Sphere-on-pillar optical nano-antennas

We propose an optical nano-antenna consisting of a pair of sphere-on-pillar structures. Experiments show that the controlled fabrication of metallic nanospheres on the tip of carbon nanotubes (CNTs) is effective, and numerical investigation revealed that a pair of such structures are capable to convert free space radiation into an intense near-field; hence can function as an optical antenna. The fabrication process, electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB), are based on nanofluidic mass delivery at the attogram scale using metal-filled CNTs. Under the irradiation of a high energy electron beam of a transmission electron microscope (TEM), the encapsulated metal is melted and extruded out from the tip of the nanotube; generating a metallic sphere. In the case that the encapsulated materials inside the CNT have a higher melting point than that of the beam energy can reach, electromigration-based mass delivery is an optional process to apply. Under a low bias (2-2.5V), spherical nanoparticles are formed on the tips of nanotubes. The optical properties of the nano-antenna are analyzed numerically using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties. Sphere-on-pillar optical antennas carry a great potential for bio-sensing, tip-enhanced spectroscopy applications, and interfacing integrated nanophotonic circuits.