Learning Forum

[Anish_Mondal]

I am currently working on the radiation pattern of some grating structures in 3D FDTD. I use the far-field display setting of a 2D Frequency Domain Field and Power Monitor, which is placed above the grating structure. When I vary the monitor's distance from the grating, I see a significant change in the far-field output. Why does this happen? And how to get a more accurate far-field projection. (do I keep the distance less or more?)

Thank you

[Amrita Pati]

Hello Anish,

Far field projections are generally used for isolated structures as opposed to periodic ones. The farfield functions assume the fields to be 0 outside the monitors by default. Therefore, if the monitor is not big enough to have all of the radiation pass through it then the farfield results will be inaccurate. In such a case, the distance of the monitor from the structure might also affect the projections depending upon the amount of radiation passing through the monitor at each location.

But more importantly, for periodic structure like gratings in your case, you will have to consider a few other things. If you have a definite number of unit elements in the grating, you may enable the “Assume Periodic option in far field projections”. For more information, please refer to the following link:
https://optics.ansys.com/hc/en-us/articles/360034914793-Understanding-the-Assume-Periodic-option-in-far-field-projections

If you have a grating extending infinitely, then you should use grating projections. More information can be found on:
https://optics.ansys.com/hc/en-us/articles/360034394354

Thank you.

[Anish_Mondal]

Sorry for my miscommunication. I am indeed using an isolated structure.