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Broadband high-efficiency dielectric metasurfaces for the visible spectrum a,1 a a a,b a,1 Robert C. Devlin , Mohammadreza Khorasaninejad , Wei Ting Chen , Jaewon Oh , and Federico Capasso aHarvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; and bUniversity of Waterloo, Waterloo, ON N2L 3G1, Canada Contributed by Federico Capasso, July 19, 2016 (sent for review May 31, 2016; reviewed by Stefan Maier and Lei Zhou) Metasurfaces are planar optical elements that hold promise for The ability to realize high-efficiency dielectric metasurfaces is overcoming the limitations of refractive and conventional diffractive critically dependent on the optical properties of the constituent optics. Original dielectric metasurfaces are limited to transparency material, as characterized by the complex refractive index, ñ = n + ik. windows at infrared wavelengths because of significant optical The material should have negligible absorption loss (k ≈ 0) with absorption and loss at visible wavelengths. Thus, it is critical that relatively high refractive index (n 2) at visible wavelengths. new materials and nanofabrication techniques be developed to Although a negligible absorption is necessary for high transmission extend dielectric metasurfaces across the visible spectrum and to efficiency, a high refractive index ensures strong confinement of enable applications such as high numerical aperture lenses, color the light, which ultimately allows full control over the phase of the holograms, and wearable optics. Here, we demonstrate high perfor- exiting wavefront (phase change