Capretti, Antonio (2013) Linear and Nonlinear Plasmonics. [Tesi di dottorato]

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Abstract

In the present Thesis, the electromagnetic properties of metal nanostructures are theoretically and experimentally investigated, for applications ranging from chemical sensing to integrated optical devices. Collective resonances of the conduction electrons occur on the surface of metal particles with nanoscale sizes, if visible or infrared light interacts with them. These resonances, usually referred to as Localized Surface Plasmons (LSPs), are able to confine the incident light into regions of sub-wavelength dimensions. The electric field in proximity of the metal surface can be orders of magnitude higher than the incident field. This effect, usually known as field enhancement, is traditionally used to increase the cross-section of optical phenomena, for instance the Raman scattering and the harmonic generation, by means of specific planar aggregates of metal nanoparticles, such as arrays and nanolenses. In the first part of this Thesis, novel configurations of plasmonic nanolenses are engineered in order to optimize the field enhancement and the spectral response of the metal nanoparticles, and general design rules are derived. Specifically, metal nanostructures composed by two different interacting metals, such as gold and silver, are studied. These devices, usually called heterostructures, feature very specific spectral properties, derived from the different LSP frequencies of the two metals. Their properties in planar arrangement for sensing applications are studied. Furthermore, an optimization method is applied to the engineering of plasmonic nanolenses. As a result, novel configurations are found that maximize the field enhancement in a selected probing point. The radiative coupling between the metal nanoparticles is found to be under-estimated in comparison with the near-field coupling, that was traditionally studied in the literature. These results are experimentally validated by surface-enhanced Raman scattering measurements. In the second part of the Thesis, the nonlinear optical properties of metal nanoparticles are investigated, for both fundamental and practical purposes. Nonlinear Plasmonics is a promising field for the realization of integrated optical devices, but the origin of the second-order nonlinearities from metal nanostructures is not completely understood. Consequently, the relative contributions of the two main sources of nonlinearities, namely the metal surface and the bulk, are here investigated with the main goal to accurately design novel devices based on plasmonic nonlinear effects. First, the analytical solution of second-harmonic scattering from metal nanospheres is developed, by expanding the fields and the sources in vector spherical wavefunctions. For the first time, both the bulk and surface sources are considered in a full-wave approach. Then, second-harmonic generation measurements are performed on gold colloids, and multipolar contributions are found to be significant for large-size particles. The experimental results are combined with numerical calculations, and the relative contributions of surface and bulk sources are estimated for gold nanoparticles. Eventually, second-harmonic generation experiments are performed on planar arrays of gold nanoparticles, demonstrating the significant contributions of multipolar sources. In particular, their relative magnitude is found to be extremely sensitive to the array geometry and the particle distance. In conclusion, in the present Thesis linear and nonlinear optical processes from plasmonic nanostructures are investigated. General principles for the design of plasmonic nanolenses are found. Moreover, the relative contributions of surface and bulk second-order sources are investigated for gold nanoparticles. Planar nanolenses and arrays are experimentally and theoretically investigated, for both linear and nonlinear photonic applications.

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