Harvard SEAS develops millimeter-scale flat metalenses

28 Jan 2021

Capasso group forms 2mm achromatic metalenses that focuses RGB with mini display for AR, VR applications.

At the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), a team of researchers led by Federico Capasso, has been developing “meta lenses” that promise replace bulky curved lenses with a simple, flat surface that uses nanostructures to focus light.

In 2018, the Capasso’s team developed achromatic, aberration-free metalenses that work across the entire visible spectrum of light. But these lenses were only tens of microns in diameter, too small for practical use in virtual and augmented reality systems.

Now, the researchers have developed a two-millimeter achromatic metalenses that can focus RGB wavelengths without aberrations and developed a miniaturized display for virtual and augmented reality applications. The research is published in Science Advances.

“This state-of-the-art lens opens a path to a new type of virtual reality platform and overcomes the bottleneck that has slowed the progress of new optical device,” said Capasso, who is the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, and senior author of the paper.

“Using a new design principle, we have developed a flat lens to replace the bulky lenses of today’s optical devices,” said Zhaoyi Li, a postdoctoral fellow at SEAS and first author of the paper. “This is the largest RGB-achromatic metalens to date and is a proof of concept that these lenses can be scaled up to centimeter size, mass produced, and integrated in commercial platforms.”

Like previous metalenses, this lens uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. By engineering the shape and pattern of these nanoarrays, the researchers could control the focal length of red, green and blue color of light. To incorporate the lens into a VR system, the team developed a near-eye display using a method called fiber scanning.

Miniaturized display

The display, inspired by fiber-scanning-based endoscopic bioimaging techniques, uses an optical fiber through a piezoelectric tube.

When a voltage is applied onto the tube, the fiber tip scans left and right and up and down to display patterns, forming a miniaturized display. The display has high resolution, high brightness, high dynamic range, and wide color gamut.

In a VR or AR platform, the metalens would sit directly in front of the eye, and the display would sit within the focal plane of the metalens.

The patterns scanned by the display are focused onto the retina, where the virtual image forms, with the help of the metalens. To the human eye, the image appears as part of the landscape in the AR mode, some distance from our actual eyes.

“We have demonstrated how meta-optics platforms can help resolve the bottleneck of current VR technologies and potentially be used in our daily life,” said Li. Next, the team is planning to scale up the lens even further, making it compatible with current large-scale fabrication techniques for mass production at a low cost.

The Harvard Office of Technology Development has protected the intellectual property relating to this project and is exploring commercialization opportunities.