In a recent paper, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed a metasurface that uses very deep, very narrow holes, rather than very tall pillars, to focus light to a single spot.
Today, most metasurfaces (nanoscale structures that interact with light) use 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, giving the metasurface more versatile control of each color of light. But very tall pillars tend to fall or cling together. Instead of building tall structures, the researchers at Harvard went the other way.
The new metasurface uses more than 12 million needle-like holes drilled into a 5-micrometer silicon membrane, about 1/20 the thickness of hair. The diameter of these long, thin holes is only a few hundred nanometers, making the aspect ratio — the ratio of the height to width — nearly 30:1.
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.