Far-Field and Near-Field Investigation of Longitudinal Plasmons of AgAuAg Nanorods

We studied the localized surface plasmons of AgAuAg-nanorods (NRs) using far-field dark-field spectro-microscopy and scattering-type scanning near-field optical microscopy (sSNOM) techniques. We observe that the far-field scattering spectra of individual AgAuAg-NRs exhibit longitudinal dipolar and octupolar resonances that are mainly determined by the overall lengths of the NRs and are fairly insensitive to the relative compositions of Ag and Au. Corresponding near-field distributions measured by sSNOM further reveal plasmonic local field patterns that closely resemble those of monolithic Au-NRs. These show that the longitudinal plasmons of AgAuAg-NRs oscillate along the entire length of the NRs, although resonance widths indicate appreciable damping by the Ag-Au interfaces. The result constitutes an interesting counter-example of tunability of plasmons by composition variation and, thus, provides an important design rule for composition-tunable bimetallic plasmonic structures.