Beetle Sports Photonic Crystal Coat

Why are some coleoptera beetles blue? Because light striking the beetle's external hard parts undergoes destructive interference. Precisely how this happens is now being studied quantitatively by a team of scientists in Namur, Belgium. Electron microscope pictures of the beetle's scaly cuticle, online at Physics News Graphics, help to explain that each scale is made of alternating layers of pure chitin (high index of refraction) and mixtures of chitin and air (low index of refraction).

The resulting structure is a photonic crystal: because of wave interference, light of certain frequencies are excluded. In this case blue light is forbidden from being absorbed by the animal's shell; all blue light is reflected while other frequencies are absorbed in the cuticle, and the creature consequently has a blue appearance.

Artificial photonic crystals have been studied for many years. Often featuring a honeycomb structure, these materials are to light waves what semiconductors are to electrons: transmission in certain energy bands is permitted while other bands are forbidden. Lord Rayleigh in 1918 was the first to suggest that the iridescence of some insects arose from interference effects. And by now the photonic-crystal effect is known to occur naturally in many places, such as the opalescence of weevils and the striking colors in the peacock's tail feathers.

According to Jean Pol Vigneron at the University of Namur in Belgium (jean-pol.vigneron@fundp.ac.be), lessons learned from the beetle scale's iridescence might be applied to the manufacture of paint, clothes, paper and in simplifying the kinds of windows and windshields that currently employ interference effects. The beetle's optics might also help in designing micro-fabricated displays in which different colors could be obtained through the clever reflection, rather than by emission, of light.

Vigneron et al., Physical Review E, December 2005