Directional side scattering of light by a single plasmonic trimer

Directional side scattering of light by individual gold nanoparticles (AuNPs) trimers assembled by the atomic force microscope (AFM) nanomanipulation method is investigated in experiment and theory. The AFM nanomanipulation approach brings an active way to construct ultracompact and effective optical nanoantennas. Different configurations of the trimers are constructed in situ via AFM nanomanipulation. Unidirectional side scattering of light by a single trimer is demonstrated with a broad response bandwidth over 400 nm and directivity up to similar to 7.8 dB in experiments. The near-field plasmon coupling of the AuNPs is simulated with the 3D finite-difference time-domain method and the far-field radiation patterns are calculated by employing near-field-to-far-field transformation methods. The calculated results are in agreement with the experiments qualitatively. The physical origin is revealed intuitively by employing a simple phenomenological "two-dipole" model. The unidirectional light scattering is due to the interference between multiple plasmonic resonance modes of the trimers. The study contributes to the understanding of the optical response of complex nanostructures and optimizing nanoantenna performances for practical applications, e.g. increasing the detection efficiency of surface-enhanced spectroscopy.