Jena, Germany - An international team of researchers from Jena, Germany, Basel, Switzerland, and Kaust, Saudi-Arabia proposed a new and unique graphene application in their publication entitled, “Tunable graphene antennas for selective enhancement of Thz-emission.”, Optics Express 21 Issue 3, pp. 3737-3745 (2013), link . In the paper the authors show how a specific molecule can be forced to emit electromagnetic radiation at certain pre-defined frequencies. The groundbreaking application relies on the unique properties of graphene may ease the identification of molecules as the authors state.
The mechanism can be understood in analogy to a tuned radio and will be briefly described in the following; more in-depth information can be found here . A certain molecule, coronene, is placed inside the feed of a carefully designed graphene antenna. This can be achieved by either an atomic force microscope or a microfuidic channel in an experiment. The molecule is then transferred into an excited state by absorption of a photon in the visible or near ultra violet frequency range. At that point, the graphene antenna is tuned to a specific resonance frequency of the molecule - just like a radio - using a certain gate voltage.
Now, coronene can lose energy very easily through the transition of the graphene antenna whereas other transitions are barely affected. Technically, the radiation rates of coronene are changed due to the Purcell effect. As for general molecular systems, there are internal redistribution processes between different states of coronene. These redistributions are much faster than any radiation of the electronic states of the molecule. If one of the states in the redistribution chain looses energy much faster than the others, the molecule will mainly emit light at that specific frequency - the tuned frequency of the graphene antenna. The authors call the entire process selective emission.
The frequencies at which the molecule emits are however not in the optical domain, they are in the so-called terahertz (THz) gap, where functional devices are urgently needed. Such an application cannot be realized with conventional plasmonic structures made of metals for two main reasons: the interaction of metallic antennas also at optical frequencies and the lacking ability to tune such devices. Furthermore, the authors claim that graphene is generally better suited for applications in the THz frequency range for their relatively low losses compared to metals.
One may ask, if the predicted selective enhancement of coronene emissions is a special behavior of the very molecule. The authors argue that this is not the case since the emission properties of corenene are just an application of the fundamental generalized Planck's radiation law for luminescence. This makes the impact of the paper even stronger as it suggests that such graphene antennas may be used to identify molecules generally using their THz footprint which might give rise to spectroscopic applications in drug analysis or explosive detection.
It is still a big question of our time, if we can make final use of the fascinating properties of graphene. We have seen that researchers are working very hard to find such applications and that the use of graphene may open up entirely new possibilities for fundamental studies and engineering. Nevertheless, one thing seems to be clear: we can be very excited about upcoming developments in graphene research!
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