Researchers can now “paint” rough, flexible surfaces through the phenomenon of thin-film interference. A team at Harvard University demonstrated that ultrathin optical coatings can display strong optical interference effects on notebook paper. Such coatings comprise nanometer-thin, highly absorbing germanium films on top of reflecting gold substrates. Thin-film optical interference was found to persist when films were “deposited on substrates that have a large degree of roughness and inhomogeneity on micro- and nanoscales,” the researchers wrote. They added that many optical coatings feature “sophisticated multi-layer structures,” the majority of which have layers of transparent material that is about a quarter of the wavelength of incident light. Five pieces of paper, all coated with a 100-nm-thick layer gold, display different colors when coated with different thicknesses of germanium film. From top to bottom: a sheet with no germanium, one with a 7-nm coating, a 10-nm coating, 15 nm and 100 nm. The 100-nm germanium film is sufficiently thick that the observed color is that of bulk amorphous germanium. Courtesy of Mikhail Kats/Harvard University. “Normally when you think of thin-film interference, you think of smooth substrates,” said Mikhail Kats, a postdoctoral fellow at Harvard. “With increasing roughness, the effect is expected to fade and you just don’t get this coherent behavior.” Traditionally, thin films must be transparent and no thinner than 600 nm to exhibit this kind of interference. It results from changes in optical path length between the film and the observer, with different lengths corresponding to different colors. With significantly thinner films, however, those changes are greatly diminished. The effect could allow the coloring of metallic objects, such as logos or signs, using less material than traditional methods. “There's a famous story where, by not painting the space shuttle's fuel tank, 600 pounds were saved, because that's how much the paint weighed,” Kats said. “If you wanted to put a logo on something that you're sending up in space, or you wanted to color it, here you could do it without almost any increase in weight.” Also, these structures are easy to fabricate and analyze, making them useful in applications such as color coatings, harvesting of solar energy and anti-reflection coatings, as well as static and tunable absorbers and thermal emitters over wavelengths in the UV to IR range, the researchers wrote in the study. The research was published in Applied Physics Letters (doi: 10.1063/1.4896527). For more information, visit: www.harvard.edu.