Raman scattering images and spectra of gold ring arrays

We analyze the optical properties of mu m-sized rings of gold nanoparticles by the combination of extinction spectroscopy, surface enhanced Raman scattering (SERS) spectroscopy and microscopic dark field imaging in two variants. The imaging process relies on either SERS from methylene blue dye molecules adsorbed on the nanoparticles or elastically scattered light. Whereas elastically scattered light images are governed by the coherence and intensity of the light scattered from the particles, SERS images reflect the optical near field of the particles averaged incoherently over a surface area corresponding to the point spread function of the microscope. From the analysis of the extinction spectra, scattered light and SERS images, we find that near field interaction of the single gold nanoparticles in the rings plays a minor role. Both scattered light and SERS images are well reproduced by simple model calculations. Due to the different signal generation and coherence properties, the combination of both imaging methods is a useful means in the characterization of optical properties of nanostructured metal surfaces.