Mapping optical near-fields in gold nanobars with hollow-pyramid SNOM probes

Scanning Near-Field Optical Microscopy (SNOM) is a powerful optical imaging technique allowing sub-diffraction limited resolution. By positioning a probe in the near-field of a sample, this method allows mapping of localized electromagnetic fields which are not accessible to far-field techniques. However, the invasive character of such a measurement can lead to a significant influence of the probe, which should be studied carefully for a particular type of probe and setup configuration. In this work, we investigate an Al coated hollow SiO2 pyramid probe mounted on a standard AFM-type cantilever (WITec, Germany) to image even and odd order surface plasmon resonance (SPR) modes in gold nanobars at visible and near-infrared frequencies. Propagating surface plasmon polaritons (SPPs) on extended metal strips have been studied extensively with these probes [1], while SPRs in nanoparticles received far less attention. Our SNOM results are compared with far-field extinction spectroscopy measurements and detailed finite difference time domain (FDTD) simulations of the three dimensional probe-nanobar system. The observed near-field patterns and far-field data are in good agreement with the simulations. However, it was found that the geometry of the hollow pyramid probe introduces a non-trivial correlation between the near-field maps and the SPRs. FDTD simulations are used to elucidate the involved effects. [1] R. Zia, et al., Nature Nanotechnology 2, 426 (2007) [2] K. Imura, et al., J.Chem.Phys, 122, 154701 (2005) [3] J. Dorfmüller, et al., Nano Letters, Vol9, No6, 2372 (2009)status: publishe