Plasmonic and topological insulator nanostructures and metamaterials nanoscale near-field investigations: Experiment and theory

Multiple plasmon dipolar modes and plasmonic hotspots of infrared antenna structures and their nanoscale interactions are studied in the near-field using a scattering type scanning near-field microscope (s-SNOM) and theoretical finite domain time difference (FDTD) calculations. The enhanced evanescent fields are imaged as a function of wavelength, polarization, gap size, and tip material. It is shown that in antenna-field imaging using scattering probes, the probe tip can be considered as a load in the gap of the antenna, and the antenna response to tips can be directly understood by using nanocircuit theory. Real-space evolution of the change in dipolar resonance of a "stationary" antenna structure due to closely approaching multiple dipolar particles is investigated. Structural dimensions of triangular plasmonic antennas which possess asymmetrically distributed plasmonic dipolar modes can be tuned for large field enhancement at different wavelengths and polarizations. It is shown that cross antenna structures highly enhance Ez fields relative to ordinary bowties due to strong interparticle coupling by redistributing the fields, putting more amplitude into the Ez components near their gaps compared to bowties. Further it is shown that polarization specific excitation and detection of plasmon field distributions reveal that cross bowtie nanoantenna (crossed-bowtie) structures could be used as active infrared polarization selectors.;Bi2Se3 thin films are imaged in the near-field using spectroscopic scattering type near-field optical microscopy (s-SNOM) at mid infrared laser wavelength region (9-11microm). Single phases Bi 2Se3 thin film structures were prepared by mechanical exfoliation on silicon wafers. We report size and wavelength dependent near-field interaction contrasts in both optical amplitude and phase. We show that near-field optical imaging allows material specific identification and characterization of Bi 2Se3 exfoliated samples including the confirmation of residual tape presence or removal in stacked films. We describe an alternative "shear exfoliation" sample preparation method which reliably deposits Bi2Se 3 without the possibility of adhesive contaminants.