Fast computing with light fields

Physicists are working hard to control electronics with light waves. The oscillation of a light wave takes approximately one femto­second. Controlling electrical signals with light could make the computers of the future over a million times faster, which is the aim of petahertz signal processing or light wave electronics. The researchers have been inves­tigating how light waves can be converted to current pulses for several years. In their experiments, the researchers illu­minate a structure of graphene and gold electrodes with ultrashort laser pulses. The laser pulses induce electron waves in the graphene, which move toward the gold electrodes where they are measured as current pulses and can be processed as infor­mation.

Depending on where the laser pulse hits the surface, the electron waves spread dif­ferently. This creates two types of current pulses – real and virtual charges. “Imagine that graphene is a pool and the gold electrodes are an overflow basin. When the surface of the water is disturbed, some water will spill over from the pool. Real charges are like throwing a stone into the middle of the pool. The water will spill over as soon as the wave that has been created reaches the edge of pool, just like electrons excited by a laser pulse in the middle of the graphene,” explains Tobias Boola­kee, researcher at the Chair of Laser Physics. “Virtual charges are like scooping the water from the edge of the pool without waiting for a wave to be formed. With electrons, this happens so quickly that it cannot be perceived – a virtual charge. In this scenario, the laser pulse would be directed at the edge of the graphene right next to the gold electrodes.” Both virtual and real charges can be inter­preted as binary logic – 0 or 1.

The laser physicists have been able to demons­trate with their experiments for the first time that this method can be used to operate a logic gate – a key element in computer proces­sors. The logic gate regulates how the incoming binary infor­mation (0 and 1) is processed. The gate requires two input signals, here electron waves from real and virtual charges, excited by two syn­chronized laser pulses. Depending on the direction and strength of the two waves, the resulting current pulse is either aggre­gated or erased. Once again, the electrical signal that the physicists measure can be inter­preted as binary logic, 0 or 1.

“This is an excellent example of how basic research can lead to the deve­lopment of new techno­logy. Through funda­mental theory and its connection with the experiments, we have uncovered the role of real and virtual charges which has opened the way to the creation of ultrafast logic gates,” says Ignacio Franco from the University of Rochester. “It will probably take a very long time before this techno­logy can be used on a computer chip. But at least we know that light wave elec­tronics is a feasible techno­logy,” adds Tobias Boolakee. (Source: FAU / U. Rochester)

Reference: T. Boolakee et al.: Light-field control of real and virtual charge carriers, Nature 605, 251 (2022); DOI: 10.1038/s41586-022-04565-9

Links: Laser Physics (P. Hommelhoff), Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany • Franco Group, Dept. of Physics, University of Rochester, Rochester, USA