A pair of precision-orbiting small satellites will attempt to capture the first views ever of small-scale features near the surface of the sun that scientists believe drive the heating and acceleration of solar wind. According to Doug Rabin, a heliophysicist at NASA’s Goddard Space Flight Center, photon sieves, a technology that can focus extreme-ultraviolet (EUV) light, are expected to be able to resolve features on the sun 10 to 50 times smaller than what can be seen today with NASA’s Solar Dynamics EUV imager. Photon sieves like this are cut from a single wafer of silicon or niobium to focus extreme-ultraviolet light, a difficult wavelength to capture. Courtesy of Christopher Gunn/NASA. To be most effective, however, they must be wide, superthin, and etched with precise holes to refract light. Working in Goddard’s Detector Development Laboratory, Goddard engineer Kevin Denis developed new ways to create wider and thinner membranes from wafers of silicon and niobium. Each advancement so far has required additional steps to protect the resulting sieves, such as leaving a honeycomb of thicker material to support the membrane and prevent tearing. “It’s a sheer physical challenge to construct sieves with such precision,” Rabin said. “Their smallest features are two microns across with a two-micron gap between perforations — that’s about the size of most bacteria.” Etched from the center with ever smaller rings of holes, photon sieves are built to refract light similarly to the Fresnel lenses used in lighthouses. EUV light passing through this sieve is bent gradually inward to a distant camera. Thin membranes matter for solar science because these sieves transmit more light than thicker materials, Denis said. He and fellow engineer Kelly Johnson successfully produced an 8-cm-diameter silicon sieve, a mere 100 nm thick. Now they are experimenting with niobium membranes that can further improve light-gathering efficiency because they transmit up to seven times more light than silicon. They have successfully etched a 13-cm-diameter niobium sieve just 200 nm thick. New photon sieves consist of a honeycomb structure supporting a superthin membrane cut to focus extreme-ultraviolet light. In this sieve, the largest gaps and holes can be seen in the center hexagon, but the rest quickly become too small for the human eye to detect. Courtesy of Christopher Gunn/NASA. Photon sieves cut from materials as thick as 25 µm are already part of the technology demonstration VISORS (Virtual Super Optics Reconfigurable Swarm) CubeSat mission, expected to launch in 2024. VISORS consists of one compact satellite about the size of a briefcase outfitted with sieves to refract light onto a receiver on a second satellite 130 ft (40 m) away. Maintaining these spacecraft’s high-precision orbits and developing a sunshade are the focus of other Goddard projects. VISOR’s success could pave the way for a larger future mission, with spacecraft separation measured in kilometers, employing the greater resolution of Denis’ thinner sieves once they are ready for spaceflight. Another larger photon sieve will be used to calibrate the MUSE (Multi-Slit Solar Explorer) spectrometer expected to launch in 2027. The work was published in Physics Today (www.doi.org/10.1063/PT.3.5292).
