First things first, please make sure the simulation is periodic. You can either compare with 3by3 periods, or copy and paste the single period in a word file or other file for 3by3 periods and check if they duplicate without error.
No, Periodic structure should use grating analysis, as the farfield projection requires the fields at the monitor edge be zero, and it is not designed for periodic structure. One may use visualizer to roughly check the farfield, but not for quantify the results.
For plane wave, you just only need to have two simulations, with orthogonal polarization, and then average the result. Please refer to this https://optics.ansys.com/hc/en-us/articles/1500006149562
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