Conventional antennas are used to transmit radio or TV signals, but they can be made to work at optical frequencies if shrunk to the nanoscale. Nanoantennas possess "plasmonic modes" that can be tuned to resonate with the optical transitions in molecules nearby and it is these modes that increase the interaction between light and matter on sub-wavelength scales in the near field region of the antennas. However, although normal antennas are easy to tune, these nanodevices are not because of their small size.

Graphene, a monolayer of hexagonally arranged carbon atoms, comes into its own here. Its conductivity and optical constant can be tuned by simply applying a gate voltage applied to it. This is because the applied voltage changes the carrier (electron and hole) concentration in the material. Indeed, earlier this year, a team of scientists at Harvard, led by Federico Capasso, found that graphene placed in the nanogap of a dipole antenna acts as an electrically tuneable nanocircuit element and can therefore be used to control the resonance of the antenna.

Hybrid plasmonic device

“We have now found that a new hybrid plasmonic device that we have made by combing graphene with metallic nanostructures works over a wide wavelength range of up to 1100 nm at mid-infrared wavelengths, which is enough to switch the antenna resonance on and off over this wavelength range,” says team member Yu Yao. “The device can also be tuned in a matter of just nanoseconds.”

Capasso and colleagues fabricated their devices on a silicon wafer with a thin layer of silicon oxide as the insulating layer. A large-area graphene sheet, grown by a chemical vapour deposition (CVD) technique, was transferred onto the silicon oxide layer and the nanoantennas patterned directly onto the carbon sheet by removing specific sections of graphene.

The Harvard group says that it is now trying to use these tuneable antennas to make novel devices such as high-speed reconfigurable optical lenses, nanoscale transmitters and modulators. “Such devices might find potential applications in next-generation displays, imaging and optical communications systems,” Yao told nanotechweb.org.

The present work is detailed in Nano Letters DOI: 10.1021/nl403751p.

Further reading

First flat lens focuses light without distortion (Sep 2012)
Nanoantennas for ultrafast optical switches (May 2010)