Beetle ‘Bling’ Reveals Optical Secrets

Deep in Costa Rica’s tropical rain forests live two lustrous beetle species that are giving optics researchers new insights into the way biology can recreate the appearance of metals by means of reflected light.

Scientists at the University of Costa Rica found that the gold of the Chrysina aurigans beetle and the silver of the Chrysina limbata beetle are created by the unique structural arrangements of many dozens of layers of exoskeletal chitin in the elytron, a hardened forewing that protects the delicate hindwings that are folded underneath. A paper about the discovery appears in the first issue of the OSA's newest open-access journal, Optical Materials Express.

The beetles were captured in the university's Alberto Brenes Mesén Biological Reserve, a tropical rain forest environment.

Chrysina aurigans (left) and Chrysina limbata (right) beetle specimens display their brilliant golden and silver appearance, respectively. (Image: Eduardo M. Libby)

"The metallic appearance of these beetles may allow them to be unnoticed, something that helps them against potential predators," said William E. Vargas, physicist and study leader. The surface of their elytra "reflects light in a way that they look as bright spots seen from any direction. In a tropical rain forest, there are many drops of water suspended from the leaves of trees at ground level, along with wet leaves, and these drops and wet leaves redirect light by refraction and reflection respectively, in different directions. Thus, metallic beetles manage to blend with the environment."

To interpret the cause of this metallic look, Vargas and his team assumed that a sequence of layers of chitin appears through the cuticle, with successive layers having slightly different refractive indices. In these beetles, the cuticle, which is just 10-millionths of a meter deep, has some 70 separate layers of chitin — a nitrogen-containing complex sugar that creates the hard outer skeletons of insects, crabs, shrimps and lobsters. The chitin layers become progressively thinner with depth, forming a so-called "chirped" structure.

"Because the layers have different refractive indices, light propagates through them at different speeds. The light is refracted through — and reflected by — each interface, giving in particular phase differences in the emerging reflected rays," Vargas said. "For several wavelengths in the visible range, there are many reflected rays whose phase differences allow for constructive interference. This leads to the metallic appearance of the beetles."

This is similar to the way in which a prism breaks white light into the colors of the rainbow by refraction, but in the case of these beetles, different wavelengths, or colors of light, are reflected back more strongly by different layers of chitin. This creates the initial palette of colors that enable the beetles to produce their distinctive hues. The mystery the researchers still needed to understand in more detail, however, was how the beetles could so perfectly create the structure causing the brilliant metallic tones of silver and gold.

This device is used to carry out the direct reflectance measurements under normal incidence of nonpolarized light on the elytron (forewing) of a beetle. The allowed displacements and rotations of the probe holder allow the researchers to focus the beam on the beetle’s elytron perpendicularly. (Image: Optics Express/University of Costa Rica)

Using a device they specially designed to measure the reflection of light when it strikes the curved surface of the beetles' elytra, Vargas and his colleagues found that as light strikes the interface between each successive layer (the first interface being the boundary between the outside air and the top chitin layer), some of its energy is reflected, and some is transmitted down to the next interface.

"This happens through the complete sequence of interfaces," he said.

Because a portion of the light is reflected, it combines with light of the exact same wavelength as it passes back through layer upon layer of chitin, becoming brighter and more intense. Ocean waves can exhibit the same behavior, combining to produce rare but powerful rogue waves. In the case of the beetles, this "perfect storm" of light amplification produces not only the same colors but also the striking sheen and glimmer that we normally associate with fine jewelry.

In the two beetle species, interference patterns are produced by slightly different wavelengths of light, thus producing either silver or gold colors.

"For the golden-like beetle, the constructive interference is found for wavelengths larger than 515 nm, the red part of the visible wavelength range, while for the silver-like beetle, it happens for wavelengths larger than 400 nm — that is, for the entire visible wavelength range," he said.

"The detailed understanding of the mechanism used by the beetles to produce this metallic appearance opens the possibility to replicate the structure used to achieve it and thus produce materials that, for example, might look like gold or silver but are actually synthesized from organic media," said Vargas.

This potentially could lead to new products or consumer electronics that can perfectly mimic the appearance of precious metals. Other products could be developed for architectural applications that require coatings with a metallic appearance. Vargas notes that in the solar industry, for example, chirped multilayer reflectors could be used as back layers supporting the active or light-absorbing medium, to improve the absorption of the back-reflected light.

For more information, visit: www.opticsinfobase.org