  Chiral plasmonic nanostructures will be of increasing importance for future
applications in the field of nano optics and metamaterials. Their sensitivity
to incident circularly polarized light in combination with the ability of
extreme electromagnetic field localization renders them ideal candidates for
chiral sensing and for all-optical information processing. Here, the resonant
modes of single plasmonic helices are investigated. We find that a single
plasmonic helix can be efficiently excited with circularly polarized light of
both equal and opposite handedness relative to that of the helix. An analytic
model provides resonance conditions matching the results of full-field
modeling. The underlying geometric considerations explain the mechanism of
excitation and deliver quantitative design rules for plasmonic helices being
resonant in a desired wavelength range. Based on the developed analytical
design tool, single silver helices were fabricated and optically characterized.
They show the expected strong chiroptical response to both handednesses in the
targeted visible range. With a value of 0.45 the experimentally realized
dissymmetry factor is the largest obtained for single plasmonic helices in the
visible range up to now.
