The bright iridescent colors in butterfly wings or beetle shells come not from any pigment molecules but from how wings are formed — a naturally occurring example of what physicists call photonic crystals. Scientists can make their own colored structural materials in the lab, but it can be difficult to scale the process for commercial applications without sacrificing optical precision.
Now MIT scientists have adapted 19th-century holographic technology to develop chameleon-like films that change color when stretched. The method can be easily scalable while maintaining nanoscale optical resolution. They describe their work in a new research paper published in the journal Nature Materials.
In nature, chitin (a common polysaccharide in insects) scales are arranged like roof tiles. Essentially, diffraction gratings form, except for photonic crystals that produce specific colors, or wavelengths, of light, while diffraction gratings will produce the entire spectrum, just like a prism. Also known as optical bandgap materials, photonic crystals are “tunable,” meaning that they are precisely arranged to block certain wavelengths of light while allowing others to pass through. Adjust the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength.
Creating structural colors such as those found in nature is an active area of materials research. For example, optical sensing and visual communication applications may benefit from structurally colored materials that change hue in response to mechanical stimuli. There are many techniques for making such materials, but none of these methods can control the structure on the small scales required and scale it 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 similar in some ways to how nature produces structural color. Dive into the history of holography and learn about the late 19th century color photography technique invented by physicist Gabriel Lippmann.
As mentioned earlier, Lippmann became interested in developing a method for stabilizing the colors of the solar spectrum on a photographic plate in 1886, “in which the image remains stable and can survive in daylight without deterioration.” He achieved this goal in 1891, producing color photographs of a stained-glass window, a bowl of oranges, and a stained parrot, as well as landscapes and portraits—including a portrait.
Lippmann’s color photography process involved projecting the optical image as normal onto a photographic plate. Drop was carried out through a glass plate coated with a clear emulsion of 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, struck the mirror, and was reflected back into the emulsion.