A pinboard by
Tiansong Deng

Post-doc, University of Chicago


Control of the spatial polarization of light allows tailoring the electromagnetic response of plasmonic nanostructures. In this work, we show that focused cylindrical vector beams (CVBs) can be used to efficiently excite dark plasmon modes in highly symmetric gold nanoparticle (AuNP) dimers. Specifically, we use single particle spectroscopy and FDTD simulations to study the response of AuNP dimers excited by linearly (LP), azimuthally (AP), and radially (RP) polarized beams. Under LP excitation, the resonances correspond to in-phase coupling of the dipolar moments of the particles, with dipolar moments parallel or perpendicular to the dimer axis. These resonances are known as bright modes, as they can easily couple to light. By contrast, the field distribution of focused AP or RP beams indicates that the fields acting on the AuNPs is primarily perpendicular or parallel to the dimer’s axis, but with opposite directions at each particle. Therefore, the resonances here are out of plane coupling of dipolar moments, or so called “dark modes”. In addition, multipolar expansion of the fields associated with each scattering spectrum shows that the resonances excited by LP beams are dominated by electric dipole modes. By contrast, CVB excitation causes new modes, such as magnetic dipole and electric quadrupole modes, to be driven and that they even dominate the scattering spectra. This work opens new opportunities for spectroscopic investigation of dark modes and Fano resonances in plasmonic nanostructures, which are difficult or impossible to be excited by conventionally polarized light.