  The aim of this letter is to {present analytical method to} quantitatively
address the influence of a focusing illumination on the transmission
/reflection properties of a {metasurface illuminated by a finite-size beam}. In
fact, most theoretical and numerical studies are performed by considering an
infinite periodic structure illuminated by a plane wave. In practice, one deals
with a finite-size illumination and structure. Combination of the angular
spectrum expansion with a monomodal modal method is performed to determine the
beam size needed to acquire efficient properties of a Metasurface that behaves
as Anisotropic Plate MAP Interesting results show that the beam-size can be as
small as 5X5 periods to recover the results of a plane wave. Other results also
show that the beam-size could be used as an extrinsic parameter to enhance the
MAP performance and to finely adjust its expected properties (birefringence
and/or transmission coefficient).
