Understanding plasmons in nanoscale voids

Metallic nanoscale voids ("anti-nanoparticles") are shown to possess radically different plasmon modes to metal nanoparticles. Comparing new boundary element calculations for the first time with experiment clearly and intuitively identifies plasmon wavefunctions in spherical voids according to their atomic-like symmetries. As the spherical voids are progressively truncated, the degenerate radial modes split in energy, with intense coupling to incident light at specific optimal angles. In contrast to nanoparticles, voids embedded in metal films possess additional rim plasmon modes that selectively couple with void plasmons to produce bonding and antibonding hybridized states with significant field enhancements. These modes, which are verified in experiment, are crucial for the effective use of plasmons in antenna applications such as reproducible surface enhanced Raman scattering.