Magnetic Circular Dichroism Responses with High Sensitivity and Enhanced Spectral Resolution in Multipolar Plasmonic Modes of Silver Nanoparticles with Dimensions between 90 and 200 nm

Characteristic features of magnetoplasmonic responses in higher-order multipolar (quadrupolar and octupolar) modes of Ag nanoparticles (from 90 to 200 nm in diameter) are demonstrated for the first time using magnetic circular dichroism (MCD) spectroscopy. In optical extinction spectra, with an increase in the size of the nanoparticles, the red shift of dipolar plasmon peaks and the appearance of higher-order multipolar resonances can reasonably be observed. Aside from the dipolar and quadrupolar modes, the octupolar plasmonic extinction is very weak and almost unresolved. In contrast, strikingly, MCD shows a very sharp and intense peak (or valley) for the octupolar resonance, meaning its unique properties with high sensitivity and enhanced spectral resolution. MCD responses assignable to the quadrupolar mode have a distinct derivative-like shape, which is different from those observed for Ag nanocubes and nanodecahedra in our previous studies. We then discuss this behavior from the viewpoint of size and/or polyhedral shape inhomogeneity.