DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response

Surface plasmon resonances generated in metallic nanostructures can be utilized to tailor electromagnetic fields. The precise spatial arrangement of such structures can result in surprising optical properties that are not found in any naturally occurring material. Here, the designed activity emerges from collective effects of singular components equipped with limited individual functionality. Top-down fabrication of plasmonic materials with a predesigned optical response in the visible range by conventional lithographic methods has remained challenging due to their limited resolution, the complexity of scaling, and the difficulty to extend these techniques to three-dimensional architectures. Molecular self-assembly provides an alternative route to create such materials which is not bound by the above limitations. We demonstrate how the DNA origami method can be used to produce plasmonic materials with a tailored optical response at visible wavelengths. Harnessing the assembly power of 3D DNA origami, we arranged metal nanoparticles with a spatial accuracy of 2 nm into nanoscale helices. The helical structures assemble in solution in a massively parallel fashion and with near quantitative yields. As a designed optical response, we generated giant circular dichroism and optical rotary dispersion in the visible range that originates from the collective plasmon-plasmon interactions within the nanohelices. We also show that the optical response can be tuned through the visible spectrum by changing the composition of the metal nanoparticles. The observed effects are independent of the direction of the incident light and can be switched by design between left- and right-handed orientation. Our work demonstrates the production of complex bulk materials from precisely designed nanoscopic assemblies and highlights the potential of DNA self-assembly for the fabrication of plasmonic nanostructures.Comment: 5 pages, 4 figure

Photonic Analogue of Two-dimensional Topological Insulators and Helical One-Way Edge Transport in Bi-Anisotropic Metamaterials

Recent progress in understanding the topological properties of condensed matter has led to the discovery of time-reversal invariant topological insulators. Because of limitations imposed by nature, topologically non-trivial electronic order seems to be uncommon except in small-band-gap semiconductors with strong spin-orbit interactions. In this Article we show that artificial electromagnetic structures, known as metamaterials, provide an attractive platform for designing photonic analogues of topological insulators. We demonstrate that a judicious choice of the metamaterial parameters can create photonic phases that support a pair of helical edge states, and that these edge states enable one-way photonic transport that is robust against disorder.Comment: 13 pages, 3 figure

Cross-section comparisons of cloaks designed by transformation optical and optical conformal mapping approaches

We review several approaches to optical invisibility designed using transformation optics (TO) and optical conformal mapping (CM) techniques. TO is a general framework for solving inverse scattering problems based on mimicking spatial coordinate transformations with distributions of material properties. There are two essential steps in the design of TO media: first, a coordinate transformation that achieves some desired functionality, resulting in a continuous spatial distribution of constitutive parameters that are generally anisotropic; and, second, the reduction of the derived continuous constitutive parameters to a metamaterial that serves as a stepwise approximation. We focus here on the first step, discussing the merits of various TO strategies proposed for the long-sought 'invisibility cloak'-a structure that renders opaque objects invisible. We also evaluate the cloaking capabilities of structures designed by the related CM approach, which makes use of conformal mapping to achieve index-only material distributions. The performance of the various cloaks is evaluated and compared using a universal measure-the total (all-angle) scattering cross section. © 2011 IOP Publishing Ltd.US Army Research Office (contract no. W911NF-09-1-0539