Gold nanoantennas as probes for near-field optical microscopy

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The main purpose of this thesis is to explore a new concept of SNOM probes. In order to produce more reliable and reproducible tips, we propose to graft a single gold particle of 60 nm diameter onto the aperture of a SNOM probe. The expected advantage of such a design is a better control of the light distribution at the probe aperture, a higher light transmission by the probe and an increase of the achievable resolution in the near-field. Indeed, the single and spherical metal particle below the tip apex is responsible for a redistribution of the electromagnetic energy below the tip, the particle acting as a nano-antenna that scatters the exciting field. The present work focuses on optimizing the parameters, through numerical simulations and practical realizations. From a theoretical point of view, simulations using Maxwell's formalism have been carried out in the field of localized and unlocalized surface plasmons, suggesting the a nanoparticle grafted at the SNOM tip aperture should lead to a field enhancement. Simple models are exploited to understand the influence of the particle's dielectric profile, size and shape on the scattering properties, with the aim of exploiting resonant plasmon effects to increase the optical performance of near field probes. The results are compared to other simulations based on the Finite Dimension Time Domain method (FDTD). From an experimental point of view, a new SNOM head based on low coherence interferometry using an optical fiber is proposed to measure extremely small SNOM tips oscillation amplitudes, both in air and aqueous environments. It allows characterizing the SNOM-tip oscillation modes and amplitudes on the one hand, and, on the other hand, performing topographical measurements with a high precision both in dry and aqueous environments with typical SNOM tip vibration amplitudes of 50-100 pm. Furthermore, two methods are proposed to graft one single gold particle at the aperture of a SNOM tip. First, a chemical process based on the use of thiosilane molecules to attach one single gold particle on the silica aperture of a tip is presented. Second, the electron beam induced deposition allows depositing gold nanostructures embedded in a carbon matrix at the SNOM tip end. Finally, light scattering by individual nanometer-sized gold particles attached at the apex of fiber-based probes for near field optical microscopy is experimentally investigated. This grafted single nanoparticle at the tip apex produces an enhancement of the light throughput, both in the near and the far field, a homogenization of the diffracted light polarization and a higher accuracy of the optical contrast. The dependence of the light scattering by one single gold nanoparticle on the wavelength, the shape and the surrounding medium dielectric profile is theoretically described and experimentally investigated in the presented work, demonstrating that the plasmon resonances play a crucial role in the light scattering process in the context of scanning near-field optical microscopy.