Title: High Numerical Aperture and Broadband Achromatic Flat Lens

Abstract: Flat lenses have shown promising applications in miniaturized and ultracompact lightweight optical systems. However, it has been a great challenge in simultaneously achieving broadband achromatism and high numerical aperture. Here, we demonstrate that this long-term dilemma can be broken through by the zone division multiplex of the meta-atoms on a composite substrate possessing stepwise optical thickness. The aperture size can be freely expanded by increasing the optical thickness difference between the central and marginal zones of the substrate, free from achromatic bandwidth. The achromatic flat lens with both 0.9 numerical aperture and bandwidth of 650-1000 nm is experimentally achieved. A microscopic imaging with 1.1 {\mu}m resolution has also demonstrated. These unprecedented performances mark a substantial step toward practical applications of the flat lenses.