Hi Chuan,
I would not recommend using a mode source. The mode source is used to inject guided waveguide modes into a geometry, here you are simulating free-space propagation. Usually we would use a plane wave/Gaussian source for free space propgation. Would you be able to share a few more details like: What kind of BCs are you using? Does the simulation have a specific width in the horizontal direction? Also, I think even if you use a larger source, you should be able to see the transmission through the smaller region if you just use a small monitor. But if you could provide the above details, I can share more specific recommendations.
Regards,
Amrita
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