Near unity Raman $\beta$-factor of surface enhanced Raman scattering in a waveguide

TL;DR

This paper demonstrates near-unity Raman β-factor in waveguide-enhanced SERS, enabling highly efficient, directed Raman scattering with broad spectral enhancement, opening new avenues for integrated photonic sensors.

Contribution

It introduces a waveguide-based SERS method achieving near-unity β-factor, significantly improving directionality and efficiency of Raman signals compared to traditional SERS.

Findings

>99% efficiency in directing SERS into a single mode

10^4 times SERS enhancement over broad spectral range

Ability to image Raman transport in waveguides

Abstract

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.