"Making contacts to rods with a radius of just 25 nm is not easy and can be considered a research topic in itself," Michael Johnston of Oxford University's physics department told nanotechweb.org. "Even once you have made contacts it is difficult to separate the electrical properties of the contacts from those of the nanowire."

To get around the problem, researchers from Oxford University and the Australian National University are using a method dubbed time-resolved terahertz spectroscopy. The optically pumped apparatus can be thought of as a "contactless" conductivity probe, but that's not its only advantage.

"Our terahertz conductivity measurements have a much higher time resolution (~100 fs) than conventional techniques," explained Johnston. "Pure electrical methods are limited by the time/frequency response of the present generation of electronics."

The group measures terahertz conductivity as a function of time and as a function of frequency after injecting charge carriers into the nanowire. According to Johnston, comparing the results with theoretical models of electron motion in nanowires gives a full picture of the nanostructure's electrical properties.

"It turns out that the conductivity of nanowires is much higher than was expected," he commented. "One would imagine that the conductivity of nanowires would be severely restricted by their large surface area to volume ratio, as charges near the surface are much more likely to be trapped and scattered than those away from the surface. Instead, we found that the peak conductivity of charge carriers injected into GaAs nanowires can be as high as a third of the conductivity in the bulk."

Building on the result, the researchers discovered that they were able to tune the period of time that the nanowires were conductive by saturating the structure with optically injected charge carriers.

"In future devices, the trap density could be controlled using surface treatments or by overcoating the nanowires," Johnston enthused. "Our work shows that nanowires have tremendous potential as ultrafast switching devices and ultrafast optoelectronic devices such as fast photodetectors or modulators for use in very high bandwidth communications systems."

The researchers reported their work in Nano Lett..