A new nanopatterned structure based on sub-wavelength physics has proved its value in a real-world setting, and it could qualify for use in outer space. The device, integrated with one of the world’s most advanced solar observatories, captured pictures of the Sun’s magnetic field in a new, advantageous way: via a single snapshot with no moving parts. The demonstration, published 10 June in Science Advances, offers a promising tool for astronomy, consumer electronics, quantum optics, and other applications that involve measuring polarized light.
The structure is based on an optical metasurface, which refers to a patterned array engineered at sub-wavelength scales to manipulate the diffraction of the wavefront. That’s not unlike classical diffraction gratings, whose periodic etchings split light into different colors and directions. But metasurfaces offer another advance: They can split light into its polarized components. Researchers in the lab of Noah Rubin, professor of electrical and computer engineering at University of California, San Diego, created the device.
“What’s special about metasurfaces is you can design an array of elements that respond in one way to this polarization and in another way to that polarization,” says Rubin. “This is a capability that’s only fully emerged in recent years.”
Now the metasurface is poised to “leave the lab and go into a serious piece of scientific instrumentation, possibly for one of the first times,” he says. Integrating the component with the Dunn Solar Telescope at the National Solar Observatory in Sunspot, New Mexico, showed that it could perform key measurements that today normally rely on complex rotating components—an advance that may one send it on a space mission.
Studying our Sun is fundamental for predicting space weather and the coronal mass ejections that influence life on Earth. The key to these dynamics is magnetism, which we can detect from afar with polarized light.
Although light emitted from the Sun starts out unpolarized, magnetic fields on the Sun’s surface and in its atmosphere polarize some of the light. Measurements of the polarized light provide a set of parameters from which astronomers can deduce the magnetic field strength and direction. But the classic way of doing this involves an optical component that rotates within a camera, taking separate measurements of multiple parameters before reconstructing the full image.
This demonstration integrated the device with a ground-based telescope, but it could also be used with space-based telescopes in the future. “If you’re thinking about a space mission, you don’t want a moving component. It’s a single point of failure,” says Rubin. Moreover, measuring subtle polarization signatures requires advanced hardware to compensate for jitter in the satellite itself.
Simultaneously acquiring all of a scene’s polarization data using a passive component could greatly relax the engineering requirement of future satellite-borne imagers. That’s where a diffraction grating with special polarization behavior can shine.
How nanopillars parse polarization
Rubin and his colleagues first proposed the design for this camera in a previous conceptual paper. They added a polarization component to a widely-used optics theory that describes how light diffracts and informs the way engineers design patterns to manipulate that diffraction. With some mathematical wizardry, the researchers described a periodic surface whose elements capture discrete components of polarized wavefronts.
The team then fabricated their surface on a glass substrate. Each unit of its repeating pattern contained 144 rectangular pillars arranged in an area less than five micrometers wide. When placed within a camera lens system, each diffracted component captured a different part of incoming polarized light—recording the entire polarization picture simultaneously. “It’s like a special beamsplitter that has an arbitrary number of channels, and we can control their sensitivity,” says Rubin.
He and his colleagues built a high-performance version of their metasurface-enabled imaging device to fit with a 6-millimeter aperture lens. They teamed up with industry partner BAE Space & Mission Systems to run it through the vibration and thermal tests required to send equipment to space. Finally, they integrated their device to the light-feed of the Dunn Solar Telescope and recorded snapshots of the sun. Comparing their magnetic field images to ones taken from instruments in low-Earth orbit showed the correct order of magnitude and spatial patterns. With the researchers’ design, 70 percent of incoming light reached the metasurface and could then be received by the detector, and the device achieved near state-of-the-art contrast.
That’s exciting for more than just solar astronomy. “These are a good target for any application where we want to condense a lot of polarization elements into one device,” says Rubin. Examples include facial recognition technology for cell phones, entangled photon preparation for quantum experiments, and coronagraphs that block light from distant stars for hunting orbiting planets.
NASA intends to launch a solar monitoring mission in the 2030s. The UCSD team and their industry partners are part of an initial study to examine new approaches for instrument design. That means competing with other teams to develop new technology that NASA may select for the mission. “Ours could be it,” says Rubin.
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