Fig. 2

High numerical aperture of metalenses. (a) Schematic description of the metalens with nanoantenna arrays [55]. (b) Left panel: normalized measured intensity map in the focal plane (z = 45 μm). Right panel: intensity profile through the focus and the fitting to the Airy profile [55]. (c) schematic measurement of the liquid immersion metalens. Inset: an SEM image of meta-atoms [56]. (d) Left panel: normalized intensity profile of the focal spot from the metalens with NA = 0.9. Scale bar: 400 nm. Middle panel: The horizontal cut of left panel with its intensity normalized to its corresponding diffraction-limited Airy disk (black curve) for a same given area. Right panel: measured intensity variation at the center of the focal spot (green dots) along propagation direction [56]. (e) SEM image of the high-NA metalens and a building block of c-Si nano-bricks on a sapphire substrate [57]. (f) Focusing performance of the metalens in air (upper line) and immersion oil with n = 1.512 (lower line). Normalized intensity profile of the focal spot at the focal plane (middle panel) and intensity distribution in the focal plane along the dotted red line (right panel) [57]