Surface-enhanced vibrational spectroscopy has emerged as a powerful tool to probe the (photo)chemistry and (photo)physics of individual molecules, which may be regarded as the primary focus of the field of nanophotonics. Single molecule (SM) sensitivity relies on the excitation of localized surface plasmons which confine far-field radiation to molecular length scales, ultimately enabling submolecular imaging with chemical selectivity. Over the length scales that enable submolecular resolution, localized optical fields cannot be separated from the charge density oscillations that sustain them. As such, the interplay between plasmons, photons and electrons in plasmonic nano-junctions is of critical importance for the development of imaging modalities capable of circumventing the diffraction limit and, more generally, for gaining broader insight into the fundamental mechanisms of surface-enhanced Raman and tip-enhanced Raman scattering (SERS and TERS).
In what follows, the SM surface-enhanced parameter space is explored through linear and ultrafast optical measurements utilizing nanometer to ångstrom scale plasmonic junctions and non-resonant molecular reporters. SERS measurements carried out on the prototypical gold dimer nano-antenna highlight the interplay between the molecular Raman scattering channel(s) and coherent luminescence of the plasmon itself. Ultrafast studies utilizing 100 femtosecond laser pulses establish the operating principles of the ultrafast analog of surface-enhanced spectroscopy, which are then utilized to demonstrated time and frequency resolved coherent Raman scattering (CRS) in the single molecule limit.
The fundamental mechanism of SERS is explored by replacing the continuous-wave excitation source with a picosecond pulse train, which probes the non-equilibrium population dynamics of SM phonon states. The latter are inferred through the observed inverted Stokes/anti-Stokes ratios and verified through stimulated and coherent Raman scattering (SRS and CARS) measurements carried out on the same system. Direct plasmon-molecule energy transfer is dominant over the optically induced and thermal contributions to the vibrational population.
Finally, SM TERS measurements utilizing a metalloporphyrin functionalized silver tip are carried out in ultrahigh vacuum and ultralow temperatures (5 K). These measurements directly image the atomic lattice of a copper nitride monolayer with 1.5 Å precision. A series of measurements recorded on this system highlight the essential role of photoelectrons in the TERS process.
