Aerosols can affect everything from severe weather to air quality. Polarimeters, which characterize aerosols and cloud particles by observing how they interact with light, are among the best tools scientists have to help develop their understanding of the role played by these tiny particles in atmospheric events. Although there are many airborne polarimeters available to scientists, only a few of these instruments have ever flown in space. NASA’s Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) mission, launched in 2024, marked the first space-based science mission featuring polarimetry in over a decade. “The composition of aerosols, the shape, the size – that's something that we really need to understand better to improve climate modeling,” said Kirk Knobelspiesse, an atmospheric scientist at NASA’s Goddard Space Flight Center. Two fully fabricated flat polarimeter samples are shown. Metasurfaces could become the foundation for future, ultra-light instruments, including space-based polarimeters. Courtesy of the Capasso Group/Harvard University. A team of researchers from the Capasso Group at Harvard University, supported by NASA’s Earth Science Technology Office, recently completed an early concept study exploring a new technology for space-based polarimetry. Specifically, the team investigated whether a novel polarization-sensitive metasurface optical element might be useful for observing atmospheric particles. The study concluded that this metasurface optical element can reliably detect polarized light within the 550, 670, and 870 nm wavelengths, ideal light signatures for observing aerosols and cloud particles. “I think all of this will play well for the long-term plans of NASA,” said Federico Capasso, Robert Wallace Professor of Applied Physics at Harvard and principal investigator for this project. Metasurface optics are lighter and smaller than their traditional counterparts, making them less expensive to send into orbit. As NASA plans future Earth science missions, metasurface optics could be key to building a new generation of compact polarimeters. “The size, weight, and mass production possibility are often quoted as advantages for metasurface optics,” said Lisa Li, a former member of the Capasso Group who played a key role in manufacturing this unique metasurface. Lighter, smaller components easily produced at scale can reduce the overall cost of a science mission. What makes Capasso’s metamaterial unique is its bespoke grating pattern, etched across a silica glass substrate, that splits an observed scene into distinct polarization channels. This ability to discriminate between polarization states without bulky subsystems could allow researchers to produce a complete polarimetric system (a sorter and an imager) within a single element. Noah Rubin, a former member of the Capasso Group and a co-investigator for this project, explained that this was the key achievement of their project: proving that their metasurface grating could measure signatures of polarized light with the accuracy researchers would require from a space-ready science instrument. “We realized it would be possible to make, essentially, what we call a flat polarimeter,” said Rubin. There is still much work to be done before NASA has a flight-ready metasurface polarimeter at its disposal, but this early work produced a scientific bedrock on which future metasurface breakthroughs will rely. “I’d like to extend some of this work, some of this polarization sensitive imaging, to include infrared light, which is a very important wavelength regime for ice cloud remote sensing,” Rubin said. The research was published in Optics Express (www.doi.org/10.1364/OE.450941) and in Applied Optics (www.doi.org/10.1364/AO.480487).