Infrared Light Captured with Graphene

Near-field image of a rectangle graphene nanoresonator (Source: CIC nanoGUNE)

Near-field image of a rectangle graphene nanoresonator (Source: CIC nanoGUNE)

Researchers from CIC nanoGUNE, in colla­boration with ICFO and Graphenea, have demonstrated how infrared light can be captured by nano­structures made of graphene. This happens when light couples to charge oscillations in the graphene. The resulting mixture of light and charge oscil­lations – the plasmon – can be squeezed into record-small volumes – millions times smaller than in conventional dielectric optical cavities. This process has been visualized by the researchers now, for the first time, with the help of a state-of the-art near-field microscope and explained by theory. Parti­cularly, the researchers identified two types of plasmons – edge and sheet modes – propagating either along the sheet or along the sheet edges. The edge plasmons are unique for their ability to channel electro­magnetic energy in one dimension. The work – funded by the EC Graphene Flagship – opens new opportunities for ultra-small and efficient photo­detectors, sensors and other photonic and opto­electronic nanodevices.

Graphene-based techno­logies enable extremely small optical nano­devices. The wavelength of light captured by a graphene sheet – a monolayer sheet of carbon atoms can be shortened by a factor of 100 compared to light propagating in free space. As a consequence, this light propa­gating along the graphene sheet – a graphene plasmon – requires much less space. For that reason, photonic devices can be made much smaller. The plasmonic field concen­tration can be further enhanced by fabricating graphene nano­structures acting as nano­resonators for the plasmons. The enhanced field have been already applied for enhanced infrared and tera­hertz photo­detection or infrared vibra­tional sensing of molecules, among others.

“The develop­ment of efficient devices based on plasmonic graphene nano­resonators will critically depend on precise under­standing and control of the plasmonic modes inside them” says Pablo Alonso-Gonzalez, who performed the real-space imaging of the graphene nano­resonators with a near-field microscope. “We have been strongly impressed by the diversity of plasmonic contrasts observed in the near-field images” continues Alexey Nikitin, Ikerbasque Research Fellow at nanoGUNE, who developed the theory to identify the individual plasmon modes.

The research team has disent­angled the individual plasmonic modes and separated them into two different classes. The first class of plasmons – “sheet plasmons” – can exist “inside” graphene nano­structures, extending over the whole area of graphene. Conversely, the second class of plasmons – “edge plasmons” – can 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 are much better confined than the sheet plasmons and, most impor­tantly, transfer the energy only in one dimension. The real-space images reveal dipolar edge modes with a mode volume that is 100 million times smaller that a cube of the free-space wavelength. The researchers also measured the dispersion  of the edge plasmons based on their near-field images, high­lighting the shortened wavelength of edge plasmons compared to sheet plasmons. Thanks to their unique properties, edge plasmons could be a promising platform for coupling quantum dots or single molecules in future quantum opto-electronic 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”, adds Rainer Hillen­brand who led the project. (Source: CIC nanoGUNE)

Reference: A. Y. Nikitin et al.: Real-space mapping of tailored sheet and edge plasmons in graphene nanoresonators, Nat. Phot., online 21 March 2016, DOI: 10.1038/nphoton.2016.44

Links: CIC nanoGUNE, San Sebastian, Spain • Graphene Flagship Program, European Union

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