Surface-Enhanced Raman Spectroscopy Based on Plasmonic Slot Waveguides With Free-Space Oblique Illumination

We report a novel on-chip approach for surface-enhanced Raman spectroscopy (SERS) using a plasmonic slot waveguide. The Raman signal is excited via free-space excitation and is collected by the waveguide. A significant improvement of the signal-to-background-ratio (SBR), as compared to the case of back-scattered Raman spectroscopy with waveguide-mode excitation, is demonstrated using a silicon nitride plasmonic slot waveguide with a 7-mm access waveguide. The flexible adjustment of the incident angle in free space and modification of waveguide geometry enables the further optimization of the pump-to-Stokes conversion efficiency. The combination of high SBR and long access waveguide allows the integration with additional photonics devices (sources, filters, spectrometers) on the same chip.