Strong electric-field gradient in a nested plasmonic nanocavity for multipolar Raman spectroscopy

Plasmonic nanocavities, known for their ability to enhance light-matter interactions, are further advanced in this study through the design of a nested plasmonic nanocavity. By replacing the conventional flat metal substrate with a silver (Ag) coated nanosphere array, the nested plasmonic nanocavity achieves significant enhancements in both electric-field intensity and its spatial gradient effect under excitation of a linearly polarized beam. This plasmonic nanocavity breaks the dipole selection rule, enabling the visualization of forbidden quadrupole-Raman scattering modes in the 4-thiobenzonitrile molecules. Additionally, the strong localized electric field induces a measurable Stark effect in the dipole-Raman spectra. The nested plasmonic nanocavity thus serves as a powerful platform for probing weak molecular processes, leveraging its dual enhancement of electric-field and gradient effects to unlock new spectroscopic capabilities.