Recent Developments in Photoelectrics
Recent progress in this field has been made by Professor Holleitner, and Dr. Leonhard Prechtel, and their coworkers, at the Walter Schottky Institute of Munich University (Germany).
In their experiments, these scientists have improved the time resolution of their instrument, and have been able to detect photocurrents with only a few picoseconds duration.
The photocurrent measurements were performed using an advanced technique, called pump-probe spectroscopy. With this method, two lasers are used first to excite the graphene (pump) and then to detect (probe) the generated electrical signal.
In this study, graphene was deposited on a copper substrate and subsequently covered by a thin layer of a polymer (polymethylmetacrylate, PMMA). Then the copper was removed, using acid, and the graphene-PMMA mixture was placed on an appropriate substrate, connected to gold electric contacts. The PMMA was removed using acetone, leaving graphene suspended between the contacts. The Figure on the side shows the graphene between the two contacts hit by the laser.
Excellent Time Resolution and Terahertz Emission
In these conditions, professor Holleitner and his coworkers measured the photocurrents emitted from graphene as a function of time. The values of the current changed depending on the power of the laser used; the current increased sharply immediately after the pump, showing a peak, and then decreased slowly. The peak was detected for times as low as 4 picoseconds.
Commenting these results, Professor Holleitner said: “Now we have a confirmation of the exceptional ultrafast optoelectronic properties of graphene, we can think about many possible applications, using graphene for photodetectors or photoswitches. Furthermore, we also observed that, under excitation, graphene emits terahertz radiation. At the moment there are very few materials able to emit at this interval of frequencies; therefore, this is an issue which could have interesting technological developments in the future, in fields such as sensing and communications”
The full study about graphene ultrafast response was published on Nature Communications.
L. Prechtel et al. Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene. (2012). Nature Communications, doi:10.1038/ncomms1656. Accessed February 3, 2012.
F. Bonaccorso et al. Graphene photonics and optoelectronics. Nature Photonics, 4, 611-622 (2010). Accessed February 3, 2012.
J. S. Braley. Photodetectors. Accessed February 3, 2012.
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