19 research outputs found
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