Harvard University researchers have devised an optical coating that changes color when its thickness is varied by just a few atoms.
Scientists at the Harvard School of Engineering and Applied Sciences found that by essentially spray-painting an ultra-thin germanium coating onto gold, the surface color can be changed.
Mikhail Kats, Romain Blanchard and Patrice Genevet
|Gold films colored with nanometer-thick layers of germanium show a range of color possibilities.|
The findings were recently published in the Nature Materials Journal.
Possible applications of the coating range from jewelry to solar photovoltaic cells, the researchers say.
Coating of Many Colors
Led by Frederico Capasso, the Robert L. Wallace professor of applied physics and Vinton Hayes senior research fellow in electrical engineering at SEAS, the group showed that adding layers of germanium to the surface of a gold sample changes its color.
With a 7 nanometer-thick coating of germanium, the gold turns pink. Adding another 4 nm layer turns the coating violet; and another 4 nm makes the coating dark blue. (4 nm is less than 15 atoms thick.)
The researchers have also performed the same treatment on silver, making it appear gold, as well as a range of pastel colors.
Students Mikhail Kats, Romain Blanchard and Patrice Genevet were all members of Capasso’s team.
How does it Work?
The absorbing germanium coating traps certain colors of light while flipping the phase of others so that the crests and troughs of the waves line up closely and reflect one pure, vivid color, according to the researchers.
“We are all familiar with the phenomenon that you see when there’s a thin film of gasoline on the road on a wet day, and you see all these different colors,” said Capasso.
Eliza Grinnell / SEAS Communications
|Romain Blanchard, Mikhail Kats and Patrice Genevet are members of the research group that discovered the optical coating.|
Those colors appear because the lightwaves’ crests and troughs interfere with each other as they pass through the oil into the water below and reflect back up into the air, according to the researchers. Some colors get a boost in brightness, while others are lost.
For centuries, it was thought that that type of “thin-film interference effect” could occur only in transparent or “lossless” material, the researchers said.
But now, the team says it has exploited the completely overlooked optical phenomenon.
“In my group, we frequently reexamine old phenomena, where you think everything’s already known,” Capasso said. “If you have perceptive eyes, as many of my students do, you can discover exciting things that have been overlooked.”
Applying the Coating
The germanium film is applied through standard manufacturing techniques—lithography and physical vapor deposition, which the researchers compare to stenciling and spray-painting—so with only a minimal amount of material (a thickness between 5 and 20 nanometers), elaborate colored designs can easily be patterned onto any surface, large or small.
Patent, Commercialization in the Works
Harvard’s Office of Technology Development has filed a patent application and is working with the Capasso lab to pursue the commercialization of the new technology, either through a start-up company or through licensing to existing companies.
Application areas being explored include consumer products and optical devices, such as filters, displays, photovoltaics, detectors, and modulators.
This work was supported in part by the U.S. Air Force Office of Scientific Research and a National Science Foundation (NSF) Graduate Research Fellowship. Some of the work was performed at the Harvard Center for Nanoscale Systems, a member of the NSF-supported National Nanotechnology Infrastructure Network.