X-ray snap of butterfly wings unveils physics of colour

Scientists, using x-ray imaging techniques to study wings of the Emperor of India butterfly, have discovered two physical attributes that make them so bright and colourful.
"Over millions of years, butterflies have evolved sophisticated cellular mechanisms to grow brightly coloured structures, normally for the purpose of camouflage as well as mating," says Oleg Shpyrko, an associate professor at University of California, San Diego in the US.
"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 colour," said Shpyrko.
However, how these complex optical structures are assembled in a way that make them so bright and colourful remained a mystery.

In an effort to answer that question, Shpyrko and Andrej Singer, a postdoctoral researcher in his laboratory, went to the Advanced Photon Source at the Argonne National Laboratory in the US, which produces coherent x-rays very much like an optical laser.
By combining these laser-like x-rays with an advanced imaging technique called "ptychography," the researchers developed a new microscopy method to visualise the internal nanostructure of the tiny "scales" that make up the butterfly wing without the need to cut them apart.

The researchers said that their examination of the scales of the Emperor of India butterfly, Teinopalpus imperialis, revealed that these tiny wing structures consist of "highly oriented" photonic crystals.

"This explains why the scales appear to have a single colour," said Singer.
"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," he added.
These crystal dislocations or defects occur, the researchers say, when an otherwise perfectly periodic crystal lattice slips by one row of atoms.
"Defects may have a negative connotation, but they are actually very useful in improving materials," said Singer.
"In the evolution of butterfly wings, it appears nature learned how to engineer these defects on purpose," he added.
The study was published in the journal Science Advances.