The bright iridescent colors in butterfly wings or beetle shells come not from pigment molecules, but from how the wings are structured — a naturally occurring example of what physicists call photonic crystals. Scientists can create their own structural colored materials in the lab, but it can be challenging to scale the process up 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 maintaining 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 that photonic crystals will only produce specific colors or wavelengths of light, while a diffraction grating will produce the entire spectrum, just like a prism. Also known as photonic bandgap materials, photonic crystals are “tunable,” meaning they are arranged precisely to block certain wavelengths of light while allowing others to pass. Change the structure by changing the size of the tiles and the crystals become sensitive to a different wavelength.
Creating structural colors as found in nature is an active area of materials research. For example, applications for optical sensing and visual communication would benefit from structurally colored materials that change color in response to mechanical stimuli. There are several techniques for making such materials, but none of these methods can both master the structure at the required small scale and scale it up outside of laboratory settings.
Then co-author Benjamin Miller, a graduate student at MIT, discovered an exhibit on holography at the MIT Museum and realized that creating a hologram was in some ways similar to how nature produces structural colors. He delved into the history of holography and learned about a late 19th century color photography technique invented by physicist Gabriel Lippmann.
As we reported before, in 1886 Lippmann became interested in developing a way to record the colors of the solar spectrum on a photographic plate, “which would keep the image fixed and able to remain in daylight without deterioration.” He achieved that goal in 1891, producing color images of a stained glass window, a bowl of oranges, and a colorful parrot, as well as landscapes and portraits, including a self-portrait.
Lippmann’s color photography process involved projecting the optical image onto a photographic plate as usual. The projection was done through a glass plate covered with a transparent emulsion of very fine silver halide grains on the other side. There was also a liquid mercury mirror in contact with the emulsion, so the projected light traveled through the emulsion, hit the mirror and was reflected back into the emulsion.