Direct Measurement of Optical Resonance Modes in Gold Meta-Atoms Fabricated by Metallization of Triangle- and V-Shaped DNA Origami Templates

Despite intense interest in optical metamaterials that operate at near-infrared and visible frequencies, progress in fabricating and characterizing such materials remains a significant challenge. Since the optical properties of metamaterials are derived from the collective response of the assembly of meta-atom units, it is highly desirable to understand the construction and scattering behavior of individual meta-atoms. In this work, we utilized DNA origami templating to create gold nanostructures, including hollow nanotriangles and V-shaped nanoresonators, to serve as representative meta-atom building blocks. Metallization of these nanostructures was achieved by a three-step process involving the photoreduction of silver to create metal seeds on the origami, the electroless reduction of gold onto the seeds to create a uniform and conformal metallic coating, and subsequent chemical annealing to smooth out the structures. Optical scattering from individual metallized DNA origami was then collected with dark-field microscopy. Experimental scattering measurements were complemented by numerical simulations of the scattering spectra of both ideal-shaped nanostructures and realistic representations of the as-fabricated nanostructures. Results demonstrated that the hollow nanotriangles exhibited only a single electric resonance mode, whereas the V-shaped structures possessed both well-defined and relatively low loss electric and magnetic resonance modes, which is the desired response of the idealized split-ring resonator meta-atom. Simulation results were compared to experimentally measured polarization and orientation-dependent scattering behaviors to corroborate the behavior of these nanostructures. The ability to directly observe the complex optical scattering of these individual meta-atoms provides an opportunity to verify the desired optical response and gives direct feedback about the influence of the nanostructure shape and morphological disorder as well as metallization quality, including defects and roughness, on their optical properties.