The post is closed. Did you find the solution? From grating_S_parameter group you have the “S” paramters which are actually the E field(s). Except that you modified the group it only gives one field component.
Once you have the E fields,say Ex and Ey, or Ep and Es, you can decompose it as suggest in the first reply. for the farfield it is straight forward to get the optical intensity (0.5*E^2/impedance). Please find more information regarding to power calculation from some books. This article has some information but you may not directly use it without modification since it is for dipole.