Traditional antennas are used to transmit radio and television signals but the concept can be applied to optical frequencies too if the device is scaled down to nanometre scales. This is because antennas work by exploiting oscillating charges along the length of the device, which means that its size must be reduced to fit a resonant mode for the wavelength of electromagnetic radiation it supports.

Optical antennas can be used to control light at the nanoscale and will be a major tool for developing nanophotonics applications in the future. The devices possess "plasmonic modes" that can be tuned to resonate with the electronic transitions in molecules nearby. It is these plasmonic modes that increase the coupling between light emitted by neighbouring molecules and the antenna.

Otto Muskens of the University of Southampton and colleagues at the Donostia International Physics Center in San Sebastian, Spain, and the University of Toulouse, France, designed a plasmonic device that consists of a dipole antenna with two conductive arms (see figure). In their set-up, the photoconductive material is a semiconductor, such as silicon.

When light is shone onto the nanoantenna, it goes from being a capacitor to a conductor. Below the switching threshold, there are individual half-wavelength resonances over the two arms. Above the switching threshold, the antenna arms couple and start to conduct electricity together. This leads to a completely new fundamental mode, explains Muskens, where a half-wave resonance is formed over the entire nanoantenna. This half-wavelength resonance appears at longer optical wavelengths and the strategically placed photoconductive bridge in the nanoscale gap between the antenna arms makes the new switching scheme possible.

According to the researchers, the device might be used to make nanoantenna switches in integrated optoelectronic circuits, where single nanoantennas could help to connect electrical and optical information channels.

The team now plans to make a real nanoantenna switch in the lab. "At the University of Southampton, we are exploring different routes – either using top-down lithographic design or bottom-up synthesis of hybrid plasmonic devices," Muskens told

The work was reported in Nano Lett..