Nano Lett. http://doi.org/pb5 (2013)

The coupling of plasmonic nanostructures can lead to intense local optical fields, which can be used in devices such as sensors and photodetectors. To create and control the coupling it is necessary to place the nanostructures at a short distance from each other, with a typical gap of a few tens of nanometres. Jeremy Baumberg and colleagues at the University of Cambridge, the Nokia Research Center in Cambridge and the Center of Materials Physics in San Sebastián have now used graphene to generate well-controlled gaps between gold nanospheres and a gold substrate.

Nanospheres with diameters of 80 nm were deposited on a silicon substrate covered by a thin gold layer and a monolayer of graphene. Single particle scattering from the samples revealed a spectral doublet, which the researchers attributed to the mixing of two modes: a dipole resonance due to the electric polarization between the charge in the nanosphere and an equal and opposite charge in an image nanosphere formed in the gold layer; and a plasmon mode that is localized near the graphene layer. When the graphene layer was removed, only a peak associated with the coupling of the sphere with its image remained. Conversely, when more layers of graphene were used to create the gap only a peak associated with the gap survived.

Baumberg and colleagues also observed that although the separation between the two peaks was the same for every gold nanosphere studied, their spectral position varied accordingly to the local conductivity of the graphene layer. This suggests that a voltage applied to the graphene layer to vary its conductivity could be used to finely tune the plasmonic resonances.