Subwavelength resolution imaging requires high numerical aperture (NA) lenses, which are bulky and expensive. Metasurfaces allow the miniaturization of conventional refractive optics into planar structures. We show that high-aspect-ratio titanium dioxide metasurfaces can be fabricated and designed as metalenses with NA = 0.8. Diffraction-limited focusing is demonstrated at wavelengths of 405, 532, and 660 nm with corresponding efficiencies of 86, 73, and 66%. The metalenses can resolve nanoscale features separated by subwavelength distances and provide magnification as high as 170×, with image qualities comparable to a state-of-the-art commercial objective. Our results firmly establish that metalenses can have widespread applications in laser-based microscopy, imaging, and spectroscopy.

In this Letter, we demonstrate highly efficient, polarization-insensitive planar lenses (metalenses) at red, green, and blue wavelengths (λ = 660, 532, and 405 nm). Metalenses with numerical apertures (NA) of 0.85 and 0.6 and corresponding efficiencies as high as 60% and 90% are achieved. These metalenses are less than 600 nm-thick and can focus incident light down to diffraction-limited spots as small as ∼0.64λ and provide high-resolution imaging. In addition, the focal spots are very symmetric with high Strehl ratios. The single step lithography and compatibility with large-scale fabrication processes make metalenses highly promising for widespread applications in imaging and spectroscopy.

Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Original dielectric metasurfaces are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Thus, it is critical that new materials and nanofabrication techniques be developed to extend dielectric metasurfaces across the visible spectrum and to enable applications such as high numerical aperture lenses, color holograms, and wearable optics. Here, we demonstrate high performance dielectric metasurfaces in the form of holograms for red, green, and blue wavelengths with record absolute efficiency (>78%). We use atomic layer deposition of amorphous titanium dioxide with surface roughness less than 1 nm and negligible optical loss. We use a process for fabricating dielectric metasurfaces that allows us to produce anisotropic, subwavelength-spaced dielectric nanostructures with shape birefringence. This process is capable of realizing any high-efficiency metasurface optical element, e.g., metalenses and axicons.devices composed of subwavelength-spaced units and near-flat profiles compared with refractive optics (7-9)-have allowed unprecedented control over optical wavefronts (5) while circumventing Ohmic losses associated with plasmonic metasurfaces. Due to the scientific and technological importance of visible wavelengths, there has been increasing effort to realize DMs at these wavelengths. For example, silicon nitride DMs have been realized at a red wavelength (λ = 633 nm) (10), scattering properties of coupled TiO 2 resonators (11) have been examined, and TiO 2 binary diffraction gratings have been fabricated via top-down etching (12). However, no current DM implementation has been capable of providing arbitrary phase control of an optical wavefront while maintaining high efficiency across the entire visible spectrum (especially at blue and green wavelengths). Furthermore, the typical top-down techniques used to implement these metasurfaces can introduce significant surface roughness and make it difficult to create subwavelength sampling of a desired optical phase profile.In this paper, we demonstrate amorphous TiO 2 metasurfaces that maintain high efficiency across the entire visible spectrum. Our approach to creating DMs uses a bottom-up nanofabrication via atomic layer deposition providing high-aspect ratio, anisotropic dielectric nanostructures with minimal surface roughness. As proof of the concept that we can provide control of the phase of a wavefront from 0 to 2π, a requirement for many optical components, we produced metasurface holograms based on geometric phase. Efficient metasurfaces with metallic components operating in reflection have been demonstrated at red and nearinfrared wavelengths (13, 14) but have efficiencies of <1% and <10% at blue and green wavelengths, respectively (15, 16). Thus, the TiO 2 metasurfaces we demonstrate here provide substantial improvement with efficiencies fro...