Skip to Content
Uncategorized

Micro Carvings Make Metals Glisten In Any Colour

Optoelectronics engineers have discovered how to make gold of any colour by carving tiny shapes onto its surface

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

Keep Reading

Most Popular

individual aging affects covid outcomes concept
individual aging affects covid outcomes concept

Anti-aging drugs are being tested as a way to treat covid

Drugs that rejuvenate our immune systems and make us biologically younger could help protect us from the disease’s worst effects.

close up of baby with a bottle
close up of baby with a bottle

The baby formula shortage has birthed a shady online marketplace

Desperate parents just want to feed their babies. They’re having to contend with misinformation, price gouging, and scams along the way.

"Olive Garden" NFTs concept
"Olive Garden" NFTs concept

I tried to buy an Olive Garden NFT. All I got was heartburn.

Our newest issue spells out what you need to know about the dizzying world of digital money.

Stay connected

Illustration by Rose WongIllustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at customer-service@technologyreview.com with a list of newsletters you’d like to receive.