Grating transmission disagreement

- April 4, 2024 at 10:44 pm
  
  Davide Cassara
  Subscriber
  
  Hi everyone,
  I am encountering a problem in Lumerical that I don't really understand. I have some old simulations from a former member of my research group, these are FDTD simulations of a grating with some pillars, very basic structure. He had added two Analysis groups, each of them with a DFT monitor to compute the grating efficiency of each diffraction order. The two monitors were literally identical, just at different position on z (200 nm apart from each others). Nevertheless, the results coming from the two monitors are very different, one of them gives an efficiency of 35%, the other more than 70%. I am sure that the right one is the one with 70% efficiency because that device was fabricated and measured, yielding that efficiency. The main problem is that now I cannot access the data of that monitor (the 70% one), I can only see the figures generated by Lumerical, but if I try to access the data, to export them I get the error "Failed to calculate the data", while the other monitor works just fine in terms of exporting it. I have also tried to make the monitor equal also in terms of z position, but, as expected, it doesn't change, one is always high efficiency and the other half of it almost.
  I have tried to remake the simulation from scratch with only one monitor and I get the results of low efficiency, but my only concern is that those data were confirmed experimentally. I reached out to the original author of the simulations and he doesn't remember why he put the two monitors two years ago, but he is sure that he was able to retrieve all the data within lumerical and generate .txt files of grating order vs efficiency.
  Has anyone ever encountered a similar problem? I really have no idea why that monitor is giving so many troubles. Could it be related to the version of Lumerical he was using at that time?
  Thanks to anyone who'll respond.
- April 5, 2024 at 5:22 pm
  
  Guilin Sun
  Ansys Employee
  
  I am sorry to hear this. Usually you can open the analysis group, check the script and see what it is computing. you can also refer to this example
  Diffraction grating and copy its analysis group.
  Make sure the PML is extended outside of PML, long simulation time with small autoshutoff min in case there is strong resonance.