Particle physics research

Laser Logic Gate Accelerates Information Processing | Research & Technology | May 2022

ROCHESTER, NY, May 17, 2022 – A logic gate developed by researchers at the University of Rochester and Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg operates at femtosecond timescales, potentially enabling the processing of information at the petahertz limit. Logic gates, which are the basic building blocks needed for computation, control how incoming information in the form of a 1 or a 0 is processed. Logic gates require two input signals and produce one logic output.

In recent years, lasers capable of generating pulses of a few femtoseconds have been developed to generate ultrafast bursts of electrical currents. This is done by lighting up tiny graphene-based wires connecting two golden metals. The ultrashort laser pulse sets in motion, or excites, the electrons in the graphene and sends them in a particular direction, thereby generating a net electric current.

Laser pulses are able to generate electricity much faster than any traditional method and can do so in the absence of applied voltage. The direction and magnitude of the current can be controlled by varying the shape of the laser pulse, or changing its phase, in other words.


Synchronized laser pulses (red and blue) generate a burst of real and virtual charge carriers in graphene which are absorbed by metallic gold to produce a net current. Courtesy of Michael Osadciw/University of Rochester.


Trying to reconcile experimental measurements in Erlangen with computer simulations in Rochester, the team found that they could generate two types of charge-bearing particles – real and virtual – in gold-graphene-gold junctions. The true charge carriers are electrons excited by light that remain in directional motion even after the laser pulse has stopped. The virtual charge carriers are only set in motion when the laser pulse is activated. As such they are elusive and exist only transiently during enlightenment.

Because graphene is connected to gold, real and virtual charge carriers are absorbed by the metal to produce a net current.

The team found that by changing the shape of the laser pulse, they could generate currents where only real or virtual charge carriers play a role. This discovery, the ability to independently control two types of currents, greatly expands the design elements of lightwave electronics.

Using this augmented control landscape, the team experimentally demonstrated, for the first time, logic gates operating at the femtosecond scale.

In the experiment, the input signals are the shape or phase of two synchronized laser pulses, each chosen to generate only a burst of real or virtual charge carriers. Depending on the laser phases used, these two contributions to the currents can add up or cancel each other out. The net electrical signal can be assigned logic 0 or 1 information, resulting in an ultra-fast logic gate.

“It will probably be a very long time before this technique can be used in a computer chip, but at least we now know that lightwave electronics are practically possible,” said Tobias Boolakee, who led the experimental efforts as PhD student. student at UFA.

The study represents the culmination of more than 15 years of research by Ignacio Franco, associate professor of chemistry and physics at Rochester.

The book was published in Nature (www.doi.org/10.1038/s41586-022-04565-9).