Nano-scale engineering and nonlinear imaging of chiral plasmonic assemblies

Plasmonic nanostructures are well known for their strong interactions with light, leading to the enhancement of many different optical phenomena. Among them, a spectacular enhancement of the chiroptical effect can be observed. The differences in the interactions of chiral plasmonic nanomaterials with left-handed and right-handed circularly polarized light can be orders of magnitude larger than such differences in organic compounds. Further improvement can be achieved by switching from linear to nonlinear optical phenomena, such as second-harmonic generation (SHG) [1]. We use polarization-resolved two-photon microscopy to investigate the giant nonlinear chiroptical effect in chiral non-centrosymmetric gold nanostructures. Our samples are composed of triangular gold nano-prisms arranged into two-dimensional chiral patterns over a glass substrate. The individual nanostructures follow a 2-D octupolar design where chirality is introduced by way of breaking the mirror symmetries by interaction with adequately set lateral entities, while keeping the overall three-fold symmetry. Large chiroptical effect in SHG have been observed in experiments and confirmed by numerical simulations. Contrary to recently investigated nano-materials, such as the super-chiral surfaces studied by Valev et al. [1], our nanostructures constitute independent sub-wavelength super-chiral nano-objects, which can be arranged to form microscopic watermarks. We demonstrate efficient read-out of such watermarks using nonlinear microscopy.