Scanning near-field optical microscopy : from single-tip to dual-tip operation
Scanning Near-field Optical Microscopy (SNOM) is a powerful and widely-used method in nano- and micro-optics. In aperture SNOM, a dielectric tip with an opaque coating featuring a sub-wavelength aperture at the tip apex is used either to locally illuminate the sample under investigation, or to map the sample’s optical near-fields. In contrast to classical microscopy, SNOM provides access to the evanescent fields at the sample surface and is thus able to break the diffraction limit. In this thesis, several new near-field characterization methods were developed: By combining two SNOMs with fiber tips into a Dual-SNOM setup, samples can be near-field illuminated at a freely chosen position by the first tip, while the second tip scans the sample surface and maps the optical near-fields, thus giving access to the near-field Green’s function. The Dual-SNOM was used for polarization-selective mapping of plasmonic aperture emission and for the investigation of plasmonic modes in metal strip waveguides. Furthermore, two single-tip techniques were developed and were applied to map whispering-gallery modes (WGM) in fused silica microdisk resonators. In scanning thermocouple-probe microscopy, the sample surface is scanned by a miniature thermocouple tip, which is heated by optical absorption and thus converts the optical signal into an electrical signal directly at the measurement location. In the second mapping method for WGM, a sharp, fully metal-coated fiber tip is used as a scatterer whose position influences the resonance positions and the coupling between WGMs and their respective counter-propagating, degenerate modes. Consequently, the transmission and reflection that can be measured at the ends of the tapered fiber loop which serves for excitation of the WGMs are also modified by the tip’s presence. By correlating the transmission and reflection signals with the tip’s position, highly resolved images were obtained. As an application example of aperture SNOM, near-field mapping of non-broadening plasmonic Airy beams and of plasmonic hot-spots generated by interference of two such beams is demonstrated.