Optical Yagi–Uda antennas — a nanometre-scale design of the famous radio-wave TV antenna seen on rooftops around the world — could be a useful light source for future applications involving on-chip optical data communication. Consisting of a reflector, an active feed element and directors, the Yagi–Uda antenna acts as a highly directional emitter of electromagnetic waves. However, to date, experiments with optical versions have relied on the use of a separate bulky light source to feed the antenna, and a fully integrated miniature system would be preferable.

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Now, René Kullock and co-workers from Universität Würzburg in Germany have done just that and developed an electrically driven, integrated system that offers directional emission of infrared light (pictured) (Nat. Commun. 11, 115; 2020).

The antennas were fabricated on a glass plate by focused ion beam milling of chemically grown single-crystalline gold microplatelets. The size and the positions of the reflector, the feed element and the directors were determined by numerical simulations such that the forward-to-backward ratio became high for light emission around 870 nm. The feed element with a gap of around 25 nm in the middle was electrically connected via kinked gold wires to avoid optical field disturbance.

Light with a broad spectrum around 830 nm was generated via antenna-enhanced inelastic tunnelling of electrons over the antenna feed gap where a single gold particle was placed. To exactly place one particle into the antenna gap, the German scientists employed feedback-controlled single-particle dielectrophoresis. A water droplet containing gold particles was placed on top of the antenna structures and an a.c. electrical signal was used to move a gold particle to the region with the highest field gradient, that is, the feed gap.

Electroluminescence from the antennas was measured by applying a d.c. voltage of up to 1.8 V and collecting the emitted light via a high-numerical-aperture objective lens. In order to experimentally estimate the forward-to-backward ratio, the emission pattern of the electroluminescence was recorded by back focal plane imaging for various antennas and calculated. The obtained forward-to-backward ratio was estimated to be in the range of 6.5–9.1 dB, equivalent to that of Yagi–Uda antennas for radio-frequency waves.

“This work paves the way for optical on-chip data communication that is not restricted by Joule heating but also for advanced light management in nanoscale sensing and metrology as well as light-emitting devices”, concluded Kullock.