Nonlinear Raman Probe of Single Molecules Attached to Colloidal Silver and Gold Clusters

We review surface-enhanced linear and nonlinear Raman scattering experiments on molecules and single wall carbon nanotubes attached to colloidal silver and gold clusters. Surface-enhanced hyper-Raman scattering and surface-enhanced anti-Stokes Raman scattering from pumped vibrational levels are studied as twophoton excited Raman processes where the scattering signal depends quadratically on the excitation laser intensity. The experimental results are discussed in the framework of strongly enhanced electromagnetic fields predicted for such cluster structures in so-called “hot spots.” The electromagnetic enhancement factors for Stokes, pumped anti-Stokes, and hyper-Raman scattering scale as theoretically predicted, and the field strengths in the hot spots, it is inferred, are enhanced of the order of 10. From our experiments we claim a very small density of hot spots (0.01% of the cluster surface) and lateral confinement of the strong field enhancement within domains that can be as small as 10 nm. Effective cross sections of the order of 10−16 cm and 10−42 cm s for Stokes and pumped anti-Stokes scattering, respectively, are adequate for oneand two-photon Raman spectroscopy of single molecules.