We all know how striking butterfly wings can be with their shimmering emerald greens, pearly whites and fiery oranges as they catch the light, but there’s something truly special about the male Pierella luna butterfly. In fact, this butterfly is so unique that it is inspiring a new generation of photonic materials that could be used to prevent counterfeiting.
Pierella luna may look a little drab at first glance, but males have an iridescent patch on their wings that appears to change color depending on the angle at which they are viewed. This optical trick is the result of microscopic ridges on the wing scales which behave as a diffraction grating. Diffraction gratings are a series of closely spaced slits on a surface that split white light into its individual wavelengths of color and send these colors in different directions, acting as a “super prism.” But what is interesting is that the ordering of the colors seen is the reverse of normal; usually, greater angles produce more red, but for the butterfly bigger angles result in more blue light being seen. This is known as reverse color diffraction.
The butterflies are able to achieve this optical trick because the scales on this unusual patch are curled upward at 90 degrees to the wing, meaning that the diffraction grating is perpendicular to the wing.
“There are a number of organisms that employ some kind of diffraction in nature to create colorful appearances, but none of them use the same approach as this butterfly,” MIT Professor Mathias Kolle told ABC News.
Kolle’s team and colleagues from Harvard saw the potential applications that a material capturing these unique features could have, so they set out to produce a photonic surface that is capable of mimicking this reverse color diffraction.
As described in the Proceedings of the National Academy of Sciences, to produce their bio-inspired material, they etched a series of tiny upright plates spaced 500 nanometers apart into transparent epoxy resin that mimic the scales of a Pierella luna’s wing. Each little plate also features tiny scallops, or ridges, on the edges of the surface which cause the diffraction. The researchers are also able to change the height and spacing between both the plates and the ridges to create different effects.
Because the material is difficult to recreate, it could be added to banknotes or passports to make them difficult to counterfeit. According to the researchers, the complex iridescent patch produced by the material could be a suitable replacement for the existing holograms used on these products. Furthermore, they could even be used to alter the way that light enters solar cells or leaves LEDs, improving efficiency, but they haven’t yet tested whether the materials could be used in such a way.