Direct imaging of surface-enhanced Raman scattering in the near field.

By using near-field scanning microscopy/spectroscopy, we show that surface-enhanced Raman scattering (SERS) of rhodamine 6G deposited on self-affine silver colloidal film is localized to small, down to less than 200 nm, portions of the film. The locus of the SERS signals ("hot spots") does not necessarily reside in special topographic elements such as interstices and between nanoparticles. The local SERS enhancement is estimated to be over 3 orders of magnitude higher compared to the far-field measurements. Near-field imaging of SERS directly validates the theory of the optical response of self-affine fractal objects.