Guilin Sun
Ansys Employee
Sources can only work with proper boundary conditions in order to fulfill physics. Mode source has limited size so it uses PML or sometimes PEC; Plane wave is infinite so periodic-type BC or symmetry-type ( PEC/PMC in some cases) BC in both sides is used. Please refer :Ansys Insight: FDTD Solutions 的光源类型及其典型应用：初学者
Please note that for mode (and Gaussian) source, its geometry size can be as large as the simulation spans. This is because the mode property controls the physical size.
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