Ultrathin Coatings Exploit Optical Phenomenon

By exploiting an overlooked optical phenomenon, ultrathin metal surfaces were made to shine vibrant hues. The coatings that change color with only a few atoms’ difference in thickness could provide new possibilities for sophisticated optical devices, consumer products and new visual arts techniques.

“Instead of trying to minimize optical losses, we use them as an integral part of the design of thin-film coatings,” said Mikhail A. Kats, graduate student and lead author of a study on the work. “In our design, reflection and absorption cooperate to give the maximum effect.”

Gold films colored with nanometer-thick layers of germanium. Federico Capasso’s lab at the Harvard School of Engineering and Applied Sciences (SEAS) discovered that atomically thin films can be tailored to reflect a particular range of dramatic, vivid colors using nanophotonics. Courtesy of Mikhail Kats, Romain Blanchard and Patrice Genevet.

The demonstration that atomically thin films can be tailored to reflect a particular range of dramatic, vivid colors is the third recent nanophotonics-related work to emerge from the lab of Harvard School of Engineering and Applied Sciences (SEAS) professor Federico Capasso (See also: Plasmon Wave Propagates for 80 µm with No Diffraction and Phase-Shifted Flat Lens Creates Perfect Image).

For centuries it was thought that thin-film interference effects could not occur in opaque materials.

“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, the Robert L. Wallace professor of applied physics and Vinton Hayes senior research fellow in electrical engineering at SEAS.

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. Some colors get a boost in brightness, while others are lost.

Romain Blanchard, Mikhail Kats and Patrice Genevet, members of Federico Capasso’s research group at Harvard SEAS. Courtesy of Eliza Grinnell, SEAS Communications.

His team exploited this effect using a light-absorbing germanium film that is much thinner than the wavelength of light, enabling one to still see large interference effects, Capasso said. “In this particular case, there was almost a bias among engineers that if you’re using interference, the waves have to bounce many times, so the material had better be transparent,” he said.

The absorbing germanium-coated gold sheet trapped certain colors of light while flipping the phase of others so that the crests and troughs of the waves lined up closely, reflecting one pure, vivid color.

A difference of only a few atoms’ thickness across the coating was sufficient to produce dramatic color shifts. The germanium film was applied using lithography and physical vapor deposition manufacturing, which the researchers compared to stenciling and spray painting. With only a minimal amount of material — a thickness between 5 to 20 nm — elaborate colored designs can easily be patterned onto any size surface.

“Just by changing the thickness of that film by about 15 atoms, you can change the color,” Capasso said. “It’s remarkable.”

The different colors in this photograph are a result of tiny variations in thickness: Just 10-15 atoms of germanium separate the pink color from the violet, and another 10-15 atoms change the color from violet to dark blue. Courtesy of Mikhail Kats and Lulu Liu.

The same treatment was also performed on silver, making it appear gold as well as a range of pastel colors.

Harvard’s Office of Technology Development has filed a patent application and is working with Capasso’s lab to commercialize the technology for optical devices such as photovoltaics, displays, filters, detectors and modulators.

“In my group, we frequently reexamine old phenomena, where you think everything’s already known,” he said. “If you have perceptive eyes, as many of my students do, you can discover exciting things that have been overlooked.”

The study appeared online in Nature Materials (doi: 10.1038/nmat3443).

For more information, visit: www.seas.harvard.edu