Guilin Sun
Ansys Employee

This is a very interesting discovery! 

I did not find the reference paper for stiching method. But I guess it may not conisder the off-axis effect: it may assume that the propagation has the same axis as the incident or the desired diffraciton order. 

I guess this is a theoretical problem. You have verified with direct simulation so such stiching method might need to be improved.

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