"Active control of the optical properties of materials represents one of the most fundamental aspects of photonics. It is crucial to deepen our understanding on light-matter interactions, and for the development of novel technologies. Of special importance is the interaction of light with metal surfaces, leading to surface plasmon polaritons (SPPs). SPPs can give rise to giant local field enhancements in subwavelength volumes. Thanks to recent developments in plasmonics, electromagnetic fields can nowadays be controlled in subwavelength volumes and on ultra-fast time scales. However, research has been limited to optical and near-IR frequencies. The lack of studies at lower frequencies originates from the weak confinement of SPPs at these frequencies. Our objective is to overcome these limitations, demonstrating for the first time ultra-fast control in subwavelength dimensions of plasmonic resonances at THz frequencies. In particular, we plan to control localized surface plasmon polaritons (LSPPs), arising from the coherent oscillation of charges in particles. For this purpose, we will fabricate semiconductor particles supporting LSPPs, which will be controlled optically at very low fluences. We will show that the spatial distribution of giant field enhancements can be tuned with unprecedented accuracy by demonstrating its optical switching by several orders of magnitude. Moreover, we will study for the first time the active coupling of plasmonic resonances in periodic arrays of semiconductor particles. These arrays will constitute the plasmonic analogue to phased array antennas. The project will open new horizons in fundamental research, as well as in applications such as THz modulators and sensors."