Surface Enhanced Raman Scattering and Chemical Interface Damping in Plasmonic Slot Waveguides

To enhance the Raman scattering efficiency of photons by molecules, various techniques relying on either stimulated or surface enhanced Raman scattering (SERS) have been developed. But they are either limited by the poor control of scattered light, narrow bandwidth of resonance frequency, or restricted area of field enhancement. Here we present a unique waveguide approach to achieve broadband enhanced Raman scattering of molecules with precisely controlled propagation direction. We have demonstrated that 99% of the Raman photons can be coupled into the waveguide. As a corallary to this result we also observe strong chemical interface damping by adsorbed molecules.<br/><br/>We report directional broadband Raman scattering of light by monolayer of molecules which are chemically coated onto plasmonic slot waveguide<b>^[</b><b>¹</b><b>^]</b>. The waveguides are decorated with optical antennas that allow light to couple in and out of the waveguide with approximately 30% efficiency. We have been able to spatially resolve the coupling in of the excitation laser and coupling out of Raman scattering. This enables us to precisely investigate how the scattered Raman photons from molecules couple into the waveguide, propagate and couple out via the antennas. We have experimentally determined the fraction of spontaneous Raman scattering coupled into a plasmonic waveguide (beta factor). The near-unity Raman beta factor is due to the largely enhanced spontaneous Raman scattering rate into the waveguide mode. The enhancement mechanism can be understood analogously to fluorescence emission enhanced by the Purcell effect, which is due to increased vacuum fluctuations and increased density of states. While Raman scattering in highly localised metallic hotspots offers high enhancement factors for a few molecules, here, a plasmonic waveguide offers predictable broadband enhancement for many molecules with a greatly improved interaction volume compared to other SERS approaches. The ability of waveguide-SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics with applications in gas and bio-sensing<b>^[</b><b>²</b><b>^]</b>. A side effect of the strong Raman enahncement in these plasmonic waveguide is the chemical interface damping due to the adsorbed molecules, which here are larger than plasmonic propagation losses. We quantify this effect for a range of molecular monolayers and discuss the physical origin of the additional scattering.<br/><br/><b>References</b>:<br/>1. Fu, M. <i>et al.</i> Near-unity Raman β-factor of surface-enhanced Raman scattering in a waveguide. <i>Nat. Nanotechnol.</i> <b>17</b>, 1251–1257 (2022).<br/>2. Khurgin, J. B. Near-unity Raman beta factor underpins high sensitivity. <i>Nat. Nanotechnol.</i> 1–1 (2022) doi:10.1038/s41565-022-01233-x.