Direct Detection of Optical Resonance Modes in Meta-Atoms Fabricated By Metallization of DNA Origami Templates

Despite the intense and growing interest in optical metamaterials, progress in fabricating such materials is still limited. Metamaterials consist of a periodic or aperiodic arrangement of meta-atom building blocks. Even though the optical properties of metamaterials are derived from the collective response of the meta-atoms arranged in 2D or 3D assembly, it is highly desirable to understand the scattering patterns of individual meta-atoms, which will help in designing metamaterials with predictable optical properties. To date, most efforts have been put into top-down processes such as lithography-based techniques to create metamaterial building blocks for the application at optical frequencies. However, these techniques are generally expensive and have low throughput. DNA origami-based nanofabrication can potentially overcome these limitations. In this work, we fabricated various DNA origami templated gold nanostructures, including hollow nanotriangles, V-shaped nanoantennas, C and U-shaped structures, and rectangular tiles. Metallization is achieved by a two-step process involving photoreduction of silver to create metal seeds, followed by electroless reduction of gold. Optical scattering from individual metallized DNA origami was measured using dark-field microscopy. Experimental scattering measurements are complemented by numerical calculations of the scattering spectra of both ideal nanostructures as well as realistic DNA origami-templated nanostructures to incorporate the effect of roughness and disorder present in the meta-atoms. While simulations of the hollow nanotriangle demonstrate only an electric resonance mode, the V-shaped nanoresonator possesses both the electric and magnetic resonance modes as is present in the idealized split-ring resonator meta-atom. Simulation results were compared to experimentally measured polarization and orientation-dependent scattering behavior of these nanostructures. By comparing the scattering pattern of the gold hollow nanotriangle to that of the V-shaped nanostructure, we are able to verify the presence of an electric resonance in the nanotriangle, and the presence of both electric and magnetic modes in the V-shaped structure. The ability to directly observe the complex optical scattering of these individual nanostructures provides the opportunity to verify simulation results as well as to provide a means of direct feedback about the influence of nanostructure quality, including defects and roughness, on the optical response.