Electrodynamics of surface-enhanced Raman scattering

TL;DR

This paper develops a scalar potential-based theoretical framework for surface-enhanced Raman scattering (SERS), analyzing electrostatic and electrodynamic limits, and explores how Coulomb interactions influence resonances and scattering behavior.

Contribution

It introduces a scalar potential formulation for SERS, including Coulomb interactions, and applies it to spherical inclusions to better understand resonance effects and scattering singularities.

Findings

Coulomb interactions suppress unphysical resonant infinities.

Effective restoring-force constants are modified, enabling dual resonance exploration.

Approximate models show shifts in singularities related to dielectric properties.

Abstract

We examine SERS from two perspectives: as a phenomenon described by the Laplace Equation (the electrostatic or Rayleigh limit) and by the Helmholtz Equation (electrodynamic or Mie limit). We formulate the problem in terms of the scalar potential, which simplifies calculations without introducing approximations. Because scattering is not usually calculated this way, we provide the necessary theoretical justification showing that the scalar-potential description is complete. Additional simplifications result from treating the scatterer as a point charge q instead of a dipole. This allows us to determine the consequences of including the longitudinal (Coulomb) interaction between q and a passive resonator. This interaction suppresses the mathematical singularities that lead to the unphysical resonant infinities in first and second enhancements. It also modifies the effective restoring-force constant of a resonant denominator, which permits us to explore the possibility of dual resonance through a molecular pathway. We apply the formalism to spherical inclusions of radius a for q located at polar and equatorial positions. For small a, of the order of 1 nm or less, the low-l multipole terms are important. For the more relevant case of radii of the order of 10 nm and larger, the q-sphere interaction can be approximated by a model where q interacts with its image charge for a dielectric plane, and the singularity shifts in a discrete manner from \epsilon(\omega)=-2 to \epsilon(\omega)=-1. These results are supported by more accurate calculations taking retardation into account, although the use of only one spherical-harmonic term does not fully represent the difference between forward- and backscattering.