In the present Thesis, the electromagnetic properties of metal nanostructures are theoretically and experimentally investigated, for applications ranging from chemical sensing to integrated optical devices. Collective resonances of the conduction electrons occur on the surface of metal particles with nanoscale sizes, if visible or infrared light interacts with them. These resonances, usually referred to as Localized Surface Plasmons (LSPs), are able to confine the incident light into regions of sub-wavelength dimensions. The electric field in...