We study the near-field optical behavior of Fabry-Pérot resonances in thin metal nanowires, also referred to as quasi one-dimensional plasmonic nanoantennas. From eigenmodes well beyond quadrupolar order we extract both, propagation constant and reflection phase of the guided surface plasmon polariton with superb accuracy. The combined symmetry breaking effects of oblique illumination and retardation allow the excitation of dipole forbidden, even order resonances. All measurements are supported by rigorous simulations of the experimental situation.

We numerically study the spectral response of 'U'-shaped split-ring-resonators at normal incidence with respect to the resonator plane. Based on the evaluation of the near-field patterns of the resonances and their geometry-dependent spectral positions, we obtain a comprehensive and consistent picture of their origin. We conclude that all resonances can be understood as plasmonic resonances of increasing order of the entire structure. In particular, for an electrical field polarized parallel to the gap the so-called LC-resonance corresponds to the fundamental plasmonic mode and, contrary to earlier interpretations, the electrical resonance is a second order plasmon mode of the entire structure. The presence of further higher order modes is discussed.

Recent advances in nanolithography have allowed shifting of the resonance frequency of antennas into the optical and visible wavelength range with potential applications, for example, in single molecule spectroscopy by fluorescence and directionality enhancement of molecules. Despite such great promise, the analytical means to describe the properties of optical antennas is still lacking. As the phase velocity of currents at optical frequencies in metals is much below the speed of light, standard radio frequency (RF) antenna theory does not apply directly. For the fundamental linear wire antenna, we present an analytical description that overcomes this shortage and reveals profound differences between RF and plasmonic antennas. It is fully supported by apertureless scanning near-field optical microscope measurements and finite-difference time-domain simulations. This theory is a starting point for the development of analytical models of more complex antenna structures.

We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.When nanoscopic metallic structures are illuminated at adequate frequencies, the incoming radiation can couple to charge density oscillations and excite so-called localized surface plasmon polaritons (LSPPs). Recently, the nearfield enhancing qualities of LSPPs have been realized to hold promise for a bounty of novel applications in optics and photonics. 1 These applications often rely on the fine details of LSPPs and their interactions with nanostructures. For example, in the fields of ultra sensitive bio(chemo)detectors 2,3 or plasmonic metamaterials, 4-6 direct near-field optical microscopy of LSPPs would be of great benefit. While spectroscopic far-field properties of LSPP resonances are routinely accessible, real-space near-field information is difficult to obtain.To assess LSPPs of real nanostructures, microscopy techniques are required that are capable of spatially resolving the relevant structure sizes. One approach is the subsequent investigation by means of atomic force microscopy (AFM) of chemical or mechanical changes induced in suitable substrates by the excitation of optical eigenmodes. 7,8 A second approach uses electron energy loss or induced cathode-luminescence scanning electron microscopy to map LSPPs with nanometer resolution. 9,10 This technique requires vacuum compatible samples.For many applications in plasmonics, an all-optical detection of LSPPs with ultimate spatial resolution is called for. Characterizing local optical fields under ambient conditions has been achieved with AFM by carrying nanoscopic optical probes to the immediate vicinity of the nanostructures. [11][12][13][14][15][16] However, such an optical probe has to perform contradicting tasks: as an "optical nano-antenna", its reception/emission efficiency improves with larger size, 17 and as a near-field detector, the achievable spatial resolution improves with smaller size, which can also reduce parasitic interference and coupling effects between probe and sample. 14 In this communication, we demonstrate that these competing demands can be concerted by polarization control of the exciting and scattered radiation, even with off-the-shelf AFM tips as optical probes. We are able to clearly map dipolar and quadrupolar LSPPs, both in phase and in amplitude.In apertureless scanning near field optical microscopy (aSNOM), parasitic background signal from bulk scattering in the sample and tip is often suppressed by anharmonic lockin detection techniques, 18,19 and homodyne, heterodyne, or pseudoheterodyne inte...