Advantage of multi-mode sapphire optical fiber for evanescent-field SERS sensing.

An unclad, multi-mode single crystal sapphire fiber was used as a platform, and immobilized colloidal Ag nanoparticles (NPs) were used as enabler, for evanescent-field fiber-optic sensing via surface-enhanced Raman scattering (SERS) of Rhodamine 6G (R6G) solution. The dependence of the measured Raman intensity on NP coverage density (to a maximum of 120  particles/μm²) as well as the coverage length (to a maximum of 6 cm) was investigated. We demonstrate the utility of SERS-active sapphire fibers for sensitive measurements (10⁻⁸ M R6G). We further reveal, with the aid of theoretical analysis, that multi-mode fiber offers a significant advantage compared to its single-mode counterpart because the former allows two orders of magnitude higher particle coverage density than the latter to maximize SERS benefit, while maintaining the dominance of Raman gain despite the competitive interplay of NP-induced absorption and scattering loss along the interaction path length.