Direct Near-Field Optical Imaging of Higher Order Plasmonic Resonances

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 near-field 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)detectors2,3 or plasmonic metamaterials,4-6 direct near-field optica