Hello, thank you for your questions and sharing your model screenshots! Based on what you shared, a PML boundary could be a good choice since there is a high reflection at metal boundaries.
However, I would consider instead expanding the simulation region if that is possible. If the evanescent fields of the mode interact with the boundary (either metal or PML), it can introduce artifacts into your simulation. Even a PML boundary can have a non-negligible reflection. Therefore, we generally recommend that the field intensity should be around 10^-10 at PML boundaries.
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