Quantum Effects Enhance the Performance of Optical Antennas

According to a study published in Science Advances, physicists from Würzburg have demonstrated a nanometer-sized light antenna with electrically adjustable surface features. This breakthrough could potentially lead to the development of faster computer chips.

Today's computers have reached their physical limits in terms of speed. Semiconductor components typically run at a maximum useful frequency of a few gigahertz, which is equivalent to several billion computer operations per second.

Plasmonic resonators, sometimes known as “light antennas,” appear to be a potential method for reaching this increase in speed. These are nanometer-sized metal structures where light and electrons interact. Depending on their geometry, they can interact with various light frequencies.

The challenge is that plasmonic resonators cannot yet be effectively modulated, as is the case with transistors in conventional electronics. This hinders the development of fast light-based switches.

Dr. Thorsten Feichtner, Physicist, Universität Würzburg

Charged Optical Antennas: University of Würzburg Breaks New Ground

A JMU research team, in collaboration with Southern Denmark University (SDU) in Odense, has now made a significant step forward in light antenna modulation: they have achieved electrically controlled modulation, paving the way for ultra-fast active plasmonics and thus significantly faster computer chips.

Instead of modifying the entire resonator, the researchers focused on altering its surface properties. This breakthrough was achieved by electrically connecting a single resonator, specifically a gold nanorod. While the concept is simple, it required sophisticated nanofabrication techniques, including the use of helium ion beams and gold nanocrystals.

Professor Bert Hecht, who leads the JMU Chair of Experimental Physics (Biophysics), oversaw the development of this unique production technology. Advanced measuring techniques, including the use of a lock-in amplifier, were crucial in detecting the subtle yet significant effects on the resonator’s surface.

The effect we are making use of is comparable to the principle of the Faraday cage. Just as the electrons in a car struck by lightning collect on the outside and the occupants inside are safe, additional electrons on the surface influence the optical properties of the resonators.

Dr. Thorsten Feichtner, Study Leader, Universität Würzburg

Surprising Quantum Effects

Until recently, optical antennas were almost always described using classical models, where the metal's electrons were thought to stop abruptly at the nanoparticle's edge, much like water hitting a harbor wall. However, the Würzburg scientists' experiments revealed variations in resonance that cannot be explained by traditional models. Instead, electrons "smear" over the boundary between metal and air, creating a smooth, gradual transition akin to a sandy beach met by the sea.

To explain these quantum phenomena, theorists at SDU Odense created a semi-classical model. It combines quantum features into a surface parameter, allowing the computations to be performed using classical methods.

By perturbing the response functions of the surface, we combine classical and quantum effects, creating a unified framework that advances our understanding of surface effects.

Luka Zurak, Study First Author and Physicist, Universität Würzburg

New Field of Research with Great Potential

The new model can replicate the results, but it is unclear which of the numerous quantum processes are involved at the metal surface.

“But with this study, it is now possible for the first time to specifically design new antennas and exclude or amplify individual quantum effects,” added Thorsten Feichtner.

In the long term, the researchers foresee additional applications: smaller resonators could enable highly efficient optical modulators, which may be applied in future technologies. Additionally, the system could be used to investigate the role of surface electrons in catalytic processes, offering new insights into energy conversion and storage methods.

Journal Reference:

Zurak, L., et al. (2024) Modulation of surface response in a single plasmonic nanoresonator. Science Advances. doi.org/10.1126/sciadv.adn5227.