Khan, Adnan Daud (2013) INVESTIGATION OF FANO RESONANCES IN SYMMETRIC AND ASYMMETRIC THREE DIMENSIONAL PLASMONIC NANOSTRUCTURES. [Tesi di dottorato]

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Abstract

Line shaping of localized surface plasmon oscillations in plasmonic nanostructures is a fundamental and application driven research in biomedicine, sensing, energy and optical communication. Such metallic nanostructures with careful arrangement can also support Fano like resonances. These resonances usually crop up from the coupling and interference of a non-radiative mode (dark mode) and a continuum (bright mode) of radiative electromagnetic waves and are distinguished from their Lorentzian counterpart by a distinctive asymmetric line shape and they are typically more sensitive to the geometry of the nanoparticle and changes of the refractive index of the environment. The role of the symmetry breaking in the coupling between the plasmon modes of the individual parts of the layered and neighboring metallic nanoparticles is very important. The reduction of the symmetry of the system relaxes the dipole coupling selection rules resulting in a mixture of dipole and higher order modes. The understanding of this coherent mode coupling will enable the engineering of plasmon line shaping. The focus of this work is to examine theoretically various symmetric and asymmetric plasmonic nanostructures to study the effect of Fano resonances. The proposed nanostructures include spherical, cubical, elliptical, cylindrical and conical geometries with multicomponents as well as nanoparticle pairs (dimer). These nanostructures are designed in such a way that the broad superradiant and narrow subradiant plasmon modes overlap in energy, resulting in a strong Fano resonance in the optical spectrum, which is characterized by a sharp dispersion than the conventional plasmon resonances. Different kinds of new symmetry breaking schemes have been introduced in the layered Fano resonators, which mixes plasmonic modes of distinct angular momenta and provides a set of unique and higher order tunable Fano resonances. The generation of multiple Fano resonances with large modulation depths in the asymmetric Fano resonators are greatly appropriate for multi-wavelength surface-enhanced Raman scattering and plasmon line shaping by modifying the plasmon lines at various spectral locations simultaneously. In addition, the tunable strong Fano like resonance obtained in the conical dimer resonator can be useful for plasmon induced transparency and the local near fields in the “hot spots” are essential for the near field applications. Among all the Fano resonators, the polarization dependent conical Fano resonators have shown striking properties such as high tunability of Fano resonances, as well as the control on resonant electromagnetic field enhancement and scattering direction. Eventually, the optical responses of all the symmetric and asymmetric Fano resonators are well replicated using a mass-spring mechanical analogy and analytical model of Fano line shape. The observations in this thesis may lead to new opportunities to tailor near and far-field optical properties of plasmonic nanoparticles for specific applications, such as high performance surface-enhanced spectroscopy, electromagnetic induced transparency, biosensing, plasmon line shaping, lasing, nonlinear and switching.

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