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Metalens setup
University of Washington rigged up an experimental setup to capture an image of flowers through a metalens (mounted on a microscope slide) and visualize it through a microscope. (UW Clean Energy Institute Photo / Matt Hagen)

Imagine a camera that captures pictures on a flat surface, without any need for a glass lens.

Such cameras already exist, thanks to exotic materials known as metasurfaces. They’re not yet ready for prime time, but a new approach that relies on heavy-duty computational processing could soon get them there.

University of Washington researchers show how it could be done in a paper published last week by the journal Science Advances. If the technique can be commercialized, it could turn metasurface-based lenses, or metalenses, into the next big thing in ultrathin cameras and microscopes.

Metasurfaces are 2-D analogs of metamaterials, electronic devices that are engineered to bend light waves in unusual ways. The first metamaterials were touted as potentially leading to Harry Potteresque “invisibility cloaks,” but they’re actually more useful as antennas and sensors.

“Metalenses are potentially valuable tools in optical imaging, since they can be designed and constructed to perform well for a given wavelength of light,” lead author Shane Colburn, a UW doctoral student in electrical engineering, said in a news release. “But that has also been their drawback: Each type of metalens only works best within a narrow wavelength range.”

A full-color image might look sharp in one wavelength — say, in the red part of the spectrum — but look blurry or distorted in the blue part of the spectrum. To get around that issue, the UW team designed a metalens that produced uniformly blurry images across the full visual spectrum, using nanometer-scale pillars of silicon nitride arranged on a glass surface.

Metalens performance
For the color image of flower buds at far left, a traditional metalens (second from left) captures images with strong aberrations and blurring. The UW team’s modified metalens (third from left) yields an image with similar levels of blurring for all colors. The team removed most of these aberrations using computational filtering, producing an image (right) with high structural similarity to the original. (UW Photo / Colburn et al.)

Because the images were distorted by the same amount in a wide range of wavelengths, they could be corrected using computational processing to produce a clearer full-color image.

The team’s imaging system was only 200 micrometers thick — which is in the range of the thickness of a human hair, or about 2,000 times thinner than a conventional cellphone camera.

The promise of metasurfaces is quickly coming into focus: At least two other teams of researchers have reported technological advances in metalenses in the past month or so. So it may not be long before flat-surface imaging goes “meta.”

In addition to Colburn, the authors of “Metasurface Optics for Full-Color Computational Imaging” include Alan Zhan and senior author Arka Majumdar.

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