Enhancement and subwavelength confinement of light in plasmonic nanostructures

This work includes the fabrication of plasmonic nanostructures as well as the investigation of their optical properties in the near-field and in the farfield. As expressed in the title of this thesis the focus lies in the subwavelength confinement and enhancement of light in such nanostructures. The experimental work is supported by intensive numerical simulations using the Green's tensor method. The spectral properties of these structures are studied in detail, with the objective to optimize their coupling to dipole emitters. The structures investigated in this thesis include isolated subwavelength apertures and periodic arrays of subwavelength apertures in metallic films, plasmonic nanoantennas and radiatively coupled plasmonic particle pairs. The subwavelength apertures and periodic aperture arrays are fabricated using focused ion beam milling. The fabrication of plasmonic dipole and bowtie nanoantennas with nanoscale dimensions is more sophisticated and represents a significant part of this thesis. To guarantee a reproducible fabrication method for nanoantennas – and especially their tiny gap – a process with a resolution better than 20 nm is necessary. This resolution is achieved with electron-beam writing and a carefully developed lift-off process. This way, large fields of antennas with different geometries (length, gap) can be fabricated in gold and silver. For the measurements, a phase sensitive near-field optical microscope has been devised to measure the full complex near-field of the fabricated plasmonic nanostructures. Furthermore two spectroscopic experiments have been developed: A simple transmission setup using a commercial inverse microscope for extinction measurements and a more sophisticated homebuilt dark-field microscopic setup to measure the scattering spectra of single plasmonic nanostructures. The near-field in and around the fabricated apertures is investigated experimentally. The comparison of the measured amplitude and phase patterns with the simulations reveals which field components are effectively measured by the experimental setup. These measurements provide a detailed characterization of the imaging mechanisms in near-field microscopy. It is further shown that the near-field distributions of periodic arrays of subwavelength apertures contain information about the surface modes excited in these arrays. The near-field distribution and the spectral response of plasmonic dipole and bowtie antennas are investigated numerically in great detail as a function of different geometrical parameters. Also the polarization sensitivity of a dipole antenna is discussed and a simple analytical model proposed. Intensity enhancement between two and three orders of magnitude are obtained in dipole antennas, depending on their geometry. The spectral response of these structures can be tuned over the entire visible spectrum, up to the near infrared. The simulation results are compared with spectral extinction measurements on fabricated antenna ensembles in dense arrays and with scattering spectra of single antennas. The qualitative comparison between calculated and measured results is excellent. Furthermore, near-field measurements and confocal measurements are performed on antenna arrays and the results are compared with the polarization sensitivity calculations. The difficulties associated with near-field measurements on nanostructures in terms of radiative probe-sample coupling are discussed by comparing the measured near-field patterns with numerical simulations. Finally, the interaction of nanofabricated antennas with molecules is investigated in the context of fluorescence and Raman scattering. The antenna geometry is optimized to enhance the respective optical transition. Fluorescence correlation spectroscopy indicates a strong interaction between the antenna and the dynamics of the fluorescence, with important variations of the fluorescence life-time. For Raman spectroscopy, it is shown that antennas can be designed to selectively enhance one specific vibrational level. These preliminary results pave the way for the design and controlled fabrication of plasmonic nanostructures to modify optical interactions in molecular systems.


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