There are several issues in your scripted simulation file:
1: the substrate should be included for a small portion, like a fraction of wavelength (when no SPP). Currently you are simulating the substratge suspended in air, which forms a cavity;
2: the distance from source to the up surface is too long. A fraction of wavelength is good.
3: you used periodic boundary conditions but the substrate is not large enough to be outside of the periodic BCs. FDTD is a discrete method that ExEyEz and HxHyHz are not collocated:
Therefore you will need to extend the substrate to outside of the PBC.
You current settings and PEC material use much less memory, but the long distance and THz simulation needs longer simulation time to have the pulse propagate to the bottom.
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