It is not known for me what exactlythe electric intensity inside the material you want. Do you want to check the static field ? if so, you will need to make this material to be semiconductor. Please refer to this tip with proper material parameters:
Tips for creating a new semiconductor material
if you want to find mode intensity inside the waveguide with such material at a given wavelength please use FDE.
If you want to know the electric field intensity in the material at any wavelength you can simulate it with a broadband source in FDTD.
If the above are not what you expect, please elaborate more about the device and its working principle.