Why absorbed power depends on the position of plane wave source?

Hello!

I’m PhD student and is currently using FDTD to solve for a light absorption problem. The geometry of simulation looks like this: a plane wave (633nm) falls on a gold probe. I used the analysis group Pabs_adv to determine the power absorbed by probe. In the script of Pabs_adv I normalize the power: I multiply by the input intensity and the area of the plane wave source (Input power). Input intensity is 1*e^10 W/m^2, and area of source is 4 um^2. And I have a question - why is absorbed power depends on the distance between the probe and the source, despite the fact that is a plane wave. When I move the plane wave source, I get different values of Pabs.

I have attached the FDTD file with my simulation.

Thanks in advance!

only plane wave with tip 1 um distance with 2D.fsp (299.4 KB)

Hello @ElAChernyh,

Thank you for the question. This issue is due to the fact that you are using a finite plane wave source, so diffraction will occur as the wave propagates (see this page for more information). This diffraction is likely what is causing the change in Pabs.

If you increase the y span of your FDTD region and the source it should better approximate an infinite plane wave. For example, here are the results for a small source (y span of 2 um) and a larger source (y span of 7 um) as a function of the source’s x position:
image
We can see that the results are much more consistent with a larger source (absorption is lower due to the normalization you are using).

I hope this helps. Let me know if you have any questions.