Should I make sure that fields have fully decayed by the end of the simulation?

Date	Category
2023-10-24 17:36:08	Simulation Troubleshoot

When early termination happens, you may sometimes get a warning that the fields remaining in the simulation at the end of the run have not decayed down to the pre-defined shutoff value. This should usually be avoided (that is to say, Simulation.run_time should be increased), but there are some cases in which it may be inevitable. The important thing to understand is that in such simulations, frequency-domain results cannot always be trusted. The frequency-domain response obtained in the FDTD simulation only accurately represents the continuous-wave response of the system if the fields at the beginning and at the end of the time stepping are (very close to) zero. That said, there could be non-negligible fields in the simulation. Yet, the data recorded in a given monitor can still be accurate if the leftover fields are no longer passing through the monitor volume. From the point of view of that monitor, fields have already fully decayed. However, there is no way to automatically check this. The accuracy of frequency-domain monitors when fields have not fully decayed is also discussed in one of our FDTD 101 videos.

The primary use case in which you may want to ignore this warning is when you have high-Q modes in your simulation that would require an extremely long run time to decay. In that case, you can use the ResonanceFinder plugin to analyze the modes, as well as field monitors with apodization to capture the modal profiles. The only thing to note is that the normalization of these modal profiles would be arbitrary and would depend on the exact run time and apodization definition. An example of such a use case is presented in our high-Q photonic crystal cavity case study.