Low and high frequency electromagnetic forces: numerical modelling and practical applications

The understanding and manipulation of nanoscale analytes is a central theme in nanotechnology and has been one of the main driving forces behind the development of this field as we know it today. Furthermore, such a technology has the potential to also revolutionize a variety of other disciplines such as quantum science and technology, manufacturing, point-of-care devices and personalized medicine, and is therefore of the utmost importance for the technological development of our society.In this framework, this thesis focuses on a specific kind of interactions -- those between electric fields and particles of matter -- as one of the main phenomena that enable the generation of suitable forces on nanoscale analytes. Surprisingly, while these electromagnetic forces rely on oscillating electric fields, no treatment of both low and high frequency forces has been developed yet. In this context, the beginning of this thesis provides a unified framework to understand the generation of forces created by both low and high frequency electric fields, with a particular emphasis on those forces described within the dipolar approximation. This is the first treatment of such kind and arguably represents the most important theoretical contribution of this thesis. The analysis is subsequently extended to the inclusion of higher multipolar orders into optical forces calculations: in particular, I review three different approaches for the calculation of the electromagnetic multipoles and use them to compute the scattering cross section and optical force acting on different spherical particles. Notably, I also provide a guide to determine how many multipolar orders need to be considered when performing these calculations, setting clear limits on the validity of the dipolar and quadrupolar approximations.After this theoretical beginning, the central part of the thesis is first dedicated to the numerical characterization of a device, comprised of electrically connected plasmonic structures, able to simultaneously control both low and high frequency electric fields. In particular, I numerically demonstrate the improved manipulation and sensing capabilities of this system, before explaining its rich scattering spectra as arising from the electric current coupling between its different plasmonic components, therefore providing the first demonstration of electrical coupling in planar Fano resonant systems. Next, I explore the practical difficulties related to the cleanroom fabrication of such a complex device, and the unexpected challenge I faced when developing a dedicated fabrication process that would counteract the detrimental effects of surface forces. This was eventually realized by employing an organic silane self-assembled monolayer as an adhesion layer. Finally, I experimentally demonstrate the use of this device to trap and sense nanoparticles, bovine serum albumin and rhodamine, thus extending the applications of tweezing devices to molecules having masses of only a few hundreds of Da and providing a powerful platform to probe matter at the nanoscale.The thesis ends by providing an outlook over future lines of research that can carry on the work presented here. Specifically, the assembly of colloidal particles is discussed as one of the next logical steps to perform after trapping, and first preliminary results are provided on the use of DNA nanotubes on patterned substrates to perform template-assisted assembly of gold nanoparticles.