Hi, thanks for your reply.

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;

What does it mean? How to set it right?

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:

Solution: Make the substrate xy area larger than the simulation range?