Metasurface-Based Camera Could Simplify Polarization Imaging for Vision Systems

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a compact, portable camera that can image polarization in a single shot. The miniature camera uses metasurface technology to integrate all of the optics needed for polarization in a single device.

The researchers applied matrix Fourier optics — an extension of Fourier optics — to the design of metasurface gratings. Their metasurface-based full-Stokes polarization camera is designed without conventional polarization optics and without moving parts.

The camera’s metasurface uses an array of subwavelength-spaced nanopillars to direct light based on its polarization. The light forms four images on the four quadrants of an imaging sensor, each one showing a different aspect of the polarization. Taken together, these give a full snapshot of polarization at every pixel. Courtesy of Noah Rubin/Harvard SEAS.

The SEAS team designed a metasurface that uses an array of subwavelength-spaced nanopillars to direct light based on its polarization. The polarization of light from a photographic scene is “sorted” by the specially designed subwavelength metasurface grating. When combined with a lens and a sensor, four copies of the image corresponding to four diffraction orders are formed on the imaging sensor. These copies have each, effectively, passed through a different polarizer whose functions are embedded in the metasurface. The four images can be analyzed pixel-wise to reconstruct the four-element Stokes vector across the scene.

The device is about 2 cm in length and no more complicated than a camera on a smartphone, the researchers said. With an attached lens and protective case, the device is about the size of a small lunchbox. The researchers tested the camera to show defects in injection-molded plastic objects; took it outside to film the polarization off car windshields; and even took selfies to demonstrate how a polarization camera can visualize the 3D contours of a face.

The portable polarization camera is about 2 cm in diameter and uses a metasurface with an array of subwavelength-spaced nanopillars to direct light based on its polarization. Courtesy of Eliza Grinnell/Harvard SEAS.

Although polarization is invisible to the human eye, cameras that see polarized light can provide a great deal of information about the objects with which they interact. “Polarization is a feature of light that is changed upon reflection off a surface,” researcher Paul Chevalier said. “Based on that change, polarization can help us in the 3D reconstruction of an object, to estimate its depth, texture, and shape, and to distinguish manmade objects from natural ones, even if they’re the same shape and color.”

Polarization-sensitive cameras are currently used to detect material stress, enhance contrast for object detection, and analyze surface quality for dents or scratches. However, the bulk and cost of the existing technology limits the scope of its potential application. Researcher Noah Rubin said that the SEAS camera technology could be integrated into existing imaging systems, such as the ones in cellphones or cars, enabling the widespread adoption of polarization imaging and potentially leading to new applications.

The compact polarization camera could be applied in the vision systems of autonomous vehicles; on board planes or satellites to study atmospheric chemistry; or to detect camouflaged objects. “Most cameras can typically only detect the intensity and color of light but can’t see polarization. This camera is a new eye on reality, allowing us to reveal how light is reflected and transmitted by the world around us,” professor Federico Capasso said.

The research was published in Science (https://doi.org/10.1126/science.aax1839).

Polarization, the direction in which light vibrates, is invisible to the human eye but provides a lot of information about the objects with which it interacts. Courtesy of Harvard SEAS.