Graphene Plasmons Explored for Nanoscale Control of IR Light

The ability to capture IR light with graphene nanostructures could open new opportunities for ultrasmall and efficient photodetectors, sensors, and other photonic and optoelectronic nanodevices.

Near-field image of a rectanglular graphene nanoresonator. Courtesy of CIC nanoGUNE.

When light couples to charge oscillations in graphene the result is plasmon — a mixture of light and charge oscillations — which can be squeezed into miniscule volumes that are millions of times smaller than in conventional dielectric optical cavities.

Researchers from the nanoGUNE Cooperative Research Center (CIC), in collaboration with ICFO (The Institute of Photonic Sciences) in Barcelona, and Cambridge, Mass.-based graphene company Graphenea, say they have visualized, for the first time, the creation of such plasmonic systems, and have disentangled the individual plasmonic modes and separated them into two different classes.

Using a near-field microscope to put theory to praxis, the researchers identified the two classes of plasmon as sheet and edge modes, which propagate along the sheet or along the sheet edges, respectively. The edge plasmons are unique for their ability to channel electromagnetic energy in one dimension.

Sheet plasmons, the researchers said, can exist "inside" graphene nanostructures, extending over the whole area of graphene. Conversely, edge plasmons exclusively propagate along the edges of graphene nanostructures, leading to whispering gallery modes in disk-shaped nanoresonators or Fabry-Perot resonances in graphene nanorectangles, due to reflection at their corners.

The edge plasmons were much better confined than the sheet plasmons, the researchers reported, and, most importantly, transferred energy only in one dimension. Real-space images revealed dipolar edge modes with a mode volume that was 100 million times smaller than a cube of the free-space wavelength.

The researchers also measured the dispersion of the edge plasmon based on their near-field images, highlighting the shortened wavelength of edge plasmon compared to sheet plasmon. Thanks to their unique properties, edge plasmon could be a promising platform for coupling quantum dots or single molecules in future quantum optoelectronic devices.

"Our results also provide novel insights into the physics of near-field microscopy of graphene plasmons, which could be very useful for interpreting near-field images of other light-matter interactions in two-dimensional materials,” said nanoGUNE professor Rainer Hillenbrand, who led the project.

The researchers said plasmonic field concentration can be further enhanced by fabricating graphene nanostructures acting as nanoresonators for the plasmons. The enhanced field have been already used for enhanced IR and terahertz photodetection, or IR vibrational sensing of molecules, among other applications.

The work was published in Nature Photonics (doi: 10.1038/nphoton.2016.44).