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(a) Comparison of the phase calculated by finite difference time domain simulation of the building block (nanopillar on a glass substrate) as a function of radius R, and the phase due to propagation in an isolated cylindrical waveguide, considering just its fundamental mode at = 633 nm. The areas A, B, and C represent different degrees of fitting for two methods; (b) The nanopillars selected for the design of metalenses with a focal length of 2, 4, and 6 μm respectively.
