Imaging nanometre-sized hot spots on smooth au films with high-resolution tip-enhanced luminescence and Raman near-field optical microscopy.

A novel near-field optical microscope based on a parabolic mirror is used for recording high-resolution tip-enhanced photoluminescence (PL) and Raman images with unprecedented sensitivity and contrast. The measurements reveal small islands on the Au surface with dimensions of only a few nanometres with locally enhanced Au PL. These islands appear as nanometre-sized hot spots in tip-enhanced Raman microscopy when benzotriazole molecules adsorbed on the Au surface serve as local sensors for the optical field. The spectra show that localized plasmons are the cause of both the locally enhanced Au PL and enhanced Raman scattering. This finding suggests that the dispersive background in the surface-enhanced Raman spectra can be explained simply by the enhanced Au PL in the gap. Furthermore, our results show that the surface flatness must be better than 1 nm, to provide an optically homogeneous substrate for near-field enhanced PL and Raman spectroscopy.