Negative asymmetry parameter in plasmonic core-shell nanoparticles

Plasmonic nanoparticles with a dielectric-metal core-shell morphology exhibit hybridized modes where the surface plasmon polaritons at the outer and inner surfaces of the shell couple. We demonstrate that suitably tailoring the interference of such hybrid surface plasmon polariton modes leads to composite subwavelength nanospheres with negative asymmetry parameters and strong scattering in the optical frequency range. As a result, for a low density collection of scatterers an anomalous regime occurs, where the extinction mean free path is longer than the transport mean free path. Explicit results for silver-coated nanospheres are presented.