Optical antennas collect energy from visible light waves, and concentrate it into localized forms, like an electrical current. While the idea has been around for a decade or longer certain challenges have impeded their realization. Optical antenna designs include features on the order of tens of nanometers, making building them both expensive and time consuming. In addition, the physics of optical antennas is still not well understood. Since metals do not behave like perfect conductors at visible wavelengths, the usual design rules of radio antennas can’t simply be scaled down to smaller sizes. Because of that, many research papers have performed computer simulations of optical antennas, but few have described experimental results. Finally, no method has been found to transmit or store the collected energy efficiently, without losing it to heat dissipation.

A practical optical antenna design would involve simple components that are easy to reproduce, but would give us a high degree of control over the wavelength range at which the antenna worked, or the direction in which it emitted or collected light. In this paper, Arnaud et al. present measurements of a simple optical antenna composed of a metallic nanowire laid across a waveguide.
Previous studies have indicated that it is possible to create an ion-exchange waveguide, which is a glass conduit with a slightly conductive core. It solves the problem of energy transport, since it has inherently low losses, and can couple to optical fibers, which are very well understood. While illuminating the waveguide with light would not result in energy collection, the authors show that a metallic nanoscopic wire laid across the waveguide acts as an antenna, allowing the coupling of energy between free space and the waveguide.

The authors measure the radiation pattern of the antenna by pumping light through the waveguide. Some of that light couples to the antenna and is radiated into space. They use a microscope to collect this light, but instead of creating an image of the antenna, they look far out of focus, in the pupil plane of the microscope. This allows them to read out the angles at which the radiation is emitted, and measure the directionality and efficiency of the radiation.

The authors find that even such a simple structure allows the control the emission pattern by varying the length and width of the nanowire, or by tuning the wavelength of light. While many groups have demonstrated impressive designs for optical antennas fabricated with serial lithography, or simple antennas like single nanoparticles with essentially no controllable parameters, this study represents an effective compromise in which the antenna design is simple and repeatable, yet offers a degree of control over the radiation pattern and wavelength tuning.

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