Ultrafast Light-Driven Current Source

Schematic of the experiment for ultra-fast current generation: When the light wave (red) hits the graphene, an electronic current is generated instantly. By reading and analyzing this current, FAU researchers have found that quantum mechanical interference of electron waves can be used to control current on extremely short time scales. (Source: FAU, T. Higuchi)

Controlling electronic current is essential to modern elec­tronics, as data and signals are trans­ferred by streams of electrons which are controlled at high speed. Demands on transmission speeds are also increasing as technology develops. Scientists from the Chair of Laser Physics and the Chair of Applied Physics at Friedrich-Alexander-Uni­versity Erlangen-Nuremberg FAU have succeeded in switching on a current with a desired direction in graphene using a single laser pulse within a femto­second. This is more than a thousand times faster compared to the most efficient tran­sistors today.

In gases, insulating materials and semi­conductors, scientists have already shown that it is possible to steer electrons with light waves and thus, in principle, to control current. However, this concept has not yet been applied to metals as light cannot usually pene­trate the material to control the electrons inside. To avoid this effect, physicists in the working groups of Peter Hommel­hoff and Heiko Weber used graphene. Even though graphene is an excellent conductor, it is thin enough to let some light pene­trate the material and move the electrons.

For their experiments, the scientists fired extremely short laser pulses with specially engi­neered wave­forms onto graphene. When these light waves hit the graphene, the electrons inside were hurled in one direction, like a whiplash. “Under intense optical fields, a current was generated within a fraction of an optical cycle – a half femto­second. It was sur­prising that despite these enormous forces, quantum mechanics still plays a key role,” explains Takuya Higuchi from the Chair of Laser Physics.

The researchers discovered that the current generation process in the graphene follows complicated quantum mechanics. The electrons travel from their initial state to the excited state by two paths rather than one – similar to a forked road leading to the same destination. Like a wave, the electrons can split at the fork and flow on both roads simul­taneously. Depending on the relative phase between the split electron waves, when they meet again, the current can be very large, or not present at all.

“This is like a water wave. Imagine a wave breaks against a building wall and flows to the left and the right of the building at the same time. At the end of the building, both parts meet again. If the partial waves meet at their peak, a very large wave results and current flows. If one wave is at its peak, the other at its lowest point, the two cancel one another out, and there is no current,” explains Hommel­hoff. “We can use the light waves to regulate how the electrons move and how much elec­tricity is generated.” The results are another important step in bringing elec­tronics and optics together. In the future, the method could open a door for realizing ultra­fast elec­tronics operating at optical fre­quencies. (Source: FAU)

Reference: T. Higuchi et al.: Light-field-driven currents in graphene, Nature, online 25. September 2017; DOI: 10.1038/nature23900

Link: Collaborative Research Centre SFB 953, “Synthetic Carbon Allotropes”, Friedrich-Alexander-Universität Erlangen-Nuremberg FAU, Erlangen, Germany

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