Rationale for $10^{14}$ enhancement factor in single molecule Raman spectroscopy

TL;DR

This paper extends quantum electrodynamics to explain a $10^{14}$ enhancement in single molecule Raman scattering, attributing it to local field and density of states effects near silver nanoparticles, without chemical mechanisms.

Contribution

It introduces a quantum electrodynamic model for Raman enhancement, emphasizing local field and density of states effects, and proposes experimental methods for validation.

Findings

Up to $10^{14}$ increase in Raman scattering cross-section near silver nanoparticles.

Single molecule Raman detection explained without chemical enhancement mechanisms.

Proposed experiments include transient Raman and near-field optical mapping.

Abstract

We extend the Purcell's original idea [Phys. Rev. \textbf{69}, 682 (1946)] on modification of photon spontaneous \textit{emission} rate to modification of photon spontaneous \textit{scattering} rate. We find the interplay of local incident field enhancement and local density of photon states enhancement in close proximity to a silver nanoparticle may result in up to $10^{14}$-fold rise of Raman scattering cross-section. Thus single molecule Raman detection is found to be explained by consistent quantum electrodynamic description without any chemical mechanism involved. A model of the so-called "hot points" in surface enhanced spectroscopy has been elaborated as local areas with high Q-factor at incident and scattered (emitted) light frequencies. For verification of the model we consider further experiments including transient Raman experiments to clarify incident field enhancement and scanning near-field optical mapping of local density of photon states.