In the domain of science and technology, an enduring mission has been to design electronics and information processing that function closest to the fastest timescales permitted by the laws of nature.
Synchronized laser pulses (red and blue) generate a burst of real and virtual charge carriers in graphene that are absorbed by gold metal to produce a net current. “We clarified the role of virtual and real charge carriers in laser-induced currents, and that opened the way to the creation of ultrafast logic gates,” says Ignacio Franco, associate professor of chemistry and physics at Rochester. Image Credit: (University of Rochester illustration / Michael Osadciw.
A favorable way to accomplish this goal requires using laser light to direct the movement of electrons in matter and then utilizing this control to create electronic circuit elements — a concept called lightwave electronics.
Extraordinarily, lasers at present allow bursts of electricity to be generated on femtosecond timescales — that is, in a millionth of a billionth of a second. Nevertheless, the ability to process data in these ultrafast timescales has stayed elusive.
Currently, scientists at the University of Rochester and the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have made a significant step in this direction by showcasing a logic gate — the elementary unit of computation and information processing — that works at femtosecond timescales.
The achievement has been reported in the journal Nature. It was achieved by harnessing and autonomously regulating, for the first time, the virtual and real charge carriers that make up these ultrafast bursts of electricity.
The scientists’ findings have paved the way for information processing at the petahertz limit, where one quadrillion computational processes can be processed every second. That is virtually a million times faster than present-day computers working with gigahertz clock rates, where 1 petahertz is 1 million gigahertz.
“This is a great example of how fundamental science can lead to new technologies,” says Ignacio Franco, an associate professor of chemistry and physics at the University of Rochester who, in partnership with doctoral student Antonio José Garzón-Ramírez ’21 (Ph.D.), carried out the theoretical studies that resulted in this discovery.
Lasers Generate Ultrafast Bursts of Electricity
In the last few years, researchers have understood how to manipulate laser pulses that last a few femtoseconds to produce ultrafast bursts of electrical currents. This is achieved, for example, by irradiating minute graphene-based wires joining two gold metals. The ultrashort laser pulse triggers in motion, or “excites,” the electrons in graphene and, essentially, directs them in a specific direction, thereby producing a net electrical current.
Laser pulses can create electricity much faster than any conventional technique — and achieves that without applied voltage. Moreover, the magnitude and direction of the current can be regulated just by changing the shape of the laser pulse (that is, by altering its phase).
The Breakthrough: Harnessing Real and Virtual Charge Carriers
Franco’s and FAU’s Peter Hommelhoff’s research teams have been involved for many years in turning light waves into ultrafast current pulses.
In attempting to settle the experimental measurements at Erlangen with computational simulations at the University of Rochester, the researchers had an thought: In gold-graphene-gold junctions, it is viable to produce two flavors — “virtual” and“real” — of the particles transporting the charges that make up these bursts of electricity.
- “Real” charge carriers can be defined as electrons stimulated by light that stay in the directional motion even after the laser pulse is switched off.
- “Virtual” charge carriers can be defined as electrons that only move in the net directional motion while the laser pulse is switched on. Thus, they are obscure species that only live briefly when irradiated.
Since graphene is linked to gold, both virtual and real charge carriers are absorbed by the metal to generate a net current.
Amazingly, the team learned that by altering the laser pulse’s shape, they could produce currents where only the virtual or the real charge carriers play a role. Simply put, they not only produced two current flavors, but they also understood how to exploit them autonomously, a discovery that significantly enhances the components of design in lightwave electronics.
Logic Gates Through Lasers
Using this improved control landscape, the researchers were able to experimentally show, for the first time, logic gates that work on a femtosecond timescale.
Logic gates are the elementary building blocks required for computations. They regulate how inbound information, which assumes the form of 0 or 1 (known as bits), is processed. Logic gates necessitate two input signals and produce a logic output.
In the scientists’ experiment, the input signals are the phase or shape of two synchronized laser pulses, each one selected to only produce a burst of virtual or real charge carriers. Based on the laser phases applied, these two contributions to the currents can either cancel out or add up. The net electrical signal can be allocated logical information 0 or 1, producing an ultrafast 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 is practically possible,” says Tobias Boolakee, who led the experimental efforts as a Ph.D. student at FAU.
“Our results pave the way toward ultrafast electronics and information processing,” says Garzón-Ramírez ’21 (Ph.D.), now a postdoctoral researcher at McGill University.
What is amazing about this logic gate, is that the operations are performed not in gigahertz, like in regular computers, but in petahertz, which are one million times faster. This is because of the really short laser pulses used that occur in a millionth of a billionth of a second.
Ignacio Franco, Associate Professor of Chemistry and Physics, University of Rochester
From Fundamentals to Applications
This new, possibly revolutionary technology emerged from fundamental studies of how charge can be powered in nanoscale platforms using lasers.
Through fundamental theory and its connection with the experiments, we clarified the role of virtual and real charge carriers in laser-induced currents, and that opened the way to the creation of ultrafast logic gates.
Ignacio Franco, Associate Professor of Chemistry and Physics, University of Rochester
The study by Franco signifies over 15 years of research. In 2007, as a Ph.D. student at the University of Toronto, he formulated a technique to produce ultrafast electrical currents in molecular wires revealed to femtosecond laser pulses.
This early proposal was later executed experimentally in 2013 and the comprehensive mechanism underlying the experiments was elucidated by the Franco group in a 2018 study. Since then, there has been what Franco describes as an “explosive” experimental and theoretical advance in this domain.
This is an area where theory and experiments challenge each other and, in doing so, unveil new fundamental discoveries and promising technologies.
Ignacio Franco, Associate Professor of Chemistry and Physics, University of Rochester
Journal Reference:
Boolakee, T., et al. (2022) Light-field control of real and virtual charge carriers. Nature. doi.org/10.1038/s41586-022-04565-9.