Artistic representation of the holey metalens. Image: Capasso Lab/Harvard SEAS.
Artistic representation of the holey metalens. Image: Capasso Lab/Harvard SEAS.

Metasurfaces are nanoscale structures that interact with light, with most current metasurfaces employing monolith-like nanopillars to focus, shape and control light. The taller the nanopillar, the more time it takes for light to pass through the nanostructure, providing the metasurface with versatile control over each color of light. But very tall, thin pillars are prone to falling over or clinging together, so how about going in the other direction.

In a recent paper in Nano Letters, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) report a novel metasurface that employs very deep, very narrow holes, rather than very tall pillars, to focus light to a single spot.

This new metasurface comprises more than 12 million needle-like holes drilled into a 5µm-thick silicon membrane. The long, thin holes are only a few hundred nanometers in diameter, making their aspect ratio – the ratio of height to width – nearly 30:1. This 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, a professor of applied physics and a senior research fellow in electrical engineering at Harvard SEAS, who is senior author of the paper.

“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.”

Just like the nanopillars, the holes in the metalens vary in size to focus light, and are precisely positioned over the 2mm lens diameter. This hole size variation bends the light towards 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 multi-wavelength 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 them 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 commercialization opportunities.

This story is adapted from material from Harvard SEAS, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.