Looks like there quite a few critical issues in your simulation.
There is no object in the simulation region, not even the source, nor monitors. Therefore, there simulation was not meaningful. You will need to bring the objects to inside the simulation region (the orange box).
The Pt material only has data up to about 2 um. So I am not sure if the fit has a good representation for longer wavelengths, likely not. You will probably want to find some material from literature for the longer wavelengths in order to have a good representation of the material.
Since the wavelength range is very large, the software needs to injects a long pulse. Based on the signal vs time plot, it is obvious that the simulation time is too short for the simulation. You will need to increase the “simulation time” in the FDTD region for the simulation to run reasonably.
On the other hand, I noticed that you had a very thick graphene layer (1um). This is way beyond the usual graphene thickness (~ nm). I am not sure if the graphene model will provide some good representation for graphene material to you, likely not. I guess you will need a reference to obtain some bulk material properties to represent graphene for this thickness range.
For your other questions, the background index can be changed here.
Edited file here Graphene_Dielectric_Metal.fsp (300.1 KB)
Graphene is not an easy material to work with, I can imagine that the simulation you need to deal with might have some challenges. My feedback is that you should start with something simple as an excise to learn our software. There are lot of advantages to play with some simple simulations first, easier to debug, less things to go wrong, etc. It will actually save you time down the road. I’d recommend you to play with this simple simulation, and we can discuss any questions you may have regarding this example. There is also a demo in this video which I think you should try to reproduce too. I think it isn’t the most efficient to start simulating your own model right now.
I hope this helps