Laser, graphene enable fastest light-driven current

"Under intense optical fields, a current was generated within a fraction of an optical cycle -- a half femtosecond," said physicist Takuya Higuchi.
By Brooks Hays   |   Sept. 26, 2017 at 12:37 PM
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Sept. 26 (UPI) -- Scientists in Germany have triggered the fastest light-driven current using a single laser pulse. The laser pulse successfully pushed a current of electrons across a graphene transistor, a single-atom layer of carbon.

The laser-driven current traverses the transistor in a single femtosecond, a millionth billionth of a second -- more than 1,000 times faster than the fastest commercial transistors.

Researchers have previously generated light-driven currents across gases, insulators and semiconductors, but until now, scientists hadn't been able to direct a light-powered current across a metal medium. Graphene is a semi-metal and a great conductor, but its thinness allows light waves to penetrate and generate a current.

During tests, scientists fired infinitesimally brief laser pulses with specifically tuned wavelengths across the graphene medium. The laser light waves inspired a whiplash-like movement of electrons in the desired direction.

Researchers detailed the results of their tests in a new paper published this week in the journal Nature.

"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," Takuya Higuchi, head of laser physics department at the Friedrich-Alexander University Erlangen-Nürnberg in Germany, said in a news release.

In observing the laser-induced current, researchers found the electrons follow the logic of a quantum system, taking not one but two paths between their initial and excited states. The electrons form a two-state system, traveling simultaneously along two paths to the same end.

Depending on the relation between the two electron waves, their relative phases, the two electrons can re-emerge as a larger current and disappear altogether.

"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," explained Peter Hommelhoff, a researcher with the laser physics department. "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."

By precisely tuning the laser wavelengths, scientists can control how the electrons flow through the graphed and how much electric current is generated.

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