Multiple‐Resonance‐Enhanced Raman Spectroscopy in 2D‐Material‐Capped Tapered‐Fiber Microcavity

Abstract Plasmon‐free surface‐enhanced Raman spectroscopy (SERS) assisted by charge‐transfer (CT) resonance in semiconductors has drawn considerable attention in the past decades due to the extraordinary advantages in chemical stability and homogeneity. Unfortunately, the weak confinement of excitation light in semiconductor nanostructures and 2D materials, due to either their limited refractive indexes or atomic thickness, results in the Raman enhancement by nonmetallic SERS substrates significantly lower than the noble ones. Here a novel plasmon‐free SERS probe is reported by a tapered optical fiber (TOF) coated with monolayer‐MoS₂ (ML‐MoS₂), where the TOF‐supported whispering‐gallery modes (WGMs) simultaneously regulate multiple‐resonance processes, i.e., excitonic resonance, molecular resonance, charge‐transfer resonance and fluorescence resonance energy transfer processes, in the ML‐MoS₂ and analytes. The contribution of the four processes promoted by WGM to enhancement factor of Raman intensity (EFRI) is quantitatively determined by four dye molecules with different energy levels. The Raman enhancement mechanism of WGM‐promoted multiple‐resonances is therefore revealed, for the first time. The maximum EFRI is up to 1.8 × 10⁹ for the limit of detection (LoD) down to 10⁻¹⁴ M. The ML‐MoS₂/TOF SERS probes also demonstrated their outstanding feasibility and stability facilitating to trace detection in microdroplets for practical applications in future.