Home Forums FDTD Observation point or plane

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• #58236
Luis Rodriguez
Participant

Dear all,

I am simulating the optical response of metamaterials. My question is, in a 3D simulation, shall I use an observation point or an observation plane? When I use the observation point, I get the result I may expect, but when I use an observation plane, I get something different. My intuition is that an observation plane is the right option, but results are not as expected, so I am not sure. I am simulating a substrate with metamaterials on top. Any help, and any good explanation about what are the differences between observation points and planes would be very welcome!

Luis

• #58275
Luis Rodriguez
Participant

Some additional information: I have attached the design file, and in the attached picture you can see the differences between the reflectivity calculated from an observation area and from an observation point. I am only interested on specular reflectivity

Regarding R from the observation area, I used the power spectrum option and then I calculated Pz normalized to input plane.

Regarding R from the observation point, I calculated the normalized DFT for Ex, Ey, and Ez and then R=(Ex)^2+(Ey)^2+(Ez)^2

Overall the coincidence is acceptable, but the ball should enhance the reflectivity at 0.0165 microns, and it only happens when I calculate R from the observation point. However, when I use the observation area, results are different.

I am lost! Any help would be very welcome!

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• #58336
Scott Newman
Moderator

Hello Luis,

There is one thing to keep in mind when comparing observation areas to points. This is that you are calculating R using the observation point based on the field at one specific point while using the observation area is using power values obtained by integrating over the entire area. This means that if your field has any convergence/divergence I would not expect the point and area to return the same values.

In order to move forward could you let me know what version of the product you are obtaining your results with?

Scott

+1
• #58338
Luis Rodriguez
Participant

Hi Scott,

Thanks for your feedback! I am using the 32-bits free version.

Regarding your answer, the point is that the nano structures are smaller than the wavelength, so there should not be any divergence or convergence. Indeed, looking at Ez in the reflected field, I see that is close to 0 across the spectrum, meaning that there is not diffraction due to the nano structures.

I tried to move both observation area and observation points further to meet the far field approximation and still results are very different at the target wavelength. I am clueless ðŸ™‚

Thanks,
Luis

• #58385
Scott Newman
Moderator

Hello Luis,

Perhaps I explained that poorly as convergence/divergence is only remotely related to what I was trying to get at. You are injecting a rectangular field larger than the domain so are expecting a uniform distribution across the input. The important part is to look at your observation area and note that the field and therefore the Poynting vector (what we integrate to get power) is not uniform. Because it is not uniform I would not expect something that is integrated across it (the power from the observation area) to match a measure based on one mesh point at the center of the field (observation point).

Another illustration of what I am trying to get at is to double SIZE parameter you have set doubling the transverse direction and rerun your simulation and extract your values. You should notice that the results from the observation area are effectively the same while there is significant changes due to the observation point. You are correct that the observation area is what you should be using.

As to why it is not showing the expected reflectivity I am not certain, do you have a paper you are trying to reproduce? One thing I did not is can you please confirm what materials you are using because the refractive indices seem off to me but could be my misunderstanding your material naming convention.

Please be aware the 32-bit product is an outdated release that is no longer receiving software updates and as such can only receive limited support at this time.