Absorption Calculation Problem



We are trying to run FDTD simulation where we embed Ag-silica core shell nanoparticles (Ag = core, silica = shell) inside the Si substrate of a thin-film solar cell. We are attaching the .fsp file for our simulation. We intend to embed an array of Ag-silica nanoparticles in 3 positions inside the Si substrate: (i) right underneath the Si substrate (top surface); (ii) inside the middle of the Si substrate; (iii) on the bottom surface of the SI substrate. Our goal is to find the increase in light absorbed by the Si substrate due to the effect of the Ag-silica nanoparticles. We also present our results. As you can see, for the case of the Ag-silica nanoparticle right inside the top surface of the silica (case (i)), we can see a smooth curve representing the increased optical absorption across all wavelengths. However, for the case of the Ag-silica nanoparticles in the middle of the Si substrate (case (ii)) and for the Ag-silica nanoparticles near the bottom surface of the Si substrate (case (iii)), our absorption enhancement curves show a large amount of noise which is periodic in nature and more pronounced in the longer wavelengths. We believe this is due to interference problems which is related to both the SI substrate thickness and maybe other causes we are not too sure about. Can you please help us identify why this problem is occurring and how we can fix it.



absp_analysis.lsf (227 Bytes)
embedded_particles.fsp (288.7 KB)


Hi @1130344

The best practice would be to perform convergence testing. I can see that the PML is very close to the sphere in the -z direction. You can also run multiple narrowband simulations instead of one broadband simulation.

When I checked the simulations with three structure group, I could see the ripples even with the top structure group:

However the results between structure group vary which can be explained by the strong absorption in silicon:

Please keep me updated with your thoughts.


Thank you for your reply.
You could see the ripples even with the top structure because I changed the position of the particles there and disabled it. My concern was with case(ii) only. Will running multiple narrowband simulations help reduce the oscillations? Can you please expand on what you meant by convergence testing?


Hi @1130344

Please visit the link below for a complete list of convergence testing:

Using narrowband pulse is also part of convergence testing. If the results are still the same, that means that results are converging.

I did a few test and results were almost the same. Can you please elaborate on what do you expect the results to be and why the current results are not correct? Do you have any references?



Hello @bkhanaliloo
Thank you for all the replies.
As we go deeper in to the substrate we expect the enhancement curve profile(shape) or the magnitude to change not these high periodic feature(oscillations) which could be some kind of noise(we are not sure). We don’t have any references currently. We just want to know whether this is a physical property of the structure or this has something to do with cavacity.
Thank you


Hi @1130344

Can you please study how the results vary with:

  1. Increasing the z-span of FDTD
  2. Using different PML profiles and number of layers
  3. Increasing simulation time

Generally, non-physical ripples can be caused if there was some light left inside the simulation region when simulation finishes. This can be because of poor PML absorption or early shutoff. If the results were still the same, we need to explain where the ripples are coming from. Unfortunately I do not have a clear explanation for this.



Dear @bkhanaliloo
Again Thank you so much for your time and effort. I will work on these changes and let you know the results.