The bright, iridescent colors of butterfly wings or beetle shells come not from any pigment molecule, but from the way the wings are structured – a natural example of what physicists call photonic crystals. Scientists can make their own structural colored materials in the lab, but it can be difficult 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 up 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 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 bandgap materials, photonic crystals are “tunable,” meaning they are precisely ordered to block certain wavelengths of light while letting others pass. Alter the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength.
Creating structural colors like those found in nature is an active area of materials research. Optical sensing and visual communication applications, for example, would benefit from structurally colored materials that change hue in response to mechanical stimuli. There are several techniques for fabricating such materials, but none of these methods can both control the structure at the small scales required and extend beyond laboratory parameters.
Then co-author Benjamin Miller, an MIT graduate student, discovered an exhibit on holography at the MIT Museum and realized that creating a hologram was similar in some ways to how nature produces a structural color. He delved into the history of holography and discovered a late 19th century color photography technique invented by physicist Gabriel Lippmann.
As previously reported, Lippmann became interested in developing a means of fixing the colors of the solar spectrum on a photographic plate in 1886, “by which the image remains fixed and can remain in daylight without deterioration”. He achieved this 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 was to project the optical image as usual onto a photographic plate. The projection was made through a glass plate covered with a transparent emulsion of very fine grains of silver halide on the other side. There was also a mirror of liquid mercury in contact with the emulsion, so that the projected light passed through the emulsion, hit the mirror and was reflected back into the emulsion.