  Asymmetric transmission - direction-selective control of electromagnetic
transmission between two ports - is an important phenomenon typically exhibited
by two-dimensional chiral systems. Here, we study this phenomenon in chiral
plasmonic metasurfaces supporting lattice plasmons modes. We show, both
numerically and experimentally, that asymmetric transmission can be achieved
through an unbalanced excitation of such lattice modes by circularly polarized
light of opposite handedness. The excitation efficiencies of the lattice modes,
and hence the strength of the asymmetric transmission, can be controlled by
engineering the in-plane scattering of the individual plasmonic nanoparticles
such that the maximum scattering imbalance occurs along one of the in-plane
diffraction orders of the metasurface. Our study also shows that, contrary to
the case of a non-diffractive metasurface, the lattice-plasmon-enabled
asymmetric transmission can occur at normal incidence for cases where the
metasurface is composed of chiral or achiral nanoparticles possessing 4-fold
rotational symmetry.
