Chiral plasmonic systems have been shown to exhibit large chiroptical responses, much larger than those found in molecular or solid state systems. In this Letter, we investigate the role of resonant coupling in such systems and whether the formation of collective plasmonic modes in a chiral assembly of metallic nanostructures is a necessary condition for chiroptical response. We show in experiment and simulation that off-resonant coupling between spectrally detuned nanostructures arranged with structural chirality leads to a clear but weak chiroptical response. We interpret our results in the framework of scattering between the individual constituents that in turn leads to a chiroptical farfield response. We envision that our results will allow further tuning and manipulation of chiroptical responses in plasmonic systems for tailored chiral light matter interaction.

Chirality is an important molecular property for structural analysis. Similarly, it has been shown that plasmonic chiral systems exhibit strong circular dichroism (CD) responses that can be used to determine the relative positions of their constituent plasmonic elements. Here we show that the sign of the circular dichroism spectrum in a plasmonic system can be controllably changed through small geometric perturbations that change the energetic ordering of the hybridized modes. This mechanism is distinct from geometrical changes that explicitly change the handedness of the system. In a simple system composed of two stacked L-shaped resonators we observe a reversal of the optical rotation spectral signature for small relative shifts, and we show through electromagnetic modeling and experiments on lithographically patterned samples that this is due to a rearrangement of the relative energies between modes. The plasmonic system allows for geometric perturbation along controlled directions and therefore offers more control than corresponding molecular examples. Interestingly, this strong sensitivity in the optical response encodes more spatial information into the optical spectrum, emphasizing the importance of chiral plasmonic assemblies for structural investigations on the nanoscale.

Self-assembled plasmonic structures combine the specificity and tunability of chemical synthesis with collective plasmonic properties. Here we systematically explore the effects of symmetry breaking on the chiroptical response of an assembly of plasmonic nanoparticles using simulation. The design is based on a tetrahedral nanoparticle frame with two different types of nanoparticles, where chirality is induced by targeted stimuli that change the distance along one edge of the assembly. We show that the intensity, spectral position, and handedness of the CD response are tunable with small structural changes, making it usable as a nanoscale plasmonic ruler. We then build upon this initial design to show that the symmetry breaking principle may also be used to design a chiral pyramid using a mixture of different nanoparticle materials, which affords tunability over a broad spectral range, and retrieves nanoscale conformational changes over a range of length scales. (Figure Presented).