Optical nanoantennas are emerging as one of the key components in the future nanophotonic and plasmonic circuits. The first optical nanoantennas were in a form of simple spherical nanoparticles. Recently more complex Yagi-Uda nanoantenna structures were demonstrated. These nanoantennas enhance radiation of single emitters and provide well-defined directional radiation. In this contribution, we present the novel design of the directional nanoantenna, which is excited from the propagating mode of the plasmonic waveguide. The nanoantenna design is based on the travelling wave principle, well known at RF/microwave frequencies. By properly designing the propagating parts of the nanoantenna, a very efficient coupling to free space wave impedance can be achieved. Furthermore, the control over the radiation direction and beam width is relatively easy with this nanoantenna. Compared to the previously published Yagi-Uda designs, the new nanoantenna presented in this work has directivity three times higher. 1. Introduction In the future we foresee the development of optical wireless communication networks at the nanoscale [1]. In this work we will focus on one specific problem in the development of the physical layer of such wireless networks-design of novel nanoantennas (often also referred to as optical antennas, or plasmonic antennas in the open literature). Nanoantennas are key components and enabling technology for the future wireless connectivity at the nanoscale, coupling light from external world to nanoscale circuits and vice versa. Nanoantennas have their origins in the field of physics and more precisely in near-field microscopy. Wessel in 1985 was the first to mention explicitly the analogy of local microscopic light sources to classical antennas [2]—a concept that has since been thoroughly explored. History of nanoantennas can be found in [3]. As one of the main applications for novel nanoantennas we envisage (a) on-chip and intrachip wireless Nano-links in future micro- and nanodevices and (b) single-emitter sources coupled to the directional nanoantenna. The coupling of the nanoantenna to single-emitter light sources has been investigated by several groups theoretically [4–7] as well as experimentally [8, 9]. Nanoantenna in this application is used mainly to direct and focus light. Due to the lack of true nanoscale light sources, the coupling of the emitter (e.g., quantum dot) to the nanoantenna has been achieved by proximity coupling (near-field coupling) which is rather inefficient. Ideally, one would like to connect the source and the
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