About a year ago a group of researchers at the Barcelona Institute of Photonic Sciences (ICFO) showed indirectly that graphene is capable of transforming a single photon into multiple electrons. In other words, the material could be highly effective at transforming light into electricity. Now a group of scientists from Lausanne Federal Polytechnic University (EPFL) has succeeded in demonstrating how this phenomenon occurs. The work, which has been published in the magazine Nano Letters, opens up the possibility of creating a new generation of advanced photovoltaic devices with graphene.
Scientists knew that when graphene absorbs a photon, it generates multiple electrons with enough energy to conduct an electrical current. The problem was that this phenomenon of photons converting into electricity happens so fast that conventional techniques couldn’t detect it. Now a team of researchers from various universities (EPFL in Switzerland, Aarhus in Denmark and the Elettra Centre in Italy) have developed a new method called “time- and angle-resolved photoemission spectroscopy” (trARPES) which can detect it.
The demonstration with this advanced spectroscopic technique was performed at the Rutherford Appleton Laboratory in Oxford (England). In order to capture the conversion process, the researchers placed a small graphene sample in an ultra-high vacuum (UHV) chamber and subjected it to the impact of a pulse of extreme ultra-violet (XUV) laser light with the aim of exciting its electrons to the highest possible state of energy at which an electrical current can be conducted. They then hit the graphene with a timed probe pulse, which photographed the energy presented in each electron at a certain point in time. The images were taken repeatedly, creating an animated film which captures the entire conversion process.
Energy-Flow Control in Telecommunications
Just as photons are quantisations (divisions) of electromagnetic and mechanical waves, a plasmon is the quasiparticle resulting from the quantisation of plasma oscillations. Well, just a few days ago scientists at ICFO, MIT, CNRS, CNISM and Graphanea have proven that it is possible to control the energy flow from erbium ions to photons and plasmons simply by applying an electrical voltage. Erbium ions emit light at a wavelength of 1.5 mm, known as the third telecom window,and are used in optical fibre amplifiers. (The ‘third window’ is very important in optical telecommunications because it is a region in which there is very little energy loss, favouring information transmission.)
Being able to control the electrical modulation of photon emissions in optical communications and light-emitting devices such as lasers, sensors or screens is a significant development and paves the way for the creation of new photonic nanodevices based on active plasmonics.
The ICFO researcher Frank Koppens has commented that this work “demonstrates that, thanks to graphene’s optoelectronic properties, efficient electrical control of light on a nanoscale is possible.” The results of the research could lead to a new generation of nano-sensors with applications in fields as diverse as bio-molecules, solar cells and light detectors, as well as in the processing of quantum information.
The study, published in the magazine Nature Physics, was funded by the European Commission’s Flagship Graphene, the NWO Rubicon programme, the Fundació Privada Cellex Barcelona, and the ERC and MIT MISTI-España programmes.
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