New graphene applications: Ultra-fast photodetector and terahertz generator
(Nanowerk News) 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 (see paper in Nature Communications: "Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene").
A graphene sheet stretches the small gap between two metalic contacts.
Graphene leaves a rather modest impression at a first sight. The material comprises 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
in the development of photodetectors. These are electronic components that can detect
radiation and transform it into electrical signals.
Graphene's extremely high conductivity inspires scientists to utilize it in the design of ultrafast
photodetectors. However, until now, it was not possible to measure the optical and
electronic behavior of graphene with respect 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 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.
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.
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