In the last decades, plasmonic resonant nanoantennas have created interest in a wide range of research fields that deal with light confinement on the nanoscale. One promising new research branch involves electrically switchable optical properties, which are scaled down to sub-μm size using plasmonic structures. In this thesis, samples with antenna structures whose resonances can be electrically modulated were designed, fabricated and characterised both electrically and optically. A comprehensive analytical study on the interaction of carrier modulation and optical antennas showed that shifts of the resonance wavelength depend on the antenna aspect ratio and material, and are enhanced if the surrounding medium’s permittivity is near zero. The simulation capabilities of the properties of transparent conductive oxides were successfully utilised to design an ultrathin optical solar reflector that selectively radiates visible and nearinfrared light while strongly absorbing mid-infrared light. The measured solar absorptance was 0.12 and the IR emissivity 0.79. Such selective reflectors can replace currently-used metallised quartz tiles to reduce launch costs of spacecraft. Combining electrical and optical simulation models with nanoscale resolution, a novel modulator structure was designed. By directly electrically addressing nanoantennas, a modulator was enabled to perform in transmission additionally to reflection. Reducing the ITO volume to a gap-filling removed negative impacts of the ITO background, so that the resulting modulator could freely shift the resonance of the antenna. The final structure showed a greatly enhanced amplitude modulation of 45% and a resonance shift of 38nm at 1550nm with an applied electric field of 1Vnm⁻¹. Fabricated structures showed that the placing of an ITO gap-loading can be achieved by taking into account height alignment errors of current e-beam systems. Experiments on a planar electrical modulator with a TiN-HfO2-ITO stack showed first electro-optical modulation results, which can benefit from the design developed with the simulation model. The promising results obtained in this thesis open a new pathway for electro-plasmonic modulation in a variety of structures such as tunable reflectors and transmitters in free space or on silicon waveguides.