Metasurfaces are nanoscale structures that interact with light. Today, most meta­surfaces 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. What if, instead of building tall structures, you went the other way? Now, 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.  

The new meta­surface uses more than 12 million needle-like holes drilled into a 5-micrometer silicon membrane. The diameter of these long, thin holes is only a few hundred nanometers, making the aspect ratio 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.

“If you tried to make pillars with this aspect ratio, they would fall over,” said Daniel Lim, a graduate student at SEAS. “The holey platform increases the acces­sible aspect ratio of optical nano­structures without sacri­ficing mechanical robustness.” Just like with nano­pillars, which vary in size to focus light, the holey metalens has holes of varying size precisely positioned over the 2 milli­meters lens diameter. The 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. “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 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 commerciali­zation oppor­tunities. (Source: Harvard SEAS)

Reference: S. W. D. Lim et al.: A High Aspect Ratio Inverse-Designed Holey Metalens, Nano Lett. 21, 8642 (2021); DOI: 10.1021/acs.nanolett.1c02612

Link: Capasso Group, Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, USA