Optimization of resonant all-dielectric nanoparticles for optical manipulation and light management

Sammanfattning: The resonant interaction between light and matter lies at the heart of nanophotonics research. In particular, nanoparticles that possess optical resonances in the visible spectral range have been avidly studied and employed for various technical and biological applications in the last two decades. While the most commonly employed nanoparticles are metallic ones with localized plasmonic resonances, these particles suffer from inevitable optical losses and parasitic photothermal heating. Recently, through the advent of new fabrication techniques, all-dielectric nanoparticles with high refractive index have arisen as a competitive alternative both as colloidal nanoparticles and as building blocks in metasurfaces. These particles present low-loss geometric resonances of electric and magnetic character with Q-factors comparable to plasmonic nanoparticles. Importantly, the various multipolar responses excited in these particles can be engineered to interact and give rise to highly directional scattering or light confinement. This thesis focuses on the design, modelling and optimization of resonant all-dielectric nanoparticles for nanophotonic applications through electrodynamics simulations such as finite-difference time-domain and various analytical or semi-analytical models. It is demonstrated that highly specific design of metasurfaces with silicon nanoantennas can yield close to 100% optical absorption at specific light wavelengths. The effect is a result of complete destructive interference between different multipolar excitations and can be achieved despite the low intrinsic losses of silicon. Further, this effect is exploited to propose a novel solar harvesting device using nanostructured amorphous silicon with theoretically predicted efficiencies that approach state-of-the-art thin film solar cells. Owing to their significant interaction with light and generally low losses, resonant all-dielectric particles are promising candidates for nanoscopic handles in biological systems. This thesis therefore focuses partly on optical forces and manipulation of silicon nanoparticles. The zero-backscattering Kerker  condition is investigated as an avenue to decrease radiation pressure in an optical trap. Moreover, a comparison to more conventional nanoparticle materials for optical tweezers such as gold and polystyrene is made, including photothermal effects. Lastly, the interaction of porous silicon nanoantennas with subwavelength emitters or absorbers is studied and the influence of porosity, pore size, and pore placement is elucidated.