Defect-induced activation of symmetry forbidden infrared resonances in individual metallic nanorods

We report on the observation of second order infrared plasmon resonances in lithographically prepared gold nanorods investigated by means of far-field microscopic infrared spectroscopy. In addition to the fundamental antenna-like mode, even and odd higher order resonances are observed under normal incidence of light. The activation of even-order modes under normal incidence is surprising since even orders are dipole-forbidden because of their centro-symmetric charge density oscillation. Performing atomic force microscopy and calculations with the boundary element method, we determine that excitation of even modes is enabled by symmetry breaking by structural deviations of the rods from an ideal, straight shape. a) Authors to whom correspondence should be addressed; electronic mail: [email protected] and [email protected].