Light manipulation by plasmonic nanostructures

Abstract

This thesis studies various effects based on the excitation of surfaces plasmons in various plasmonic nanostructures. We start the thesis with a general introduction of the field of plasmonics in Chapter 1. In this chapter we discuss both propagating surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs), how they are related to each other through the Bohr condition, the features of subwavelength confinement and near-field enhancement, and wave guidance through coupled LSPs. Then after the discussion of the achievements and challenges in this field (Section 1.3) we will outline the basic structure of the thesis at the end of this chapter (Section 1.4). In Chapter 2 we demonstrate a new mechanism to achieve complete spectral gap without periodicity along propagation direction based on the coupling of backward and forward modes supported by plasmonic nanostructures. We study the backward modes in single cylindrical plasmonic structures (Section 2.2) and focus on the two simplest cases: nanowires and nanocavities. Afterwards, we demonstrate how to achieve spectral gaps in coupled plasmonic nanocavities (Section 2.3). A polarization-dependent spectral gap is achieved firstly in two coupled nanocavities which support forward and backward modes respectively (Section 2.3.1). At the end we demonstrate a complete spectral gap, which is induced by the symmetry of a four-coupled-nanocavity system (Section 2.3.2). In Chapter 3 we study beam shaping in plasmonic potentials. Based on the similarity between Schrodinger equation for matter waves and paraxial wave equation for photons, we introduce the concept of plasmonic potentials and demonstrate how to obtain different kinds of potentials for SPPs in various modulated metal-dielectric-metal (MDM) structures. We investigate firstly the parabolic potentials in quadratically modulated MDM and the beam manipulations in such potentials, including polychromatic nanofocusing in full parabolic potentials (Section 3.2.1), plasmonic analogue of quantum paddle balls in half parabolic potentials (Section 3.2.2), and adiabatic nanofocusing in tapered parabolic potentials (Section 3.2.3). In the following section (Section 3.3) we show the existence of linear plasmonic potentials in wedged MDM and efficient steering of the Airy beams in such potentials (Section 3.3.2) after a brief introduction on Airy beams in free space (Section 3.3.1). In Chapter 4 we study scattering engineering by magneto-dielectric core-shell nanostructures. The introduction part (Section 4.1) gives a brief overview on the scattering of solely electric dipole (ED) or magnetic dipole (MD), and how the coexistence and interference of the ED and the MD can bring extra flexibility for scattering shaping. Afterwards, we discuss the scattering shaping by core-shell nanostructures through the interferences of electric and artificial magnetic dipoles (Section 4.2), including two examples of broadband unidirectional scattering by core-shell nanospheres (Section 4.2.1) and efficient scattering pattern shaping of core-shell nanowires (Section 4.2.2). At the end of this chapter we demonstrate polarization independent Fano resonances in arrays of core-shell nanospheres (Section 4.3.2). At the end of this thesis (Chapter 5) we summarize the results and draw the conclusions. We also discuss the challenges and possible future developments of the field of plasmonics.