Holey metalens! New metalens focuses light with ultra-deep holes

Illustration of a holey metallicens. Credit: Capasso Lab/Harvard SEAS
Metasurfaces are nanoscale structures which interact with light. Metasurfaces today use monolithic nanopillars that resemble monoliths to focus, shape, and control light. The nanostructure takes longer for light to pass through it, so the metasurface has more control over each color. However, pillars that are very tall can either fall or stick together. Instead of building tall structures, what if you did the opposite?

Researchers at Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), have developed a metasurface to direct light to a single spot.

Nano Letters published the research.

The new metasurface is made up of more than 12,000,000 needle-like holes that are drilled into a 5-micrometer silicon material. This membrane is approximately one-fifth the thickness of human hair. These thin, long holes are only a few hundred nanometers in diameter, which makes the aspect ratio (the ratio of height to width) close to 30:1.

This is the first time that holes have such a high aspect ratio are being used in meta-optics.

Federico Capasso is the Robert L. Wallace Professor in Applied Physics and Vinton Hayes Senior Fellow in Electrical Engineering at SEAS. He was also the senior author of this paper.

An SEM image using scanning electron microscopy (SEM). (left) The holes on the side I of the holey metallicens. (right) A SEM image showing the holes on the side II. Credit: Capasso Lab / Harvard SEAS

Daniel Lim, a graduate student from SEAS, and co-first author, said that if you attempted to make pillars with this aspect, they would collapse. "The accessible aspect ratio of optical structures is increased by the holey platform without compromising mechanical robustness."

The holey metalens is just like nanopillars that vary in size to focus light. It has holes of different sizes precisely placed over the 2mm lens diameter. The lens focus is bounded by the variation in the hole size.

Maryna Meretska is a postdoctoral fellow in SEAS and coauthor of the paper. "Holey metasurfaces give a new dimension for lens design by controlling light confinement and propagation over a wide parameter range and make new functionalities possible," she said. Nonlinear optical materials can be used to fill holes, which allow for multi-wavelength generation, manipulation, and control of light. Liquid crystals can also be used to modulate light's properties.

Metalenses were manufactured using standard semiconductor industry processes and materials. This allows it to be made at scale in future.

The intellectual property related to this project has been protected by the Harvard Office of Technology Development and it is being explored for commercialization.

Further exploration of Metasurfaces control polarized lighting at will

More information: Soon Wei Daniel Lim et al, A High Aspect Ratio Inverse-Designed Holey Metalens, Nano Letters (2021). Journal information: Nano Letters Soon Wei Daniel Lim et al, A High Aspect Ratio Inverse-Designed Holey Metalens,(2021). DOI: 10.1021/acs.nanolett.1c02612

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