Roles of multipolar resonances and dual plasmonic modes for SERS hot spots

The hexagonal silver-nanoparticles (Ag-NPs) array grown in self-assembled anodic aluminum oxide (AAO) nanochannels have been successfully demonstrated to show reliable enhanced Raman-scattering signal. In this work, we systematically explored the roles of multipolar resonances for Ag-NP/AAO substrates in affecting the signal amplification, the background noise, as well as the signal-to-noise ratio at three typical wavelengths for Raman spectroscopy (532 nm, 633nm, and 785 nm). In addition, a duel-resonant system consisting of Ag-NPs embedded in a sinusoidal-shaped AAO/Al substrate was also investigated. The grating structure enables the excitation of surface plasmon polaritons (SPPs) to further couple with the generated hots spots between adjacent Ag-NPs and meanwhile provides a diffraction dispersion which is beneficial for spectral detection.