Counterfeiting is a crime as old as money itself. It causes a reduction in the value of real money and can add to company losses, as they are not reimbursed for counterfeits. In 1996 Australia became the first country to have a full series of circulating polymer banknotes, which are difficult to counterfeit because they cannot be successfully reproduced by photocopying or scanning. Now scientists have discovered a way of mimicking the stunningly bright and beautiful colors found on the wings of tropical butterflies, that could help make banknotes and credit cards even harder to forge.

The striking iridescent colors displayed on beetles, butterflies and other insects have long fascinated both physicists and biologists, but mimicking nature's most colorful, eye-catching surfaces has proved elusive. This is partly because rather than relying on pigments, these colors are produced by light bouncing off microscopic structures on the insects' wings.

Mathias Kolle, working with Professor Ullrich Steiner and Professor Jeremy Baumberg of the University of Cambridge, studied the Indonesian Peacock or Swallowtail butterfly (Papilio blumei), whose wing scales are composed of intricate, microscopic structures that resemble the inside of an egg carton. Because of their shape and the fact that they are made up of alternate layers of cuticle and air, these structures produce intense colors.

Using a combination of nanofabrication procedures – including self-assembly and atomic layer deposition – Kolle and his colleagues made structurally identical copies of the butterfly scales, and these copies produced the same vivid colors as the butterflies' wings.

As well as helping scientists gain a deeper understanding of the physics behind these butterflies' colors, being able to mimic them has promising applications in security printing.

"These artificial structures could be used to encrypt information in optical signatures on banknotes or other valuable items to protect them against forgery. We still need to refine our system but in future we could see structures based on butterflies' wings shining from a £10 note or even our passports," he says.

Unlocking the nanostructure of butterfly wings also holds promise in other areas. As we reported last year, researchers hope to develop various optically-active structures, such as optical diffusers or coverings that maximize solar cell absorption by replicating the biotemplate of butterfly wings.

Intriguingly, the butterfly may also be using its colors to encrypt itself – appearing one color to potential mates but another color to predators.

Kolle explains, "The shiny green patches on this tropical butterfly's wing scales are a stunning example of nature's ingenuity in optical design. Seen with the right optical equipment these patches appear bright blue, but with the naked eye they appear green.

"This could explain why the butterfly has evolved this way of producing color. If its eyes see fellow butterflies as bright blue, while predators only see green patches in a green tropical environment, then it can hide from predators at the same time as remaining visible to members of its own species."

The results of the team’s research appear in the journal Nature Nanotechnology.