The bright iridescent colors in butterfly wings or beetle shells don't come from any pigment molecules but from how the wings are structured—a naturally occurring example of what physicists call photonic crystals. Scientists can make their own structural colored materials in the lab, but it can be challenging to scale up the process for commercial applications without sacrificing optical precision.
Now MIT scientists have adapted a 19th-century holographic photography technique to develop chameleon-like films that change color when stretched. The method can be easily scaled while preserving nanoscale optical precision. They described their work in a new paper published in the journal Nature Materials.
In nature, scales of chitin (a polysaccharide common to insects) are arranged like roof tiles. Essentially, they form a diffraction grating, except photonic crystals only produce specific colors, or wavelengths, of light, while a diffraction grating will produce the entire spectrum, much like a prism. Also known as photonic band gap materials, photonic crystals are "tunable," which means they are precisely ordered to block certain wavelengths of light while letting others through. Alter the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength.
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