Achromatic metalens focuses light regardless of polarization

23 Jan 2019

New design "doubles the efficiency of the lens", says Federico Capasso's development group at SEAS, Harvard.

Lenses are designed to work across a wide range of wavelengths and applications, from phone cameras to microscopes and sensors. Primarily they must be able to focus light regardless of its polarization.

Researchers have long believed that symmetric nanostructures such as circular pillars have been essential elements in the development of photonic devices that are not sensitive to polarization.

But now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a polarization-insensitive metalens comprising non-symmetric nanofins that can achromatically focus light across the visible spectrum without aberrations.

Such a flat lens could be used for many applications from virtual or augmented reality headsets to microscopy, lithography, sensors, and displays.

The research was led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, and published in Nature Communications.

“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 and first author of the paper. “This is the first paper that demonstrates both achromatic and polarization insensitive focusing in the visible spectrum.”

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.

Discarding incorrectly-polarized light

“This meant we were essentially discarding half of the incident light which does not possess the right polarization,” said Alexander Zhu, co-author of the study and graduate student at SEAS. 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.

Looking ahead, Capasso himself commented, “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.”

Harvard’s Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialization opportunities. This research was co-authored by Jared Sisler, and Zameer Bharwani. It was supported by the Air Force Office of Scientific Research and the Defense Advanced Research Projects Agency.

Through the conference program at Photonics West 2019, early next month, members of Capasso's group will be making several presentations on meta surfaces and related photonics science and applications.