The latest ultra-black coating combines extreme thinness with durability. It has an extremely high light absorption rate, which made it a perfect candidate for starshades, massive nanostructured devices designed to allow telescopes to see distant planets and, potentially, signs of life not of this Earth.
Detecting a planet beside a star would be like seeing a firefly next to one of those stadium floodlights: the light from the star is so very much brighter than that from the planet. Sunlight reflected off a spacecraft also causes unwanted haze, making observations more difficult.
One solution, a starshade, acts like a giant cosmic flower about half the size of a football field held between the telescope and the star. The shadow it casts is crisply defined, blocking nearly all starlight, yet allowing the gentle glow of the planet to slip around its edges and into our telescope’s view.
For the last ten years, NASA engineers have been studying several techniques to reduce the path of solar light. They shaped the starshade’s edges with ultrathin blades of amorphous metal, 300 nanometers thick, and even these scattered more light than was needed.
However, repeated tests with dark surfaces showed challenges: making the layers too thick would blunt edges and increase light scattering. Carbon nanotube coatings, for example, are on the order of a few microns thick, orders of magnitude wider than the extremely thin starshade edges. Likewise, >3D microstructured light-trapping coatings suffered from the increased thickness of the 2DCa layer, which led to a suppressed clean shadow.
In 2004, David Sheik (of ZeCoat!) designed a ‘black mirror’, which absorbs light rather than reflecting it. Using cutting-edge computation and a spiral glass-vapor deposition process, they fashioned an ultra-thin coating of stacked metal and glass layers, which trapped light in nanoscale cavities (kind of like the inverse physics of a Fabry–Perot laser cavity).
The coating was an order of magnitude thinner than the previously tested coatings. NASA’s Exoplanet Exploration Program had reviewed it (as a starshade prototype) back in 2020. Engineers confirmed the ability of coated blades to reduce stray light by 20x using a custom scatterometer developed at JPL, thereby suppressing glare enough to allow telescopes to detect faint emission from distant worlds.
To further this success, ZeCoat developed the technology under a NASA SBIR contract in 2021. They developed a process suitable for roll-to-roll thin-film deposition that applies ultra-black coatings to large sheets of polyimide.
These assembled one-meter-wide membranes will become the disks and petals of the starshade, further minimizing stray light. The result is sharper shadows, clearer pictures, and telescopes with a far-reaching ability to peer deeper into the faint glare of exoplanets.
Ultra-black coatings also have a range of potential applications beyond starshades, including scientific research, defense, and consumer tech. They could dim satellite constellations so they are barely visible in Earth’s night sky, or resaturate areas near smartphone cameras to enhance image display.