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Beetle masters optics

Study finds cells in the shell twist light in a particular way

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2:37pm, July 23, 2009
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Jeweled beetles’ resplendent shells have physicists green with envy. Intricate arrangements of cells on the beetles’ outer layers manipulate light in a special way, a study published online July 23 in Science reveals. Understanding the shell’s structure might prove useful for designing new optical devices.

Chrysina gloriosa get their greenish color from microstructures in their exoskeleton rather than from pigment. Study coauthor Mohan Srinivasarao of the Georgia Institute of Technology in Atlanta and his colleagues have found that these structures are also responsible for the beetles’ light-bending tricks. Light hitting the shell is reflected by the microstructures, and these reflections create an electric field that forms a clockwise helix. Humans cannot see this property — known as left-handed circular polarization — but can see a green hue.

To find out how the beetle can shape light in this distinctive way, Srinivasarao and his colleagues examined the beetle’s exoskeleton under high-powered microscopes.

“The details are just stunning,” Srinivasarao says. “There are all of these patterns on the beetle that you see when you look under a microscope.”

Under the scope, the beetle’s body appears as a “richly decorated mosaic of cusps and color,” the authors report. Cells with five, six and seven sides create intricate clusters. Cells gradually twist as layers get deeper, creating microstructures that look like snail shells turned on their sides. These regions — called focal conic domains — twist the reflected light to make it circularly polarized.

Researchers don’t yet know why the beetle has this light-shaping property. “Is there a point to it? It’s a very appropriate question,” says Pete Vukusic, a physicist who wrote a commentary on the new study in the same issue of Science. Such specialized light may represent a form of communication with other beetles, speculates Vukusic, of the University of Exeter in England.

Stealing the beetles’ tricks may also help researchers design materials with desirable optical properties, comments Michael Barnes of the University of Massachusetts Amherst. Although it’s too early to say what specific devices might be created with inspiration from the beetle shell, “the scientific goal is to understand the 'what' and the 'how' of micro- and nanoscale structures in natural systems,” he says, “so that we can design our own systems for specific purposes.”

What’s more, people may be able to take a lesson from the beetle in manufacturing such materials using microstructures. “The beetle doesn’t use any harmful chemicals,” Srinivasarao says. “Here is a creature that’s figured out how to do this very easily.”

Structurally, the beetle’s exoskeleton is very similar to a type of man-made material called a cholesteric liquid crystal. “Once I looked at these beetles, [the structure] looked like something I had seen before,” Srinivasarao says. Some types of liquid crystals are used in computer monitors, televisions and other displays. Cholesteric liquid crystals, which can be designed to change color with temperature and pressure, are used to pinpoint stressed areas on airplane wings, for example, Srinivasarao says.

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