Controlling the near-field oscillations of loaded plasmonic nanoantennas

Optical and infrared antennas(1-6) enable a variety of cutting-edge applications ranging from nanoscale photodetectors(7) to highly sensitive biosensors(8). All these applications critically rely on the optical near-field interaction between the antenna and its 'load' (biomolecules or semiconductors). However, it is largely unexplored how antenna loading affects the near-field response. Here, we use scattering-type near-field microscopy to monitor the evolution of the near-field oscillations of infrared gap antennas progressively loaded with metallic bridges of varying size. Our results provide direct experimental evidence that the local near-field amplitude and phase can be controlled by antenna loading, in excellent agreement with numerical calculations. By modelling the antenna loads as nanocapacitors and nanoinductors(9-11), we show that the change of near-field patterns induced by the load can be understood within the framework of circuit theory. Targeted antenna loading provides an excellent means of engineering complex antenna configurations in coherent control applications(12), adaptive nano-optics(13) and metamaterials(14).