Scales from the wings of the Emperor of India butterfly consist of “highly oriented” photonic crystals. (Photos by Andrej Singer, UC San Diego) The two key physical attributes that contribute to the brightness and colorfulness of a butterfly wing have been discovered by a team of scientists by visualizing its internal nanostructure.
Over millions of years, butterflies have evolved sophisticated cellular mechanisms to grow brightly colored structures, normally for the purpose of camouflage as well as mating. It’s been known for a century that the wings of these beautiful creatures contain what are called photonic crystals, which can reflect light of only a particular color.
Oleg Shpyrko, Associate Professor of Physics, UC San Diego
However, the exact assembly of the complex optical structures that makes the butterfly wing bright and colorful is still unknown. In an attempt to solve this mystery, Shpyrko along with Andrej Singer, who is a postdoctoral researcher at his laboratory, used the Advanced Photon Source at the Argonne National Laboratory in Illinois. This advanced photon source generates coherent X-rays in the same manner of an optical laser.
A new microscopy method was developed by the UC San Diego physicists, along with the physicists at Yale University and the Argonne National Laboratory. The researchers integrated the laser-like X-rays into an advanced imaging technique known as “ptychography” in order to develop this new microscopy technique. Using the new method, the internal nanostructure of the tiny “scales” in a butterfly wing was visualized by keeping the wing intact.
The scales of the Emperor of India butterfly, Teinopalpus imperialis, were examined by the researchers. In a recent publication of the Science Advances journal, it was reported by the researchers that the tiny structures present in a butterfly wing comprise photonic crystals that are “highly oriented.”
This explains why the scales appear to have a single color. We also found through careful study of the high-resolution micrographs tiny crystal irregularities that may enhance light-scattering properties, making the butterfly wings appear brighter.
Andrej Singer, Postdoctoral Researcher, UC San Diego
According to the researchers, the slippage of a perfect periodic crystal lattice by one row of atoms causes crystal dislocations or defects.
Defects may have a negative connotation, but they are actually very useful in improving materials. For example, blacksmiths have learned over centuries how to purposefully induce defects into metals to make them stronger. ‘Defect engineering’ is also a focus for many research teams and companies working in the semiconductor field. In photonic crystals, defects can enhance light-scattering properties through an effect called light localization. In the evolution of butterfly wings, it appears nature learned how to engineer these defects on purpose.
Singer, Postdoctoral Researcher, UC San Diego
Leandra Boucheron and Sebastian Dietze of UC San Diego, David Vine and Ian McNulty of Argonne National Laboratory, and Katharine Jensen, Eric R. Dufresne, Richard Prum and Simon Mochrie of Yale were also involved in this research. The U.S. Department of Energy’s Office of Science and Office of Basic Energy Sciences funded this research project.