Novel manufacturing methods for optical antennas

The resonant excitation of free electrons in metallic nanostructures enables extreme near field intensities along with a deep sub-wavelength localization of the electromagnetic energy. This has been exploited to enhance light-matter interaction down to the single molecule level, to localize heat, and tailor radiation into the far field, all applications of optical antennas. Importantly, the rise and advances in the field of plasmonics over the last ten years have been tightly bound to the development of nanofabrication techniques. The present thesis shows how different nanofabrication approaches can be combined and exploited to produce optical antennas with single nanometer interparticle distances, full geometric and material control and increased fabrication throughput and reliability. Different nanofabrication techniques are first developed for the manufacturing of aperture-type antennas that target applications in fluorescence enhancement for the study of biological systems. The fabrication of high-resolution (~20 nm) bowtie nanoapertures via reactive ion etching through stencils is demonstrated to be an efficient approach to upscale the lithographic fabrication of a master, the stencil, to produce a number of functional samples in an aluminum thin film. Fluorescence measurements are performed on living cells, a proof of concept of the system for the sub-wavelength illumination of dynamic lipid membranes. This initial work preludes to the investigation of the dimer in a box geometry that provides further near field enhancement and confinement in order to reach measurement volumes down to the few tens of zeptolitres and over three orders of magnitude fluorescent enhancement. A second part of the work is dedicated to the investigation and exploitation of colloidal nanoparticle assembly. Although these elements possess unique intrinsic properties, their deterministic positioning is crucial to harness their full potential. An initial study enables understanding and exploitation of the various mechanisms underlying the capillary assembly of gold nanorods onto topographic templates. Novel topographic trap geometries are realized, including three dimensional barriers and funnels to reach unity yield, nanometric positioning and 1° orientation accuracy. Further on, complex nanorod trimers of dolmen geometry are assembled and characterized, revealing the unique potential of nanoparticle assembly in creating plasmonic structures with few-nm gaps. ¿ Finally, two projects exploit the developed nanofabrication capabilities to investigate material-based opportunities for nanoantennas. First, heterostructures composed of two coupled elements made of different material, respectively gold-silver and gold-aluminum, are investigated. High-resolution electron beam lithography and sub-10 nm layer-to-layer alignment is used to produce dimers with fully controlled geometry and interparticle gap in order to reveal the underlying mechanisms of detuned plasmonic pairs. Finally, the very last section offers an outlook beyond metallic nanostructures relying on high index dielectric, silicon nanodiscs. These silicon elements rely on Mie resonances rather than their plasmonic counterpart in metals. The gradual contribution of both strong electric and magnetic dipoles in silicon structures is compared to the properties of metal discs and used to produce vivid color palettes visible under bright field microscopy and naked eye.