Let me summarize your suggestion and let me know whether there is any misunderstanding:
Step A: check the divergence's reason
take only half of the simulation region + Use thicker PML in X min/max = 64 + only take override mesh on single Z dimension (the thin layer dominates my mode) + add monitors to check the scattered light
Q1: after checking and back to final simulation, I still need to recover the override mesh back to the X/Y direction so that an integer number of grids is placed in the coupling gap, correct?
Q2: where should I place those monitors? At corresponding boundary?
Step B: Recover the simulated region and set PML accordingly and run again.
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