Terahertz radiation, which lies between the microwave and mid-infrared regions of the electromagnetic spectrum, could, for example, be employed at airports to detect items such as concealed weapons and explosives. This is because the radiation passes through clothing and packaging but is strongly absorbed by metals and other inorganic substances.

“Our study shows that we can now measure how the ultrafast electrons in graphene respond to terahertz excitation, and that this response is very sensitive to the initial doping level in the material,” team member Sufei Shi told nanotechweb.org. “The result will be very important for designing graphene-based optoelectronics devices in the future that work at these frequencies.”

Monitoring THz wave transmission change

The researchers, led by Feng Wang, used a pump laser light beam with a wavelength of 800 nm to excite their sample (a graphene field-effect transistor), while also sending in terahertz waves at approximately the same time, as a probe. “By adjusting the time delay between the pump and the probe, we were able to monitor terahertz wave transmission change through the sample,” said Shi.

Graphene is a zero-bandgap material. When it is charge neutral, it behaves like a semiconductor if excited with light. However, when it is heavily doped (that is, with a lot of charge carriers – electrons or holes), it behaves more like metal. According to the California team's new experiments, the electrons in both charge-neutral and heavily doped graphene also appear to be hotter than the rest of the material lattice. This means that graphene does not behave like a conventional semiconductor when exposed to light but that it is these hot charge carriers that determine how terahertz photoconductivity changes in graphene.

Hot charge carriers increase THz conductivity

In the charge-neutral material, the hot charge carriers increase electron and hole densities and so increase the THz conductivity, explains Shi. In highly doped graphene, however, photoexcitation does not change the overall concentration of conducting charge carriers but instead increases the rate at which electrons are scattered in the material and so reduces its THz conductivity.

“We also found that electron heating is more efficient in the charge-neutral material and that it is possible for a single photon from the pump laser to create multiple conducting carriers,” said Shi. Such a phenomenon could help bypass the so-called quantum photon-electron conversion efficiency, he says, and be exploited in efficient light energy-harvesting devices, such as solar cells.

The results will be important for fabricating new types of ultrafast and highly efficient photodetectors and energy-harvesting devices that would operate in a very different way to standard semiconductor devices because they rely on hot-electron generation, he adds.

The California team says that it is now busy looking at making switches and modulators based on the ultrafast terahertz response of graphene.

The current work is reported in Nano Letters.