Page last updated at 15:14 GMT, Friday, 24 July 2009 16:14 UK

Green beetle's super-shiny secret

Victoria Gill
Science reporter, BBC News

Green jeweled beetle
Under left circularly polarised light, the same beetle looks "super-green"

The stunning metallic green sheen of the "jewelled beetle" is produced by microscopic cells in its exoskeleton.

Researchers reporting in the journal Science have revealed that, with no green pigments, these structures make the beetle appear iridescent green.

These cells, they said, were almost identical to hi-tech liquid crystals.

The scientists found that the structures make the beetle appear very intensely green under light that is polarised in one direction.

Polarisation can be thought of as the orientation of light waves. And white light, or natural light, is essentially a mixture of randomly polarised light.

Jeweled beetle (P. Vukusic)
Helix structures in its exoskeleton makes the beetle appear green

The structures in the jewelled beetle's (Chrysina gloriosa's) exoskeleton are spiral - or helical - so they reflect light that is polarised in the same direction as that spiral.

Without any need for pigments, these "photonic" structures manipulate light to make the beetle appear bright green.

Lead author Mohan Srinivasarao, from the Georgia Institute of Technology in Atlanta, had previously studied cholesteric liquid crystals, which are used in hi-tech reflective displays.

"The two systems were remarkably similar," he told BBC News. "When I first looked at the beetle's exoskeleton (with the microscope) I thought, 'I've seen this somewhere before'."

Light trick

Having studied the exoskeleton using a highly sensitive confocal microscope, he and his team reconstructed a 3D map of its underlying structure.

They discovered that the green colour is produced by the pitch, or size, of the helices, as the distance over which the helix turns 360 degrees controls what colour of light it reflects.

Dr Srinivasarao explained that because this pitch was comparable to the wavelength of green light, it was mostly green light that was reflected and therefore seen.

He and his colleagues demonstrated this effect with a dramatic display - taking photographs of the beetle under left and right circularly polarised light.

Jeweled beetle (P. Vukusic)
Nature can show scientists how to manipulate light

Under left circularly polarised light, which matched the direction of the helices, the beetles appeared "super-green", because, as Dr Srinivasarao explained, "there weren't that many other wavelengths of light reflected back at you".

"Under natural light, the maximum intensity of the green colour can only ever be 50%," he said.

Scientists already are studying ways to use materials that have properties similar to the beetles.

Researchers in New Zealand, for example, are studying beetles to produce a thin, solid mineral, magnesium oxide, which can be ground into flakes and potentially used as a currency security measure.

This finding reveals another natural template that materials scientists can study and copy.



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