Electrically modulated light antenna paves the way for faster computer chips

Today’s computers are reaching their physical limits in terms of speed. Semiconductor components typically operate at a maximum usable frequency of a few gigahertz, which corresponds to several billion computational operations per second.

Modern systems therefore use multiple chips to distribute computing tasks, because the speed of each chip can no longer be increased. However, if light (photons) were used instead of electricity (electrons) in computer chips, they could be up to 1,000 times faster.

Plasmonic resonators, also known as “antennas for light,” are a promising solution to achieve this speed leap. They are nanometer-sized metal structures in which light and electrons interact. Depending on their geometry, they can interact with different light frequencies.

“The challenge is that plasmonic resonators cannot yet be modulated efficiently, as is the case with transistors in conventional electronics. This hampers the development of fast light-based switches,” says Dr. Thorsten Feichtner, a physicist at the Julius-Maximilians-University (JMU) Würzburg in Bavaria, Germany.

Loaded optical antennas: a new path

A research team from JMU, in collaboration with the University of Southern Denmark (SDU) in Odense, has now taken a significant step forward in the modulation of light antennas.

He succeeded in achieving electrically controlled modulation that paves the way for ultrafast active plasmonics and thus significantly faster computer chips. The experiments were published in the journal Scientific progress.

Rather than trying to modify the entire resonator, the team focused on changing its surface properties. This breakthrough was achieved by electrically connecting a single resonator, a gold nanorod – a conceptually simple idea, but one that could only be achieved using sophisticated nanofabrication based on helium ion beams and gold nanocrystals.

This unique manufacturing method was developed at the Chair of Experimental Physics (Biophysics) at JMU under the direction of Professor Bert Hecht. Sophisticated measurement techniques with a lock-in amplifier proved crucial for detecting the small but significant effects on the resonator surface.

Dr Feichtner, the study leader, explains: “The effect we exploit is comparable to the Faraday cage principle. Just as electrons from a car struck by lightning accumulate on the outside and the occupants inside are safe, additional electrons on the surface influence the optical properties of the resonators.”

Surprising quantum effects

Until now, optical antennas could almost always be described classically: the electrons in the metal simply stopped at the edge of the nanoparticle, like water on a harbor wall. But the measurements by the Würzburg scientists revealed changes in the resonance that can no longer be explained classically: the electrons “spread out” on the boundary between metal and air, resulting in a smooth, gradual transition, similar to a sandy beach meeting the sea.

To explain these quantum effects, theorists at SDU Odense have developed a semiclassical model. It embeds quantum properties into a surface parameter, so that calculations can be performed using classical methods.

“By perturbing the surface response functions, we combine classical and quantum effects, creating a unified framework that advances our understanding of surface effects,” says JMU physicist Luka Zurak, first author of the study.

A new area of ​​research with high potential

The new model makes it possible to reproduce the experiments, but it is not yet clear which of the many quantum effects are involved on the metal surface. “But thanks to this study, it is now possible for the first time to design new antennas in a targeted manner and to exclude or amplify certain quantum effects,” says Dr. Feichtner.

In the long term, the researchers are still considering other applications: smaller resonators promise high-efficiency optical modulators, which could be used technologically. In addition, the presented system also makes it possible to study the influence of surface electrons in catalytic processes. This would provide new insights into energy conversion and storage technologies.