  Inverse design of large-area metasurfaces can potentially exploit the full
parameter space that such devices offer and achieve highly efficient
multifunctional flat optical elements. However, since practically useful flat
optics elements are large in the linear dimension, an accurate simulation of
their scattering properties is challenging. Here, we demonstrate a method to
compute accurate simulations and gradients of large-area metasurfaces. Our
approach relies on two key ingredients - a simulation distribution strategy
that allows a linear reduction in the simulation time with number of compute
(GPU) nodes and an efficient single-node computation using the
Transition-matrix (T-matrix) method. We demonstrate ability to perform a
distributed simulation of large-area, while accurately accounting for
scatterer-scatterer interactions significantly beyond the locally periodic
approximation, and efficiently compute gradients with respect to the
metasurface design parameters. This scalable and accurate metasurface
simulation method opens the door to gradient-based optimization of full
large-area metasurfaces.
