Hi Henrick,
It looks like some strange modal interaction is going on with the simulation boundaries. Would you be able to share a screenshot of the simulation geometry? Sometimes if there is a gap between the boundaries and geometry (by geometry I mean substrate/superstarte here, I can't tell if there is one or if this is just a waveguide surrounded by the background material), it creates an artificial cavity which can result in these unphysical modes. I would also be interested to know what BCs are you using? Metal or PML? If you are using metal, try switching to PML and rerun the simulation.
Regards,
Amrita
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