Cave entrances often appear black and forbidding. Light enters, but little escapes, absorbed as it “bounces around” the interior.
To trap light in much the same way, engineers at the University of Notre Dame have devised a superblack material from a matrix of microscopic, sheer-walled “caves”—each just 10 microns wide. The result reflects less than 0.4% of visible light across the full spectrum. Unlike many superblack materials which are fragile and expensive, theirs is robust, low-cost and easy to customize.
Their results were published in Nature Communications.
“It’s not the color of the cave that makes it appear black, it’s the structure. The geometry of our material—tiny honeycombs—makes it highly effective at trapping light,” said Matthew Rosenberger, assistant professor of aerospace and mechanical engineering at the University of Notre Dame.
To create these light-absorbing structures, Rosenberger’s team first makes a mold out of silicon, the hard, shiny material used in microchips. This mold has a microscopic pattern of cone-shaped holes arranged like a honeycomb—the inverse of the final structure. Then, they pour soft, flexible silicone mixed with black dye into the mold. Once it hardens, they peel it out—like removing fancy ice cubes from a tray. Voilà—a surface covered in tiny cone-shaped bumps that trap light.
Unlike previous methods, the team’s platform allows them to easily engineer molds and microcavities with application-specific optical and mechanical properties.
“We can tailor each material to its specific purpose. For instance: do we care more about blackness or robustness? We can adjust our method to increase either,” said Rosenberger.
Versatility in the production of superblack materials would enable a wide range of applications, from imaging systems, to stealth technologies, to telescopes.
The team’s superblack material can also be used to obscure shapes. When they used it to cover the center of a coffee mug, the shadows and highlights that our eyes use to identify the mug’s shape were erased.
“The idea behind this project was simple—we wanted to create materials that look truly black, and the approach we had the most control over was using microstructures,” said Rosenberger. “It’s mostly about geometry—how light reflects and scatters. Unlike approaches that rely heavily on the wave nature of light, this method is easier to understand and control, which makes it more scalable and predictable.”
The lab’s research was supported by the U.S. government and an Interdisciplinary Materials Science and Engineering Fellowship. The team benefited from several Notre Dame facilities, including Notre Dame Nanofabrication, Notre Dame Integrated Imaging, as well as ND Energy Materials Characterization. ND Energy provided funding.
—Karla Cruise, Notre Dame Engineering; Photos by Wes Evard, Notre Dame Engineering