The Raman scattering of light by molecular vibrations is a powerful technique to fingerprint molecules through their internal bonds and symmetries. Since Raman scattering is weak¹, methods to enhance, direct and harness it are highly desirable, and this has been achieved using optical cavities², waveguides^3,4,5,6 and surface-enhanced Raman scattering (SERS)^7,8,9. Although SERS offers dramatic enhancements^2,6,10,11 by localizing light within vanishingly small hot-spots in metallic nanostructures, these tiny interaction volumes are only sensitive to a few molecules, yielding weak signals¹². Here we show that SERS from 4-aminothiophenol molecules bonded to a plasmonic gap waveguide is directed into a single mode with >99% efficiency. Although sacrificing a confinement dimension, we find a SERS enhancement of ~10³ times across a broad spectral range enabled by the waveguide’s larger sensing volume and non-resonant waveguide mode. Remarkably, this waveguide SERS is bright enough to image Raman transport across the waveguides, highlighting the role of nanofocusing^13,14,15 and the Purcell effect¹⁶. By analogy to the β-factor from laser physics^{10,17,18,19,20}, the near-unity Raman β-factor we observe exposes the SERS technique to alternative routes for controlling Raman scattering. The ability of waveguide SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics^7,8,9 with applications in gas sensing and biosensing.

分子振动引起的拉曼光散射是一种通过内部键和对称性对分子进行指纹识别的强大技术。由于拉曼散射很弱¹，因此非常需要增强、引导和利用它的方法，这已经通过使用光腔²、波导^3,4,5,6和表面增强拉曼散射 (SERS) ^{7,8 来实现， 9} . 尽管 SERS通过将光定位在金属纳米结构中几乎消失的小热点内提供了显着的增强^{2,6,10,11 ，但这些微小的相互作用体积仅对少数分子敏感，产生微弱信号12}. 在这里，我们展示了来自 4-氨基苯硫酚分子键合到等离子体间隙波导的 SERS 被定向到单模，效率 >99%。尽管牺牲了限制尺寸，但我们发现波导的较大感测体积和非共振波导模式使SERS 在宽光谱范围内增强了 ~10 ^3倍。值得注意的是，该波导 SERS 亮度足以对跨波导的拉曼传输进行成像，突出了纳米聚焦^13,14,15和珀塞尔效应¹⁶的作用。通过类比激光物理学中的β因子^{10,17,18,19,20}，接近统一的拉曼β-我们观察到的因素将 SERS 技术暴露于控制拉曼散射的替代路线。波导 SERS 引导拉曼散射的能力与基于集成光子学^7,8,9的拉曼传感器相关，并应用于气体传感和生物传感。