# Absorption Calculation Problem

#1

Hello
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.
case(i):

case(ii):

case(iii):

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

#2

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: