Surface-enhanced non-linear Raman scattering at the single-molecule level

Surface-enhanced hyper-Raman scattering and surface-enhanced anti-Stokes Raman scattering were studied as potential tools for non-linear single-molecule Raman spectroscopy. Experiments were performed using near-infrared excitation on crystal violet adsorbed on colloidal silver or gold clusters. Strong enhancement factors on the order of 10 ²⁰ were inferred from hyper-Raman scattering experiments on colloidal silver. Such extremely high enhancement factors overcome the inherently weak nature of the effect, and surface-enhanced hyper-Raman scattering appears on comparable intensity levels as surface-enhanced Raman scattering. Surface-enhanced anti-Stokes Raman scattering starts from vibrational levels, that are populated by the very strong surface-enhanced Raman process. Thus, the anti-Stokes Raman scattering signal depends quadratically on the excitation laser intensity. For the first time, surface-enhanced anti-Stokes and Stokes Raman scattering was detected from single molecules on colloidal gold clusters.