Electric-Field-Gradient-Selected Quadrupolar Raman Scattering in a Hybrid Plasmonic Nanocavity

In surface-enhanced Raman spectroscopy (SERS), accurate identification of molecular multipolar (e.g., electric quadrupole) Raman vibrational modes is crucial for elucidating the physical mechanisms underlying SERS enhancement beyond the electric dipole approximation as well as for detailed molecular structure analysis. This study developed a plasmonic nanosphere–nanohole hybrid nanocavity by precisely integrating a metal nanohole with a Au nanosphere, which synergistically generates a high-intensity localized electric field with a pronounced electric field gradient. Leveraging the nanocavity’s tunable plasmonic resonance and strong local field enhancement enabled single-molecule SERS detection (10–15 mol/L) under vertical excitation of a linearly polarized beam (LPB). Importantly, the intrinsic electric field gradient was exploited to efficiently and selectively excite the symmetry-forbidden quadrupolar Raman mode of the 4-thiobenzonitrile (TBN) molecules. Through power-dependent multipolar Raman spectroscopy combined with density functional theory (DFT) calculations, a quantitative nanocavity–activity relationship was established between the molecular orientation and Raman scattering intensity. This work provides a paradigm for nanoscale manipulation of plasmonic field–molecule interactions, paving the way for ultrasensitive molecular sensing technologies and further exploration of molecular excited-state dynamics and novel light–matter interaction mechanisms.