Light Pulses Quickly Drive Electronic Currents Through Graphene

Physicists have used a laser to control an electronic current through graphene within one femtosecond. According to the research team this speed could be more than a thousand times faster than the speed of current transistors.

While it’s possible to use light waves to direct electrons through semiconductors and insulating materials, according to researchers this approach has not yet been used to direct electrons — and thus control currents — through metals, because light cannot penetrate the material to reach the electrons inside.

To move electrons through metal, scientists at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) used graphene, a semimetal material that is thin enough to let some light penetrate. They fired extremely short laser pulses with custom engineered waveforms onto graphene. When the 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,” said professor Takuya Higuchi.

The current generation process in the graphene followed the principles of quantum mechanics. The electrons traveled from their initial state to the excited state by two paths rather than one, similar to a forked road where two paths protrude from the fork but lead to the same destination.

The electrons can split at the fork and flow on both paths 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.

“. . . Imagine a wave (of water) 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,” said professor Peter Hommelhoff. “We can use the light waves to regulate how the electrons move and how much electricity is generated.”

The team believes that graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material because of its broadband and ultrafast optical response, weak screening and high damage threshold.

This discovery could be a step toward realizing ultrafast electronics operating at the frequencies of light.

The research was published in Nature (doi:10.1038/nature23900).