Water droplets in burgeoning storm systems; particulates launched to the stratosphere by volcanic eruptions; ash from western wildfires drifting eastwards across the continental U.S. Aerosols such as these impact everything from severe weather to air quality. Polarimeters, which characterize aerosols and cloud particles by observing how they interact with light, are one of scientists’ best tools for understanding the massive role these tiny particles play in atmospheric events.
But while 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.
Kirk Knobelspiesse, an atmospheric scientist at NASA’s Goddard Space Flight Center, explained that creating advanced polarimeters to study the atmosphere is essential for studying Earth’s climate. “The composition of aerosols, the shape, the size – that’s something that we really need to understand better to improve climate modeling,” Knobelspiesse said.
A team of researchers from the Capasso Group at Harvard University, supported by NASA’s Earth Science Technology Office (ESTO), 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, which spawned papers in Optics Express and Applied Optics, concluded that this metasurface optical element can reliably detect polarized light within the 550, 670, and 870 nanometer 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.
Traditional optics, like lenses in a telescope, rely on a material’s bulk properties to control observed light. Metasurface optics, on the other hand, rely on complex arrays of micrometer-sized structures arranged in a grating pattern across a flat surface.The spacing and shape of these structures modifies the phase and polarization of light reaching the detector.
Also known as flat optics, 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–which 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, Rubin said, this early work sponsored by ESTO’s Instrument Incubation Program 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,” he said.
NASA’s Instrument Incubator Program (IIP), a part of ESTO, funded this study. For more information about working with NASA to develop new technologies for Earth observation, visit ESTO’s open solicitations page here.
For additional details, see the entry for this project on NASA TechPort here.
Project Lead: Federico Capasso, Harvard University
Sponsoring Organization: Instrument Incubation Program, NASA’s Earth Science Technology Office
