  In a recent experiment, the out-of-plane surface susceptibility of a
single-layer two-dimensional atom crystal in the visible spectrum has been
measured. This susceptibility gives a measurable contribution to the
reflectivity of two-dimensional materials. Here we provide a complete
theoretical description of the reflective properties, considering incoming s
and p polarized plane waves at any angle of incidence on the crystal, computing
local, reflected and transmitted electromagnetic fields. We finally connect the
microscopic polarizability to both the in-plane and the out-of-plane
macroscopic surface susceptibilities.
