  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 weak, methods to enhance, direct and harness it are
highly desirable, e.g. through the use of optical cavities, waveguides, and
surface enhanced Raman scattering (SERS). While SERS offers dramatic
enhancements 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 detect. 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 10$^4$ 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
nanofocusing and the Purcell effect. Emulating the $\beta$-factor from laser
physics, the near unity Raman $\beta$-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 photonics with applications in gas and
bio-sensing as well as healthcare.
