Butterfly wings inspire counterfeit-proof material

Harvard researchers have taken inspiration from 'colour-changing' wings of a butterfly to develop an artificial photonic material that is difficult to recreate and could allow for counterfeit-proof banknotes.
Researchers were inspired by the male Pierella luna butterfly of Latin America which uses its wings to perform an advanced optical trick known as reverse colour diffraction in order to attract a mate.
Owing to the microstructure of its wings - made up of tiny scales curled slightly upward at the end to diffract light - the butterfly appears to change colour when it's viewed from different angles.
Researchers figured out a way to use artificial photonic materials to mimic the Pierella luna's attractive light show.

They created what's known as a diffraction grating, a surface that splits white light into its individual wavelengths of colour and sends those colours travelling in different directions.
When the photonic material is viewed from one angle, it looks to be one colour, but from a different angle, the colour appears to change, according to study co-author Mathias Kolle, a member of the Harvard research team and an assistant professor of mechanical engineering at the Massachusetts Institute of Technology (MIT).

The new photonic material's microstructure could make it valuable for a range of applications, 'LiveScience' reported.
The superthin, transparent material consists of an array of microscopic plates, or scales, that mimic those that make up the Pierella luna's wing.

Each plate is about 18 micrometres tall - about one-fifth the diameter of a human hair - and each features a scalloped, or ridged, edge. The ridges on each plate look like tiny lines running through the material and are spaced about 500 nanometres apart.
All of these features - both the plates themselves and the ridges running through them - can be manipulated to create different optical effects, Kolle said.
The material is also fairly difficult to recreate, Kolle said, and believes it could be used to make more secure banknotes or passports.
If used for such purposes, it would lend these printed objects a so-called "optical signature," he said.

The ability to tune the material to specific wavelengths could also make it valuable for producers of solar cells or light-emitting diodes (LEDs) used inside consumer electronic devices.
Both of these products need to be as efficient as possible in the ways they absorb or release light, the researchers said.