Multipolar Resonances of Ag Nanoparticle Arrays in Anodic Aluminum Oxide Nanochannels for Enhanced Hot Spot Intensity and Signal-to-Background Ratio in Surface-Enhanced Raman Scattering

Hot spots generated through the electrodynamic coupling of the local plasmons in metallic nanoparticles play an important role in enhancing surface-enhanced Raman spectroscopy (SERS) signal. However, the accompanied increasing penetration field inside the nanoparticles is known to be one of the mechanisms contributing to the continuum background. Here, we systematically explored the multipolar resonances for SERS substrates comprised of Ag-nanoparticle (Ag-NP) arrays grown in anodic aluminum oxide nanochannels and their roles in affecting the signal amplification and the background noise at three typical wavelengths for Raman spectroscopy (532, 633, and 785 nm). By varying the diameters of the Ag-NP arrays, different orders of electric-quadrupole (EQ) modes are excited at each studied wavelength. Compared to the dipolar resonance, the second-order EQ mode was found to exhibit stronger hot spot intensity confined at the exposed side of the nanogaps, which are beneficial to serve as active sites interacting with analyte molecules. More importantly, a significant suppressed SERS background was achieved for these higher-order modes due to their small penetration depth inside the particles. The enhanced signal-to-background ratio for the second EQ mode was validated among the three Raman wavelengths, which provide promising design guideline for SERS-active substrates with higher sensitivity.