Surface Plasmon Field-Enhanced Raman Scattering Based on Evanescent Field Excitation of Waveguide-Coupled Surface Plasmon Resonance Configuration

We propose a bimetallic waveguide-coupled surface plasmon resonance (WCSPR) configuration to enhance Raman scattering with an evanescent field excited using surface plasmons. The WCSPR configuration is a five-phase Kretschmann resonance setup composed of a K9 prism/inner Ag film/MgF₂ film/outer Ag film/water structure. Incident angle-dependent SERS spectra were measured in the evanescent field on this WCSPR configuration using an in-house-built angle-dependent SPR-SERS microspectrometer. The SERS signal obtained under evanescent field excitation at the SPR angle was 20× higher than that collected using the conventional SPR configuration. The experimental results also proved that the waveguide-coupled surface plasmons in this evanescent field-enhanced SERS spectroscopy setup had electric field penetration depth of at least 500 nm, which is longer than the penetration depth for conventional surface plasmons. High-quality SERS signals can still be obtained after adding a 500 nm MgF₂ film to the WCSPR configuration due to the deeper penetration depth. This advantage makes it possible to use Ag with a stronger electric field enhancement capability instead of an inert noble metal such as Au in the experiment without worrying about the oxidation of Ag. The enhancement factor of this WCSPR configuration was 6.2 × 10⁷. The lowest detectable concentration for the SERS signal of 4-mercaptopyridine reached 1.0 × 10^–10 M using the WCSPR configuration. The WCSPR configuration has great potential for label-free sensing and detection of macromolecules and biomolecules.