A Compact Full-Stokes Polarization Camera

The portable polarization camera is about two centimeters in diameter and uses a metasurface with an array of subwavelength spaced nanopillars to direct light based on its polarization. (Source: E. Grinnell, Harvard SEAS)

When the first full-length movie made with the advanced, three-color process of Techni­color premiered in 1935, The New York Times declared “it produced in the spectator all the excitement of standing upon a peak … and glimpsing a strange, beautiful and unexpected new world.” Techni­color forever changed how cameras  and people saw and experienced the world around them. Today, there is a new precipice – this one, offering views of a polarized world.

Polari­zation is invisible to the human eye but visible to some species of shrimp and insects. But it provides a great deal of infor­mation about the objects with which it interacts.  Cameras that see polarized light are currently used to detect material stress, enhance contrast for object detection, and analyze surface quality for dents or scratches. However, like the early color cameras, current-generation polari­zation-sensitive cameras are bulky. Moreover, they often rely on moving parts and are costly, severely limiting the scope of their potential appli­cation.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a highly compact, portable camera that can image polari­zation in a single shot. The miniature camera – about the size of a thumb – could find a place in the vision systems of autonomous vehicles, onboard planes or satellites to study atmo­spheric chemistry, or be used to detect camou­flaged objects. “This research is game-changing for imaging,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics. “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.”

“Polari­zation is a feature of light that is changed upon reflection off a surface,” said postdoc Paul Chevalier. “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 man-made objects from natural ones, even if they’re the same shape and color.” To unlock that powerful world of polari­zation, Capasso and his team harnessed the potential of meta­surfaces, nanoscale structures that interact with light at wavelength size-scales.

“If we want to measure the light’s full polarization state, we need to take several pictures along different polari­zation direc­tions,” said Noah Rubin, graduate student in the Capasso Lab. “Previous devices either used moving parts or sent light along multiple paths to acquire the multiple images, resulting in bulky optics. A newer strategy uses specially patterned camera pixels, but this approach does not measure the full polari­zation state and requires a non-standard imaging sensor. In this work, we were able to take all of the optics needed and integrate them in a single, simple device with a meta­surface.”

Using a new under­standing how polarized light interacts with objects, the researchers designed a metasurface that uses an array of subwave­length spaced nanopillars to direct light based on its polari­zation. The light then forms four images, each one showing a different aspect of the polari­zation. Taken together, these give a full snapshot of polari­zation at every pixel. The device is about two centimeters in length and no more complicated than a camera on a smartphone. With an attached lens and protective case, the device is about the size of a small lunch box. 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.

“This tech­nology could be integrated into existing imaging systems, such as the one in your cell phone or car, enabling the wide­spread adoption of polari­zation imaging and new appli­cations previously unfore­seen,” said Rubin. “This research opens an exciting new direction for camera techno­logy with unpre­cedented compactness, allowing us to envision appli­cations in atmo­spheric science, remote sensing, facial recognition, machine vision and more,” said Capasso. The Harvard Office of Tech­nology Development has protected the intel­lectual property relating to this project and is exploring commerciali­zation oppor­tunities. (Source: Harvard SEAS)

Reference: N. A. Rubin et al.: Matrix Fourier optics enables a compact full-Stokes polarization camera, Science 365, eaax1839 (2019); DOI: 10.1126/science.aax1839

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

 

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