Sylvie Rana
Thank you for your reply. I will try my best to describe my problem.
I am trying to simulate a metamaterial based reflector, with an aim to be used for future RIS applications. My design is intended to reflect the incoming plane wave (for now I am simulating for normal incidence) to a specified desired direction towards a target presumably located at the far field distance. In this context, I would like to answer your questions:
No, the top surface of the metasurface is facing the +ve Z-direction, and the ground plane(pec) located at Z=0. I have applied the incident plane wave from a height of 20*lambda (this is required for my problem) from the top surface of the metasurface, directly above the middle of the metasurface. The plane wave has a k vector of -1 (minus one), so that it can travel in the -ve Z direction and illuminate the metasurface top layer. The metasurface is situated on x-y plane.
I am using plane wave incidence for my metasurface having finite number of unit cells, as I assume that the metasurface is going to be illuminated by plane wave (in my problem statement this is expected)  radiated from an antenna (say horn), situated a far field distance.
Thus, I am not expecting S-paramters right now. Rather, I would like to get the far field scattering pattern from the metasurface due to plane wave illumination.
I didnt get any other option to get the correct radiation pattern at my desired direction. I tried plotting the bistatic RCS too, besides rE plot, which gives me the same kind of scattering pattern. Kindly suggest me what else can give me the correct scattering pattern for the given problem. My main goal is to get most scattering at the desired direction for now. However, I might need to get the amount of power recieved at the target end (scattered power from the RIS), once I achieve the correct scattering pattern at desired direction, with scattered power at unwanted directions reduced as far as possible.
I have successfully simulated the unit cell of the finite array metasurface in my design with Floquet ports and Master/Slave boundaries, and obtained the desired characteristics. Hopefully, I am fine with this portion of my work. 

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