  Helical dichroism (HD), based on the interaction between chiral plasmonic
nanostructures and light with orbital angular momentum (OAM), has attracted
researchers in a wide range of fields from the viewpoint of fundamental physics
and applications. However, the relation between the HD and the excited plasmon
modes has been poorly understood in experiments. Because of the weak chiral
interaction between the chiral structures and OAM light, the structure size had
to be much larger than the incident light wavelength to obtain a sufficient HD
signal in an experiment, resulting in a complex superposition of higher-order
plasmon modes. Recently, we experimentally demonstrated that a twisted gold
nanorod dimer, one of the simplest 3D chiral plasmonic structures, exhibits
giant circular dichroism due to strong plasmon coupling between the nanorods,
followed by the hybridization of dipole mode. In this study, we reveal that the
HD of this nanorod dimer appears due to the hybridization of quadrupole plasmon
mode rather than dipole mode. Furthermore, the measurement of the HD signal can
be achieved by using the array of the twisted dimers. The dependence of the HD
on the incident light wavelength exhibits that the HD sign changes around the
quadrupole plasmon resonance, which is in good agreement with the simulation.
These results open new avenues for the profound understanding of the
light-matter interaction with respect to angular momentum.
