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When it comes to metal, you can have any colour as long as its silver. That’s because the sea of electrons within most metals absorbs and emits light over the entire optical range. Rather than silver, metals are actually colourless.

There are exceptions, of course. Gold absorbs blue light and this produces its characteristic yellow colour. And copper absorbs blue and green light making it look a reddy orange.

Other colours are hard to come by without coating the metal or carving a diffraction grating onto its surface to produce a characteristic interference pattern, like those from a compact disc.

But now there’s another way thanks to some interesting work by Jianfa Zhang at the University of Southampton and a few pals. Their idea is to carve a different type of repeating pattern on to the surface of a metal.

These patterns are smaller than the wavelength of visible light. Instead of causing the light to interfere, they work by changing the properties of the sea of electrons in the metal–in particular its resonant frequency. This alters the frequency of light it absorbs and reflects.

This is the same technique that researchers have been using for some time to build invisibility cloaks . The idea is that by carefully building repeating patterns of subwavelength structures, researchers can tailor the way a “metamaterial” can steer light.

But instead of creating 3D structures that steer light as it passes through the material, Zhang and co carve the relevant structures onto the surface to control the way light is absorbed and reflected.

The structures that do the trick are tiny rings carved into the surface. The team calculate that they can make gold or aluminium appear almost any colour simply by varying the size and depth of these rings. They’ve even demonstrated the technique on a thin layer of gold (see picture above).

What’s interesting about the technique is that it gives engineers a way to control the colour, indeed the entire spectral response, of the metal without changing its other properties, such as its conductivity, hardness and lustre. That’s hard to do using coatings.

Jiang and co mention a couple of applications. An obvious one is to give high value goods an additional aesthetic appeal. In other words, to make hi-tech jewellery. Another idea, given the difficulty of reproducing the effect, is as an anti-forgery mechanism on bank notes and credit cards.

There are surely a myriad other applications. Suggestions in the comments section please.

Ref: arxiv.org/abs/1011.1977: Controlling the Colour of Metals: Intaglio and Bas-Relief Metamaterials

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