Experimental Evidence of a Twofold Electromagnetic Enhancement Mechanism of Surface-Enhanced Raman Scattering

Abstract

The electromagnetic enhancement mechanism is a major contributor to surface-enhanced Raman scattering enhancements, which are a direct consequence of the roughness present on noble metal surfaces. The electromagnetic enhancement mechanism is a twofold phenomenon that involves the enhancement of both the incident excitation and scattered Raman fields. In this paper, we report a direct observation of the double-enhancement mechanism using a Ag nanorod array/SiO₂ dielectric layer/Ag mirror multilayer thin-film “local plasmon resonator”. The effect of light interference was controlled by adjusting the film thickness of the SiO₂ phase control layer (PCL), and the absorption rate in the Raman scattering wavelength range was tuned from 90 to 0%. In addition to the characteristic Raman peak of an aqueous solution of 4, 4-bipyridine, the background of the Raman scattering spectrum was also enhanced. By examining the relationship between the background Raman emission and the absorption spectrum, we demonstrated that the intensity of the background emission is closely related to the surface-enhanced Raman scattering (SERS) enhancement. We further illustrated that the Raman scattered field and background were enhanced when the absorption was high in the wavelength range of the scattering field. The present results not only suggest that the PCL layer may increase the intensity of plasmon-mediated broadband emission but also provide us with sufficient evidence for a twofold SERS enhancement mechanism.