Surface-enhanced Raman imaging of fractal shaped periodic metal nanostructures

Surface-enhanced Raman scattering (SERS) from Rhodamine 6G (R6G) homogenously adsorbed on fractal shaped 170-nm-period square arrays formed by 50-nm-high gold nanoparticles (diameters of 80, 100, or 120 nm are constant within each array), fabricated on a smooth gold film by electron-beam lithography, is characterized using high-resolution Raman microscopy with polarized excitation. Linear reflection spectroscopy verifies that all nanostructures exhibit resonances close to the 532 nm excitation wavelength used for Raman microscopy. The SERS images feature diffraction-limited (∼0.35 μm) bright spots corresponding to local SERS enhancements of up to ∼120 (relative to that from a smooth gold film), which are influenced by array boundaries, particle diameter, excitation polarization, and detected wavelength. We use six main Raman lines of the R6G spectrum for characterization of multiresonant local-field enhancements that are related to constructive interference of surface plasmon polaritons partially reflected inside the array boundaries. © 2009 Optical Society of America