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February 01, 2012

Ultra-fast photodetector and terahertz generator in graphene

Technische Universitaet Muenchen - Extremely thin, more stable than steel and widely applicable: the material graphene is full of interesting properties. As such, it is currently the shining star among the electric conductors. Photodetectors made with graphene can process and conduct both light signals and electric signals extremely fast. Upon optical stimulation, graphene generates a photocurrent within picoseconds (0,000 000 000 001 second). Until now, none of the available methods were fast enough to measure these processes in graphene. Professor Alexander Holleitner and Dr. Leonhard Prechtel, scientists at the Technische Universitaet Muenchen (TUM), have now developed a method to measure the temporal dynamics of this photo current.

Nature Communications - Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene
Ultrafast photocurrent circuitry for graphene




Graphene, a two-dimensional layer of carbon atoms, is a promising building block for a wide range of optoelectronic devices owing to its extraordinary electrical and optical properties, including the ability to absorb ~2% of incident light over a broad wavelength range. While the RC-limited bandwidth of graphene-based photodetectors can be estimated to be as large as 640 GHz, conventional electronic measurement techniques lack for analysing photocurrents at such frequencies. Here we report on time-resolved picosecond photocurrents in freely suspended graphene contacted by metal electrodes. At the graphene–metal interface, we demonstrate that built-in electric fields give rise to a photocurrent with a full-width-half-maximum of ~4 ps and that a photothermoelectric effect generates a current with a decay time of ~130 ps. Furthermore, we show that, in optically pumped graphene, electromagnetic radiation up to 1 THz is generated. Our results may prove essential to build graphene-based ultrafast photodetectors, photovoltaic cells and terahertz sources.
The physicists first developed a method to increase the time resolution of photocurrent measurements in graphene into the picosecond range. This allowed them to detect pulses as short as a few picoseconds. (For comparison: A light beam traveling at light speed needs three picoseconds to propagate one millimeter.)

The central element of the inspected photodetectors is freely suspended graphene integrated into electrical circuits via metallic contacts. The temporal dynamics of the photocurrent were measured by means of so-called co-planar strip lines that were evaluated using a special time-resolved laser spectroscopy procedure – the pump-probe technique. A laser pulse excites the electrons in the graphene and the dynamics of the process are monitored using a second laser. With this technique the physicists were able to monitor precisely how the photocurrent in the graphene is generated.

At the same time, the scientists could take advantage of the new method to make a further observation: They 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. The special thing about THz radiation is that it displays properties shared by both adjacent frequency ranges: It can be bundled like particle radiation, yet still penetrates matter like electromagnetic waves. This makes it ideal for material tests, for screening packages or for certain medical applications.

8 pages of supplemental information

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