Learning Forum

[Guilin Sun]

 Boundary conditions are vital for FDTD simulation, as they act as initial conditions in solving Maxwell's equations.

How to know the boundary conditions are correct?

First, you will have a good understanding of your device: Periodic or isolated?

Second, what results do you want to obtain?

Third, what the illumination source will be used in experiment?

Periodic boundary works for the case when the device has periodicity. In addition, the source must be infinitely large, eg, the plane wave. In such case Periodic, Bloch, and Symmetry BCS can be used. The results are usually the transmission, reflection, absorption, and sometimes scattering. The best example is Diffraction grating.

PML: PML is one of the successful core BCs that drives FDTD popular. It functions as the absorbing material without reflection (in theory) , similar to microwave chamber, see photo in wiki

https://en.wikipedia.org/wiki/Radiation-absorbent_material

It truncates open area or infinitely large object/space to be limited for simulation. The best example is scattering nano particles: Mie scattering (FDTD)

In general, it works for isolated objects, and also mostly is used in source injection axis. This is the most popular BC in FDTD simulations, and almost every simulation needs it.

However, periodic structure may not always use periodic BCs. The best example is OLED simulation: https://support.lumerical.com/hc/en-us/articles/360042225754-OLED-Methodology

To know if the boundary conditions are correct or not, one way is to check the result and see if it is expected.

One thing to keep in mind: while FDTD can guarantee the correctness in math (eg, periodic BCs are applied correctly), it is the user's responsibility to guarantee the correctness in physics. Boundary conditions must match with structure AND source.

Please refer those articles:

Periodic boundary conditions in FDTD and MODE

Bloch boundary conditions in FDTD and MODE

PML boundary conditions in FDTD and MODE

Symmetric and anti-symmetric BCs in FDTD and MODE

The FDTD Solver Region - Setup Tips - Boundary Conditions Tips

The FDTD Solver Region - Solver Region Settings - Boundary Conditions Tab

Broadband Fixed Angle Source Technique (BFAST)

 

 

Note: BFAST has its own built-in periodic conditions, which override the default BCc user sets, by default.

[Rony Das]

What are the boundary conditions when BFAST is used in LUMERICAL FDTD between X And Y Axis?

[Guilin Sun]

BFAST has its own built-in boudary conditions in the periodic axes. You do not need to set them specifically. However, if the source has only titled in one plane, for example in xz plane, which hshould use the default BFAST-specific BC, if the device has symmetry property in YZ plane, you can still set it accordingly, without use of BFAST BC:

https://optics.ansys.com/hc/en-us/articles/360034382694-Symmetric-and-anti-symmetric-BCs-in-FDTD-and-MODE

https://optics.ansys.com/hc/en-us/articles/360034902273-Broadband-Fixed-Angle-Source-Technique-BFAST- 

[Rony Das]

[Guilin Sun]

The boundary conditions are correct. The top-left warning means the simulation is diverged. Please refer to this article https://optics.ansys.com/hc/en-us/articles/11277217507603-Troubleshooting-diverging-simulations-in-FDTD

if you have more questions please write a new post.