@Guilin Sun Thank you so much for your detailed reply. I will try your suggestions and let you know how they turn out.
I have 2 quick questions:
1) I have yet to try Metallic BC to see if it still diverges. To clarify, that would be replacing just the PML layers (z+ and z-) with Metallic BC, and leaving the other 4 BCs (x+, x-, y+, y-) as Periodic BC, correct? (as opposed to all 6 BCs being Metallic).
2) I can try extending the length of the pulse to decrease the wavelength range. Just to check, this can also be achieved be de-selecting the 'Optimize for Short Pulse' Option? (as opposed to manually setting the time domain options)