Learning Forum

[francis.granger43]

Hello,

I am looking to calculate fields outside of a closed surface using a box of monitors and the far field projection functions. I tried something founds here: https://support.lumerical.com/hc/en-us/articles/360034915613-Far-field-projections-from-a-box-of-monitors, but it does not work very well with a substrate. I was wondering if a similar script exists only with half the box ?

Thanks a lot

[Guilin Sun]

Yes you are right that when there is a substrate, we cannot use a 6 monitors doing the far field projection, as it requires that all monitors be located in the same homogeneous material.
Depending on the source radiation, you may try one of the two methods:
1: using large simulation region and only one plane monitor:
https://support.lumerical.com/hc/en-us/articles/360042713433-FDTD-vs-stackdipole-halfspace-emission-in-a-multilayer-stack
2: using 5 box monitors above the substrate:

The analysis group has 6 monitors but you can modify the script to multiply zero to the monitor on the substrate. The result is 3D radiation pattern. Since the result is already there, you can modify the script to get what you want.
for example, if z2 monitor lies on the substrate, you can modify the script like this
temp = farfieldexact("x2",x,y,z,i) + farfieldexact("y2",x,y,z,i) + 0*farfieldexact("z2",x,y,z,i);
It will cancel its contribution to the farfield.

Please try.

[francis.granger43]

Thank you for your quick reply.
So far I used method 1 but because the plane monitor has to be as wide as possible, the simulation is quite time consuming.
Concerning method 2, I used the Analysis group: far field from closed box. I tried by only using half of the box (upper half) but it seems that the result is not as good as using a single monitor. In my case, the half box lies on the substrate.

[francis.granger43]

Thanks, I found out how to get similar results using the two methods. Is there a rule to know the minimum distance between the monitors and the source ? I think I read few wavelength but I was wondering if you have more information about it ?
Thank you.

[Guilin Sun]

When the material which holds the monitors has loss such as vacuum, or the loss is extremely small such as ima(refractive_index) is on the order of -6, use larger distance is good, as it can minimize the numerical integration error. But too large the distance (and consequently the monitor sizes) can hinder the simulation speed. In general, a distance of about one wavelength or more is good.

[Jz_Young]

Dear Dr. Sun In terms of the solution you proposed for inhomogeneous structure, by cancelling one monitor in the substrate (method 2), I find out that the projection pattern will not be convergent anymore. For example, the far-field projection pattern varies depending on the size of the box ´╝ê5 monitors), see: https://forum.ansys.com/discussion/33989/far-field-projection-with-box-of-monitors-dipole-source#lates
There are no structure objects in the simulation. A dipole source is at the center of the cancelled monitor (z1), the far-field pattern changes with the size of the cubic monitor box.
Bests Jingzhong

[Guilin Sun]

This will depend on the dipole polarization. If the polarization is the strongest along the cancelled monitor, the error can be large. Please try other dipole polarization to do the convergence. Please note that dipole is very special. This method is a remedy in the case of substrate existence. If there is no substrate, please use the regular method.

[Jz_Young]

Dear Dr. Sun Thanks for your explanation. In above convergence test, the dipole polarization is oscillating in-plane of the cancelled monitor. The resultant far-field patterns are significantly different with respect to the size of open monitor box.
As you suggested, I tried out-plane dipole polarization relative to the cancelled monitor, giving rise to the distinguished far-field projection patterns as shown in the attached images, based on different sizes of the open monitor box.
In the practical simulation, the dipole is embedded in the substrate, and the substrate is infinite along x and y, and the open monitor box is in vacuum space. Therefore, the source is outside of the box, leading to a worse error of far-field integration I guess...Do you have any suggestion to compensate for this case?
Bests Jingzhong

[Guilin Sun]

This can be very challenge, as the monitors need to pick up all the fields correctly. In particular when the field is very strong in one direction, a minor change of the monitor size or any small leak of the fields (eg, not picked up by the monitor) can cause large difference. However, the monitor must be located in a uniform material. This is a must to have accurate result in theory for far field projection. Anything violates this will lead to errors. You may need to compromise the accuracy and simulation efficient, if you cannot use one very large DFT monitor to capture the fields completely.

[francis.granger43]

Hi, I also have small differences when I change the size of the box of monitors (see images below). The monitors are located in a uniform material (n=1). I also do not understand the symmetry of the farfield since I would expect something perfectly circular.

[francis.granger43]