Multipolar Surface Magnetoplasmon Resonances in Triangular Silver Nanoprisms studied by MCD Spectroscopy

Magneto-optical responses of triangular silver nanoprisms are demonstrated for the first time with magnetic circular dichroism (MCD) spectroscopy. In Ag nanoprisms with the mean aspect ratio ranging from 6.3 to 8.8, optical absorption exhibits a strong main peak at 650–800 nm that is ascribed to the in-plane dipolar localized surface plasmon resonance (LSPR), along with other two peaks at 450–500 nm and ∼330 nm, which can be assigned to the in-plane and out-of-plane quadrupolar LSPR, respectively. A derivative-like MCD signal is observed at the energy of in-plane dipolar LSPR, but the position crossing zero is not in correspondence with the absorption maximum. In contrast, interestingly, the MCD response for in-plane or out-of-plane quadrupolar LSPR shows a simple (single) peak that corresponds to the position of its absorption maximum, suggesting that MCD spectroscopy is very effective for easy identification (or rapid determination) whether the LSPR modes are dipolar or quadrupolar. The agreement between the energies of quadrupolar LSPR maxima of absorption and MCD spectra can be explained within the context of a model on the Lorentz force imparted on the collective electrons that circularly move in the nanoprism under an applied magnetic field. We believe the present results will bring a wide range of impact to the field of active plasmonics and magnetoplasmonics.