Dielectric microsphere coupled to a plasmonic nanowire: A self-assembled hybrid optical antenna

Hybrid mesoscale-structures that can combine dielectric optical resonances with plasmon-polaritons are of interest in chip-scale nano-optical communication and sensing. This experimental study shows how a fluorescent microsphere coupled to a silver nanowire can act as a remotely-excited optical antenna. To realize this architecture, self-assembly methodology is used to couple a fluorescent silica microsphere to a single silver nanowire. By exciting propagating surface plasmon polaritons at one end of the nanowire, remote excitation of the Stokes-shifted whispering gallery modes (WGMs) of the microsphere is achieved. The WGM-mediated fluorescence emission from the system is studied using Fourier plane optical microscopy, and the polar and azimuthal emission angles of the antenna are quantified. Interestingly, the thickness of the silver nanowires is shown to have direct ramifications on the angular emission pattern, thus providing a design parameter to tune antenna characteristics. Furthermore, by employing three-dimensional numerical simulations, electric near-fields of the gap-junction between the microsphere and the nanowire is mapped, and the modes of nanowire that couple to the microsphere is identified. This work provides a self-assembled optical antenna that combines dielectric optical resonances with propagating-plasmons and can be harnessed in hybrid nonlinear-nanophotonics and single-molecule remote sensing.