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Beetle perfects artificial opal growth

16 Dec 2003

Perfect synthetic opals could be produced by copying a technique used by a small beetle, say researchers in this week’s Nature.

Researchers at the University of Oxford, UK, have discovered what they say is the first example of an opal-type photonic crystal structure in an animal. The intricate three-dimensional structure occurs in small beetle just a few centimetres long. If the beetle’s self-assembly process can be emulated, the team says it could lead to a simpler and cheaper way of producing artificial opals. (Nature 426 786).

“The interesting thing is that this has been found in a living organism,” said researcher Andrew Parker. “This means that the beetle must have cells that are making the structure, which gives us something to copy. There is a whole manufacturing process going on which starts with a series of chemicals and ends with a perfect opal structure.”

The opal-making animal is the weevil Pachyrhynchus argus, a small beetle found in forests in north-eastern Australia. Its body appears a metallic green color from all angles thanks to a photonic crystal structure that resembles opal.

The vivid color comes courtesy of thin, flat scales which occur in patches over the beetle’s body. The scales consist of an outer shell and an inner structure that contains layers of 250 nm diameter transparent spheres.

“The spheres are arranged in hexagonal-close packing order,” explained Parker. “The scales contain the opal structure. There are tens of layers packed on top of each other in a single scale.”

The scales produce the green color by thin-film reflection. “Because we have stacks of spheres instead of flat layers, we have a three-dimensional structure where you can effectively form layers in many directions,” he said. “The reflections from each of these layers are superimposed and you get a color-averaging effect which appears green.”

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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