October 4, 2021 at 5:42 pmGuilin SunAnsys Employee
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
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:
Note: BFAST has its own built-in periodic conditions, which override the default BCc user sets, by default.
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