Source shifting in frequency during parameter sweep



I am trying to model the reflected amplitude of Chi3 for a thin film as a function of film thickness. The thin film is
that is on a SiO2/Si substrate so I want to use lumerical to account for any reflections/interference effects that might affect the reflected THG signal. I’m basing my model off the harmonic.fsp example and look at the simulated spectrum amplitude at the frequencies of interest. I have found that when I change the thickness of the material, the source
jumps around significantly as can be seen in this figure (used the original example with Chi2 and varied the mesh and material thickness simultaneously):

Is this a problem with the Fourier Transform? I’ve increased the mesh accuracy to 6 and it doesn’t seem to help. I’ve also turned off auto-shutoff thinking that maybe I need to collect spectra that all have the same simulation length. Do you have any suggestions?



Hi Nathan,

I have a few questions:

  • By “mesh thickness” in the plot do you mean the thickness of the chi3-material film?
  • Are you plotting the spectrum from the fields recorded at a time monitor?

I am not sure this is an issue related to the Fourier transform. I think I would need to take a look at your simulation file to give you more specific feedback.


Thanks for the response! Yes, I was changing the mesh thickness and the chi3-material thickness at the same time, so the y-axis label is the same for both. I found that it is better to use a frequency monitor instead of a time monitor and my problem went away.