Scientists at Vienna University of Technology (TU Vienna) have developed light detectors composed of graphene, demonstrating how remarkably fast the material converts light pulses into electrical signals, a capability that could considerably improve date flow between computers. When data is transmitted by light pulses (for instance in fiber optic cables), the pulses must be converted into electrical signals, which can be processed by a computer. In certain materials, light can cause electrons to leave their positions and travel through the material freely. Light makes the electrons flow on a sample of graphene. (Images: Vienna University of Technology) “Light detectors which convert light into electronic signals have been around for a long time. But when they are made of grapheme [a honeycomblike carbon structure made of only one layer of atoms], they react faster than most other materials could,” said the university’s Alexander Urich, who investigated the optical and electronic properties of graphene with colleagues Thomas Müller and Karl Unterrainer. The scientists had shown previously that graphene can convert light into electronic signals with remarkable speed. However, the reaction time of the material could not be determined — the photoelectric effect in graphene is so fast that it cannot be measured by the usual measuring methods. But now, sophisticated technological tricks could shed some light on the properties of graphene. Alexander Urich and Thomas Müller with a chip containing graphene. At TU Vienna, the researchers fired laser pulses at a graphene photodetector in quick succession and measured the resulting photocurrent. If the time delay between the laser pulses changed, the detector’s maximum frequency could be determined. “Using this method, we could show that our detectors can be used up to a frequency of 262 GHz,” Müller said. This corresponds to a theoretical upper bound for data transfer using graphene photodetectors of more than 30 GB/s. It has yet to be determined to what extent this is technically feasible, but this result clearly shows the remarkable capability of graphene and its potential for optoelectronic applications. Karl Unterrainer, Thomas Müller and Alexander Urich of the Vienna University of Technology. The main reason for the fact that graphene photodetectors can operate at such high frequencies is the short life span of the charge carriers in graphene. The electrons that are removed from their fixed position and that contribute to the electrical current settle down at another fixed position after a few picoseconds. As soon as this happens, the graphene photodetector is ready for another light signal, which frees new electrons and creates the next electrical signal. The fast reaction time of graphene is one more item on the list of remarkable properties of this material. In graphene, charge carriers can travel extremely far without being disturbed. Graphene can absorb light in a huge spectral range, from infrared to visible light — unlike standard semiconductors, which can absorb only a small part of the spectrum. For more information, visit: www.tuwien.ac.at