In Qing dynasty China, artisans augmented decorative pieces by incorporating iridescent kingfisher feathers—a technique known as tian-tsui. Scientists at Northwestern University's Center for Scientific Studies in the Arts have used high-energy X-ray imaging to achieve unprecedented nanoscale resolution of the unique structure of those feathers, presenting their findings at the annual meeting of the American Association for the Advancement of Science.
As previously reported, nature is the ultimate nanofabricator. The bright iridescent colors in butterfly wings, soap bubbles, opals, or beetle shells don’t come from any pigment molecules but from how they are structured—naturally occurring photonic crystals. In nature, scales of chitin (a polysaccharide common to insects), for example, 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. In the case of kingfisher feathers, the color is due to the microscopic ridges that cover the parallel rows of keratin strands that grow along the central shaft.
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. They are used in optical communications as waveguides and switches, as well as in filters, lasers, mirrors, and various anti-reflection stealth devices.