The Yagi-Uda antenna is familiar to many people, at least in its radio-wave form: most old TV aerial antennas, with their spiky rectangular or triangular form, are this type of antenna. Invented by Shintaro Uda and Hidetsugu Yagi in the 1920s, it has very directional characteristics, both in absorptive and emissive versions.
To be useful at optical frequencies, Yagi-Uda antennas must be vastly miniaturized, shrunk to the nanometer scale. Researchers at the University of Würzburg (Germany) have now laid the optical foundation for this technology, describing 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 experimental physicist Bert Hecht. The results could be useful for designing future directional antennas for light that could be used to exchange data on a computer chip between different processor cores with little loss and at the speed of light.
What does a Yagi-Uda antenna for light look like? "Basically, it works in the same way as its big brothers for radio waves," says René Kullock, one of the researchers. 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 so-called 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.
Some time ago, the Würzburg physicists were already able to demonstrate that the principle of an electrically driven light antenna works. But to make a relatively complex Yagi-Uda antenna, they had to come up with some new ideas. "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," says Hecht.
In a next step, the physicists positioned a gold nanoparticle in the active element so that it touched one wire of the active element while keeping a distance of 1 nm to the other wire. The gap is so narrow that electrons can cross it when voltage is applied using quantum tunneling, explains Kullock. This charge motion generates oscillations at optical frequencies in the antenna that are emitted in a specific direction. As in its larger radio-wave counterparts, the directional accuracy of light emission of the new optical Yagi-Uda antenna is determined by the number of antenna elements.
Much work still needs to be done before the new invention is ready to be used in practice. First, the physicists must create a receiver counterpart; second, they must boost the devices' efficiency and stability.
1. René Kullock et al., Nature Communications (2020); doi: 10.1038/s41467-019-14011-6