The metal thin films can be modeled simply using RF and wave-optics modules of COMSOL multiphysics. For microwave and THz waves, just the conductivity of the metal entered then by putting magnetic permeability and electric permittivity equal to 1 you use it as a "material" and apply it to a proper "geometry" (layer with specific thickness). I must mention that the conductivity is measured experimentally and is reported in many papers.

You can model thin films as 2D planes and then using "transition boundary conditions" or "impedance boundary conditions" you can introduce them to Comsol.

In more advanced cases you must use Drude(and Lorentz) model gotten from experimental data available.