Everyone is familiar with conventional antennas that are used to transmit radio or TV signals, but these can be made to work at optical frequencies as long as the device is shrunk to the nanoscale. Nanoantennas possess "plasmonic modes" that can be tuned to resonate with the optical transitions in molecules nearby. It is these plasmonic modes that increase the coupling between light emitted by neighbouring molecules and the antenna. However, while TV and radio antennas are easy to tune, these nanodevices are not because of their small size.

A team led by Federico Capasso at Harvard has now shown that the resonance of nanoantennas containing graphene can be tuned in the mid-infrared part of the electromagnetic spectrum by simply applying a voltage.

Electrically tunable nanoconductor

Graphene is a monolayer of hexagonally arranged carbon atoms. A gate voltage applied to graphene can change the carrier (electron and hole) concentration in the material and change its conductivity and optical constant. Capasso and colleagues found that graphene placed in the nanogap of a dipole antenna acts as an electrically tunable nano-circuit element and can therefore be used to control the resonance of the antenna.

“From an optics point of view, the antenna focuses incoming light down to a very bright sub-wavelength hotspot,” explained team member Yu Yao. “This leads to strong light–graphene interaction, therefore, allowing us to tune the antenna resonance over a broad wavelength range.”

The researchers 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 chemical vapour deposition (CVD), was transferred onto the silicon oxide layer and the nanoantennas patterned directly onto the carbon sheet.

The devices could find applications in multi-analyte sensors and optoelectronics. Indeed, the Harvard team has already made broadband high-speed optical modulators from their antennas that operate in the mid-infrared range.

The scientists say that they are now busy looking at different antenna structures to further increase the tuning range. “We are also trying to find out how metasurfaces (arrays of hundreds of nanoantennas) based on these tunable devices behave,” Yao told nanotechweb.org.

Details of the research can be found in Nano Letters.