Probe-sample optical interaction: size and wavelength dependence in localized plasmon near-field imaging.

The probe-sample optical interaction in apertureless near-field optical microscopy is studied at 633 nm and 808 nm excitation wavelengths using gold nanodisks as model systems. The near-field distributions of the dipolar and quadrupolar surface plasmon modes have been mapped successfully using metal coated probes with different polarization combinations of excitation and detection except when the incident and the scattered light polarizations are chosen to be parallel to the probe axis. For the parallel polarization of the incident and the scattered light, the pattern of the near-field distribution differs from the inherent plasmon mode structures of the sample, depending sensitively on the sample size and excitation energy. For a given excitation energy, the near-field amplitude shifts from one pole to the other as the sample size increases, having nearly equal amplitude at the two poles when the plasmon resonance peak spectrally overlaps with the excitation energy.