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Beetles inspire tunable micromirrors

23 May 2007

The unusual outer shell of a South American beetle, which can control both the polarization and wavelength of reflected light, could lead to a new breed of tunable micromirrors.

A South American beetle with an unusual shell made up of stacked nanolayers could provide the inspiration for making tunable micromirrors for optical applications, say physicists in the UK. Sharon Jewell and a colleague at the University of Exeter have shown that the beetle's novel microstructure can control both the polarization and wavelength of reflected light – a first in the animal world.

Many members of the Plusiotic family of beetles have iridescent-looking shells that reflect only left-handed circularly polarized light. Some of these beetles have shells containing fibrous chitin embedded in a protein matrix – a structure that is very similar to the helicoidal microstructure in a so-called cholesteric liquid crystal.

In a cholesteric liquid crystal, all the molecules in one plane of the structure are aligned and the molecules in each successive plane are rotated slightly with respect to those in the previous plane. This creates a series of parallel helices within the liquid crystal, with the molecules gradually turning through a full circle (a distance known as the "pitch") over a certain length of the helix.

Recently, scientists have used the beetle Plusiotic resplendens as a blueprint to make tunable optical diodes for use in liquid crystal-based lasers. Using optical scanning and transmission electron microscopy (TEM), Jewell and colleagues have now studied the shell microstructure of several beetles, including P. boucardi, that have green-coloured outer shells (or elytra).

The researchers optically characterized the shells by using a spectrometer to measure the intensity of light reflected from the shell as a function of wavelength. They also carried out a more detailed analysis of the underlying beetle-shell structure using TEM on a cross-section of the shells.

Jewell and co-workers then compared the results from the microscopy studies, combined with the optical spectra, to data from models produced using a multilayer optics modelling code. The results suggest that the beetle's shell consists of an upper layer of cuticular wax followed by a helicoidal layer with a short pitch of around 310 nm. Underneath this layer is another helicoidal layer with a longer pitch of 370 nm.

The researchers also observed that the reflection of light from the beetle shell was finely tuned to three specific wavelengths of 519 nm (green), 588 nm (orange) and 620 nm (red).

"The structures studied may lead to the fabrication of similar, self-assembled microstructures through the use of polymerizable liquid crystals," Jewell told nanotechweb.org. "These may have applications in reflective optical displays."

The Exeter team now plans to recreate the beetle shell structures in the lab. "It would also be interesting to study the optical properties of the beetle's eyes to explore any possible link between this selective reflection and the creature's visual system," added Jewell.

The work was reported in the New Journal of Physics.

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