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Meso- and nano-structured metal-dielectric interfaces for plasmonic nanofocusing

With increasing demand for nanoscale optical devices, the ability to confine the light on length scales smaller than those allowed by the diffraction limit of light has begun to attract enormous interest. A possible solution to this problem is offered by Plasmonics. 
Under certain conditions, a metal-dielectric interface supports Surface Plasmons (SPs), i.e. electromagnetic excitations strongly coupled to oscillations of free electrons in the metal. Thanks to these excitations, electromagnetic energy can be confined in a sub-wavelength volume close to the metal surface. This opens up the way for a wide range of opportunities and applications, from photovoltaics to biosensing. 
The focus of this thesis was to engineer metal-dielectric interfaces in order to excite plasmonic hotspots, i.e. nanometer-sized regions where the electromagnetic field is strongly enhanced. As hinted above, this can be achieved by conveniently meso- and/or nano-structuring the metal surface. The properties of metal-dielectric interfaces emerging from these studies are of particular interest in the field of molecular sensing, but are also of more fundamental interest. 
Different classes of devices are proposed. We started from digital plasmonic gratings, studying their excitation modes thoroughly. We then moved to nanostructures supporting a Localized Surface Plasmon Resonance (LSPR), specifically plasmonic nanoantennae, which are of special interest for the enhancement of well-established sensing techniques such as SERS. Coming to nanofocusing, we studied plasmonic wedges – which provide adiabatic nanofocusing at their ridge – as well as bull’s eye/Archimedean spiral structures, which can generate and focus Surface Plasmons carrying orbital angular momentum (OAM). Finally, we proposed a non-trivial integration of the above-mentioned effects, in the form of an Archimedean spiral coupled to nanoantenna resonators. 
In parallel, we collaborated with the company A.P.E. Research to develop a new characterization instrument, called EllipsSNOM. It consists in implementing a SNOM head on a J.A. Woollam Co. Inc. Variable Angle Spectroscopic Ellipsometer (VASE). This challenging task was pursued in order to get simultaneous control of the far field and the near field properties of the structures; ellipsometry and SNOM microscopy are indeed both essential techniques to achieve a full characterization of plasmonic nanodevices.