Q analysis and Transmission box for Purcell factor


I am working on calculating the Purcell factor using power ratio manually without using “Purcell” in FDTD. And also looking for resonant modes and its Q factor( Vertical Q) in a photonic crystal.

I have 2 Questions while simulation.

First, I tried to calculate 3D Q factor using (high) Q Analysis in Analysis group. But after running the simulation, at the result view, “Q” of the Q analysis shows a weird graph. I attached the graph below. I don’t know what the problem is.
Also, I tried to measure 2D Q factor(Vertical Q) using Q Analysis in Analysis group same as above. This also has the same problem.

Second, I wanted to calculate the Purcell factor with Transmission box in Analysis group. I tried to use this data to calculate the Purcell factor divide by the data from Transmission box without the structure.
The graph of the transmission box, it shows negative power. the structure is a passive cavity, So I think there should be no negative power. Because negative power means power flow into the structure.

Q factor graph

transmission box

Here is my .fsp File



Hello @lilatop,

In regard to your question on transmission. The z span of your monitors is too small, and so the evanescent fields are distorting your results. The monitors should be well away from the structure so the radiation they see is being actually being radiated to the PML. It is likely though I haven’t run your simulation the simulation is too short and is ending before the auto-shutoff is reached.

The Q analysis groups are explained in that link, please make sure that the conditions for the time monitors are being met. I believe that your simulation has gotten too complicated too quickly. I would recommend first finding the resonant modes which can be done using a dipole cloud. Then think about how to excite the modes of interest, and consider whether you need so many dipoles? These will complicate your calculations, and so limiting the number of them will make the analysis cleaner. Which sort of Q factor do you have? Use the appropriate analysis group.

Also it would be quite simple to use the Purcell enhancement returned by the dipole given that you are not using lossy or dispersive materials. Why the need to use the transmission monitors and so many sources?

To get a better sense of what is going on you might want to run three short simulations with a single dipole using ,x,y,z orientation. Look at which modes are excited, how slowly they decay, and get the Purcell enhancement for a dipole emitter at the center of the cavity.

Kind Regards,