Optical Metasurface Captures Solar Magnetic Fields

A nanopatterned optical metasurface has captured images of the Sun’s magnetic field in a single snapshot without moving parts, demonstrating a technique that could simplify instruments for future space missions, according to research published 10 June in Science Advances [1]. The device, integrated with the Dunn Solar Telescope at the National Solar Observatory in Sunspot, New Mexico, splits incoming light into its polarized components using an array of 144 rectangular pillars within a repeating pattern less than five micrometers wide [1]. Noah Rubin, professor of electrical and computer engineering at University of California, San Diego, led the team that fabricated the surface on a glass substrate and fitted it to a 6-millimeter aperture lens [1]. “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,” Rubin said [1]. Traditional solar magnetic field measurements rely on rotating optical components that record multiple parameters sequentially before reconstructing a full image [1]. The metasurface approach acquires all polarization data simultaneously, a passive method that avoids the single-point failure risk of moving parts on satellites [1]. “If you’re thinking about a space mission, you don’t want a moving component. It’s a single point of failure,” Rubin said [1]. The team worked with BAE Space & Mission Systems to subject the device to vibration and thermal testing required for space-qualified hardware [1]. During the telescope demonstration, 70 percent of incoming light reached the metasurface and was received by the detector, achieving near state-of-the-art contrast [1]. The resulting magnetic field images matched the order of magnitude and spatial patterns recorded by instruments in low-Earth orbit [1]. Rubin noted that metasurfaces can serve as a special beamsplitter with an arbitrary number of channels whose sensitivity can be controlled [1]. “This is a capability that’s only fully emerged in recent years,” he said [1]. The broader class of computational imaging techniques has advanced rapidly in recent decades. Modern ptychography, for instance, reconstructs complex-valued images from coherent diffraction patterns by scanning a probe across a specimen with overlapping illumination and applying iterative phase-retrieval algorithms, yielding quantitative, aberration-free phase images [3]. While ptychography addresses lens limitations through computation, the metasurface approach tackles polarization measurement through sub-wavelength engineering of the optical element itself [1][3]. NASA plans to launch a solar monitoring mission in the 2030s, and the UCSD team is part of an initial study examining new instrument designs [1]. “Ours could be it,” Rubin said [1]. Beyond solar astronomy, the researchers see applications in facial recognition, entangled photon preparation for quantum experiments, and coronagraphs that block starlight to hunt for orbiting planets [1]. “These are a good target for any application where we want to condense a lot of polarization elements into one device,” Rubin said [1].