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Now, University of Utah chemists have discovered that nature already has designed photonic crystals with the ideal, diamond-like structure: They are found in the shimmering, iridescent green scales of a beetle from Brazil.

The study by Michael Bartl, an assistant professor of chemistry at the University of Utah and University of Utah chemistry doctoral student Jeremy Galusha and colleagues is published in the journal Physical Review Letters. The scales are made of chitin, which forms the external skeleton, or exoskeleton, of most insects and is similar to fingernail material.

The scales are affixed to the beetle’s exoskeleton. Each measures 200 microns (millionths of a meter) long by 100 microns wide.

Bartl says the beetle was interesting because it was iridescent regardless of the angle from which it was viewed — unlike most iridescent objects — and because a preliminary electron microscope examination showed its scales did not have the structure typical of artificial photonic crystals.

The Utah team’s study is the first to show that “just as atoms are arranged in diamond crystals, so is the chitin structure of beetle scales,” he says. Galusha determined the 3-D structure of the scales using a scanning electron microscope.

He cut a cross section of a scale, and then took an image of it. Then he used a focused ion beam — sort of a tiny sandblaster that shoots a beam of gallium ions — to shave off the exposed end of the scale, and then took another image, doing so repeatedly until he had images of 150 cross-sections from the same scale.

Then the researchers ‘stacked’ the images together in a computer, and determined the crystal structure of the scale material: a diamond-like architecture, but with building blocks of chitin and air instead of the carbon atoms in diamond, according to a University of Utah press release.

Next, Galusha and Bartl used optical studies and theory to predict optical properties of the scales’ structure. The prediction matched reality: green iridescence.

Many iridescent objects appear that way only when viewed at certain angles, but the beetle remains iridescent from any angle.

Bartl says the way the beetle does that is an “ingenious engineering strategy” that approximates a technology for controlling the propagation of visible light. — Our Bureau

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