How to find the transmission spectrum of a plasmonic gold frame

I am trying simulate Figure 2(a) from this paper.srep02105.pdf (1.3 MB).
I attached my simulated transmission monitor.


This structure is simulated in COMSOL software as illustrated in Nature paper and I want to simulate it In Lumerical FDTD.

My result is different from paper result(figure 2(a)) and get confused why?
Here is the project file for my simulation:
au-frame_sub1.87.fsp (247.1 KB)
Thanks for any help

Hi. I’ve looked at your question and have discussed it with some of my peers. There are a few issues in the setup that we’ve noticed and we are checking them one by one. We’ll get back to you soon with some answers.

Hi. We were able to modify the file and get pretty decent result for transmission. The transmission spectrum that we got is shown below,

This is very close to the figure in the published article. The dip looks slightly shifted but that is probably due to a coarse mesh and you can just to some convergence testing to correct it. Here is what we did to fix the simulation,

  1. The index of the substrate dielectric was changed to 3.5 (from 1).

  2. In order to ensure that the PML does not introduce any reflection in the simulation volume, we placed the simulation boundary approximately half a wavelength away from the gold layer both at the top and at the bottom.

  3. The number of layers in the PML was increased to ensure that the PML itself was thick enough to properly absorb the transmitted light. We ended up using 64 layers, a much lager number than the default 8 layer.

  4. Finally, and this is probably the most important point, we changed the material model for gold. In the paper, they used the drude plasma model to create the gold material at THz frequencies. FDTD has a built-in material type for the drude plasma model called “Plasma”. We used this material model and used the values provided in the paper to create the material model.

You can see that the resulting model is quite different from the model you had created,

One key point to note is that the real part of permittivity in the new model is much larger and also negative. This is an important requirement for the creation of surface plasmons. With the new model and the other modifications in place, the transmission data looks quite reasonable.

Here is the modified file: au-frame_mod.fsp (259.1 KB)


Thank you so much. This has been of great help.
I have 2 question:
1-Why did you locate source in substrate?
2-you said:we placed the simulation boundary approximately half a wavelength away from the gold layer both at the top and at the bottom.
What do you mean?(minimum half a wavelength or maximum half a wavelength).
Thank you.

There was no particular reason to place the source inside the substrate except that we wanted to only see the transmission through the gold grid and avoid any reflection on the top surface of the substrate.

If you are using the older “legacy” PML, then the rule is that you need a minimum distance equal to half of the maximum wavelength between your structure and the PML boundary. This is not a mandatory requirement anymore with the new stretched coordinate PML. However, in some cases where the PML generates noise (such as this one) you can follow this rule to get rid of the noise.


Thank you so much.

Hi friends,
I have question:
How can I find transmission phase like fig 2(d).

I do not have any idea for finding it.

Thanks for helps.

Hi @m.dezyani, I believe you should be able to use the s-parameter analysis group to calculate the s-parameters for the structure (transmission and reflection) and since the s-parameters are complex, you can get the transmission phase from there. Please take a look at this KB page for some details about the analysis group: You can find an example of the use of this analysis group here: Please note that in its current from the analysis group will work with normal incidence only. If you eventually need to simulate for oblique incidence then some modifications will be needed.

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