Enhancing the plasmonic fields by a high refractive index dielectric coating for surface enhanced spectroscopies

For stabilization and functionalization, plasmonic nanoparticles (NPs) are often coated with dielectric shells, yet suffer from the weakening of local electric field enhancement. Herein, we numerically demonstrate that, instead of being weakened, the local electric fields can be further enhanced using an appropriate high refractive index dielectric coating. Because the high refractive index dielectric shell owns strong Mie resonances that can participate in the bonding plasmon hybridization, it can avoid excessive diminishing of light absorption when the shell becomes thick. Meanwhile, it can sustain a great leap of the local electric fields reaching the surface, which follows the boundary conditions at the interface within electrodynamics. The two mechanisms can work synergistically to offset the decay of the local electric fields inside the shell. It is also worth noting that a dielectric shell with too high a refractive index may badly suppress the light scattering by the core-shell NPs, going against the light coupling effect. These findings arc of significant importance in extended applications of surface enhanced spectroscopies, especially for shell-isolated NP-enhanced Raman spectroscopy and surface enhanced fluorescence.