Surface Plasmon-Coupled Directional Emission Enhanced Raman Scattering Based on Waveguide Coupling Surface Plasmon Resonance Configuration

Abstract

In this paper, we introduce a method called waveguide coupling surface plasmon-coupled directional emission Raman (WCSPCR), which utilizes a bimetallic waveguide-coupled surface plasmon resonance structure (WCSPR) to enhance the directional emission Raman spectrum by utilizing excited surface plasmons in the evanescent field. We further enhance Raman scattering by utilizing localized and propagating surface plasmons. The further enhanced Raman scattering was directionally emitted due to the resonance effect of the propagating surface plasmons in the WCSPR structure. We obtained angle-dependent directional emission Raman scattering spectra using an in-house angle-dependent SPR-SERS microspectrometer. Compared to a traditional SPR structure, the WCSPR structure showed a superior electromagnetic field and a narrower emission angle. This led to an eight-fold increase in the directional emission Raman signal and a narrower full width at half-maximum of the directional emission angle under the WCSPR configuration. This could further improve the directional emission of Raman signals and enhance the collection efficiency of Raman signals. To further enhance the directional emission Raman signal, we incorporated silver nanoparticles (AgNps) into the WCSPR configuration, creating a WCSPR/4-Mpy/AgNps sandwich structure. Under this configuration, localized and propagating surface plasmons are effectively coupled, resulting in a significant enhancement of the directional emission Raman signal for 4-mercaptopyridine. The combination of local and propagating surface plasmons further amplified the AgNp-assisted SERS signal in the WCSPR/4-Mpy/AgNps sandwich structure, the amplified SERS signal was directionally emitted due to the interaction with the propagating surface plasmons, yielding a signal strength 30 times that of the traditional SPR structure. These findings are consistent with electric field simulations. The WCSPCR method shows promise in enhancing SERS signals, simplifying SERS instruments, and may have applications in biochemical testing.