Learning Forum

[mohamed.elshorbagy]

I have experienced an issue using the file "usr_reflection_angled.fsp" in the Lumerical support article "Tips for accurately measuring reflection in an FDTD simulation". 

I have set the source angle to zero and the material of the rectangle to PEC. Running two simulations with mesh accuracy 6 and 7 respectively yields the blue and red graphs shown in the figure.

The reflectivity changes with wavelength, and rises above 1 at some wavelengths. The reflectivity is also mesh-dependent. I would expect the reflectivity to stay at a constant 1 and be mesh independent. I would greatly appreciate your assistance in resolving this issue since I am trying to investigate the effect of surface roughness on reflectivity.

[Guilin Sun]

This is due to numerical errors, such as limited mesh size, source injection, PML etc. Discrete FDTD computation is mesh dependent, and can approach theoretical result when the mesh size approach zero and PML is sufficiently thick to absorb all energy.  

You can use the montor in between the source and the front surface.

The error is already on the order of 10^-4, quite small. Please refer this link for more information about converging test https://optics.ansys.com/hc/en-us/articles/360034915833-Convergence-testing-process-for-FDTD-simulations

[Pankaj Kumar Sahoo]

Hi,

I am using s-parameter script from the metalens example (https://optics.ansys.com/hc/en-us/articles/360042097313-Metalens) to simulate the reflection and phase of gratings with varying period and fillfractor. Unfortunately, I am getting reflection values (more than 1) from the R_Gn result of R monitor as 379.3, 5.1, 7.6, 6.7, 1.4, for some values of grating period and fillfactor. I am using the same setup as in the example except the metasurface. I am using the script file provided in the website and modified as per my simulation. Kindly suggest what could be the issue. If needed i can share my script and setup files.

Regards

Pankaj

 

[Guilin Sun]

Hi Pankaj,

Later please write a new post as the forum is for one question one answer.

 

Every device is different. So the settings from one simulation do not guarantee the other simulation. It seems the causes R>1 could be:

1: the simulation is going to diverge or already diverged.

2: the simulation time is not long enough: due to large autoshutoff min (default 1e-5), or the set time is shoort. Please increase the simulation time 10 or more times, and autoshutoff min 1e-6 or smaller.

3: the device has higher diffraction ,with diffraction angle larger than 70deg. Please use visualizer to approximately project the farfield with 10by10 period, or use grating example to get accurate result: Diffraction grating

If the diffraction is large, you will need to use Steep Angle PML, more number of layers.

 

In addition, please first test mesh accuracy 2, not 6 or 7. If mesh accuracy 2 does not diverge, most likely it is due to PML. Please refer  Troubleshooting diverging simulations in FDTD