Unlocking anisotropic nanocavities via radial vector beam for visualizing quadrupolar Raman scattering

Conventional surface-enhanced Raman spectroscopy (SERS), which operates predominantly under the electric-dipole selection rule, is inherently limited in probing symmetry-forbidden vibrational modes associated with multipolar Raman scattering, due to insufficient local electic field gradients and the polarization sensitivity of plasmonic nanostructures. Herein, we construct a plasmonic nanocavity consisting of a gold nanorod (AuNR) on a silver (Ag) substrate and employ a tightly focused radial vector beam (RVB) as the far-field excitation source. Theoretical calculations confirm that the axisymmetric polarization of RVB overcomes the polarization dependence of the anisotropic nanocavity and generates continuous hotspots with significantly enhanced electric field intensity and a steeper field gradient compared to linearly polarized beam (LPB) excitation. Experimentally, we successfully activate and detect the quadrupolar Raman scattering from 4-thiobenzonitrile (TBN) molecules within the plasmonic nanocavity, including modes that remain forbidden under LPB excitation. Power-dependent measurements reveal a distinct nonlinear enhancement behavior for the quadrupolar Raman mode and identify an optimal excitation window that balances signal enhancement against rising background noise. Moreover, the observable anti-Stokes Raman spectra under RVB irradiation directly corroborate the substantial local field enhancement within the nanocavity. This work establishes a versatile RVB-driven nanocavity platform for accessing multipolar molecular vibrations, advancing the frontiers of molecular spectroscopy and nanophotonic sensing.