Not smooth curve and calculating modes.

fdtd

#1

I am simulating plasmonic nanorods on a substrate , here is my simulation:

Here is my curve ( scattering cross section)

Why isn’t it smooth? I thought it was meshing problem , I increased meshing size ( still not smooth)
I increased frequency points to 300 ( still not smooth)

Any idea?

Second question is , I want to calculate the mode ( E^2) Is it better to put my monitor just under the nanoparticles or in the middle? And are they positioned in a right way in the simulation?

Thanks!


#2

Hi Ahmed,

The ripples in the scattering cross-section are probably due to an early termination of the simulation. If the fields inside the simulation region have not decreased enough, it is possible that some artifacts (such as ripples) show up in the frequency-domain data. That would be my guess, but if you share your simulation file I can take a look and check if there are any other issues.

Regarding the position of the monitor to get E^2, I guess it depends on what you are interested in. My guess is that the purpose of the rods is to enhance the fields near the surface of the substrate (such as in this plasmonic solar cell example). Then you probably want a monitor just below the rods, inside the substrate. You can always look at several monitors at different positions and determine which one makes more sense for your analysis.


#3

If the simulation really terminated early, how can I fix this?

I changed the geometry a bit, and I got ripples at the right :


#4

Hi Ahmed,

Sorry for the long wait. I checked your simulation file and it seems like the simulation time is long enough to trigger the autoshutoff, so probably there is no early termination of the simulation. However, I noticed there was a small region where the nanorods overlap with the substrate. Is this intended? Typically, one would have the scattering object just sitting on top of the substrate. Also, the substrate is usually assumed to be much larger than the scattering object so it typically extends all the way into the PML.

For comparison with your previous results I simulated the case where the rods do not overlap with the substrate and the substrate extends all the way to the PML: nanorods_modFG.fsp (308.0 KB). The scattering cross section looks like this:

Before checking if there are any numerical artifacts in the cross section I think we should make sure the structure is correctly simulated.

Some additional changes I made to the simulation are the following:

  • The mesh inside the TFSF should be uniform along the directions perpendicular to the injection direction as mentioned here.
  • For simplicity I centered all the structures, the source, the simulation region, etc. at 0.
  • Used antisymmetric boundary conditions along the x direction.