Researchers led by Federico Capasso at the Harvard John A. Paulson School of Engineering and Applied Science (SEAS) have developed a 2-mm achromatic metalens capable of focusing RGB colors without aberrations. The researchers also developed a miniaturized display for virtual reality and augmented reality applications. Capasso’s lab was successful in previously developing achromatic aberration-free metalenses in 2018. At just tens of microns in diameter, though, the lenses were too small for practical use in VR and AR applications. “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,” Capasso said. He served as senior author on the paper describing the work. The AR imaging result using the full-color near-eye fiber scanning display, which shows an RGB-color virtual image floating in a real-world scene. Courtesy of Zhaoyi Li, Harvard University. As with previous versions of developed metalenses, the new lens uses arrays of titanium-dioxide nanofins. These serve to focus wavelengths of light and eliminate chromatic aberration. In engineering the shape and pattern of these nanoarrays, the team controlled the focal length of red, green, and blue light. To incorporate the lens into a VR system, the team used fiber scanning to develop a near-eye display. The researchers said the display is inspired by fiber-scanning-based endoscopic bioimaging techniques and uses an optical fiber housed in a piezoelectric tube. When that tube is electrified, the fiber tip scans in four directions, creating display patterns and forming a miniaturized display. The display exhibited high resolution, high brightness, high dynamic range, and a wide color gamut. In the case of an AR or VR platform, the metalens is situated directly in front of the eye with the fiber-scanning display positioned in the focal plane of the lens. Patterns generated by the display are focused onto the retina, thereby forming the virtual image. In an AR mode, the image would appear to the eye as part of the landscape. “Using new physics and 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.” The research was published in Science Advances (www.doi.org/10.1126/sciadv.abe4458).