Two-Photon Microscopy with a Double-Wavelength Metasurface Objective Lens Ehsan Arbabi, † Jiaqi Li, ‡,¶ Romanus J. Hutchins, § Seyedeh Mahsa Kamali, † Amir Arbabi, ∥ Yu Horie, † Pol Van Dorpe, ‡,¶ Viviana Gradinaru, ⊥ Daniel A. Wagenaar, ⊥ and Andrei Faraon* ,† † T. J. Watson Laboratory of Applied Physics and Kavli Nanoscience Institute, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States ‡ IMEC, Kapeldreef 75, B-3001 Leuven, Belgium ¶ Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium § Department of Physics and Astronomy, University of Missouri Columbia, Columbia, Missouri 65211, United States ∥ Department of Electrical and Computer Engineering, University of Massachusetts Amherst, 151 Holdsworth Way, Amherst, Massachusetts 01003, United States ⊥ Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125, United States * S Supporting Information ABSTRACT: Two-photon microscopy is a key imaging technique in life sciences due to its superior deep-tissue imaging capabilities. Light-weight and compact two-photon microscopes are of great interest because of their applications for in vivo deep brain imaging. Recently, dielectric metasurfaces have enabled a new category of small and lightweight optical elements, including objective lenses. Here we experimentally demonstrate two-photon microscopy using a double-wavelength metasurface lens. It is specifically designed to focus 820 and 605 nm light, corresponding to the excitation and emission wavelengths of the measured fluorophors, to the same focal distance. The captured two- photon images are qualitatively comparable to the ones taken by a conventional objective lens. Our metasurface lens can enable ultracompact two-photon microscopes with similar performance compared to current systems that are usually based on graded-index-lenses. In addition, further development of tunable metasurface lenses will enable fast axial scanning for volumetric imaging. KEYWORDS: Optical metasurface, flat optics, multiwavelength lens, two-photon microscopy T wo-photon microscopy is widely used for deep tissue imaging in various areas of life sciences. 1−4 The method utilizes the lower scattering of near-IR light inside tissues, the higher transverse and lateral resolution, and the lower level of background fluorescence in two-photon excitation to form high-quality images hundreds of microns deep inside tissues. 3,4 Development of compact low-weight two-photon microscopes for in vivo imaging of brain activity has been of great interest in recent years. 5−10 For compactness and low-weight, most of these systems use graded index objective lenses with optical qualities inferior to the conventional refractive objectives. Dielectric metasurfaces are a recent category of diffractive devices 11−13 that enable high-end optical elements like blazed gratings 14 and lenses 15−18 with high efficiencies, and with a thin and lightweight form factor. Metasurface devices integrated in thin layers 19,20 and on membranes 21 have milligram and microgram weights. Therefore, the weight of optics will not be a significant factor in the total weight of systems that employ metasurface optics. In addition, because of their novel capabilities 22−30 and manufacturability with conventional nanofabrication techniques, metasurfaces have attracted a great deal of attention in the past few years, especially for imaging applications. 16,17,31 Fluorescence mi- croscopy is an especially suitable area for meta-lenses as the fluorescence bandwidth is usually limited and predetermined. For instance, meta-lenses have recently been utilized to capture single-photon fluorescence images of diamond nanocrystals with embedded silicon vacancy emitters where the effects of the finite fluorescence bandwidth were also studied. 18 Despite this, meta-lenses have not previously been employed for multiphoton fluorescence microscopy. The reason lies in the fact that they are conventionally designed for a single operation wavelength, while in fluorescence microscopy the focal positions at the excitation and emission wavelengths can be far apart due to the chromatic dispersion. 32−35 This can significantly reduce the excitation-collection efficiency in the system. Received: April 28, 2018 Revised: July 12, 2018 Published: July 17, 2018 Letter pubs.acs.org/NanoLett Cite This: Nano Lett. 2018, 18, 4943-4948 © 2018 American Chemical Society 4943 DOI: 10.1021/acs.nanolett.8b01737 Nano Lett. 2018, 18, 4943−4948 Downloaded via HANYANG UNIV on March 4, 2019 at 01:56:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.