Direct measurement of dispersion relation for surface plasmon-polaritons on silver nanoantennas

We investigated the degree to which the dispersion relation of surface plasmon-polaritons excited on silver nanoantennas depends on length. To accomplish this, dispersion measurements for individual silver nanoantennas with lengths from 220 nm to 2.5 μm were performed using angle-resolved electron energy-loss spectroscopy (AREELS) and spatially resolved EELS (SREELS). AREELS enabled measurements of the dispersion relation extending into the high-energy region of 2.6 eV for a 2.5-μm-long silver nanoantenna, without the need to excite Fabry-Perot-type resonances. Our experiments showed that the dispersion relation of silver nanoantennas that have the same diameter is independent of their individual lengths.