Experimental Near-Field Characterization of Plasmonic Nanostructures

Abstract

The spatial resolution of conventional optical technologies is fundamentally limited by diffraction due to the wave nature of light. A promising platform to overcome this diffraction limit in nanophotonic applications is the field of plasmonics, which involves so-called surface plasmon polaritons (SPPs). A SPP results from the coupling between an electromagnetic wave and charge density oscillations in a metal. Essentially, SPPs are bound electromagnetic waves, which propagate along an interface between the metal and a dielectric and exhibit an evanescent character in the direction perpendicular to this interface. This characteristic conditions a high confinement and enhancement of the involved electromagnetic fields at the metal-dielectric interface. Shrinking the SPP propagation degree of freedom to one dimension in plasmonic waveguides or localizing the SPPs completely on plasmonic nanostructures can further increase the field confinement and enhancement. These properties qualify plasmonic nanostructures and waveguides for various applications in nanophotonics. While plasmonic waveguides are promising candidates for the realization of nanophotonic circuits, plasmonic nanostructures are successfully applied for biological and chemical sensing or in nanoscale quantum optics. Due to the bound nature of the SPPs, conventional optical microscopy only allows for their indirect observation through leakage radiation from propagating SPPs or scattered radiation from localized SPPs (LSPPs). However, there are near-field imaging techniques available, which allow for probing the evanescent fields of the SPPs perpendicular to the metal-dielectric interface directly with a resolution far beyond the diffraction limit. <br> In this work, two different near-field imaging techniques, namely electron energy-loss spectroscopy (EELS) and scattering-type scanning near-field optical microscopy (s-SNOM) are applied on plasmonic structures. A brief introduction to the fabrication of plasmonic nanostructures is presented in chapter three including the preparation of solid and nanoporous thin gold films and patterning of the first via focused ion beam milling. In this context, a new approach for the fabrication of freestanding structured thin gold film is presented. Chapter four gives an introduction into the fundamentals and the experimental setups of the abovementioned near-field imaging techniques. In chapters five to seven, these techniques are utilized to investigate different plasmonic sample systems ranging from extended array structures featuring thin-film SPPs over one dimensional plasmonic waveguide structures up to LSPPs occurring on random nanoporous gold structures. As a key feature, EELS allows to investigate the full spectral near-field response of plasmonic samples. Additionally, the highly focused electron beam acts as a white light source for SPPs and is capable of exciting dark plasmonic modes. An exclusive advantage of the s-SNOM technique is the simultaneous detection of the near-field amplitude and phase, which is in particular beneficial for the investigation of propagating SPPs along plasmonic slot waveguides in chapter six. The near-field studies applied in this work, reveal some unique insights into the functionality of the investigated plasmonic sample systems, which can not be achieved by other state of the art experimental techniques.