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Optical metasurfaces.

Flat & adjustable "metalenses" now one step closer to reality

Researchers at Linköping University have developed optical metasurfaces in conductive plastic that can be used in sensors, biomedical imaging, video holograms, and ultimately even cameras.

Calle Rosenqvist
CALLE@KAMERABILD.SE
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In a 2016 article Kamera & Bild reported on a research group at Harvard University that succeeded in developing a new type of flat lens - a lens that would compete with traditional lenses. At the Canon Expo 2023 concept fair in Japan, Canon also showcased a prototype of flat lenses with nanopillars.  

Dongqing Lin and Magnus Jonsson, Linköping University.

The design with flat lenses, however, is not entirely simple. The tests conducted with metalenses created in, among other things, titanium dioxide and so-called nano-fins, open up for high precision but difficult focusing afterward. But now Linköping University announces that the research group that in 2019 began using plastic - that is, polymers - to dynamically change the focal point on the so-called metasurfaces created in the metalens. 

Now the research group with the main author of the article, Dongqing Lin and Magnus Jonsson, professor of applied physics at Linköping University, announces in an article published in Nature Communications that they have managed to improve the performance of their flat lenses up to ten times, by controlling the distance between the antennas that create the metasurface itself - and have managed to create a resonance where the antennas help each other and enhance light interaction. 

The phenomenon is called "collective lattice resonance" and allows metasurfaces of conductive polymers to achieve sufficient performance to function well with light. The researchers have managed to regulate these for infrared light, but not yet for visible light - which will also be the next problem to solve.

Here is an illustration of the ultra-thin metalens. The lens consists of nanofins on a glass substrate and focuses the incoming light on a single point.

In laboratory tests from 2016 and the company "Metalenz," the metalenses were 30 percent sharper than a research microscope, which is impressively sharp considering both size and manufacturing cost. At that time, it was not known what the possibility of mass production looked like, although it was believed it could be solved relatively easily. By 2018, all problems had been solved, and the next step was to reduce the lens size.

The technology works by controlling the speed of different wavelengths (colors) using a sort of labyrinth for the different colors. Through so-called nano-fins in titanium dioxide, the colors can be adjusted separately, thereby curbing chromatic aberration. The light can then be focused with an accuracy less than the wavelength of the light, which opens up for higher resolution.

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