This thesis reports advances in terahertz time-domain spectroscopy, relating to the development of new techniques and components that enhance the experimentalist’s control over the terahertz polarisation state produced by photoconductive emitters. It describes how utilising the dynamic magnetoelectric response at THz frequencies, in the form of electromagnons, can probe material properties at a transition between two magnetically ordered phases. Additionally, preliminary investigations into the properties of materials exposed to extreme terahertz optical electric fields are...