Optical Nano-antennas direct single-molecule emission
Fernando D. Stefani
Antennas have been used for over a century to control the emission and reception of radio and microwave radiation. An optical equivalent is of great interest as it will enable unique nano-scale control of both the absorption and emission of individual emitters .
Here we will discuss results obtained by dynamically coupling single molecules to an optical monopole antenna (as shown in Fig. 1), precisely tuned to resonance . First we employ single molecules to directly map, in the near field, the antenna mode field and the dependence of the plasmon resonance on the antenna length . A locally enhanced field is observed at the antenna apex that is confined within 25 nm and leads to an increased excitation of the molecules.
In emission, the radiative properties of a molecule, for example the excited state lifetime  and emission spectrum , can be manipulated. In the second part, we demonstrate experimentally how an optical antenna controls the emission direction of a single molecule (Fig. 2) . The angular emission is determined by the antenna design regardless of the orientation of the molecule. This clearly exhibits the role of the plasmon resonance in the emission process and provides a straightforward guideline to arbitrarily direct single-molecule emission with optical antennas, an interesting prospect for efficient antenna-based nano-sized sensors and light sources .
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