Learning Forum

[ferruccio.pisanello]

Hi,

I want to calculate scattering from a nano-particle on an optical fiber due to the light modes guided inside the fiber (sketch attached). Would it be correct to use the scattering cross-section analysis group for this problem?

Most of the scattering examples in the App gallery use TFSF and the **** analysis group is placed outside the TFSF. What should be the correct simulation methodology in this particular case?

Any help/suggestions are appreciated. Thanks

[Kyle]

The TFSF source is used to substract the "incident field" (field with no scatterer) from the "total field" (field in the simulation with the scatterer), which gives the "scattered field". The built-in TFSF source does this for a plane wave source. In general, you can get the scattered fields with a different source by:
Running the simulation with no scatterer and recording the E/H fields with monitors to get the incident field
Running the simulation again with the scatterer present, recording the fields to get the total field
Subtracting the field results from the first simulation from the field results of the second simulation to get the scattered fields
Performing your post-processing with the scattered field results
The example on this page uses this approach: Plasmon Mode Excitation. You won't be able to use the analysis groups directly because you're running two different simulations, but you can look at the analysis group scripts to see how the post-processing is performed.

[ferruccio.pisanello]

Hi Thanks for the explanation and for sharing the example.
Regards

[ferruccio.pisanello]

Hi kjohnson Thanks for your reply.
In the example, Plasmon Mode Excitation, in script plasmon_mode_excitation.lsf, for the absorption calculation, I think the total field should be the input to the function total_power()? In the file, I downloaded from lumerical website, the scattered file is used as input. I have this confusion. Can you please comment on this?
px1 = total_power(E_x1_scat, H_x1_scat, sp.power(:, 1), "x");
Regards

[Kyle]

In this section the script is adding up the total scattered power, which is why it is using the scattered fields. Typically for plane wave TFSF sources the scattered power is normalized to the source intensity to get the cross section: https://support.lumerical.com/hc/en-us/articles/360034902133-Understanding-source-normalization-in-the-TFSF-source. In this script is looks like the scattering cross-section is normalized with respect to he average intensity:
scat_cross_radial = Pscat*sp.power(:, 1)/ave_intensity_radial;