Moiré Metasurfaces Extend Focal Length Tunability

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.

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).