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

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\ua0 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

Electromagnetic Energy Distribution in Resonant Quasi Porous Silicon Nanostructures

Geometric resonances in high refractive index dielectric nanoantennas enhance the local density of optical states, increasing the decay rate of electric and magnetic dipolar emitters. Due to low losses, dielectric antennas exhibit less quenching than their plasmonic counterparts. However, enhanced fields associated with these resonances, in contrast to plasmonic ones, are confined to the particle interior, complicating efficient coupling strategies. Previous research has focused on emitters either placed next to dielectric antennas or incorporated into them during fabrication. Making the nanoantenna porous enables access to the internal fields while being flexible during fabrication. Herein, a model porous silicon antenna is analyzed, and the available electromagnetic energy within it is investigated. A porosity of 30% achieves an optimal balance between antenna quality and energy available in the pores. To minimize disturbance to the optical modes, the pores should be no larger than 5% of the size of the antenna. Moreover, the magnetic dipole resonance of these structures is remarkably robust to perturbations and is thus a promising target for applications due to its tolerance to fabrication error. Calculations show that nonradiative decay is important for electric emitters despite relatively low material losses, while magnetic dipole decay rate enhancement is completely dominated by radiative yield