Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers.

Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.