Tracking Molecular Shear at Metal Surfaces Using Enhanced Lamb Wave Scattering in Plasmonic Nanocavities

Extreme plasmonic confinement to the nanoscale can be used to probe the configuration of molecules at metallic surfaces. Exploring low-frequency (<i>h</i>ν < <i>k</i>_B<i>T</i>) inelastic light scattering from molecular-monolayer-filled plasmonic nanocavities reveals additional low-frequency excitations not previously observed. We identify these as terahertz Lamb shear modes in the nanogap, exhibiting cross sections even larger than the surface-enhanced Raman scattering (SERS) of the vibrating molecules. Comparing different molecules and metals shows the influence on these Lamb modes of surface binding of the molecular monolayer as well as the strong impact of damping. The large occupation of such modes at room temperature implies their role across many fields, from electrochemistry, molecular electronics, and thermoelectrics to photocatalysis and sensing.