  The broken mirror symmetry in subwavelength photonic systems has manifested
many interesting chiroptical effects such as optical rotation and circular
dichroism. When such systems are placed periodically in a lattice form, in
addition to intrinsic chirality, extrinsic chirality also takes part, and the
overall effect depends not only on the basis and lattice but also the
excitation configuration. Here, we study planar chiral nanohole arrays in
square lattice that support Bloch-like surface plasmon polaritons (SPPs) and
clarify how the system geometry and the excitation contribute to circular
dichroism. By using temporal coupled mode theory (CMT), the dissymmetry factor
and the scattering matrix of the arrays are analytically formulated.
Remarkably, we find the dissymmetry factor depends only on the coupling
polarization angle and the in-coupling phase difference between the p- and
s-polarizations. Besides, the upper limit of the dissymmetry factor at +/-2 can
be reached simply by orienting the lattice of the arrays for properly exciting
the Bloch-like SPPs and at the same time making the basis mimic two orthogonal
and relatively displaced dipoles, demonstrating the interplay between extrinsic
and intrinsic chirality. The models have been verified by numerical simulations
and experiments, yielding the dissymmetry factors to be 1.82 and 1.55,
respectively, from the proposed dual slot system.
