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Honolulu, Hawaii, United States, 2006/09/20 - Because they are ready made, common in nature, and because they show a very high complexity, biological photonic-crystal structures will be an essential tool for building a useful knowledge of inhomogeneous optical media.
Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. They can be engineered to produce a variety of optical filtering functions. The growing efforts of physicists and materials scientists to fabricate photonic (nano)crystals were motivated mainly by the potential application of these materials in optical computing, the manufacturing of more efficient lasers, and other exciting new phenomena, like those arising from the application of disturbances such as shock waves. The manufacturing of large-area photonic crystals operating in the visible spectrum is still a challenging and expensive task, given present-day laboratory techniques. However, as with so many other materials, nature has already found a solution. Because they are ready made, common in nature, and because they show a very high complexity, biological photonic-crystal structures will be an essential tool for building a useful knowledge of inhomogeneous optical media.
A group of researchers in Hungary (Research Institute for Technical Physics and Materials Science) and Belgium (Facultés Universitaires Notre-Dame de la Paix Namur) examined the two faces (ventral and dorsal) of the Brazilian butterfly Cynophrys remus. The dorsal face, metallic blue, is clearly conspicuous, while the ventral face, a dull pea green is most probably inconspicuous in the green of the habitat, i.e. a cryptic color.
"We showed that both were structural colors (from interferences, not pigments), so that it proves that light interference can produce dull colorations as well as bright metallic glances" Dr. Jean-Pol Vigneron, one of the researchers, explained to Nanowerk. "The way it happens is related to the amount of order and disorder found in the scales geometry, different on the ventral and dorsal faces: the dorsal face of the butterfly wings is made of scales which bear a single photonic monocrystal which spans the whole scale surface, very coherent, with both short-range and long-range orders; the ventral face has scales with many small photonic crystallites, with a few different orientations, providing a few distinct colors (yellow, green, blue-green) which mix to provide the optimized pea-green cryptic color found. The crystallites have short-range order, but the sizes and locations of the crystallites are somewhat random, providing a long-range disorder. This disorder leads to the dull appearance. The dull structural coloration is indeed surprising, because intuitively, structural, interferential origin has been generally related to bright metallic colors."
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By Michael Berger, Copyright 2006 Nanowerk LLC.