I am using Lumerical FDTD Solutions 8.15.786 on Windows 7 64-bit. I have a geometry that contains a semi-infinite slab of air on top of a semi-infinite slab of silver with a slit perforated on it. There is air inside the slit and I use a TFSF source that spans a wavelength range from 400 nm to 1200 nm. All the boundaries are PML and I pay particular attention to have some space between TFSF boundaries and the simulation domain boundaries. On top of that I use a uniform simulation mesh in all directions without any mesh-override region. The TF/SF is excited from the air side, and it propagates down the slit perforated on the infinite metal slab.
My question is about how to implement the correct PML settings so that there are no reflections from the bottom of the metal, since I try to simulate a case with a metal of infinite thickness. However, when I use the modal fields to calculate the forward and backward propagating mode amplitudes, I found out that there is also substantial reflection from the bottom of the metal in a wavelength range between 850 nm to 1200 nm.
I tried out various approaches to remedy this issue: increasing the number of PML layers, changing the PML type from standardized to stabilized since there are material boundaries on the PML. However, these solutions did not work and in one of the cases(I think it was the one using standard PML) I got a diverging simulation warning even though I increased the number of PML layers. I use the streched coordinate PML that is available in the new releases of this software.
The simulation geometry in the YZ cross-section and the plot of the unnormalized reflection coefficients against the wavelength are attached as images. In these attached graphs A’s are the amplitudes of the forwards propagating modes, whereas B’s are amplitudes of backward propagating modes. I would appreciate it if you could comment on why there is reflection and how to alleviate it.
Plot of A(forward) B(backward) propagating mode coefficients
EDIT: The simulation files are attached to this post.
Slit_f2a_PML7.fsp (256.4 KB) This file is the latest simulation configuration I use
Slit_f2a_PML1.fsp (250.2 KB) This file is the original file I sent to support