TFSF and Periodicity.


Dear all,

It’s mentioned that TFSF source shouldn’t be used in periodic structures (Example: nano spheres array on a substrate). And Plane Wave source should be used instead.

If this is the case, how can I use the cross section analysis group? (Example: Calculating Scattering from the nano spheres array on a substrate) ?

Thank you.


Hi. You can use TFSF sources in periodic systems. However, there are some restrictions on how to use it. Please check out this KB page to learn about the proper use of TFSF sources in FDTD,

TFSF source in periodic boundaries


Dear aalam,
Thanks for your reply, just to make things clear :

I want to simulate a plasmonic solar cell that has array of nanospheres, I want to calculate the scattering cross section, should I use periodic FDTD ( the region covered is just one particle) + TFSF source?

And where should I put the cross section? around 1 particle or around the whole array?

Thank you.


I’ve already read that, just to make sure I get it right. I can use the TFSF source in condition that I place it in the right (periodic unit cell) =( the unit cell which is repeating) , right?


Sorry for the tardy reply. Please find my response to your questions below,

Yes. Since you have a periodic array of nanoparticles, you should use a periodic FDTD. However, using the TFSF source is not going to be right choice in most cases (including your plasmonic solar cell). If you look at the “TFSF source in Periodic Boundaries” page I mentioned above, you will notice a comment at the top of the page saying “The TFSF source should only be used in situations where scattering in the specular (zeroeth order) direction is of interest”. In all other cases a plane wave source is the more appropriate choice.

Regarding your use of the scattering cross-section analysis group, I have discussed this with my colleague @kchow since I myself am not an expert on the topic. What I have learned is that the figure of merit “scattering cross-section” is more appropriate when simulating scattering from a single scatterer rather than a periodic structure. In such a case (single particle), since you will use PML boundary condition you will also use the TFSF source. In a periodic structure like yours, the more meaningful result is probably the grating order transmission which can be calculated using the corresponding analysis group. Please take a look at this example in KB (Grating order transmission) and let me know if you have any questions.