MIT scientists create color-changing film inspired by 19th century holography

Real-time video recording an 8×6-inch stretch of structural color pattern featuring a wreath in homage to the work of 19th century physicist Gabriel Lippmann.

The bright colors in butterfly wings or beetle shells come not from any pigment molecules, but from how they are structured—a naturally occurring example of what physicists call photonic crystals. Scientists can manufacture their own structurally colored materials in the laboratory, but it can be a challenge to scale up the process for commercial applications without compromising optical precision.

Now MIT scientists have adapted 19th-century holographic photography techniques to develop a chameleon-like film that changes color when stretched. This method can be easily scaled while maintaining nanoscale optical precision. They describe their work in a new paper published in the journal Nature Materials.

In nature, chitin scales (a common polysaccharide in insects) are arranged like tiles. Basically, they form a diffraction lattice, except that a photonic crystal produces only a certain color, or wavelength, of light, whereas 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 allowing others to pass. Change the structure by changing the size of the tiles, and the crystals become sensitive to different wavelengths.

Creating structural colors like those found in nature is an active area of ​​ingredient 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 them can control the structure at the small scale required and scale up outside of a laboratory setting.

Advertisement

Enlarge / Gabriel Lippmann in the Sorbonne laboratory for research physics.

Then co-author Benjamin Miller, a graduate student at MIT, came across an exhibit on holography at the MIT Museum and realized that making holograms was similar in some ways to how nature produces structural colors. He delved into the history of holography and learned about the late 19th century color photography technique invented by physicist Gabriel Lippmann.

As we previously reported, Lippmann became interested in developing a way of assigning the colors of the solar spectrum to photographic plates in 1886, “where the image is fixed and can remain in daylight without deterioration.” He achieved that goal in 1891, producing color images of stained glass windows, bowls of oranges, and colorful parrots, as well as landscapes and portraits—including self-portraits.

Lippmann’s color photography process involves projecting an optical image as usual onto a photographic plate. The projection is carried out through a glass plate which is coated with a transparent emulsion of very fine silver halide grains on the other side. There is also a liquid mercury mirror in contact with the emulsion, so that the projected light passes through the emulsion, hits the mirror, and is reflected back into the emulsion.

Real-time stretching of the structural color material integrated as a colorimetric pressure sensor in the bandage. This video was shot outdoors to show the strong color response under natural lighting.

#MIT #scientists #create #colorchanging #film #inspired #19th #century #holography