Learning Forum

[NathanHale]

Hello!

I have been having a re-occurring problem when doing reflection and transmission simulations in Lumerical.

The results I get are wavy. I have used the tip on the webpage about moving the reflection monitor in front of the source, but I can't seem to solve this issue.

Underneath are some examples of this.

the first figure is a reflection example from aluminium having a nanogroove.

The other is a very thin, anisotropic layer along with a figure of how it should look from transfer matrix methods.

Any help on the origin of these waves and how to minimize them would be appreciated.

Aluminium

[Guilin Sun]

For the first two results, it could be due to
1: simulation time is too short & autoshutoff min is too large. Remedy: increase the simulation time, set smaller autoshutoff min, and let the autoshutoff level to terminate the simulation, eg, the progress percentage should be smaller than 100%. I would suggest to be smaller than 95% for easy use.
2: the pml might be too thin. The effectiveness of PML is due to its total thickness and absorbing parameters. For easy use, Lumerical FDTD provides different PML types that users can choose: PML boundary conditions in FDTD and MODE For the case of smaller mesh size, you may need to increase the number of PML layers. Although in general we recommend half-wavelength thick PML. In some cases it may need thicker PML. You can roughly measure it using the toolbar "ruler".
3: it might also be due to the geometry is not well resolved and the result actually smooths some peaks.
The last graph might be due to different cause. It is likely due to physics: There is a change of diffraction orders. eg, shorter wavelength may have more diffraction orders than the longer wavelength, and at some wavelength (it might not be recorded) the diffraction angle reaches 90 deg. Please use grating analysis to check the grating orders: Diffraction grating
The kink can also be due to Wood's anomaly https://support.lumerical.com/hc/en-us/articles/360041620514-Active-THz-metamaterial This is also physical. You may find more info from this paper https://opg.optica.org/ao/fulltext.cfm?uri=ao-4-10-1275&id=13932
I would suggest to practise the above suggestions. If you have a specific question, please write a new post.