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Researchers Produce Fisheye Metalens

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CAMBRIDGE, Mass., Sept. 23, 2020 — Researchers at MIT and the University of Massachusetts Lowell have developed a wide-angle metalens capable of producing 180° panoramic images. The design could be adapted for a wide range of applications, such as in smartphones and laptops, medical imaging devices, virtual reality glasses, and wearable electronics.

The lens is a single, flat, millimeter-thin piece of glass, covered on one side with miniscule structures that precisely scatter incoming light to produce panoramic images, just as a conventional curved, multielement fisheye lens assembly would. The lens works in the infrared part of the spectrum, though the researchers say that the design could be modified to also capture images in the visible spectrum.
3D artistic illustration of the wide-field-of-view metalens capturing a 180° panorama of MIT's Killian Court and producing a high-resolution monochromatic flat image. Courtesy of Mikhail Shalaginov et al.
3D illustration of the wide-field-of-view metalens capturing a 180° panorama of MIT's Killian Court and producing a high-resolution monochromatic flat image. Courtesy of Mikhail Shalaginov et al.

“This design comes as somewhat of a surprise, because some have thought it would be impossible to make a metalens with an ultrawide-field view,”  said Juejun Hu, associate professor in MIT's Department of Materials Science and Engineering. “The fact that this can actually realize fisheye images is completely outside expectation.”  

Scientists have previously designed metalenses that produce high-resolution and relatively wide-angle images of up to 60°. To expand the field of view farther would traditionally require additional optical components to correct for aberrations or blurriness.

Hu and his colleagues instead came up with a design that does not require additional components. The metalens is a single, transparent piece made from calcium fluoride with a thin film of lead telluride deposited on one side. The team then used lithographic techniques to carve a pattern of optical structures into the film.

Each structure, or “meta-atom,” is shaped into one of several nanoscale geometries, such as a rectangular or bone-shaped configuration, that refracts light in a specific way. For instance, light may take longer to scatter or propagate off one shape versus another — a phenomenon known as phase delay. In a conventional fisheye lens, the curvature of the glass creates a distribution of phase delays that ultimately produces a panoramic image.

“We have designed the backside structures in such a way that each part can produce a perfect focus,” Hu said.

On the front side, the team placed an optical aperture or opening for light.

“When light comes in through this aperture, it will refract at the first surface of the glass, and then will get angularly dispersed,” said Mikhail Shalaginov, postdoctoral associate at MIT. “The light will then hit different parts of the backside from different and yet continuous angles. As long as you design the backside properly, you can be sure to achieve high-quality imaging across the entire panoramic view.”

The team used the imaging setup equipped with the metalens to snap pictures of a striped target. They then compared the quality of the images taken at various angles across the scene, and found the lens produced images that were crisp and clear, even at the edges of the camera’s view, which spans nearly 180°.

In another study, the team designed the metalens to operate in the near-infrared range using amorphous silicon nanoposts as the meta-atoms. They then plugged the lens into a simulation used to test imaging instruments where they then loaded a scene of Paris, composed of stitched-together black-and-white images, to make a panoramic view.

“The key question was, does the lens cover the entire field of view? And we see that it captures everything across the panorama,” said Tian Gu, research scientist in Hu’s lab. “You can see buildings and people, and the resolution is very good, regardless of whether you’re looking at the center or the edges.”

To make a similar flat fisheye lens for visible light, Hu said the engineers may need to design smaller optical features than are in place currently to better refract that range of wavelengths. The material of the lens would also need to change. The general architecture of the lens would remain the same.

“Currently, all 3D sensors have a limited field of view, which is why when you put your face away from your smartphone, it won't recognize you,” Gu said. “What we have here is a new 3D sensor that enables panoramic depth profiling, which could be useful for consumer electronic devices.” 

The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.0c02783).

Photonics.com
Sep 2020
GLOSSARY
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
nanophotonics
The study of how light interacts with nanoscale objects and the technology of applying photons to the manipulation or sensing of nanoscale structures.
Research & TechnologyopticslenseslensMITmetalensmetasurfaceimaginginfrarednear infraredNIRIRnanophotonics

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