Metasurface Tech Enables Polarization Imaging on Almost Any Camera

JOEL WILLIAMS, ASSOCIATE EDITOR
joel.williams@photonics.com

Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a metasurface attachment that can turn almost any camera or imaging system, even off-the-shelf systems, into polarization cameras.

The technology developed in the Capasso Lab at Harvard could, in theory, establish a mechanism to turn a standard camera into a polarization camera, albeit without the capacity to identify material compositions.

The attachment uses a metasurface of subwavelength nanopillars to direct light based on its polarization and compiles an image that captures polarization at every pixel. As with previously developed metasurface technology, the optical devices can be manufactured using existing lithographic production infrastructure.

A metasurface attachment introduced by the Capasso Lab at Harvard University has implications for conventional cameras, as well as more advanced imaging devices used in machine vision applications. The grating is mounted just in front of the front face of a chosen objective lens in a tube that also houses a bandpass filter and a field stop. This is shown implemented (top), and as a schematic (bottom). Courtesy of Capasso Lab/Harvard SEAS.

“The addition of polarization sensitivity to practically any camera will reveal details and features that ordinary cameras can’t see, benefiting a wide range of applications from face recognition and self-driving cars to remote sensing and machine vision,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the study.

In 2019, Capasso and his team developed a compact, portable camera that used a metasurface to image polarization in a single shot. In this research, the team explored how to generalize the concept of a polarization camera

“After building the specialized polarization camera, we wanted to go more in depth and investigate the design rules and trade-offs that govern pairing a special polarization component with a conventional camera system,” said Noah Rubin, a graduate student at SEAS and co-first author of the study.

To demonstrate those design rules, the researchers attached the polarization metasurface to an off-the-shelf machine vision camera, simply screwing it on in front of the objective lens in a small tube that also housed a color filter and field stop. From there, all they needed to do was point and click to get polarization information.

According to Rubin, there isn’t necessarily one “typical” polarization camera; polarization itself is described by a number of different parameters, and, as such, its characterization necessitates several measurements. Optical systems for polarization imaging differ in how such measurements are acquired. This can be done serially in time, one after the other, either with a rotating polarization element or an electrically switchable one, such as a liquid crystal.

“Alternatively, the beam can be divided among several paths each with separate polarization-analyzing optics. This permits simultaneous data acquisition, but with an overall bulky optical setup,” Rubin said. “Our work is in this spirit but manages to combine many functions simultaneously into a single optical component — this being the metasurface — which splits light into separate parts of the camera aperture while simultaneously analyzing its polarization state.”

The nanopillars direct light based on polarization, which forms four images, each showing a different aspect of the polarization. The images are then put together, giving a full snapshot of polarization at every pixel.

“We hope that these metasurface polarization gratings can be an enabling component for new imagers and even telescopes that may present benefits in terms of size and temporal resolution over existing solutions,” Rubin told Photonics Media. “As an attachment, however, we hope that there may be some application of this work to the consumer domain, where these devices can be used analogously to a ‘camera filter’ for ease of polarization imaging, including in mobile phones.”

The attachment could also be used to improve machine vision in vehicles or in biometric sensors for security applications.

“This metasurface attachment is extremely versatile,” said Paul Chevalier, a postdoctoral research fellow at SEAS and co-first author of the study. “It is a component that could live in a variety of optical systems, from room-size telescopes to tiny spy cameras, expanding the application space for polarization cameras.”

The research was published in Optics Express (www.doi.org/10.1364/OE.450941).