Metalens Design Uses Millions of Holes to Focus Light into Single Point

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed a metalens that uses deep narrow holes, rather than tall pillars, to focus light to a single point.

The metasurface uses more than 12 million needle-like holes drilled into a 5-μm silicon membrane, about 1/20 the thickness of a human hair. The diameter of these holes is only a few hundred nanometers, making the aspect ratio nearly 30:1. According to the researchers, it is the first time that holes with such a high aspect ratio have been used in meta-optics.

“This approach may be used to create large achromatic metalenses that focus various colors of light to the same focal spot, paving the way for a generation of high-aspect ratio flat optics, including large-area broadband achromatic metalenses,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper.

Artistic representation of a holey metalens. Courtesy of the Capasso Lab/Harvard SEAS.

The lens focuses incident infrared light into a diffraction-limited spot. Rather than shaping the metasurface optical phase shift alone, the team engineered both transmitted phase and amplitude profiles simultaneously by inverse-designing the lens’ effective index profile. This approach improves the impedance match between the incident and transmitted waves, thereby increasing the focusing efficiency.

“If you tried to make pillars with this aspect ratio, they would fall over,” said Daniel Lim, a graduate student at SEAS and co-first author of the paper. “The holey platform increases the accessible aspect ratio of optical nanostructures without sacrificing mechanical robustness.”

As with nanopillars that vary in size to focus light, the metalens’ holes are variably sized and precisely positioned over the 2-mm lens diameter. The hole size variation bends the light toward the lens focus.

“Holey metasurfaces add a new dimension to lens design by controlling the confinement and propagation of light over a wide parameter space and make new functionalities possible,” said Maryna Meretska, a postdoctoral fellow at SEAS and co-first author of the paper. “Holes can be filled in with nonlinear optical materials, which will lead to multiwavelength generation and manipulation of light, or with liquid crystals to actively modulate the properties of light.”

The metalenses were fabricated using conventional semiconductor industry processes and standard materials, allowing it to be manufactured at scale in the future.

The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring opportunities for commercialization.

The research was published in Nano Letters (www.doi.org/10.1021/acs.nanolett.1c02612).