  Flat lenses enable thinner, lighter, and simpler imaging systems. However,
large-area and high-NA flat lenses have been elusive due to computational and
fabrication challenges. Here, we applied inverse design to create a multi-level
diffractive lens (MDL) with thickness <1.35{\mu}m, diameter of 4.13mm, NA=0.9
at wavelength of 850nm. Since the MDL is created in polymer, it can be
cost-effectively replicated via imprint lithography.
