Source: Spectroscopy Now.

Photocurrent pumped and probed

When graphene is stimulated optically it produces a photocurrent on a time scale of mere picoseconds. A German research team has now used the pump-probe method of time-resolved laser spectroscopy to take a snapshot of this process as it happens.

Graphene, the single layer graphite-like material that earned the 2011 Nobel Prize for Physics for its developers in Manchester, England, continues to offer intriguing glimpses into a world of future optoelectronic devices. Indeed, photodetectors composed of this carbon allotrope can both conduct and process light signals and electric signals extremely quickly. Graphene absorbs about 2 percent of incident light across a range of wavelengths. Until now, observing such a very rapid process in terms of photocurrent was not possible.

Alexander Holleitner and Leonhard Prechtel of the Technische Universitaet Muenchen (TUM), Germany have now devised an approach to measuring how this photocurrent changes over such very short time scales. The work could allow researchers to investigate graphene¡¯s high conductivity and other properties in much finer detail than was previously possible. The work could open up a whole range of applications and lead to the development of viable optoelectronic devices based on graphene.

Holleitner and Prechtel, who both work at the Walter Schottky Institut at TUM, have collaborated with members of the Cluster of Excellence Nanosystems Initiative Munich (NIM), physicists from the Universität Regensburg, Germany (Dieter Schuh), Eidgenössische Technische Hochschule Z¨¹rich (ETH), Switzerland (Werner Wegscheider), Rice University, Houston, Texas, USA (Pulickel Ajayan) and Shinshu University in Wakasato, Japan (Li song). They explain that the central element of the photodetector that they are studying contains a freely suspended graphene integrated into an electrical circuit via metallic coplanar stripline contacts. Time-resolved laser spectroscopy involved exciting electrons in the co-planar stripline circuit and monitoring the result of the stimulation with a second laser.

Terahertz observations

The technique has the added advantage of allowing the researchers to make other observations simultaneously. They have now found evidence that graphene, when optically stimulated, emits radiation in the terahertz (THz) range. This lies between infrared light and microwave radiation in the electromagnetic spectrum (0.3 to 3.0 THz; 0.1 to 1.0 mm). ¡± Various graphene-based terahertz sources and detectors have been proposed, as the frequency of plasma waves, the gap of graphene nanoribbons, and the tunable bandgap in bilayer graphene lies in the terahertz range,¡± the team explains.

Terahertz radiation is useful for penetrating matter for materials testing, scanning suspect packages at borders and in medical imaging. THz radiation is also at the heart of modern body scanners used at airports to reveal objects hidden beneath a passenger¡¯s clothes, for instance.

The researchers explain that the action occurs at the interface between the graphene and the metal stripline connectors. ¡°We demonstrate that built-in electric fields give rise to a photocurrent with a full-width-half- maximum of about 4 picoseconds and that a photothermoelectric effect generates a current with a decay time of about 130 ps,¡± they say. Additionally, given that the optically pumped graphene generates electromagnetic radiation up to 1 THz there might be wide applications for this discovery. ¡°Our results may prove essential to build graphene-based ultrafast photodetectors, ultrafast photoswitches, photovoltaic cells and terahertz sources,¡± the team concludes.

Related links

Nature Commun, 2012, online: ¡°Time-resolved ultrafast photocurrents and terahertz generation in freely suspended  rapheme¡±