Simulating S and P Polarised EM Waves

I am trying to simulate a plane wave with either S or P polarisation incident upon a nanoparticle on a substrate at various angle of incidence (phi and theta). Following the Scatterer on a Substrate model I have managed to do this for a S polarised plane wave using the follwing port conditions:


INCIDENT
x| -sin(phi)*exp(-i*(kbx*x+kby*y))
y| cos(phi)*exp(-i*(kax*x+kay*y))
z| 0

Propogation| abs (kaz)

TRANSMITTED (Through Substrate)

x| -sin(phi)*exp(-i*(kbx*x+kby*y))
y| cos(phi)*exp(-i*(kbx*x+kby*y))
z| 0

Could somebody please explain how this is derived for the model as the pdf doesnt really explain this. Also how would i modify this to have P polarisation incident?

Many Thanks Josh