Hi Abha,
You can try the "steep" PML profile. Even though you have normal incidence, you could have diffration orders in different directions as it is a periodic geometry. Just to double check, you mention that there is a particle on top of the structure, is the whole geometry (including the particle) periodic? You can use periodic BCs with plane waves only when the geometry is periodic.
lambda is the source wavelength. If you are using a wideband source then the best practice is to use the longest wavelength.Â
Would you be able to share a screenshot of the geometry? That will help me provide more specific information.
Regards,
Amrita
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