Extrinsic chirality of non-concentric plasmonic nanorings

We show how extrinsic chirality, i.e. the optical activity of achiral media exhibited at oblique light incidence, can be achieved in plasmonic nanorings by symmetry breaking. We demonstrate that even a small, 5% offset of an inner hole of a 190 nm gold ring results in a measurable circular dichroism signal in the near-infrared region. By using computer simulations, we show that optical activity arises upon excitation of a symmetric dipolar localized surface plasmon resonance mode due to the appearance of co-aligned electric and magnetic dipole moments

Plasmon hybridization and field confinement in multilayer metal-dielectric nanocups

Large-area arrays of dispersed multilayer gold-dielectric nanocups were fabricated by colloidal lithography and studied by extinction spectroscopy. Hybridization of the elemental plasmons of the individual nanocups gave rise to new resonance peaks in the visible and near-infrared regions of the extinction spectrum. Transmission electron microscopy was used to confirm the fabricated structure geometry, and the optical properties of the arrays were studied by UV-vis-NIR spectroscopy and finite-difference time-domain (FDTD) simulations. The nature of the resonances was elucidated from E-field plots and charge plots showing clear hybridized modes. We observe a dominant hybridized dipolar mode combining a bonding and antibonding mode at the two caps. A high-energy antibonding (antisymmetric) quadrupolar mode of an individual nanocup is revealed through hybridization with an elemental mode on the second nanocup. A low-energy tunable cavity mode with a very small mode volume is observed in the near-IR range. © 2013 American Chemical Society