Customer Spotlight - Electrically-Driven Yagi-Uda Antennas for Light

For the first time, physicists from the University of Würzburg have successfully converted electrical signals into photons and radiated them in specific directions using a low-footprint optical antenna that is only 800 nanometers in size. At the heart of this structure is a single CTAB-coated gold nanoparticle from Nanopartz.

Nano antennas for data transfer

Directional antennas convert electrical signals to radio waves and emit them in a specific direction, allowing increased performance and less signal interference. This scheme originated in radio technology and it exists a big interest in adapting it for light which allows much higher bandwidths and smaller structure sizes. After all, almost all Internet-based communication utilizes optical light communication. Such antennas for light could then be used to directly exchange data between individual processor cores with little loss and at the speed of light. However, in order to enable antennas to operate with the very short wavelengths of optical light, they have to be shrunk to nanometer scale.
Würzburg physicists have now laid the foundation for this technology in a pioneering publication: In the scientific journal "Nature Communications" they showed for the first time how to generate directed infrared light using an electrically driven Yagi-Uda antenna made of gold. The antenna was developed by the nano-optics working group of Professor Bert Hecht, who holds the Chair of Experimental Physics 5 at the University of Würzburg. The name "Yagi-Uda" is derived from the two Japanese researchers, Hidetsugu Yagi and Shintaro Uda, who invented the antenna in the 1920s.

The Yagi-Uda antenna concept

What does a Yagi-Uda antenna for light look like? "Basically, it looks and works the same way as its big brothers for radio waves ," explains Dr. René Kullock, a member of the nano-optics team. “There, an AC voltage is applied to a rod antenna, electrons in the metal start to oscillate and as a result electromagnetic waves are radiated in all directions. For Yagi-Uda antennas additional passive elements – the so-called reflectors and directors – are added and indirectly excited via the driving antenna." says Kullock. "This results in constructive interference in one direction and destructive interference in all other directions." Accordingly, when operated as a receiver such antennas amplify the detection of radiation from a specific direction.

Creating directional light: The world's first electrically powered Yagi-Uda antenna for light was built at the University of Würzburg's Department of Physics.

Transition to the optical regime

By extending this concept to the optical regime the researchers now faced two major challenges: speed and size. There are in principle no AC generators available which are fast enough to oscillated with optical frequencies and the structures have to be really tiny.
Fortunately, the Würzburg physicists were already able to solve the first problem in a previous work by utilizing quantum tunneling to generate light in a single optical antenna. But in order to fabricate the relatively complex Yagi-Uda design, they had to come up with some new ideas. In the end, they succeeded thanks to a sophisticated production technique: "We bombarded gold with gallium ions which enabled us to cut out the antenna shape with all reflectors and directors as well as the necessary connecting wires from high-purity gold crystals with great precision," explains Bert Hecht.

Spherical gold nanoparticles from Nanopartz play a crucial role

In a next step, the physicists positioned a single CTAB-coated gold nanoparticle from Nanopartz in the active element. They applied an AC voltage to use the effect of dielectrophoresis and at the same time monitored the scattering spectrum of the antenna as feedback. The particles are sucked in and the antenna comes to life once the spherical particle touches one wire of the active element while keeping a CTAB-mediated distance of only one nanometer to the other wire. "This gap is so narrow that electrons can cross it when voltage is applied using an effect known as quantum tunneling," explains Kullock. This probabilistic process generates charge oscillations with frequencies up to the optical regime. These excitations are then emitted via the antenna in a specific direction - thanks to the reflector and director elements.
However, much work still needs to be done before the new invention is ready to be used in practical applications. Firstly, the physicists have to work on the counterpart that receives light signals. And secondly, they have to boost the efficiency and stability.

Funding

This work was funded by the VW-Foundation (Grant 93437) and the German Research Foundation (HE 5618/4-1).

More information

Please contact Prof. Dr. Bert Hecht, Department of Physics, University of Würzburg, Phone +49 931 31-85863, hecht@physik.uni-wuerzburg.de or visit www.nanoscale-optics.de.

Article Citation

Kullock, R., Ochs, M., Grimm, P. et al. Electrically-driven Yagi-Uda antennas for light. Nat Commun 11, 115 (2020). https://doi.org/10.1038/s41467-019-14011-6

Nanopartz Products Used for this Research

The products used for this research are our Accurate Spherical Gold Nanoparticles with a CTAB capping agent. CTAB is a micelle that provides a strong positive charge for the spherical gold. The product is found at Gold Nanoparticles, part number AC11-30-CTAB-DIH-100-1.