Modeling Advice

I have been trying to simulate the scattering of an antenna above a thin layer of gold ~100 nm. I am a plane wave source, the plane wave is focused using a mirror. After the mirror, the focused plane wave is used to illuminate the antenna and layer of gold.  The simulation is using frequencies in the ~ THz, where the skin depth of Au is comparable to the thickness of the layer. Any suggestions for simulating the Au layer?