Credit: © 2008 ACS

Many everyday objects are transparent to terahertz radiation, including clothing and plastic. This makes for an attractive security tool. Terahertz sources are also a challenge to make. One method is ultrafast optical pumping of a semiconducting film. The relevance of the nanoscale geometry of such films, however, has been relatively unexplored.

Now James Scott of Cambridge University and co-workers1 have shown that a nanotube geometry can aid in producing terahertz emission. They focused on the ferroelectric material, lead zirconium titanate (PZT). PZT has several characteristics of a good terahertz emitter: a strong piezoelectric response and a large carrier concentration gradient of a factor of 100 over 20 nm. Terahertz emission is only observed, however, when PZT is fashioned into nanotubes. The authors speculate that this is the result of a mobility effect: PZT may have a higher surface mobility than bulk mobility. The nanotube geometry, with its high ratio of surface area to volume, thus increases overall mobility, which in turn aids emission.

The authors further note that they are able to modulate the near-surface electrical parameters of the nanotubes. This leaves open the possibility of achieving voltage-driven emission, something that would be especially useful for devices.