Point-Contact Tip-Enhanced Raman Spectroscopy: Picoscale Light-Matter Interactions within Plasmonic Cavities

Near-field Raman spectroscopy has found widespread applications in nanoscience and nanotechnology. Recent advances in tip-enhanced Raman spectroscopy (TERS), performed under ultrahigh vacuum and low-temperature conditions, have achieved atomic-scale Raman imaging by precisely controlling subnanometer plasmonic fields, known as picocavities. This technique also enables the observation of Raman scattering in previously inaccessible regimes, including quantum point contacts formed with a single atom or molecule. In this Mini-Review, we highlight the versatility of point-contact TERS not only for atomic-scale chemical analysis but also for a unique platform to explore light-matter interactions in nonequilibrium quantum systems. In particular, point-contact TERS underscores that Raman scattering in picocavities is highly sensitive to picoscale structural changes. This provides new opportunities for characterizing atomic-scale structures, controlling photoinduced reactions, and advancing single-molecule optoelectronics and optomechanics using picocavities, with the potential for investigating ultrafast phenomena by integration of nonlinear spectroscopy with point-contact TERS.