Metalenses are Growing Up

Metalenses – flat surfaces that use nano­structures to focus light – are poised to revolutionize everything from micro­scopy to cameras, sensors, and displays. But so far, most of the lenses have been about the size of a piece of glitter. While lenses this size work well for some appli­cations, a larger lens is needed for low-light conditions, such as an imaging system onboard orbital satel­lites, and VR appli­cations, where the lens needs to be larger than a pupil. Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences SEAS have developed an all-glass, centimeter-scale metalens in the visible spectrum that can be manu­factured using conven­tional chip fabrication methods.

Illustration of an all-glass, large-diameter metalens working at visible wavelengths made with conventional computer chip manufacturing methods. (Source: Capasso group, Harvard SEAS)

“This research paves the way for wafer level cameras for cell phones, where the CMOS chip and the meta­lenses can be directly stacked on top of each other with easy optical alignment because they are both flat,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics at SEAS. “In the future, the same company can make both the chip and the lenses because both can be made using the same tec­hnology: litho­graphy.”

“Previously, we were not able to achieve mass-production of centi­meter-scale metalenses at visible wavelengths because we were either using electron-beam litho­graphy, which is too time consuming, or i-line stepper litho­graphy, which does not have enough resolution to pattern the required subwavelength-sized structures,” said Joon-Suh Park, a Ph.D. candidate at SEAS. To mass produce a centi­meter-scale metalens, the researchers used deep-ultra­violet (DUV) projection litho­graphy, which is commonly used to pattern very fine lines and shapes in silicon chips in everything from computers to cell phones. The technique can produce many meta­lenses per chip, each made of millions of nanoscale elements with a single shot of exposure, like taking a photograph.

The researchers eli­minated the time-consuming depo­sition processes that were required for previous metalenses by etching the nano­structure pattern directly onto a glass surface. It is the first mass-producible, all-glass, centi­meter-scale metalens in the visible spectrum. While this lens is chromatic, meaning all the different colors of light don’t focus at the same spot, the researchers are working on large-diameter achro­matic metalenses. (Source: Harvard SEAS)

Reference: J.-S. Park et al.: All-Glass, Large Metalens at Visible Wavelength Using Deep-Ultraviolet Projection Lithography, Nano Lett., online 14 November 2019; DOI: 10.1021/acs.nanolett.9b03333

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

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