and Physical Sciences Research Council (EPSRC) targeting the development of a new generation of nanotechnology-enabled materials.

Equally applicable to other metals such as silver and aluminium, this breakthrough opens up the prospect of colouring metals without having to coat or chemically treat them. This could deliver valuable economic, environmental and other benefits.

The technique could be harnessed in a wide range of industries for anything from manufacturing jewellery to making banknotes and documents harder to forge.

"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.

"This is the fundamental principle we have exploited in this project. By embossing metals with patterns only around 100 nanometres 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 colour is created. The technique can be used to produce a wide range of colours 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.

However, the concept may be scaled for industrial production using such processes as nano-imprint, whereby large areas are stamped out from a master template in a manner comparable to CD/DVD production.

"We've filed a patent application to cover our work," Professor Zheludev says, "and we're currently talking to a number of organisations about taking our breakthrough towards commercialisation."
-end-
Notes for Editors

The breakthrough has been achieved as part of the 'Nanostructured Photonic Metamaterials' initiative, a six-year EPSRC-supported project at the University of Southampton's Centre for Photonic Metamaterials (http://www.metamaterials.org.uk/). Receiving just over £5 million in EPSRC funding and due to run until December 2015, this initiative is building on the previous 'Nanophotonics Portfolio Partnership' which received nearly £4 million in EPSRC support.

* 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.

100 nanometres is about ¼ of the width of a human hair.

The following papers have now been published on this work:

J. Zhang, J. Y. Ou, N. Papasimakis, Y. Chen, K. F. MacDonald, and N. I. Zheludev, "Continuous metal plasmonic frequency selective surfaces" Optics Express 19, 23279 (2011) [doi: 10.1364/OE.19.023279]

J. Zhang, J. Y. Ou, K. F. MacDonald, and N. I. Zheludev, "Optical response of plasmonic relief meta-surfaces" Journal of Optics 14, 114002 (2012) [doi:10.1088/2040-8978/14/11/114002]

"Method and device for controlling the color of metals" (US patent application 13/182,084 (13/07/2011); GB application 1011720.8 (13/07/2010))

About EPSRC

EPSRC is the main UK government agency for funding research and training in engineering and the physical sciences, investing more than £850 million a year in a broad range of subjects - from mathematics to materials science, and from information technology to structural engineering. www.epsrc.ac.uk

About the University of Southampton

The University of Southampton is a leading UK teaching and research institution with a global reputation for leading-edge research and scholarship across a wide range of subjects in engineering, science, social sciences, health and humanities.

With over 22,000 students, around 5000 staff, and an annual turnover of over £350 million, the University of Southampton is acknowledged as one of the country's top institutions for engineering, computer science and medicine. We combine academic excellence with an innovative and entrepreneurial approach to research, supporting a culture that engages and challenges students and staff in their pursuit of learning.

The University is also home to a number of world-leading research centres, including the National Oceanography Centre, Southampton, the Institute of Sound and Vibration Research, the Optoelectronics Research Centre, the Centre for the Developmental Origins of Health and Disease, the Mountbatten Centre for International Studies and the Southampton Statistical Sciences Research Institute

About the Optoelectronics Research Centre

The Optoelectronics Research Centre (www.orc.southampton.ac.uk) at the University of Southampton is one of the largest university-based research groups entirely devoted to optoelectronics in the world, and has maintained a position at the forefront of photonics research for over four decades. Its long and well established track record in the fields of optical fibre, lasers, waveguides, devices and optoelectronic materials has fostered innovation, enterprise, cross-boundary and multi-disciplinary activities. It has built strong links with industry, research institutions and universities around the world, from informal collaborations to large-scale funding. Several spin out companies, including Southampton Photonics Inc, Stratophase Ltd, Mesophotonics Ltd and ChG Southampton Ltd, have commercialised the fruits of ORC research; while its extensive outreach programme has brought lasers and optical fibres into schools across the country.

For more information contact:

Kevin MacDonald,kfm@orc.soton.ac.uk, tel 023 8059 3085

ORC media contact: Deanna Standen, University of Southampton, tel: 023 8059 4509, e-mail: ds3r10@orc.southampton.ac.uk

Images are available from the EPSRC Press Office. Contact: 01793 444404, e-mail: pressoffice@epsrc.ac.uk

Image details and suggested captions (These numbered images show the different stages of the process):

1 Gold.jpg: Gold before the technique has been applied to it.

2 Ionmilling.jpg: 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 into the chamber).

3 Design.jpg: Electron microscope image of the design imposed on the gold (600x magnification scanning electron microscope image of the nanofabricated design. The letters comprise arrays of 170 nm diameter rings etched to varying depths in the gold surface ).

4 Detail.jpg: Closer look at detail of the metamaterial design forming part of the letter 'E' (scanning electron microscope image at 20,000x magnification).

5 Goldcolour.jpg: The process completed (optical microscope image of the metamaterial sample under white light illumination. The sub-wavelength metamaterial nanostructuring produces uniform colours dependent on the etch depth).

Engineering and Physical Sciences Research Council

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