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Devices and structures

Devices and structures

Quantum physics sets a speed limit for fastest possible optoelectronic switch

26 Apr 2022 Isabelle Dumé
fastest possible optoelectronics
An international research group has found a maximum value for how fast a computer can become. (Courtesy: © Oliver Wolf – TU Graz)

Optoelectronic switches can operate up to 1000 trillion times a second – a rate of 1 petahertz – before quantum processes wreck their effectiveness, say researchers in Germany and Austria. The result places a fundamental speed limit on classical information processing, while the experimental techniques used to achieve it could help physicists obtain a better understanding of a wide range of phenomena with applications in coherent electronics.

While today’s computer chips are faster than ever before, the laws of quantum physics limit how much faster they can get. Traditionally, increases in processing speed have come from shrinking transistors and other chip components so that data has a shorter distance to travel. The physical limit here is the size of an atom.

Another approach is to increase the switching rate. One way to do this is to use light rather than a transistor to control the flow of current – for example, by applying a laser pulse to excite the electrons from the valence band of a semiconducting material into the conduction band so that the material becomes a conductor. The energy required for this excitation depends on the semiconductor and corresponds to light frequencies in the infrared to visible range, which ultimately sets the maximum switching speed possible with these materials.

Higher-frequency light

In the new work, a team led by Martin Schultze of Austria’s Graz University of Technology studied a dielectric, lithium fluoride, rather than a semiconductor because the excitation energy of dielectrics is much higher. This enabled the researchers to use higher-frequency light and thereby achieve even faster data transmission. However, as team member Marcus Ossiander explains, there is a drawback: most dielectric materials cannot conduct electricity without breaking.

To overcome this problem, the researchers increased the frequency of the switching light pulse to the extreme ultraviolet range and decreased its duration to one femtosecond (10-15 s). This is so short that the dielectric does not have time to break. By bombarding a sample of the dielectric with this ultrashort laser pulse, they excited the electrons into the conduction band where the particles can move freely. They then applied a second, slightly longer pulse to accelerate these excited electrons in the desired direction, creating an electric current that they could detect thanks to electrodes connected to both sides of the material.

“The method we employed – sampling ultrafast currents injected by extreme ultraviolet radiation and then driven by light fields – allows us to follow what electrons do when the dielectric switch is operated,” explains Ossiander, who is now a postdoctoral researcher at Harvard University in the US. “Our technique allows us to push electrons from the valence band to the conduction band in a dielectric within one femtosecond – that is, we switched the dielectric from an insulator to a conductor at the speed needed to realize a petahertz switch.”

Combination of two rules in physics

By tracking what the electrons do after they are put in the conduction band, Ossiander and colleagues showed that if switching times got any faster, the electrons would be pushed into regions of the band structure where they would “harm” the signals the researchers were trying to transmit. Ossiander attributes this result to a combination of two factors: the band structure of the material and the Fourier limit. As the light pulses used in switching become shorter, the Fourier limit means that the pulses must stretch across a broader range of wavelengths. As such, the pulses end up promoting electrons into regions where they have undesirable effects.

“Switching beyond one petahertz could cause the electrons to go to band structure regions in which they would suddenly begin to move in a direction that opposes the electric field we apply, which is of course terrible for the switching performance,” Ossiander explains. “By comparing the band structure of lithium fluoride with that of other materials, we were able to show that we pretty much reached the maximum of what is theoretically possible for any existing material before these reversal effects occur.”

A switch the size of an apartment

Real-world applications of the new maximum-speed switch are still a long way off, notes Ossiander. “Currently, the experimental set-up we used for this experiment is roughly the size and cost of a one-bedroom apartment – and this to realize a single switch,” he says. “It will still take us a while to miniaturize the lasers and other parts of the apparatus and construct the millions of parallel switches required for a processor at a size and cost that can be integrated into a smartphone, for example.”

The good news is that according to Ossiander, the technique developed in this work can be applied to study most materials. This should ultimately allow researchers to analyse phenomena such as ultrafast charge carrier transport, elastic and inelastic electron scattering and cooling of (quasi-) free charge carriers that are important for implementing coherent electronics. Ossiander tells Physics World that because the team has already demonstrated the approach for lithium fluoride, which has the largest band gap (the distance between the valence band and conduction band) of all known materials, applying it to other materials “will be relatively straightforward”.

And that is not all: Ossiander adds that the method should enable researchers to measure the electromagnetic field of laser pulses – coveted information that only a handful of techniques can currently provide. The way the laser field evolves after interacting with materials can reveal the time-resolved nonlinear polarization light created in them and therefore can also reveal how bound carriers behave in solids, explains Ossiander. “It will thus provide the full picture of the comportment of electrons interacting with light,” he concludes.

The research is detailed in Nature Communications.

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