Plasmon resonance differences between the near- and far-field and implications for molecular detection

The localized surface plasmon resonance (LSPR) of a nanoplasmonic particle is often considered to occur at a single resonant wavelength. However, the physical measures of plasmon resonance, namely the far-field measures of scattering, absorption, and extinction, and the near-field measures of surface-average or maximum electric field intensity, depend differently on the particle polarizability, and hence may be maximized at different wavelengths. We show using analytic Mie theory that the differences in peak wavelength between the near- and far-fields can reach over 200 nm for nanoparticle sizes commonly used in spectroscopy applications. Using finite element analysis, we also consider the effect of varying particle shape to spheroidal geometries, and consider polarization dependence. Using the quasi-static and extended quasi-static approximation, we show that the differences between the near- and far- field measures of plasmon resonance can be largely explained by radiation damping effects. We suggest that accounting for these differences is relevant both for optimizing device design, and for improving fundamental understanding of surface-enhanced mechanisms such as surface-enhanced Raman spectroscopy (SERS).