Metasurfaces are ultrathin optical elements that are highly promising for constructing lightweight and compact optical systems.For their practical implementation,it is imperative to maximize the metasurface efficiency.Topology optimization provides a pathway for pushing the limits of metasurface efficiency;however,topology optimization methods have been limited to the design of microscale devices due to the extensive computational resources that are required.We introduce a new strategy for optimizing large-area metasurfaces in a computationally efficient manner.By stitching together individually optimized sections of the metasurface,we can reduce the computational complexity of the optimization from high-polynomial to linear.As a proof of concept,we design and experimentally demonstrate large-area,high-numerical-aperture silicon metasurface lenses with focusing efficiencies exceeding 90％.These concepts can be generalized to the design of multifunctional,broadband diffractive optical devices and will enable the implementation of large-area,high-performance metasurfaces in practical optical systems.