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Moiré Metasurfaces Extend Focal Length Tunability

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Researchers from Tokyo University of Agriculture and Technology (TUAT) have demonstrated that moiré metalenses can tune focal length along a wider range than previously seen. Moiré metalenses are tiny patterned lenses composed of artificial meta-atoms. 

To keep these metalenses thin and compact enough for their desired uses, such as in MEMS devices, they have a limited focal tuning range. Focal length refers to the angle of view and the strength of the magnification, which is dictated by lens shape.
Schematic drawing of working principle. Courtesy of Kentaro Iwami/TUAT.
Schematic drawing of working principle. Courtesy of Kentaro Iwami/TUAT.

Convex lenses have positive focal length and bring light rays to a single point; concave lenses have negative focal lengths and disperse light rays. Combined, they create a more complete and sharper image, though tuning the focal length from negative to positive in something as compact as a metalens is difficult, according to Kentaro Iwami, associate professor at TUAT’s department of mechanical systems engineering.

“We found that wide-focal-length tuning from convex to concave can be achieved by rotational moiré metalenses,” Iwami said.

The researchers developed metalenses with high-contrast artificial “meta” atoms composed of amorphous silicon octagonal pillars. They created a moiré pattern by overlaying one metalens on the other and rotating them, allowing them to use infrared light to tune the focal length of the lenses.

The researchers plan to demonstrate wide-focal-length tuning at a visible wavelength and improve the quality of the lens with the ultimate goal of realizing an ultracompact imaging system.

“Metalenses have attracted a lot of interest because they are so thin and lightweight, and could be used in ultracompact imaging systems, like future smartphones, virtual reality goggles, drones, or microbots,” Iwami said.

The research was published in Optics Express (www.doi.org/10.1364/OE.411054).
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Published: January 2021
Glossary
metalens
A metalens, short for "metasurface lens," is a type of optical lens that uses nanostructured materials to manipulate light at a subwavelength scale. Unlike traditional lenses made of glass or other transparent materials, metalenses do not rely on the curvature of their surface to refract or focus light. Instead, they use carefully engineered patterns of nanostructures, such as nanoscale antennas or dielectric structures, to control the phase and amplitude of light across the lens's surface....
focal length
The focal length of a lens is the distance between the lens's optical center (or principal point) and the image sensor or film when the lens is focused at infinity. In simple terms, it is the distance from the lens to the point where parallel rays of light converge or appear to diverge after passing through the lens. For converging lenses (convex lenses), which are thicker in the center, the focal length is considered positive. For diverging lenses (concave lenses), which are thinner in the...
Research & TechnologymetalensesmetalensMoirélensesOpticsTokyo University of Agriculture and Technologyfocal lengthAsia-PacificOptics ExpressTech Pulse

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