Nanoantenna effect has potential for optical switching

A team investigating the properties of very small antennae has produced a fast optical transistor capable of changing the physical state of a metal oxide from insulator to conductor.

The team is looking at nanostructures that interact strongly with light; an area of interest because it could allow optical devices to be used for a very wide variety of applications, for example in computing and medicine. Such structures are smaller than the wavelength of visible light, yet allow light energy to be concentrated to a very high level.

Working with teams with specialised capabilities at Salford University and the University of the Basque Country in San Sebastien, Spain, the Southampton researchers used gold nanoantennae to achieve a phase transition in vanadium oxide (VO₂). At room temperature, VO₂ is an insulator, but if heated above 68°C its structure changes and it conducts electricity; this makes it potentially useful in optical transistors, because light energy can be used to rate its temperature.

A team at Salford University that specialises in thin-film deposition fabricated a 50nm-thick film of VO₂ on a sheet of glass that had previously been coated with fluorine-doped tin oxide (this ensured that the vanadium oxide was very smooth, with only 5nm roughness). On top of this layer, the team fabricated gold nanoantennae using electron-beam lithography to deposit a 45nm thick layer, the unwanted parts of which were removed with acetone to leave semi-cylindrical structures 80nm wide and with length varying between 160 and 360nm.

Back at Southampton, the samples were exposed to picosecond pulses from a Yb-fibre laser producing energy with a wavelength of 1060nm. Although the temperature of the samples was well below the phase transition point, the areas directly below the antennae underwent phase transition and became conducting. This effect was very localised; further away from the antennae the VO₂ remained insulating.

The researchers explain their work in a paper in the journal Light, Science and Applications. Lead author Prof Otto Muskens said: "The nanoantenna assists the phase transition of the vanadium dioxide by locally concentrating energy near the tips of the antennae. It is a lightning rod effect. Antenna-assisted switching thus results in a large effect while requiring only a small amount of energy."

Calculations by theoretical modellers in San Sebastian showed that the switching only required picojoules of energy. "If we are able to actively tune a nanoantenna using electrical or optical signal we could achieve transistor-type switches for light with nanometre-scale footprints for data communication," Muskens added. "Such active devices could also be used to tune the antenna’s light-concentration effects leading to new applications in switchable and tunable antenna-assisted processes."