Butterfly Beauty Perfected by Defects

JAMES SCHLETT, EDITOR, james.schlett@photonics.com

Having trouble accepting your physical flaws, whether they are cellulite or blemishes? Try being like a butterfly and let your “defects” help you shine.

New findings published in Science Advances on the microscopic structures on the wings of the Kaiser-i-Hind butterfly (Teinopalpus imperialis) are providing insights into how these insects appear so colorful. Using a powerful x-ray microscope at the Advanced Photon Source of the U.S. Department of Energy (DOE), researchers studied the light-scattering photonic crystals found in the scales of a wing.

The Kaiser-i-Hind butterfly (Teinopalpus imperialis). Courtesy of Anagoria: Museum für Naturkunde Berlin.

While scientists have long known that these photonic crystals create an iridescent appearance, the researchers from University of California, San Diego; Yale University; and the DOE’s Argonne National Laboratory found their scales appear as a patchwork of lattices or domains with varying orientations. This orientation is believed to contribute to the wings’ ability to brilliantly sparkle while maintaining uniform color.

“In a typical photonic crystal the reflected wavelength is dependent on the direction at which you look at it and ultimately is connected to the periodicity of the photonic structure along this particular direction. We have shown that in Teinopalpus imperialis the photonic crystals are oriented with the respect to the scale surface,” said Andrej Singer, of UC San Diego, who led the research.

A close-up look at a butterfly wing reveals a patchwork map of lattices with slightly different orientations (colors added to illustrate the domains). These structures result in the wings’ sparkling appearance. Courtesy of Ian McNulty.

Additionally, the researchers discovered edge dislocations, or defects, in the photonic crystals, which may be the result of strain relaxation as they grow. These defects, which had never been seen before, affect the wings’ brightness because there are millions of these domains and each one is reflecting light differently. Singer pointed out that, at least with artificial photonic crystals, it has been shown defects can localize light, enabling them to appear brighter.

“The discovery of the edge dislocations in the biological photonic crystals is particularly remarkable because artificially manufactured topological defects in photonic crystals lead to interesting optical properties such as Anderson localization of light,” the researchers said in the paper.

An optical micrograph of a single wing scale, where (a) shows one set of hexagonal peaks and an indication of single-domain scattering, and (b) shows two sets of peaks with varying orientations. Courtesy of Andrej Singer.

The findings could help develop photonic devices with certain mechanical, structural and photonic properties. Singer said, “Defects can be used to define the propagation direction of light in photonics crystals, and we hope that a thorough understanding of the defect engineering in natural photonics crystals may have an impact on defect engineering in artificial structures.”