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Here, using low-temperature optical scanning tunneling microscopy (STM), weinvestigate inelastic light scattering (ILS) in the vicinity of a single-atomquantum point contact (QPC). A vibration mode localized at the single Ag adatomon the Ag(111) surface is resolved in the ILS spectrum, resulting fromtip-enhanced Raman scattering (TERS) by the atomically-confined plasmonic fieldin the STM junction. Furthermore, we trace how TERS from the single adatomevolves as a function of the gap distance. The exceptional stability of thelow-temperature STM allows to examine distinctly different electron transportregimes of the picocavity, namely in the tunneling and quantum point contact(QPC) regimes. This measurement shows that the vibration mode localized at theadatom and its TERS intensity exhibits a sharp change upon the QPC formation,indicating that the atomic-level structure has a crucial impact on theplasmonic properties. To gain microscopic insights into picocavityoptomechanics, we scrutinize the structure and plasmonic field in the STMjunction using time-dependent density functional theory. The simulations revealthat atomic-scale structural relaxation at the single-atom QPC results in adiscrete change of the plasmonic field strength, volume, and distribution aswell as the vibration mode localized at the single atom. These findings give aqualitative explanation for the experimental observations. Furthermore, wedemonstrate that strong ILS is a characteristic feature of QPC by continuouslyforming, breaking, and reforming the atomic contact, and how the plasmonicresonance evolves throughout the non-tunneling, tunneling, and QPC regimes.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity