  It is shown theoretically that a nonchiral, two-dimensional array of metallic
spheres exhibits optical activity as manifested in calculations of circular
dichroism. The metallic spheres occupy the sites of a rectangular lattice and
for off-normal incidence they show a strong circular-dichroism effect around
the surface plasmon frequencies. The optical activity is a result of the
rectangular symmetry of the lattice which gives rise to different polarizations
modes of the crystal along the two orthogonal primitive lattice vectors. These
two polarization modes result in a net polar vector, which forms a chiral triad
with the wavevector and the vector normal to the plane of spheres. The
formation of this chiral triad is responsible for the observed circular
dichroism, although the structure itself is intrinsically nonchiral.
