A Broadband Achromatic Metalens

These newly designed nanostructures on the surface of a metalens can focus light regardless of its polarization, doubling the efficiency of the lens. (Source: Capasso Lab, Harvard SEAS)

We live in a pola­rized world. Much of the light we see and use is partially polarized, meaning its electric field vibrates in specific directions. Lenses designed to work across a range of appli­cations, from phone cameras to micro­scopes and sensors, need to be able to focus light regardless of its polari­zation. Researchers believed that symmetric nano­structures such as circular pillars were essential building blocks to develop photonic devices that are not sensitive to polari­zation. Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences SEAS have developed a polari­zation-insen­sitive metalens comprised of non-symmetric nanofins that can achro­matically focus light across the visible spectrum without aber­rations.

This flat lens could be used for every­thing from virtual or augmented reality headsets to micro­scopy, litho­graphy, sensors, and displays. “By making this lens polari­zation insen­sitive, we have doubled the effi­ciency of the metalens from previous itera­tions,” said Wei Ting Chen, a research associate at SEAS. “This is the first paper that demon­strates both achromatic and polari­zation insen­sitive focusing in the visible spectrum.” The research was led by Federico Capasso, the Robert L. Wallace Pro­fessor of Applied Physics.

In previous research, Capasso, Chen and their team demon­strated that arrays of titanium dioxide nanofins could equally focus wave­lengths of light and eli­minate chromatic aber­ration, but those lenses could only focus a circu­larly polarized light. “This meant we were essen­tially discarding half of the incident light which does not possess the right polari­zation,” said graduate student Alexander Zhu. In this latest design, the researchers changed the layout of the nanofins, posi­tioning each pillar so that it is either parallel or perpen­dicular to its neighbor.

“This new design gives us a lot of freedom to tune the geome­trical para­meters of the metalens, which allows us to better achieve achromatic focusing across the entire visible range,” said Chen. “Next we aim to maximize effi­ciency and make much larger-size achro­matic metalenses to bring them into everyday life for a wide range of appli­cations,” said Capasso. Harvard’s Office of Tech­nology Develop­ment has protected the intel­lectual property relating to this project and is exploring commerciali­zation oppor­tunities. (Source: SEAS)

Reference: W. T. Chen et al.: A broadband achromatic polarization-insensitive metalens consisting of anisotropic nanostructures, Nat. Commun. 10, 355 (2019); DOI: 10.1038/s41467-019-08305-y

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

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