March 26, 2013
An exerpt from Nature's "Photonics: Trick of the light " Neil Savage discusses Antoine Moreau's publication Controlled-reflectance surfaces with film-coupled colloidal nanoantennas (abstract from publication below), doi:10.1038/nature11615
Glowing for gold: metal nanoparticles turn the Lycurgus Cup red when light is shone through it.
Gold can be used to build materials with properties that seem to defy common sense. One such metamaterial attracts light from an area larger than itself. The material, built at Duke University in Durham, North Carolina, consists of a thin film of gold covered with a transparent polymer, with silver nanocubes scattered over the polymer surface at a density of up to 30 million cubes per square millimetre. With the right spacing between the underlying metal and the silver cubes, plasmon resonance alters the electromagnetic properties of the thin film in an area 30 times as large as the cube itself, cancelling out its ability to reflect light**. So carefully covering about 3% of a surface could lead to the absorption of all the light striking that surface. “All the light gets trapped under the cube,” explains Antoine Moreau, a nanophotonics researcher at Blaise Pascal University in Clermont-Ferrand, France, who worked with the Duke team. Such total absorption might be useful for turning light into heat and then generating electricity in thermo-photovoltaics. Engineering the material to control which wavelength it captures could lead to a nano-ink for security printing on currency or seals of authenticity.
The first three-dimensional metamaterials that work with infrared or visible light were created in 2008, and they are still expensive and time consuming to produce. But the Duke process relies on self-assembly, selecting the chemical and physical properties of the components so they form the desired structure, just as Edel's sensors do. In fact, much of the appeal of recent work with nanoparticles is that they rely on potentially cheap 'bottom-up' assembly."
**Controlled-reflectance surfaces with film-coupled colloidal nanoantennas
Abstract: Efficient and tunable absorption is essential for a variety of applications, such as designing controlled-emissivity surfaces for thermophotovoltaic devices1, tailoring an infrared spectrum for controlled thermal dissipation2 and producing detector elements for imaging3. Metamaterials based on metallic elements are particularly efficient as absorbing media, because both the electrical and the magnetic properties of a metamaterial can be tuned by structured design4. So far, metamaterial absorbers in the infrared or visible range have been fabricated using lithographically patterned metallic structures2, 5, 6, 7, 8, 9, making them inherently difficult to produce over large areas and hence reducing their applicability. Here we demonstrate a simple method to create a metamaterial absorber by randomly adsorbing chemically synthesized silver nanocubes onto a nanoscale-thick polymer spacer layer on a gold film, making no effort to control the spatial arrangement of the cubes on the film. We show that the film-coupled nanocubes provide a reflectance spectrum that can be tailored by varying the geometry (the size of the cubes and/or the thickness of the spacer). Each nanocube is the optical analogue of a grounded patch antenna, with a nearly identical local field structure that is modified by the plasmonic response of the metal’s dielectric function, and with an anomalously large absorption efficiency that can be partly attributed to an interferometric effect10. The absorptivity of large surface areas can be controlled using this method, at scales out of reach of lithographic approaches (such as electron-beam lithography) that are otherwise required to manipulate matter on the nanoscale.