Sorry to reply you late as our team is busy in other urgent projects.
NO need to upload anything as the forum does not allow file load and does not allow us downloand and check the files by forum policy.
I think it might be some reasons for the discrepancy. Is there any higher order diffraction from each element? is there sufficent number of elements? are the simulation settings including source are correct without any issue?
The summation method uses pure plane wave results whereas in FDTD simulation a plane wave will be affected by the aperture (if any) or PML, which will cause diffraction. and sometimes the diffraction interacts with the metalens and can create secondary (or higher-order) images.
Please check the simulation files and see if there is anything to improve Not every design can lead to expected results.
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