Simulations of directionality effects and optical forces in plasmonic nanostructures

Doktorsavhandling, 2011

With the rapid development of nanoscience and nanotechnology, surface plasmonics based on metal nanoparticles and nanostructures gain increasing interest, not only for fundamental scientific studies, but also for optical and sensor applications. At the nanoscale, the physical and chemical properties of metal particles, especially their optical properties, strongly depend on size and shape, as well as on the surrounding media and structures. By modifying those features, one may design novel functional materials and devices. This thesis deals with investigations of light-induced effects at the nanoscale, focusing on optically induced forces on plasmonic nanostructures and angular distribution of light due to the particle-substrate interactions. Nanoparticles trapped within an optically induced potential energy well (e.g. using optical tweezers) will interact with each other by mutual forces. This can lead to the self-organization of the nanoparticles within the trap, an effect known as optical binding. In this thesis I have theoretically investigated the optical forces between the metal nanoparticles for different polarization configurations. In addition to the optical forces, there are other contributions to the total interaction forces between the particles, such as the Coulomb repulsion and van der Waals attraction. These contributions have been investigated in the case of two strongly interacting gold particles through the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). The thesis also contains a study of optical manipulation (trapping, rotation and spinning) of elongated plasmonic particles, such as rods, dimers and micrometer long nanowires. The other main theme of the thesis is the angular distribution of light scattered from the nanoparticles and the nanowires. The scattering properties of these structures are strongly affected by the presence of the substrate. It is shown that the dipolar angular distribution of light scattered from a single dipolar emitter in a homogeneous medium is highly modified when the emitter is brought close to a glass substrate (and even more so when the emitter is brought close to a thin metal film). In addition to nanoparticles, the thesis deals with nanowires, structures that support plasmonic Fabry-Perot resonances due to multiple reflection of the plasmons propagating along their interfaces. It is shown that nanowires can scatter light in a rather narrow angular distribution thanks to the phase retardation of the plasmon propagating along the wire, which is dictated by the wire diameter, length and surrounding medium.