Metasurface Corrects Chromatic Aberrations

An SEM image of the metacorrector. The refractive index of each nanopillar can be tuned and controlled so that all wavelengths are brought to the same focal point. (Source: Harvard SEAS)

Today’s optical systems use tech­nology that hasn’t changed much since the mid-1700s. Compound lenses, invented around 1730, correct the chromatic aberrations that cause lenses to focus different wavelengths of light in different spots. While effec­tive, these multi-material lenses are bulky, expensive, and require precision polishing or molding and very careful optical alignment. Now, a group of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences SEAS has developed a meta­corrector, a single-layer surface of nano­structures that can correct chromatic aber­rations across the visible spectrum and can be incor­porated into commer­cial optical systems, from simple lenses to high-end micro­scopes.

The meta­corrector eli­minated chromatic aber­rations in a commercial lens across the entire visible light spectrum. The device also works for the super-complex objectives with as many as 14 conven­tional lenses, used in high-resolution micro­scopes. “Our meta­corrector tech­nology can work in tandem with tradi­tional refractive optical components to improve perfor­mance while signi­ficantly reducing the complexity and footprint of the system, for a wide range of high-volume appli­cations,” said Federico Capasso, the Vinton Hayes Senior Research Fellow in Elec­trical Engineering at SEAS.

In previous research, Capasso and his team demon­strated that meta­surfaces, arrays of nano­pillars spaced less than a wavelength apart, can be used to mani­pulate the phase, amplitude and polari­zation of light and enable new, ultra-compact optical devices, including flat lenses. This research uses the same principles to tune and control the effective refrac­tive index of each nano­pillar so that all wave­lengths are brought by the meta­corrector to the same focal point. “You can imagine light as different packets being delivered at different speeds as it propa­gates in the nano­pillars. We have designed the nano­pillars so that all these packets arrive at the focal spot at the same time and with the same temporal width,” said Wei Ting Chen, a Research Associate in Applied Physics at SEAS.

“Using meta­correctors is funda­mentally different from conven­tional methods of aber­ration correc­tion, such as cascading refrac­tive optical components or using dif­fractive elements, since it involves nano­structure engi­neering,” said Alexander Zhu, a graduate student at SEAS. “This means we can go beyond the material limi­tations of lenses and have much better perfor­mances.” Next, the researchers aim to increase effi­ciency for high-end and miniature optical devices. 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.: Broadband Achromatic Metasurface-Refractive Optics, Nano Lett., online 13 November 2018; DOI: 10.1021/acs.nanolett.8b03567

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

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