Local fields at the surface of noble-metal microspheres

Abstract

Enhancement of the Raman scattering and fluorescence emission on noble metals (Ag, Cu, and Au) is believed to be caused in part by large local fields at the incident wavelength on the surface of metallic microstructures, such as colloidal suspensions and surface roughness on electrodes and thin films. For metallic spheres immersed in water, calculations are made of the integrated near-field intensity efficiency ($Q_{\mathrm{NF}}$) and that part associated only with the radial field component as a function of incident wavelength (200- 1200 nm) and sphere radius (0- 300 nm) which exceeds the usual Rayleigh limit and extends well into the Lorenz-Mie region. The calculated wavelength and radius dependencies of ${\mathrm{Q}}_{\mathrm{NF}}$ are compared with those for the better-known efficiencies: extinction ($Q_{\mathrm{E}}$), far-field scattering ($Q_{·\mathrm{sca}}$), and absorption ($Q_{\mathrm{abs}}$). The peak values of these efficiencies have been evaluated when the incident wavelength is in resonance with dipolar and multipolar surface-plasmon modes of Ag, Cu, and Au spheres of varying radii immersed in water.