Inverse Design of Multifunctional Metasurface Based on Multipole Decomposition and the Adjoint Method

The functional and optimal design of optical metasurfaces poses great challenges, particularly in situations requiring multifunctionalities. The conventional forward design relies on a priori knowledge, which limits the development of metasurfaces with customized functions. Inverse design has achieved major breakthroughs in developing new optical functionalities but has been hindered by inevitably invoking a full-wave electromagnetic solver, subjected to low efficiency, slow convergence, and being hard to implement. Here, we propose a general and computationally efficient inverse-design framework based on multipole decomposition and adjoint optimization. By this method, we demonstrate an achromatic metalens working across the visible and near-infrared band with a bandwidth over 400 nm. We also show the design of the metasurface for multiplexed focusing at the RGB wavelength, and the metasurface for stimulated emission depletion (STED) microscopy, which generates a tightly focusing point at the excitation beam wavelength and doughnut-shaped focus for the STED beam wavelength. Our scheme could be further improved to include more geometrical degrees of freedom and show great applications in free-space diffractive optics and deep diffractive neural network.