Chiral Plasmonic Fiber Tip-Enhanced Raman Nanospectroscopy

Tip-enhanced Raman spectroscopy (TERS) suffers from a trade-off between excitation efficiency and background interference, limiting the sensitivity and obscuring higher-order Raman transitions. To overcome this challenge, we introduce a chiral plasmonic fiber tip (CPFT) fabricated via fused tapering and rotational stretching that is internally excited by the fiber vector fundamental mode. By breaking the structural symmetry of the plasmonic fiber tip, the CPFT enables constructive interference of surface plasmon polaritons at the tip apex, producing a tip hotspot with enhanced electric-field intensity and gradient. This design not only amplifies the electromagnetic field but also suppresses far-field background noise, achieving a signal-to-noise ratio 4-fold higher than linearly polarized beam side excitation. Using the CPFT-based TERS platform, we visualized dark-state Raman modes including electric-quadrupole and magnetic-dipole transitions. This approach offers a strategy for high-contrast nanoscale spectroscopy, paving the way toward highly sensitive, low-noise, next-generation TERS systems.