Direct Near-Field Observation of Orientation-Dependent Optical Response of Gold Nanorods

The orientation-dependent optical response of short gold nanorods (length less than 100 nm) has been directly observed in the near-field, mapping the in-plane and out-of-plane vector components selectively using interferometric apertureless near-field scanning optical microscope. For the gold nanorods dispersed randomly on oxide-coated silicon wafer, the optical amplitude and phase contrast that are characteristic of the longitudinal and transverse mode dipolar plasmon resonances have been clearly resolved when the long axes of the nanorods are aligned parallel and perpendicular to the electric field of the laser, respectively. The near-field amplitude ratio of the longitudinal to the transverse plasmon mode is much smaller than the corresponding ratio of the scattering cross section, indicating the more efficient coupling of the longitudinal mode to the far-field than the transverse mode. This near-field amplitude ratio increases with the length-to-width aspect ratio of the nanorods, and electromagnetic simulation suggests a similar trend in the scattering cross section. In addition, by choosing the polarization of the laser light such that either the probe or the sample is preferentially excited, the near-field profiles of the dipolar surface plasmon modes induced by the incident light and by the field localized at the probing tip are identified. In accordance with the reciprocity relations of the tip–sample optical coupling, identical near-field optical amplitude and phase contrast have been obtained when the plasmon modes are excited by the incident field and by the field localized at the tip.