Mesoscopic and Microscopic Strategies for Engineering Plasmon‐Enhanced Raman Scattering

Abstract

AbstractSurface plasmon resonance (SPR) in noble metal nanoparticles and nanostructures offers an efficient means to transport and localize the energy of light into some nanoscale space regions called hot spots, where the electromagnetic field is enhanced by many orders of magnitude upon the incident light. This local field enhancement can induce very huge enhancement of Raman signal for a molecule embedded within the hot spot, which has tremendous applications in surface‐enhanced Raman spectroscopy (SERS) and tip‐enhanced Raman spectroscopy (TERS). Here, a discussion is presented on how to engineer this SPR‐enhanced Raman scattering from both the mesoscopic and microscopic levels. The mesoscopic level focuses on engineering and optimizing the geometric and physical configurations of plasmonic nanoparticles in order to have as large as possible electromagnetic field enhancement factor in the hot spot. The microscopic level focuses on investigating the light–molecule interaction (both chemical and physical) in the microscopic level, either classical or quantum, in order to have deep and complete understanding of the key microscopic issues influencing the Raman scattering and then exploring microscopic means to further enhance the Raman scattering as large as possible. Although in many situations these two scopes can be considered separately, there are also many situations where these two scopes need to be considered together. A prominent example, discussed here, is the critical role of molecule Rayleigh scattering in a plasmonic nanogap. Furthermore, several important issues are pointed out that need attention and caution in exploring and evaluating the quantitative SPR‐based Raman enhancement, including the quantum plasmonics correction, surface and interface electron scattering correction, and the validity of classical electromagnetics and electrodynamics approaches used in single and few atom scale plasmonics.