High‐Efficiency, Extreme‐Numerical‐Aperture Metasurfaces Based on Partial Control of the Phase of Light

High‐quality flat optical elements require efficient light deflection to large angles and over a wide wavelength spectrum. Although phase gradient metasurfaces achieve this by continuously adding phase shifts in the range of 0–2π to the electric field with subwavelength‐sized scatterers, their performance is limited by the spatial resolution of phase modulation at the interface. Here, a new class of metasurfaces is introduced, where the phase shifts cover less than the full 0–2π range, offering significant advantages. This approach allows the realization of metasurfaces with more compact and less mutually‐interacting scatterers, and thus more precise phase modulation, and advances the performance limits of metasurfaces to domains significantly beyond those of the full coverage phase gradient approach. Applying this concept to both plasmonic and dielectric surfaces, large phase gradients resulting in high‐numerical‐aperture immersion metalenses (NA = 1.4) with near diffraction‐limited resolution (≈0.32λ) at visible wavelengths are demonstrated. This concept enables added functionalities such as broadband performance and wavelength de‐multiplexing on a single layer, surpassing the theoretical cross‐polarization transmission efficiency limit for single‐layer plasmonic metasurfaces, and yields 67% efficiency for dielectric metasurfaces. This work paves the way toward realizing high‐resolution flat optical elements and efficient plasmonic metadevices.