British and German researchers have developed a new type of flexible plastic film. These 'polymer opal films' can change colors under certain conditions. For example, they could easily help you to see if some kind of perishable food items are not good anymore because the packaging would change color. They also could be used to detect fake money or in other defense and aerospace applications. But read more...
You can see above an example of a polymer opal film (Credit: School of Physics and Astronomy, University of Southampton, via Softpedia).
At the University of Southampton, this research was led by Professor Jeremy Baumberg and the members of his Quantum Optoelectronics lab. Baumberg worked with other scientists at the Deutsches Kunststoff-Institut (DKI) in Darmstadt.
Now here is are some lengthy details about these films. "These 'polymer opal films' belong to a class of materials known as photonic crystals. Such crystals are built of many tiny repeating units, and are usually associated with a large contrast in the components' optical properties, leading to a range of frequencies, called a 'photonic bandgap,' where no light can propagate in any direction. Instead, these new opal films have a small contrast in their optical properties. As with other artificial opal structures, they are also 'self-assembling,' in that the small constituent particles assemble themselves in a regular structure. But this self-assembly is not perfect, and though meant to be periodic, they have significant irregularities. In these materials, the interplay between the periodic order, the irregularities, and the scattering of small inclusions strongly affect the way the light travels through these films, just as in natural opal gem stones, a distant cousin of these materials. For example, light may be reflected in unexpected directions that depend on the light's wavelength."
And here is another paragraph from the Optical Society of America press release. "Photonic crystals have been of interest for years for various practical applications, most notably in fiber optic telecommunications but also as a potential replacement for toxic and expensive dyes used for coloring objects, from clothes to buildings. Yet much of their commercial potential has yet to be realized because the colors in manmade films made from photonic crystals depend strongly on viewing angle. If you hold up a sheet of the opal film, Baumberg explains, 'You'll only see milky white, unless you look at a light reflected in it, in which case certain colors from the light source will be preferentially reflected.' In other words, change the angle, and the color changes."
This research work has been published in Optics Express under the name "Nanoparticle-tuned structural color from polymer opals" (Vol. 15, Issue 15, Pages 9553-9561, July 2007). Here is a link to the abstract. "The production of high-quality low-defect single-domain flexible polymer opals which possess fundamental photonic bandgaps tuneable across the visible and near-infrared regions is demonstrated in an industrially-scalable process. Incorporating sub-50nm nanoparticles into the interstices of the fcc lattice dramatically changes the perceived color without affecting the lattice quality. Contrary to iridescence based on Bragg diffraction, color generation arises through spectrally-resonant scattering inside the 3D photonic crystal. Viewing angles widen beyond 40º removing the strong dependence of the perceived color on the position of light sources, greatly enhancing the color appearance. This opens up a range of decorative, sensing, security and photonic applications, and suggests an origin for structural colors in Nature."
For more information, you can read the full text of this technical paper (Link on the abstract page, PDF format, 9 pages, 572 KB). You also can read "Polymer opal films shed light on spoiled foods" (Ahmed ElAmin, PackWire.com, July 24, 2007) or "Polymer Opal Films Identify Counterfeit Money and Rotten Food" (Lucian Dorneanu, Softpedia, July 26, 2007).
Sources: Optical Society of America, via EurekAlert!, July 23, 2007; and various websites
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