Longitudinally polarized fields in near-field imaging systems
The evolution path of nanotechnologies moves through the domain of microscopy. Among all the kinds of imaging tools available today, optical imaging systems are of primary importance. When using optical imaging systems, we take profit of the information remotely delivered by light to the detector, after interaction with the sample. Nevertheless, as light moves away from the place of interaction, a part of this information is inevitably lost. With the help of suitable probes, light in close proximity of the illuminated sample can be detected. In this way, an extraordinary richness of information about the sample becomes accessible and we can manage to "see the invisible". Light confined in proximity of the sample is called near-field and near-field microscopy is the branch of microscopy dealing with the detection of such an optical field. In the domain of near-field microscopy, the rigorous electromagnetic and the optical description of light meet together. When images are formed by detecting optical fields close to illuminated samples, the vectorial and electromagnetic nature of light can not be neglected. In this thesis we focused our attention on the special case of electromagnetic fields oriented parallel to the optical axis (longitudinal fields). The subject is particularly intriguing, as longitudinally polarized fields cannot be detected using standard microscope devices. We mainly considered the interaction of longitudinal fields with microfabricated, fully metal-coated quartz nano-probes, and managed to give a description of their role in the near-field imaging process. The work will be presented as follows. In the first chapter, an introduction to Scanning Near-field Optical Microscopy (SNOM) is given. Starting from the concept of super-resolution in paraxial imaging systems, we introduce the concept of near-field microscopy. For two of the best known SNOM techniques we demonstrate that a resolution beyond the capabilities of classical microscopes can be, in principle, achieved. Moreover, we present a rigorous electromagnetic description of a simplified SNOM system, and stress the necessity of adequate tools for the understanding and the interpretation of near-field images. The second chapter is devoted to the description of the SNOM system with which the main part of the experimental work has been performed. The near-field microscope is particularly interesting since it employs an innovative kind of probes: microfabricated, metal-coated quartz probes with no aperture at their apex. The presence of a full metal coating has strong effects in the near-field imaging characteristics of the probes. Results form standard imaging tests are also presented in this chapter. We demonstrate that fully-metalized probes can actually couple evanescent fields and are capable to attain super-resolution. Microfabricated probes can be used either as light sources or as light collectors. In the third chapter we study their optical response when 3D-oriented fields are collected. We find a non trivial selective collection behavior for longitudinally and transversely polarized fields. Moreover, the lateral resolution of our system shows a clear dependence on the polarization state of the collected field. A phenomenological explanation of this effect, involving a polarization-dependent coupling mechanism of light into the probes, is proposed. In the fourth chapter we study the behavior of fully-coated transparent probes as light emitters. With the help of a three-dimensional numerical model, we characterize the guiding properties of the probes and analyzed the near-field and far-field emitted patterns. A set of experimental measurements confirms the results expected by simulations and validates our phenomenological model describing the interaction of light with the probes. In addition, the essential role of longitudinal fields in an illumination-mode near-field scanning system is underlined. We managed to directly measure the longitudinally-polarized, ultra-small hot spot produced at the metallic apex, after injection of a radially polarized mode into the probe. An alternative way of producing longitudinally polarized light source for near-field imaging is proposed in the fifth chapter. With a set of theoretical calculations, we demonstrate that Surface Plasmon Polaritons (SPP) excited on structured metallic thin films can be exploited as non-radiating, strongly localized light sources. The study presented in this chapter is purely computational: no practical implementation of the proposed device has been performed yet. A conclusive summary of this work is presented in the sixth chapter.