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Optical Characterization of Plasmonic Nanostructures: Near-Field Imaging of the Magnetic Field of Light Supervisor's Foreword

Book Contribution - Chapter

The interaction of light with nanoscale objects has been one of the hot topics in the field of photonics in the past few years. More specifically, one of the promising directions is the field of plasmonics, which deals with the interaction of light with metals. Illumination of metallic nanoparticles results in the excitation of so-called surface plasmons - collective oscillations of the free electrons in the metal, driven by the incoming light. At certain frequencies, different resonant modes can be excited, which results in significant enhancement and localization of the light in the close vicinity of the nanostructure - the particle is basically acting as an antenna at optical frequencies. Such local field enhancements (also called hot spots) have shown promising applications in various areas, for example for single molecule detection, bio- and chemical - sensing, all-optical chips, cancer diagnostics and treatment, etc. Further development of those applications requires detailed understanding of the full picture of the light-matter interactions. Since light is an electromagnetic wave, the response of a material to illumination is determined by the interaction of the incoming electric and magnetic fields with the medium. Thus, for the plasmonic structures, depending on the specific application, it is crucial to characterize the resonance wavelengths for the different resonant modes, the charge and current distributions in the particles, as well as the electromagnetic near-field distribution of the light in the vicinity of the structure. Different methods, each with its own scope, advantages and disadvantages, already exist for imaging most of those parameters.Still, one of these parameters, namely the magnetic near-field distribution of photonic structures, has received far less attention from the optics society. The reason is, that at optical frequencies natural materials interact mainly with the electric component of the electromagnetic field of light and negligibly weakly with the magnetic one. Therefore, the magnetic light-matter interactions are first, not of crucial importance for those materials and second, very difficult to measure.However, recently new classes of artificial materials, so-called metamaterials, have been developed, which enhance and exploit these typically weak magnetic light-matter interactions to offer extraordinary optical properties. For example, such materials could refract light in a direction, opposite to the one in all conventional materials. This can allow realization of exotic devices, such as, an invisibility cloak, flat lens with no resolution limit, etc. Therefore, in the last years, a need has appeared for both optical magnetic sources and for detectors of the magnetic field of light and tremendous efforts have been invested in this direction.The main goal of this PhD is to offer new experimental possibilities for near-field imaging of the magnetic field of light with sub-wavelength resolution and to implement the technique for studying different plasmonic nanostructures.The experimental technique which we use is scanning near-field optical microscopy (SNOM). This is a method, based on the scanning of a probe in the near-field of the sample. Depending on the type of probe and measurement configuration, different field components can be accessed with sub-wavelength resolution. Imaging of the different electric field components is nowadays a well-developed procedure. However, it is still an ongoing challenge to experimentally access the magnetic field components, which interact much weakly with materials. Recently, it has been shown that the normal (relative to the sample surface) magnetic field component can be accessed by a split-ring aperture probe. To fill in the last standing gap in the full electromagnetic mapping of the near-field of photonic devices, in our work we focus on imaging of the lateral (tangential) magnetic field component of the light. We demonstrate that the metal-coated SiO2 hollow-pyramid circular aperture probe of a SNOM can be used as a detector for the lateral magnetic field of light. Moreover, we suggest that it might also be used as a tangential magnetic dipole source.After demonstrating the applicability of the aperture hollow-pyramid SNOM to image the lateral magnetic field of light on a basic plasmonic nanobar sample, we apply the technique to study plasmonic antennas with different geometries - bar, ring, disk and more complex geometries based on bar building blocks. Similar nanoantennas have recently attracted a lot of interest for sensor applications and for enhancement of non-linear and magneto-optical effects. However, direct experimental visualization of their magnetic near-fields was, until now, lacking. Therefore, the obtained magnetic near-field maps are essential for understanding and further developing reinforced magnetic lightmatter interactions in plasmonic nanomaterials.
Pages: VII - VIII
Number of pages: 2
Publication year:2016