Nano directional antennas as photon sources
Measuring only 800 nanometers across, the optical Yagi-Uda antenna presented in the Nature Communications journal under the title “Electrically-driven Yagi-Uda antennas for light” is akin to a shrunk-down nanoscale version of the commonly found directional TV antennas that convert electrical signals to radio waves. For the classic TV Yagi-Uda antenna, when an AC voltage is applied to the driven antenna element, electrons in the metal vibrate and the antennas radiate electromagnetic waves. Though the RF signal is emitted mostly unidirectionally through the selective superposition of the radiated waves using so-called reflector and director elements, creating a constructive interference in one direction and destructive interference in all other directions. The analogy stops there.
feed element with kinked connectors and three directors on
a glass substrate. Credit: Department of Physics / JMU.
The paper reports a “complex electro-optical nanosystem” consisting of multiple antenna elements with precisely adjusted positions and resonances as well as a sophisticated electrical subsystem to achieve highly directed light emission via inelastic tunnelling. Experimenting with various numbers of antenna elements, the authors reported forward-to-backward (FB) light emitting ratios of up to 9.1 dB (for an antenna with one reflector and three directors) and even 13.2 dB of FB ratios when scaling up the antenna to 15 elements.
Some time ago, the Würzburg physicists were already able to demonstrate the practicality of an electrically driven light antenna. But in order to make a relatively complex optical Yagi-Uda antenna, they had to come up with new fabrication techniques to achieve the accurate placement of a nanoparticle in close proximity with the two connector patches driving the electrical signal through the emitter element of the antenna. The researchers used advanced focused-ion beam milling (FIB) to cut out the antenna shape with its reflector and directors golden patches as well as the necessary connecting wires from high-purity gold crystals.
Because the light generation is achieved via antenna-enhanced inelastic tunnelling of electrons over the antenna feed gap, the researchers had to be able to control that tunnel gap accurately. For this purpose, they used feedback-controlled dielectrophoresis to precisely place single surface-passivated gold nanoparticles in the antenna gap. This novel fabrication method enabled the authors to reproducibly create various antennas designs with extreme precision, positioning a gold nanoparticle in the active element so that it touches one wire of the active element while keeping a distance of only one nanometer to the other wire.
Next, the researchers want to design the equivalent receiver antenna so they could effectively create bidirectional data transfers. This would be very useful for optical on-chip data communication but also for advanced light management in nanoscale sensing and metrology, the authors conclude.
They also anticipate that their new method of placing nanoparticles with extreme accuracy could serve to deposit functional or optically active particles such as nano-diamonds or quantum dots, possibly yielding quantum-optical applications such as single-photon sources or quantum-sensing devices.
Universität Würzburg – www.uni-wuerzburg.de
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