gold color made from naturally occurring photonic crystal structures in scales made of chitin
Posted by Jennifer Ouellette: […] Butterfly wings get their color from naturally occurring photonic crystal structures in scales made of chitin, a polysaccharide that shows up in all kinds of insects. Those scales are arranged like tiles on a roof, except they measure a mere tens of micrometers across. Last September, New Scientist reported that a group of researchers have measured the structure and optical characteristics of the photonic crystals in butterfly wings for the very first time. They did it by studying electron microscope images of the scales. It turns out that each side of the wing contains different photonic structures: a metallic blue produced by single crystals, and a dull-ish green that results from a more random arrangement of crystals. Precisely ordering the lattice structure is critical to achieving the most brilliant colorful effects — and to controlling the propagation of light at the desired frequencies. Which is why telecommunications applications rely on manmade photonic crystals rather than nature’s more random arrangements.
Natural photonic crystals
Still, butterfly wing crystals can produce green, yellow and blue colors, depending on their overall effect, and the researchers managed to generate red reflections as well. That’s significant because such a palette could be used in flat panel displays, simply by mounting an array of crystals only tiny MEMS arms to change their orientation. So any given “pixel” could produce red, green or blue. A September 1, 2006 paper in Optics Letters by a team of Swiss scientists described a similar approach using diffraction gratings and piezoelectric polymers (which contract whenever an electric voltage is applied) to faithfully reproduce a fuller range of colors than can currently be achieved in conventional displays, whether they be standard TVs, LCDs, or plasma screens. (For instance, they can’t reproduce the blues observable in the sky or in the sea.) Manuel Aschwanden of the Swiss Federal Institute of Technology in Zurich headed the project, and described the grating as having one side molded into something that looks for all the world like microscopic pleated window shades.
More frivolously, copying the structure of butterfly wings is giving rise to spiffy new kinds of make-up, giving a whole new meaning to the term “butterfly effect.” For instance, L’Oreal offers eye shadow, lipstick and nail polish featuring these iridescent effects, bringing nature’s beauty to the cosmetics counter. This is achieved by stacking nanoscale layers of materials like mica, silica or liquid crystals, of varying thickness to give each material a specific refractive index. For instance, a stack 80 nanometers high produces blue, while one 120 nanometers high produces red. In the package, though, the stuff just looks white; the colors appear when the makeup is applied and exposed to light. There are the usual concerns about using nanoparticles in cosmetics, when little is known to date about potential health risks, but that hasn’t dampened the enthusiasm for such novelties. Yet.
Researchers at the University of Toronto have developed a new elastic type of photonic crystal that changes color with the application of pressure. It also mimics the structure of butterfly wings and opal (the gemstone is another common example of a naturally occurring photonic crystal): it resembles a 3D honeycomb. They hope to develop the material further in hopes of using it to, for example, capture full-color fingerprints. The obvious advantage is the enhanced contrast and sensitivity to detail, making it easier to analyze prints for identification purposes. But any impression picked up by the elastic photonic crystal is visible immediately in bright hues, with no need to first convert that raw data into electrical signals for computer analysis. The Toronto material could also be used as pressure sensors in consumer electronics or airbag deployment — or just for children’s toys.
Imagine a toddler being able to squeeze a toy and watch the color change right in front of his/her eyes! Imagine the wonder the child would feel, especially when s/he was old enough to realize that it wasn’t magic, but a one that arises from Nature itself, that man has seen fit to copy and put to good use. I think even Hopkins would be suitably impressed at what the scientific study of a simple butterfly’s wing has wrought. So it seems fitting to close by quoting another Hopkins’ poems, “God’s Grandeur.”
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