Farfield intensity of a simple plane wave source


#1

Hi,

I have a simple question, I simulated just a simple plane wave source in pml boundaries and placed a monitor in front of that source. Why when I visualize the intensity in farfield, the intensity drops to the order of 6x10^(-10). while if I look at the nearfield it shows the correct value of 1.
The medium refractive index is just 1.

Thanks


#2

Dear @nzaraee

Near field shows total transmission (which is equal to 1), but far field shows electric field intensity (|E|^2) as a function of θ and Φ. Farfield3dintegrate command can be used to integrate over a cone (or entire hemisphere) to calculate total transmission. For your case, if you integrate over entire hemisphere, you will get a value of 1.

Please see the link below for more information on how to do these calculations:
https://kb.lumerical.com/en/index.html?solvers_far_field_projections_power_integration.html

I hope this was useful.
Thanks


#3

Hi,

Thank you very much for your reply.
The first image attached is my simulation setup( just simply a periodic
boundary region, plane wave, a monitor in front of it).
after running it, I visualize the nearfield E from monitor and look at the
abs value which is as expected 1 everywhere(image attached)
However, when I visualize the farfield from this monitor and look at the
abs value the max intensity at the center of hemisphere is on the order of
(10^-8)(image attached)
My question is that even though there is no loss in the propagation path,
why when the source propagates to farfield, it drops to such low values
compared to the initial value of 1.
Does FDTD consider loss for propagation?

Thanks,


#4

The reason I’m doing this simulation is that I’m trying to simulate a
single sphere placed on a layered substrate and look at the farfield data
at the monitor behind the source( meaning I want to have both the
reflection from the layered substrate and the scattering from the sphere.
There are some problems
1- I cant illuminate it with plane wave, because pml boundary is used in
fdtd region( If I use a wide enough fdtd region can I trust the farfield
data and how much the edge effect is important in farfield?)
2- I cant use periodic boundary, because I want the response form a single
sphere,not a period of spheres( does this periodicity affect the farfeild
if they are far enough?)

Thanks


#5

Dear @nzaraee

No, it is considered as a propagation in free space. The link that I attached before explains how you can extract power in farfield. One things to remember is that farfield gives you intensity as a function of theta and phi, and if you want to calculate total transmitted (which will be equal to 1 in your case), you will need to integrate over entire hemisphere. This is explained in the link I provided earlier.

For illuminating a single object with plane wave you will need to use TFSF source. Mie scattering application example will be very useful for your review, to learn more about scattering problems and also how to calculate farfield spectra.

Please let me know if you have further questions and I will be glad to be of a help.
Thanks