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NIM nanosystems initiative munich

Tuesday, 31 January, 2012

Ultra-fast photodetector and terahertz generator

New applications for graphene

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 light signals and also electric signals extremely fast. Within picoseconds (10-12 seconds) the optical stimulation of graphene generates a photocurrent. 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), now developed a method to measure the temporal dynamics of this photo current.

Graphene leaves a rather modest impression at a first sight. The material comprises of nothing but carbon atoms ordered in a mono-layered “carpet”. Yet, what makes graphene so fascinating for scientists is its extremely high conductivity. This property is particularly useful for the development of photodetectors. These are electronic components that can detect radiation and transform it into electrical signals. 

Its extremely high conductivity inspires scientists to utilize graphene to design ultra-fast photodetectors. However, until now it was not possible to measure the optical and electronic behavior of grapheme with regard to time, i.e. how long it takes between the electric stimulation of graphene and the generation of the respective photocurrent. 

Alexander Holleitner and Leonhard Prechtel, scientists at the Walter Schottky Institut of the TU Muenchen and members of the Cluster of Excellence Nanosystems Initiative Munich (NIM), decided to pursue this question. 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 10-12 seconds.

The central element of the inspected photodetectors is freely suspended graphene integrated into electrical circuits via metallic contacts. The temporal dynamics of the photocurrent was 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 with 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, if 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.

The research was funded by the German Research Foundation (DFG), the Excellence Cluster Nanosystems Initiative Munich and the Center for NanoScience (CeNS). Physicists from Universität Regensburg, Eidgenössische Technische Hochschule Zürich, Rice University and Shinshu University also contributed to the publication.


Original publication: 

Time-resolved ultrafast photocurrents and terahertz generation in freely suspended grapheme. Leonhard Prechtel, Li Song, Dieter Schuh, Pulickel Ajayan, Werner Wegscheider, Alexander W. Holleitner.





Press Release as pdf



Prof. Alexander W. Holleitner

Technische Universität München
Walter Schottky Institut 
Zentrum für Nanotechnologie und Nanomaterialien 

Am Coulombwall 4a, 85748 Garching, Germany 
Tel.: +49 89 289 11575 – Fax: +49 89 289 11600 
E-Mail: holleitner(at)wsi.tum.de
Internet: www.wsi.tum.de 


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