Near unity Raman β-factor of surface enhanced Raman scattering in a waveguide

The Raman scattering of light by molecular vibrations offers a powerful technique to ‘fingerprint’ molecules via their internal bonds and symmetries. Since Raman scattering is weak1, methods to enhance, direct and harness it are highly desirable, e.g. through the use of optical cavities2, waveguides3–6, and surface enhanced Raman scattering (SERS)7–9. While SERS offers dramatic enhancements6,15,22,2 by localizing light within vanishingly small ‘hot-spots’ in metallic nanostructures, these tiny interaction volumes are only sensitive to few molecules, yielding weak signals that are difficult to detect10 . Here, we show that SERS from 4-Aminothiophenol (4-ATP) molecules bonded to a plasmonic gap waveguide is directed into a single mode with > 99% efficiency. Although sacrificing a confinement dimension, we find > 104 times SERS enhancement across a broad spectral range enabled by the waveguide’s larger sensing volume and non-resonant mode. Remarkably, the waveguide-SERS (W-SERS) is bright enough to image Raman transport across the waveguides exposing the roles of nanofocusing11–13 and the Purcell effect14. Emulating the e-factor from laser physics15–17, the near unity Raman -factor observed exposes the SERS technique in a new light and points to alternative routes to controlling Raman scattering. The ability of W-SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics7–9 with applications in gas and bio-sensing as well as healthcare.