Directional Optical Travelling Wave Antenna Based on Surface Plasmon Transmission Line

Highly-directional optical travelling wave antennas based on Ag nanowires are created, whose half-power beamwidth is achieved as small as 20 degrees. The radiation direction can be effectively adjusted over a range of up to 45.6 degrees by changing the feeding surface plasmon modes, and they have an ultra-broad bandwidth due to their nonresonant characteristic. A theoretical model based on mode coupling theory is constructed that can give a full description of the radiation properties, such as the radiation pattern, beam solid angle, radiation efficiency, and working bandwidth. The optical travelling wave antennas are easy to fabricate, control, and integrate with optical transmission lines; thus, they should have great potential applications in nanophotonic devices.