Today I get sometime to review this post again, and have the following comments:

Since most often the metalens simulation involves Lumerical FDTD/RCWA and Zemax OpticStudio, some people may get confused when reading the articles from both websites. The basic work flow is from Lumerical article: https://optics.ansys.com/hc/en-us/articles/360042097313-Small-Scale-Metalens-Field-Propagation

The first step is to get the theoretical, ideal phase distribution (space) desired for the metalens with given focal length. We call this as "target phase profile". If it is ideally spherical, cylinderical, or any other phase distribution that can be analytically obtained, or from a table, it is easy to directly get the phase distribution, with any number of sampling you want. If one cannot directly get the phase, you will need Zemax to create the desired phase. The phase data in OpticStudio is in the Surface Phase tool.

Please note that in this step, the sole goal is to get the target phase distribution. Therefore you can use whatever the tools you have at hand. If you use Zemax, OpticStudio has fixed sampling points, seemly 2^n+1 where n=4,5,6,.... the integer number. Here the OpticStudio is not used to simulate the metalens, again it is to get the desired target phase!

The rest is the same as the examples. When the design of the metalens is for normal incidence, in Zemax it has to be illuminated with collimate light, and in FDTD it has to use plane wave with incident angle zero for simulation.

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