Gaussian beam illuminated on thin films

Hi, I got a question about setting gaussian beam in the wave-optical module.

My structure composes of a thin Fe layer (5 nm) on the SiO2 substrate, laser wavelength 800 nm.

The COMSOL tutorial "Nanorod" shows we can directly set the gaussian beam conveniently using the scattering wave formulation, and surrounding PML are adopted to absorb any reflection.

However, I am not sure if this method is correct since my structure is not a isolated scatter, but a large thin film, and I am sure how to set gaussian beam in the port function.

Any suggestions are quite welcome.