Controlled-reflectance surfaces with film-coupled colloidal nanoantennas

Randomly adsorbing chemically synthesized silver nanocubes, each of which is the optical analogue of a grounded patch antenna, onto a nanoscale-thick polymer spacer layer on a gold film results in a metamaterial surface with a reflectance spectrum that can be tailored by varying the geometry. Nanocube antennas In many photonic applications ranging from sensors to energy-harvesting devices, a perfectly absorbing material is desired. Previously, perfect absorbers of infrared or visible light have been made by using lithography to create patterned structures on metallic surfaces, but this approach is expensive and difficult to scale up. Antoine Moreau et al . have developed an attractively simple method, in which silver nanocubes produced by wet chemistry are randomly distributed across a polymer-coated gold surface. Each cube acts as a nanoantenna to counter the reflectance of the metal surface. These cubes are simple and cheap to produce and can be easily spread and attached to the surface, so that large areas can be covered. They provide a means of controlling the colour of the reflected light, and the efficient optical response of the cubes suggests that mixed cube populations with controlled sized dispersion could be used to adjust the absorption at will. Efficient and tunable absorption is essential for a variety of applications, such as designing controlled-emissivity surfaces for thermophotovoltaic devices 1 , tailoring an infrared spectrum for controlled thermal dissipation 2 and producing detector elements for imaging 3 . 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 design 4 . So far, metamaterial absorbers in the infrared or visible range have been fabricated using lithographically patterned metallic structures 2 , 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 analogu