| Resumo : |
This doctorate thesis describes a theoretical analysis of optical and dispersion forces in silicon nano-optomechanical devices, which are of great relevance for attaining the scientific understanding of the physical concepts behind them, for achieving the appropriate design and the technological approaches of practical and efficient devices, as well as for devising the full range of novel functionalities and their useful applications. By means of classical electrodynamic theory, this thesis demonstrates, for the fisrt time, the physical equivalence between the stress tensor formalism and the energy-based methods, both employed to compute the optical forces. A common misinterpretation found in the specialized literature during the process of computing the optical gradient forces between photonic devices, by using distinct numerical solvers, is identified and corrected. A derived version of the energy-based methods is applied to propose a novel silicon nano-optomechanical device, based on the concept of a cross-slot waveguide, which has a new mechanical degree of freedom and is controlled by using solely the optical source properties. In another original contribution of this thesis, the Minkowski stress tensor formalism is applied to generalize the optical forces, in order to take into account arbitrary background media, opening the possibility to use these nano-optomechanical devices also in biologic and optofluidic applications. Furthermore, a new and efficient method of optimizing radiation-pressure induced optical forces inside the dielectric waveguides is theoretically proposed and numerically demonstrated. Preliminary results show that the proposed method is a valuable tool that can be applied to optimize Stimulated Brillouin Scattering (SBS) effects during the design of new photonic-phononic devices. For the first time, it is shown that this optimal geometry is physically related to the minimization of longitudinal momentum. On the other hand, this thesis also reports that, due the nanoscale dimension of these optomechanical devices, the dispersion forces (composed by the Casimir and van der Waals forces) play an important role on the devices' functionalities and, therefore, must be taken into account during their design, fabrication and operation. The rigorous and complete analysis realized on this thesis significantly modify the current understanding on the subject. Besides that, it is also highlighted that the nano-optomechanical devices can be a feasible way to measure the Casimir force optically. |