Nano Antennas for Data Transfer

Date transfer with directed infrared light using an electrically driven Yagi-Uda antenna made of gold. (Source: Dept. Physics, U. Wuerzburg)

Directional antennas convert electrical signals to radio waves and emit them in a particular direction, allowing increased performance and reduced inter­­ference. This principle, which is useful in radio wave technology, could also be interesting for minia­­turised light sources. After all, almost all Internet-based communication utilises optical light communi­­cation. Directional antennas for light could be used to exchange data between different processor cores with little loss and at the speed of light. To enable antennas to operate with the very short wave­­lengths of visible light, such direc­tional antennas have to be shrunk to nanometre scale.

Now, Würzburg physicists describe 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 Bert Hecht 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.

What does a Yagi-Uda antenna for light look like? “Basically, it works in the same way as its big brothers for radio waves,” explains René Kullock, a member of the nano-optics team. An AC voltage is applied that causes electrons in the metal to vibrate and the antennas radiate electro­magnetic waves as a result. “In the case of a Yagi-Uda antenna, however, this does not occur evenly in all directions but through the selective super­position of the radiated waves using special elements, the reflectors and directors,” says Kullock. “This results in constructive inter­ference in one direction and destructive inter­ference in all other directions.” Accor­dingly, such an antenna would only be able to receive light coming from the same direction when operated as a receiver.

Applying the laws of antenna technology to nano­meter scale antennas that radiate light is techni­cally challenging. Some time ago, the physicists were already able to demons­trate that the principle of an electrically driven light antenna works. But in order to make a relatively complex Yagi-Uda antenna, 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.

In a next step, the physicists positioned a gold nano particle in the active element so that it touches one wire of the active element while keeping a distance of only one nanometre to the other wire. “This gap is so narrow that electrons can cross it when voltage is applied using a process known as quantum tunnel­ling,” explains Kullock. This charge motion generates vibrations with optical frequencies in the antenna which are emitted in a specific direction thanks to the special arrange­ment of the reflectors and directors.

The Würzburg researchers are fascinated by the unusual property of their novel antenna that radiates light in a particular direction although it is very small. As in radio wave antennas, the direc­tional accuracy of light emission of the new optical antenna is determined by the number of antenna elements. “This has allowed us to build the world’s smallest electrically powered light source to date which is capable of emitting light in a specific direction,” Hecht details. However, much work still needs to be done before the new invention is ready to be used in practice. Firstly, the physicists have to work on the counter­part that receives light signals. Secondly, they have to boost the effi­ciency and stability. (Source: U. Wuerzburg)

Reference: R. Kullock et al.: Electrically-driven Yagi-Uda antennas for light, Nat. Commun. 11, 115 (2020); DOI: 10.1038/s41467-019-14011-6

Link: Nano-Optics and Biophotonics (B. Hecht), Julius-Maximilians-Universität Würzburg, Wuerzburg, Germany

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