Metallic and dielectric nanostructures with various plasmonic and dielectric resonances are the fundamental of all nanophotonics research. The design of different geometries for these nanostructures have always been one of the main topics in this field. Based on different theoretical models and phenomena, various geometries have been tested out in the past decades. However, the pursuit of better and faster design method is still one of the major concerns. The first part of the thesis presents the direct-design process of a 3D plasmonic antenna-reactor for strong field enhancement and temperature gradient. It is shown how the understanding of a phenomenon can serve as the guideline of optimizing the geometry. In the second chapter, we further discuss the possibility of utilizing computer algorithm to optimize geometries more complex and less intuitive without a clear physical insight.